HABIB-UR-REHMANprr.hec.gov.pk/jspui/bitstream/123456789/1110/1/736S.pdfUsing Picrolonic acid and...
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SYNERGIC EXTRACTION OF RARE EARTH
ELEMENTS USING PICROLONIC ACID AND
OTHER NEUTRAL
OXO-DONORS
HABIB-UR-REHMAN
INSTITUTE OF CHEMISTRY
UNIVERSITY OF THE PUNJAB, LAHORE
PAKISTAN
2009
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SYNERGIC EXTRACTION OF RARE
EARTH ELEMENTS USING PICROLONIC
ACID AND OTHER NEUTRAL
OXO-DONORS
HABIB-UR-REHMAN
A THESIS SUBMITTED TO THE UNIVERSITY OF THE PUNJAB
IN FULFILLMENT OF THE REQUIREMENT FOR THE DEGREE OF
DOCTOR OF PHILOSOPHY
IN
CHEMISTRY
INSTITUTE OF CHEMISTRY
UNIVERSITY OF THE PUNJAB, LAHORE PAKISTAN
DECEMBER 2009
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Dedicated to my Parents
&
Family
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CERTIFICATE
This is to certify that the thesis entitled “Synergic Extraction of Rare Earth Elements
Using Picrolonic acid and other neutral oxo-donors” submitted by Mr. Habib-ur-Rehman is
based on the original research done under our direct supervision in fulfillment of the
requirement for the degree of the Doctor of Philosophy in Chemistry. We have personally
gone through all the data / results / materials reported in the manuscript and certify their
correctness / authenticity. We further certify that, the material included in this thesis has not
been used in part or full in a manuscript already submitted or in the process of submission in
partial / complete fulfillment of the award of any other degree from any other institution. We
also certify that the thesis has been prepared under our supervision according to the
prescribed format and we endorse its evaluation for the award of Ph.D. degree through the
official procedures of the university.
(Prof. Dr.Muhammad Jamil Anwar) Research Supervisor Institute of Chemistry University of the Punjab, Lahore Pakistan
(Dr. Akbar Ali.) Deputy Chief Scientist, Research Co-Supervisor PINSTECH, Nilore, Islamabad Pakistan
Dated:________________
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Declaration
Except where specific reference has been made to other sources, the work presented
in this thesis is the original work of the author. It has not been submitted, in whole or in part,
for any other degree.
(HABIB-UR-REHMAN) PINSTECH, Nilore, Islamabad Pakistan
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Abstract
Extensive work is being carried on the extraction of rare earth elements due to their
special chemical, metallurgical, optical, magnetic and nuclear properties and their use in
advanced technologies as well as in nuclear industry. Different chemical processes are being
applied for the extraction of rare earth elements from their ores and their mutual separation
on laboratory scale as well as on commercial basis. However, these processes are facing
problems such as large number of stages due to low separation factor, low efficiency and
waste management. Keeping in view of these problems, in the present research work, a
synergic extraction system comprising of picrolonic acid as an acidic chelating agent and
oxygen based neutral donors, for the extraction / separation of rare earth elements has been
studied.
Synergic extraction of Ce(III), Nd(III), Eu(III), Tb(III), Tm(III) and Lu(III) as
representative of trivalent lanthanides, using picrlonic acid (1-p-nitrophenyl-3-methyl-4-
nitro-5-pyrazolone, HPA, pKa = 2.52) as acidic chelating agent with crown ether such as 18-
crown-6 (18C6), Benzo-15crown-5 (B15C5), 12crown4 (12C4) as neutral oxo-donors in
chloroform from aqueous buffer solution of pH 1-2 having ionic strength 0.1 mol L-1 (K+/H+,
Cl-) has been studied. Radiotracer technique using their appropriate radio-isotopes prepared
in the research reactor of PINSTECH such as Ce141, Nd147, Sm151, Eu152/154, Tb160, Tm170,
Lu177, Hg 203, Fe59 etc., were used for the quantification of metal ions in the aqueous and
organic phases. Quantitative extraction (>98%) of these metal ions was observed only using
HPA and B15C5 synergic mixture at pH 2 within five minutes and the extraction was
increased with the increase in ionic radii of lanthanide ions. Composition of the extracted
species was determined by slope analysis method and found to be Ln(PA)3.nS, where Ln
represent lanthanide ion, PA conjugate base of HPA molecule and S as neutral oxo-donor.
The value of n is 1 and 2. Among the various cations and anions tested for their influence on
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the extraction these lanthanides only Fluoride, oxalate, Cu(II) , Fe(II) and Zn(II) had some
deleterious effect. The proposed synergic system presented clean separation of lanthanide
ions from mono, and various divalent metal ions especially alkali and alkaline earth metal
ions.
The effect of other neutral donors such as trioctylphosphineoxide (TOPO),
triphenylphosphineoxide (TPPO), tributylphosphate (TBP) and triphenylphosphate (TPP) was
also studied on the extraction of Eu(III). Quantitative extraction of Eu(III) was observed with
TOPO, TPPO and TBP from aqueous phase of pH2. Synergic adduct composition was found
to be Eu(PA)3TBP, Eu(PA)3.2TOPO and Eu(PA)3.2TPPO by slope analysis method. On the
basis of the estimated values of the synergic coefficient, and extraction constants (log Kex),
the oxo-donor effect was found in the order of TOPO>TPPO>TBP.
The effect of various diluents such as 1-octanol (ONL), 1-hexanol (HNL), 1-butanol
(nBNL), 2-butanol (2-BNL), n-butylether (BE), dichloroethylether (DCEE), acetylacetone
(ACAC), diisobutylketone (DIBK), cyclohexanone (CHN), benzene, toluene on the
extraction of Eu(III) from aqueous solution of pH 1-2 using HPA as extractant has been
studied. The extraction of Eu(III) using benzene and toluene was found to be negligible, with
1 & 2-butanol it was low (< 50%), where as with the other diluents studied, the extraction
was quantitative at pH 2. On the basis of log Kex, the solvents can be arranged with respect to
their extractability in the order ACAC > DIBK > BE > DCEE > ONL > HNL > CHN.
To find the trend of lanthanide extraction within the series, three solvents CHN, ONL
and DCEE as representative of ketones, alcohols and ethers, respectively, were selected for
the extraction of Ce(III), Tb(III) and Lu(III) using HPA as chelating agent from aqueous
solutions of pH 1-2, quantitative extraction was observed at pH2 and their extraction order
was found to be Ce(III)>Tb(III) >Lu(III).The composition of the extracted adduct was found
to be M(PA)3 in CHN, ONL and M(PA)3.HPA in DCEE by slope analysis method.
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The synergic mixture comprising HPA and B15C5 in benzene and toluene separately
were studied for the extraction of Eu(III) from aqueous solution of pH 1-2 and quantitative
extraction was observed at pH 1 with both the solvents. On the basis of their estimated values
of synergic coefficient and log Kex, benzene was found to be better solvent than toluene. The
composition of the synergic adduct was found to be Eu(PA)3.2B15C5 and proposed to be a
sandwich type complex having one crown ether molecule on either side of the metal chelate
bound to the central metal only through three oxygen atoms.
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Acknowledgement
All praises are for Almighty Allah who blessed me with the wealth of knowledge and
enabled me to complete this task. I feel privileged and pleasure to express my profound and
cordial gratitude to acknowledge the worthy guidance, inspiration and encouragement of my
learned supervisors, Prof. Dr. Muhammad Jamil Anwar, Pro-Voice Chancellor, University
of the Punjab, Lahore and Dr. Akbar Ali, Deputy Chief Scientist, Chemistry Division,
Pakistan Institute of Nuclear Science and Technology (PINSTECH), Islamabad, Pakistan. I
wish to thank Prof. Dr. Saeed Ahmad Nagra, Director, Institute of Chemistry, University of
the Punjab, for his support.
My special thanks are due to Mrs. Wasim Yawar, Head Central Analytical Facility
Division, PINSTECH, for her valuable guidance and constant support during research work. I
am also thankful to Dr. Ishrat Rehana, Head Spectroscopy group, CAFD, PINSTECH, for her
continuous support. I would like to thank Mr. Shafaat Ahmad, Director ACL, former Head
Central Analytical Facility Division, PINSTECH, for his kind permission and sincere
suggestions to carry out this work within and outside the division, without which I would not
have been able to complete this study.
I am thankful to all the Heads of Chemistry Division, PINSTECH, especially Dr.
Muhammad Mufazzal Saeed, Dr. Shujaat Ahmad, Dr. Jamshed Hussain Zaidi and Dr. Riaz
Ahmad, for providing me the opportunity and necessary facilities to carry out this research
work.
I wish to express my gratitude to all those who contributed towards the completion of
this work. I would like to thank my friends Dr. Muhammad Daud, Dr. Shahid Parvez, Dr.
Muhammad Wasim and Dr. Munir Ahmed for their valuable suggestions and technical help.
Special thanks are due to all the staff members of Radiochemical Separation Group,
of Chemistry Division at PINSTECH, especially Mr. Nizakat Hussain and Mr. M. A. Rizvi,
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for their continuous support during this research work. Reactor Operation Group of this
institute deserves special thanks for the provision of irradiation facility during the course of
studies. I am thankful to Mr. Ghulam Murtaza, Scientific Assistant of our Laboratory, for his
help. In the end, I would like to thank Mr. Ishfaq Aziz, Scientific Assistant, who helped me
considerably in the preparation of this dissertation by typing the manuscript.
Lastly, my profound gratitude goes to my wife for her moral support, generosity
and sharing burden of my responsibilities that enabled me to complete this work. My
affections are for my late parents and for my children who gave me the spirit and
determination throughout this period. Finally, I am grateful to Pakistan Atomic Energy
Commission and PINSTECH, for granting me the permission and providing the necessary
facilities to complete this research work.
HABIB-UR-REHMAN
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Symbols and abbreviations 12C4 12-crown-4 15C5 15-crown-5 18C6 18-crown-6 21C7 21-crown-7 24C8 24-crown-8 AAS atomic absorption spectrometry ACAC acetylacetone B15C5 Benzo-15-crown-5 C6H6 Benzene CCl4 Carbon tetra chloride CHCl3 Chloroform CHN cyclohexanone D Distribution Ratio D2EHIBA Di-(2-ethylhexyl) isobutyricamide D2EHPA 2-ethylhexylphosphoric acid DB18C6 dibenzo-18-Crown-6 DBSO dibutylsulfoxide DCEE dichloroethyl ether DCH18C6 dicylohexano-18-crown-6 DCHSO dicyclohexyl-sulfoxide DEHP di(2-ethyl-hexyl) phosphoric acid DHA Dihexylamide DHHA dihexylhexaneamide DHSO dihexyl sulfoxide DOSO Dioctylsulfoxide DPSO Diphenyl sulfoxide E Percent Extraction ETA-AAS Electro-thermal atomization-atomic absorption spectrometer H3C(CH2)5COOH Heptanoic acid HBTFA 4, 4, 4-tri-fluoro-1-phenyl-1, 3-butan-dione HFAA Hexafluoroacetylaceton HOX 8-hydroxy-quinoline HPA Picrolonic acid HPBI 3-methyl-4-benzoyl-5-isoxazolone HPMAP Phenyl-3-methyl-4-acyl-5-pyrazolone HPMPP 1-phenyl-3-methyl-4-pivaloyl-5-pyrazolone HPMTFP 1-Phenyl-3-methyl-4-trifuoroacetyl pyrazolone-5 HTTA Thenoyltrifluoroacetone Ibnl 2-butanol ICP-AES Inductively coupled plasma atomic emission spectrometry IR Infra Red MIBK Methylisobutylketone NAA Neutron activation analysis nBE n-butyl ether nBNL 1- butanol nHNL hexanol NMR Nuclear Megnatic Resonance ONL 1-octanol
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PINSTECH Pakistan Institute of Nuclear Science and Technology PMR Proton Megnatic Resonance PSO Petroleum sulfoxides REEs Rare earth elements S.F Synergistic factor SCA Salicylic acid TBP Tributylphosphate TBPO Tributylphosphineoxide TOA Trioctylamine TOPO Trioctylphodsphineoxide TPP Triphenylylphosphate TPPO Triphenylylphosphineoxide TRAMEX Tertiary Amine Extraction XRF X-ray fluorescence Γ Separation Factor
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Table of Contents CHAPTER -1 1. INTRODUCTION 1
CHAPTER-2 2. SOLVENT EXTRACTION 12
Distribution Ratio (D) 13
Percent Extraction (E) 13
Separation Factor (γ) 14
2.1. Distribution Law 14
2.2. Process of Extraction 16
2.2.1. Chemical Interactions in the Aqueous Phase 16
2.2.2. Distribution of Extractable Species 17
2.2.3. Chemical Interaction in Organic Phase 20
2.3. Extraction Systems 20
2.3.1. Types of Inorganic Extractable Complexes 20
2.3.1.1. Coordination Complexes 21
2.3.1.1.1. Simple or Monodentate Complexes 21
2.3.1.1.2. Chelate or Polydentate Complexes 21
2.3.1.2. Ion association complexes 24
2.4. Extraction Equilibria 26
2.4.1. Extraction of Metal Chelates 26
2.4.1.1. Effect of the Reagent 28
2.4.1.2. Effect of Reagent Concentration and pH 28
2.4.1.3. Effect of Metal Ion Concentration 29
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2.4.1.4. Effect of the Organic Solvent 29
2.4.1.5: Selectivity in Chelate Extractions 30
2.5. Kinetic Factors in Extraction 32
2.6. Methods of Extraction 33
2.6.1. Batch Extraction 34
2.6.2. Continuous Extraction 35
2.6.3. Countercurrent Extractions 36
2.7. Factors influencing the extraction efficiency 36
2.7.1. Choice of Solvent 37
2.7.2. Acidity of the Aqueous Phase 37
2.7.3. Salting-out Agents 38
2.7.4. Oxidation State 39
2.7.5. pH 40
2.7.6. Masking 40
2.7.7. Backwashing 41
2.8. Synergic extraction 41
2.8.1. Methods used for the study of synergistic Extraction 43
2.8.1.1. Slope Analysis Method 44
2.8.1.1.1. Extraction with acidic ligand 44 2.8.1.1.2, Synergistic Extraction 45
2.8.1.2, Job’s Method 46
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CHAPTER-3
3. LITERATURE REVIEW 48
3.1. Use of Picrolonic acid (HPA) in Copmlexation / Extraction 52
3.2. Use of Crown Ethers in Extraction of REEs 55
CHAPTER-4
4. EXPERIMENTAL 64
4.1. Apparatus 64
4.2. Materials 64
4.3. Buffer Solutions 65
4.4. Chemicals/reagents used to study anions and cations effects 65
4.5. Preparation of Radionuclides 66
4.5.1. Calculation of The Activity of Radiotracer 67
4.6 Experimental Procedures 69
4.6.1. Extraction procedure 69
4.6.2. Effect of Shaking Time on Extraction of REEs 70
4.6.3. Synergistic Extraction with Mixture of HPA and B15C5 70
4.6.4. Effect of REEs Concentration 71
4.6.5. Composition of Extractable Organometallic Complex 71
4.6.6. Effect of Neutral Ligands on the Extraction of REEs 72
4.6.7. Effect of Anions on the Extraction of REEs 72
4.6.8. Effect of Cations on the Extraction of REEs 73
4.6.9. Effect of Solvents on the Extraction of REEs 73
4.6.10. Extraction of other Matel ions with HPA + B15C5 74
4.6.11. Back Extraction of REEs 74
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4.7. Acid dissociation equilibria of HPA 74
CHAPTER-5
RESULTS AND DISCUSSION 76
5.1 Extraction of REEs with HPA and crown ethers 76 5.1.1 Effect of pH of Aqueous Phase 76
5.1.2. Effect of Equilibration Time 77
5.1.3. Effect of Metal Ion Concentration 77 5.1.4. Composition of Synergic Adduct 78 5.1.4.1. Effect of pH Variation 78 5.1.4.2. Effect of HPA Concentration Variation 78
5.1.4.3. Effect of Crown Ether Concentration Variation 78
5.1.5. The anions effect 82 5.1.6. The Cations Effect 83
5.1.7. The Selectivity of Extraction System 83
5.1.8. Acid dissociation constant 84
5.2. Solvent effect 105
5.2.1. Composition of the Extracted adduct 107
5.2.1.1. Effect of HPA Concentration 108
5.3. Extraction of Rare Earth Elements in Different Solvents 114
5.3.1. Extraction of Ce(III), Tb(III) and Lu(III) in Octanol 114
5.3.2. Extraction of Ce, Tb and Lu in Cyclohexanone 117
5.3.3. Extraction of Tb and Lu in DCEE 120
5.4. Synergistic Extraction of Eu(III) in Benzene and Toluene 124
5.4.1. Effect of pH of Aqueous Phase 124
5.4.2. Composition of Synergic Adduct 124
5.4.2.1. Effect of pH Variation 125
5.4.2.2. Effect of HPA Concentration Variation 125
5.4.2.3. Effect of Crown Ethers Concentration Variation 125
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5.5. Effect of Neutral donors 131
5.5.1 Composition of the Synergistic Adducts 131
5.5.1.1. Effect of pH 132
5.5.1.2. Effect of HPA Concentration 132
5.5.1.3. Effect of Concentration of Neutral Donors 132
5.6 Conclusion 141
CHAPTER-6
REFERENCES 143
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APPENDIX-A
List of Publications
1. Habib-ur-Rehman, Akbar Ali, Jamil Anwar and Wasim Yawar. Synergistic
extraction of Ce(III), Eu(III) and Tm(III) with a mixture of picrolonic acid and
benzo-15-crown 5 in chlofororm. J. Radioanal. Nucl. Chem. 267(2) 421-425
(2006).
2. Habib-ur-Rehman, Akbar Ali, Jamil Anwar and Shafaat Ahmed: Synergistic
extraction of Nd(III), Tb(III) and Lu (III) with a mixture of picrolonic acid and
benzo-15-crown 5 in chlofororm Radiochim. Acta. 94, 475-480 (2006).
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List of Tables
Table 2. 1 Metal Extraction Systems 23
Table 2.2 Ion Association Systems 25
Table 4.1 Radioisotopes prepared in PARR-1 68
Table 5.1 Equilibrium constants of the synergistic extraction of lanthanide (III)
ns with (HPA + B15C5)/CHCl3
80
Table 5.2 Effect of various anions on the extraction of lanthanide ions with 0.01
mol dm-3 (HPA + B15C5)/CHCl3 from aqueous solution at pH 2
85
Table 5.3 Extraction of lanthanide (III) ions in the presence of various cations
with 0.01 mol dm-3 (HPA + B15C5)/CHCL3 from pH 2 aqueous
solution
86
Table 5.4 Extraction of various metal ions with 0.01 moldm-3 (HPA + B15C5) /
CHCl3 from pH 2.0 aqueous solution
87
Table 5.5 Slope with correlation coefficients, for the extractin of Eu(III) from
different solvents from Fig. 20
107
Table 5.6 Slope with correlation coefficients, for the extractin of Eu(III) from
different solvents from Fig. 21
108
Table 5.7 Extractin constants for Eu(III) extraction in different solvents.
110
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List of Figures
Fig. 1 Extraction of Ce(III) with B15C5, HPA and HPA+B15C5 42 Fig. 2 Extraction of Ce(III) and Nd(III) with B15C5, HPA and HPA+B15C5
in chloroform
88
Fig. 3 Extraction of Eu(III) and Tb(III) with B15C5, HPA and HPA+B15C5
in chloroform
89
Fig. 4 Extraction of Tm(III) and Lu(III) with B15C5, HPA and HPA+B15C5
in chloroform
90
Fig. 5 Dependence of metal ion extraction on its concentration by (HPA+B15C5) from
pH 2 buffer solution in chloroform
91
Fig. 6 log D as a function of pH for Ce(III) and Nd(III) with (HPA+B15C5)/CHCl3 92
Fig. 7 log D as a function of pH for Eu(III) and Tb(III) with (HPA+B15C5)/CHCl3 93
Fig. 8 log D as a function of pH for TmIII) and Lu(III) with (HPA+B15C5)/CHCl3 94
Fig. 9 log –log plot of (D-DCE) related to Ce(III) and Nd(III) vs.[HPA];
[B15C5] = 0.005 mol dm-3, pH=2.0
95
Fig. 10 log –log plot of (D-DCE) related to Eu(III) and Tb(III) vs.[HPA];
[B15C5] = 0.005 mol dm-3, pH=2.0
96
Fig. 11 log –log plot of (D-DCE) related to Tm(III) and Lu(III) vs.[HPA];
[B15C5] = 0.005 mol dm-3 , pH=2.0
97
Fig. 12 log-log plot of (D-DHPA) related to Ce(III) and Nd(III) vs. [B15C5];
[HPA] = 0.005 mol , dm-3 , pH=2.0
98
Fig. 13 log-log plot of (D-DHPA) related to Eu(III) and Tb(III) vs. [B15C5];
[HPA] = 0.005 mol dm-3 , pH=2.0
99
Fig. 14 log-log plot of (D-DHPA) related to Tm(III) and Lu(III) vs. [B15C5];
[HPA] = 0.005 mol dm-3 , pH=2.0
100
Fig. 15 D-DHPA)/[B15C5] vs [B15C5] at [HPA] =0.005 mol dm-3, pH2.0
101
Fig. 16 (D-DHPA)/[B15C5] vs [B15C5] at [HPA] =0.005 mol dm-3,, pH2.0 102
Fig. 17 (D-DHPA)/[B15C5] vs [B15C5] at [HPA] =0.005 mol dm-3, pH2.0 103
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Fig. 18 Dependence of pKa of HPA on the concentration (v/v) of 1,4-dioxane in water 104
Fig. 19 Extraction of Eu(III) as a function of pH with HPA in different solvents 106
Fig. 20 log D as a function of pH for Eu(III) with (HPA) in ACAC, DIBK, ONL, Nbe,
CHN, nHNL and DCEE
112
Fig. 21 log – log plot of D related to Eu(III) vs. HPA concentration in ACAC, DIBK,
ONL, Nbe, CHN, nHNL and DCEE
113
Fig. 22 Extraction of Ce, Tb and Lu as a function of pH with HPA in octanol 116
Fig. 23 log D as a function of pH for Ce, Tb and Lu (HPA) in octanol 116
Fig. 24 log – log plot of D related to Ce, Tb and Lu vs. HPA concentration in octanol 117
Fig. 25 Extraction of Ce, Tb and Lu as a function of pH with HPA in cyclohexanone 118
Fig. 26 log D as a function of pH for Ce, Tb and Lu with (HPA) in cyclohexanone 119
Fig. 27 log – log plot of D related to Ce, Tb and Lu vs. HPA concentration in
cyclohexanone
120
Fig. 28 Extraction of Tb and Lu as a function of pH with HPA in DCEE 121
Fig. 29 log D as a function of pH for Eu, Tb and Lu with (HPA) in DCEE 122
Fig. 30 log – log plot of D related to Eu, Tb and Lu vs. HPA concentration in DCEE 123
Fig. 31 Extraction of Eu (III) with HPA, B15C5 and HPA+ B15C5 in Benzene 128
Fig. 32 Extraction of Eu (III) with HPA, B15C5 and HPA+ B15C5 in Toluene 128
Fig. 33 Effect of pH on the extraction of Eu (III) with HPA+B15C5 in Benzene
and Toluene
129
Fig. 34 log – log plot of (D-DCE) related to Eu(III) vs. HPA concentration in
Benzene and Tolueneat constant concentration (0.01 mol dm-3 ) B15C5
129
Fig. 35 log – log plot of (D-DHPA) related to Eu(III) vs. B15C5 concentration in benzene
and toluene at constant concentration (0.01 mol dm-3 ) HPA
130
Fig. 36 Extraction of Eu (III) with HPA, TOPO and HPA+TOPO in chloroform 136
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xix
Fig. 37 Extraction of Eu (III) with HPA, TPPO and HPA+TPPO in chloroform 137
Fig. 38 Extraction of Eu (III) with HPA, TBP and HPA+TBP in chloroform 137
Fig. 39 Extraction of Eu (III) with (0.01mol dm-3 ) HPA, TPP and HPA+TPP in chloroform
138
Fig. 40 Effect of pH on the extraction of Eu (III) with (HPA+S) (S= TBP, TOPO and
TPPO) in chloroform
138
Fig. 41 log – log plot of (D-DS) related to Eu(III) vs. HPA concentration in chloroform
at constant concentration TOPO, TPPO and TBP
139
Fig. 42 log –log plot of (D-DHPA) related to Eu(III) vs. [S] Neutral donors(TOPO, TPPO
and TBP) concentration into chloroform at constant concentration HPA
139
Fig. 43 Extraction of Eu(III) with isomolar mixture of HPA and TPPO 140
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CHAPTER – 1 INTRODUCTION
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1. INTRODUCTION
The rare earths are special group of elements placed within the periodic table. This
is a group of seventeen elements with atomic numbers 21, 39, and 57-71. The name
lanthanide is reserved for the elements 58-71. The name rare earths is misnomer, because
they are neither rare nor earths. The early Greeks believed that every thing in the world
was made of four elements: air, earth, fire and water. The earths were substances, which
could not be changed by the temperature then available to the scientists. The first rare
earth was discovered in the early part of the nineteenth century, and resembled the
common earths, which were oxides of magnesium, calcium and aluminum. Since the rare
earths were found to be very rare minerals, they were thus called rare earths. They are not
rare; since cerium is reported to be more abundant in the earth crust than tin, while
yttrium is more abundant than lead and some other elements of this family are also more
abundant than the platinum group elements. All these elements form trivalent bonds and
when their salts are dissolved in water, they ionize to form trivalent ions and the solutions
exhibit very similar chemical properties. The elements scandium, yttrium, lanthanum, and
actinium in the III column of the extended periodic table show similar properties in
aqueous solution. Yttrium and lanthanum are always found associated with the rare earths
in nature.
Rare earths are widely distributed in the earth crust and exhibit a great diversity in
the geological type of deposits. These occur as important constituents of more than 100
minerals and in trace quantity in many others [1]. The rare earth elements can be broadly
placed in two groups i.e., ‘light’ (cerium group) and ‘heavy’ (yttrium group).
The cerium group consists of La, Ce, Pr, Nd, Pm Sm, Eu and Gd while, the
yttrium group comprises of of Y, Tb, Dy, Ho, Er, Tm, Yb and Lu. Monazite, bastnasite,
and zenotime are major ores of rare earth elements. Monazite is orthophosphate of
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essentially the cerium group elements. Bastnasite is a fluorocarbonate of the same group
and zenotime is the orthophosphate of the heavy group of rare earths but less abundant
than monazite.
Modern placer deposits, particularly the beach deposits in the form of heavy
mineral sands are the major source of monazite in the world and occur in Australia,
Brazil, India, South Africa and USA. Rare earth reserves are widely distributed in the
world but major reserves are found in China, USA and India. China holds almost 80% of
rare earth reserves, USA 11% and India 5% [1].
About 76500 tons of rare earth metals calculated as oxides are currently consumed
in the world per year [1]. This quantity is divided among a dazzling variety of
applications with reference to their special chemical, metallurgical, optical, magnetic and
nuclear properties. Most of the traditional applications of rare earths in industry are based
on their similar chemical properties due to essentially identical outer electronic
configuration of their atoms. For majority of the purposes, mixed compounds of these
elements are sufficient. Some of the major uses of this category are rare earth chloride for
mish metal production and cracking catalyst, oxides for glass polishing, fluorides for
manufacture of arc carbon and metallurgy [2-5]. Metallurgical applications of rare earth
metals and alloys form a sizable outlet [6-10]. The most common uses of rare earth
elements are as phosphors, permanent magnet, batteries and in petroleum industries.
Rare earth phosphors have been extensively used in color TV screens, computer
monitors, fluorescent lights and medical X-ray photography [11-13]. Recent innovations
in the area of phosphors are trichromatic and superdeluxe lamps. Both of them employ
rare earths (RE) ions such as Eu3+, Ce3+, and Tb3+ as activators in an oxide, aluminate or
borate lattice.
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Permanent magnets are being used extensively in industrial and commercial
appliances. They are used in micro motors, capacitors in computers, audiovisual,
automobiles and household electronics. Permanent magnets are also used in limiting
motors in industrial robots, military and space technology. The Sm-Co magnets are used
in miniature earphones. Nd2Fe14B magnet currently constitute over 25% of world wide
market used in high technology, notably in stepper motors for computers peripherals and
consumers electronic industry [14].
The presence of the alloying elements in aluminum brings down its electrical
conductivity sharply. Rare earth elements (REEs) have low solid solubility and similar
electronic structure to that of aluminum. In addition, REEs form inter-metallic bonds with
some of the alloying elements present in aluminum matrix. This results in strengthening
of Al matrix without affecting ductility and little decrease in electrical conductivity [15-
16].
Similarly, addition of REEs to steel has strong influence on sulfur and / or sulfides
[17]. This results in cleaner steels with the alteration of the shape and distribution of
sulfides and oxysulfides [18]. High-purity individual lanthanides are being used as major
components in lasers, phosphors, magnetic bubble memory films, refractive index lenses,
fiber optics, and superconductors [19, 20].
Although RE-based superconductors are well known, their importance became
greatly enhanced after the discovery of high-temperature superconductors with rare earth
and cupric oxide as major constituents [21]. The role of trace REE in environment is not
clear, as there are conflicting reports such as reports from China indicating that REs are
used as nutrients for getting higher crop yield [22]. Toxicity of Nd salts in mice increased
in the order: chloride < propionate < acetate < 3-sulfonicotinate < sulfate < nitrate [23]
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and terbium group elements have lesser toxicity than other REE above or below them in
the periodic table [24].
The abundance of REEs in geological materials like rocks and minerals for
elucidating pathogenesis and in the evaluation of the coefficients of partitioning of REEs
between various minerals and melts received major impetus with the availability of
various powerful analytical techniques [25, 26].
Rare earths are of great significance in nuclear technology. Discovery,
exploration, and utilization of nuclear fission is one of the mankind’s greatest intellectual
achievements of 20th century. At present nuclear energy is most conveniently supplied in
the form of electrical energy. Nuclear energy is obtained from fission of uranium. A device
in which fission energy is produced in controlled manner from nuclear fuel is called
nuclear reactor. RE elements have high capture cross-section for thermal neutrons. Due to
this property, these are important in nuclear technology in two ways. Bearing high thermal
neutron cross-section, they may be used as reactor control rods. Due to the great capacity
of europium for neutron absorption, it may be used as europium bearing control rods.
Cerium and yttrium hydrides were successfully tried as neutron moderators because of
their thermal stability. In this way, these may be important component in reactor materials.
On the other hand, due to their high absorption cross-section for thermal neutrons,
these elements if present in nuclear fuel will absorb the neutrons and slow down the
reaction. In this way they are poison for nuclear fuel. If their concentration is too high,
these affect the nuclear fuel by decreasing its thermal conductivity, increasing internal
temperature and changing melting point. As, fission products migrate up and down during
fission, due to which temperature gradient change, which leads to fuel cracking.
At present the nuclear energy is based almost on thermal fission of uranium.
Separation of trans plutonium actinides from the rare earth fission products, which are
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produced about 40% of all thermal- neutron induced fission of U235 and Pu239 is very
important for any nuclear fuel cycle designed either for actinide production or burn-up for
electricity generation due to their high thermal neutron absorption. After some time, it is
necessary to reprocess the burnt fuel for the separation of fission products. The
technology of spent fuel reprocessing started in 1944 and has been continuously evolving
in the last six decades. A major credit for the success of reprocessing must go to the Oak
Ridge National Laboratory and the Knolls Atomic Power Laboratory who were pioneer in
the development of reprocessing.
The role of analyst in the preparation of high purity nuclear fuel is extremely
important. The neutron economy of uranium fueled nuclear reactors may be significantly
impaired by the presence of rare earth impurities at the level of parts per million (ppm)
which have high absorption cross section for thermal neutrons [27]. Trace impurities
specially those which have high neutron cross section not only hinder in fission reaction
but can also alter the metallurgical characteristics of uranium metal.
Determination of rare earths at ppm level in uranium is very difficult task [28]. A
number of techniques are needed to determine over 40 metallic impurities in uranium,
used as fuel. The techniques include carrier distillation, solvent extraction and
precipitation as separation methods followed by the estimation of these impurities by DC-
arc emission spectrometry, atomic absorption spectrometry (AAS), X-ray fluorescence
(XRF), neutron activation analysis (NAA) and inductively coupled plasma atomic
emission spectrometry (ICP-AES). These instrumental methods suffer from lack of
sensitivity for the determination of REEs. Therefore, direct determination of impurities in
the uranium is often hampered by matrix and spectral interferences [29]. In the last
decade, several approaches have been developed for trace metal analysis using different
instrumental techniques.
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An electro-thermal atomization-atomic absorption spectrometric (ETA-AAS)
method [30] was used for trace metal analysis by preparing matching matrix standards for
accurate analysis. This technique is less attractive due to single element analysis.
For determination of rare earths in uranium by NAA, prior separation of uranium
is required [28]. This technique is very sensitive for determination of rare earths but
procedure requires a lot of manipulations, which are time consuming.
In XRF, various uranium lines with high mass absorption coefficient affect the
estimation of REEs. Precision and accuracy are probably suffered from interfering lines
of other elements [33,34].
ICP-AES has grown into a major technique to analyze trace metals in uranium. In
this technique uranium solution having REEs impurities is directly aspirated. Multiple
standard addition method may also be used in this technique. In ICP-AES analysis, the
high uranium concentration (up to 35mg/mL) leads to enormous amount of more or less
strong emission lines, completely burying the trace element emission. ICP-AES is an
excellent technique but do not provide detailed results to achieve the conclusion [31, 32].
Therefore, separation and pre-concentration is required. Usually column or extraction is
applied. Comparing mass spectrometer (MS) detection with respect to AES detection,
spectral interferences are reduced and better detection limits are obtained. The high mass
range is not expected to give any problem. However, the mass range 16-80 suffers
polyatomic ions interference. Due to compromised operating conditions and space effect,
the lowest mass exhibit low sensitivities. Ionization suppression effects on analytes are
large in the presence of uranium. Therefore, uranium separation is still required with
liquid-liquid extraction, with which excellent detection limits are obtained.
Despite the availability of the more selective, modern methods of measurement,
successful solution to many analytical problems depends heavily on separation process.
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Many separation processes of vital interest to the analytical chemists have been
successfully translated to plant scale operation. A review of the literature reveals the
valuable assistance of the solvent extraction technique to many fields of analysis
especially to radiochemistry, colorimetry and for preconcentration in conjunction with
many of the more sophisticated instrumental techniques because of its ease, simplicity,
selectivity, speed and wide scope [35-37].
In solvent extraction, the metal or metals of interest are selectively complexed by
the ligand from feed solution with an organic extractant and after that, these metal ions
are striped from the organic phase to aqueous phase. Separation of rare earths by
extraction is not only time saving but also enhance resolution and reproducibility.
Solvent extraction continued to attract the immense interest of analytical chemists
with the broad variety of topics, from large scale separations and purification processes
for the preparation of carrier free trace level isotopes and new extraction procedures using
familiar reagents to developing and testing new reagents and study of the kinetics as well
as thermodynamic aspects of chemical reactions.
Prior to trace metal analysis in any matrix, liquid-liquid extraction with selective
organic complexing ligand i.e. Tributylphosphate (TBP), Dihexylamide (DHA) and
thenoyltrifluoroacetone (HTTA) is applied for the separation of metal of interest from the
matrix in organic solvents, i. e. carbon tetrachloride, toluene, benzene and chloroform.
Repeated extractions are required for trace metal analysis in uranium. It is also desirable
to avoid excessive pretreatment of sample before analysis [38]. Selective extraction is
possible for small amount of metal ions, which can be separated from large amount metal
ions.
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For the separation of rare earths, four extraction processes namely,
Tributylphosphate (TBP) process, Versatic acid process, Di-2-ethylhexylphosphoric acid
(D2EHPA) process and Rhone-Poulenc process are of commercial importance [39].
In TBP process, 50% TBP in trimethylbenzene is used as extractant and the
separation factor (SF) for the neighboring rare earths from La – Sm appeared to be 2 – 1.5
and rare earths heavier than Sm can not be separated. Versatic acid process uses 50 %
versatic acid in trimethylbenzene and SF from La – Pr varies from 3 -1.8 and for heavier
REEs is similar to TBP system. D2EHPA gives high separation factors from Sm-Lu and
widely used for preparing Eu concentrates. For Rhone-Poulenc Separation process is not
known but they have outlined a comprehensive manufacturing procedure that uses variety
of extractant for the extraction of REEs. This brief introduction of commercial processes
shows that no simple and efficient process is available for the separation of REEs. A
typical extraction plant may contain 75-80 stages for (1) solvent loading section (2)
countercurrent separation section, (3) stripping section and (4) purification of extractant
and among which ~50 stages are required for the extraction section only due to the low
separation factor for these extractants [39].
The extraction procedures that have been developed at the laboratory stage use
mainly neutral, anionic and acidic types of extractant.
Organo-phosphorus compounds such as trioctylphosphineoxide (TOPO),
tributylphosphineoxide (TBPO) and TBP etc. [40-42], alkylsulfoxides, dioctylsulfoxide
(DOSO), Di-n-butylsulfoxide (DBSO) etc. [43, 44] and N, N, dialkylamides such as
Dihexylhexaneamide (DHHA), Di-(2-ethylhexyl) isobutyricamide (D2EHIBA) are
classified as neutral extractants [45-47]. The main feature of this class of extractants is
their use in the separation of light actinides such as U and Pu which can exist in IV, V and
VI oxidation state in aqueous medium. As, most of the rare earths exist in III oxidation
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state, so these extractants have little application for the separation of rare earths. Some
macromolecules having cyclic structures such as crown ethers and calixarenes have also
been used for rare earth separation [48, 49]. The methods based on neutral extractants for
rare earth separation uses high acid and salt concentration which causes waste problems
in nuclear industry.
REEs are hydrolysable metals, amines are the extractants that have the potential
for their extraction from basic media [50, 51]. The extraction of lanthanide/actinides by
amine extractants suffers from many of the same limitations as the neutral
organophosphorus extractants.
Roelandt has discussed briefly the application of the TRAMEX (Tertiary Amine
Extraction) process for the purification of Cm242. Primary and quaternary amines are
indicated to be useful for REEs extraction in alkaline medium [38].
A variety of diluents have been used but their nature seems to have little effect on the
extraction efficiency or separation factor. β-diketones [thenoyltrifluoroacetone (HTTA),
Hexafluoroacetylaceton (HFAA)], 4-acyl pryazolones, salicylic acid, and organo-
phosphoric acid or their thio-derivatives [52-55] are the extractants that belong to this
category. Some work has been done evaluating the extraction of the subject metal ions by
sulfonic acids, but this class of liquid cation exchanger exhibit little selectivity, and they
have not proven useful for REEs separations. The separation factors for the REEs by
HTTA / benzene system varies from 1.18 to 9.1 [56]
The phenomenon in which two extractants taken together extract a metal ion
species with a much higher efficiency as compared to the normal additive effect of these
extractants (separately) is called synergism. From the first observation of this
phenomenon of synergism by Blake in 1958, extensive work has been carried out on the
synergistic extraction of 5f elements [57].
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In the nuclear fuel industry, synergistic extraction has been recommended for the
recovery of the REE. In the extraction of trivalent lanthanides by various mixtures of
extractants, it has been observed that the synergistic enhancement (S.E.= log [KD (1,2)
/KD(1) + KD(2)) in many cases is very high, of the order of 105. This is one of the reasons
for the great interest shown in the synergistic extraction of REEs.
Synergic extraction systems comprising of various acidic extractants HTTA, 1-
phenyl-3-methyl-4-acyl-5-pyrazolone (HPMAP), salicylic acid (SCA), oxine, HDEHP
etc. and various neutral donors TBP, TOPO, DOSO, trioctylamine (TOA),
methylisobutylketone (MIBK), etc. are mostly used for the extraction of REEs. Mathur
has published a review on the synergic extraction of trivalent actinides and lanthanides
with 160 references [58]. The author has suggested that in synergic systems adducts or
mixed ligand complexes are formed that can be used for the optimum separation of
lanthanides from each other. It is clear from this literature overview that inspite of having
so much research work done in this field, no simple, efficient and economical method for
rare earth separation is available and need of such a method still exist.
The class of chelating extractants which have received the most attention in the
recent years have the basic structure of 4-acylpyrazolone [59-61]. Because of their
increased acidity (relative to β-diketones) and various synthetic modifications which can
be made to their basic structure, these extractants present some possibilities for the
improved separation procedures for the f-block elements [54]. This is of great interest for
the treatment and recycling of industrial wastes and particularly nuclear one.
Picrolonic acid (1-p-nitrophenyl-3-methyl-4-nitro-5-pyrazolone, HPA, pKa =
2.52) belongs to pyrazolone family extractants with strong acidity, having capability for
the extraction / mutual separation of lanthanides from acidic aqueous solutions. Literature
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shows that very little work [62-65] has been done for the separation of lanthanides by
using this reagent.
The f-elements cations are “hard acid” – that is, their bonding in complexes is
rather well described by an electrostatic model and they show strong preferences for oxo-
donor atoms [66].
In view of the importance of the rare earth elements and the synergic extraction
systems comprising picrolonic acid as acidic chelating agent and oxygen based neutral
donors (oxo-donor), this study was proposed for the present research program.
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CHAPTER – 2 SOLVENT EXTRACTION
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2. SOLVENT EXTRACTION
Although solvent extraction as a method of separation has long been known to the
chemists, only in recent years it has achieved recognition among analysts as a powerful
separation technique. Liquid-liquid extraction, mostly used in analysis, is a technique in
which a solution is brought into contact with a second solvent, essentially immiscible
with the first, in order to bring the transfer of one or more solutes into the second solvent.
The separations that can be achieved by this method are simple, convenient and rapid to
perform; they are clean as much as the small interfacial area certainly precludes any
phenomena analogous to the undesirable co-precipitation encountered in precipitation
separations.
Solvent extraction is one of the most extensively studied and most widely used
techniques for the separation and pre-concentration of elements [67-69]. The technique
has become more useful in recent years due to the development of selective chelating
agents [70-73] for trace metal determination. With proper choice of extracting agents, this
technique can achieve group separation or selective separation of trace elements with high
efficiencies. In analytical applications solvent extraction may serve the following three
purposes:
i) Preconcentration of trace elements
ii) Elimination of matrix interference
iii) Differentiation of chemical species.
The procedure is applicable to both, trace and macro levels. A further advantage
of solvent extraction method lies in the convenience of subsequent analysis of the
extracted species. If the extracted species are coloured, as is the case with many chelates,
spectrophotometric methods can be employed. Alternatively, the solution may be
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aspirated for atomic absorption or ICP-emission spectrometric analysis. If radiotracers are
used, radioactive counting techniques can be employed.
Before going in detailed discussion of fundamental principles of extraction, the
three mostly used terms for expressing the effectiveness of extraction processes are being
defined below. These terms are basic for understanding of theoretical as well as practical
considerations of the subject.
Distribution Ratio (D)
The distribution of a solute between two immiscible solvents can be described by
the distribution ratio “D”.
2
1
][][
AAD = (2.1)
Where [A] represents the stoichiometric or formal concentration of a substance A and the
subscripts 1 and 2 refer to the two phases. Since in most cases, two-phase system is of
analytical interest, an organic solvent and aqueous are involved, D will be understood to
be;
Aq
Org
AA
D][][
= (2.2)
The subscripts org and Aq refer to the organic and aqueous phases respectively.
Percent Extraction (E)
The more commonly used term for expressing the extraction efficiency by
analytical chemist is the percent extraction “E”, which is related to “D” as
OrgAq VVD
D+
=+
=100
V[A]V[A]V100[A]
(E)Extraction %AqAqOrgOrg
OrgOrg (2.3)
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Where V represent solvent volume and the other quantities remain as previously defined.
The percent extraction may be seen to vary with the volume ratio of the two phases as
well as with D.
It may also be seen from equation (2.3) that at extreme values of “D”, “E”
becomes less sensitive to changes in “D”. For example, at a phase volume ratio of unity,
for any value of D below 0.001, the solute may be considered quantitatively retained in
the aqueous phase whereas for D values from 500 to 1000, the value of “E” changes only
from 99.5 to 99.9%.
Separation Factor (γ)
Since solvent extraction is used for the separation of different elements and
species from each other, it becomes necessary to introduce a term to describe the
effectiveness of separation of two solutes. The separation factor γ is related to the
individual distribution ratios as follows:
γB
A
AqOrg
AqOrg
AqAq
OrgOrg
DD
BBAA
BABA
===][][][][
][][][][
(2.4)
where A and B represent the respective solutes.
In those systems where one of the distribution ratios is very small and the other
relatively large, complete separations can be quickly and easily achieved. If the separation
factor is large but the smaller distribution ratio is sufficiently large then less separation of
both components occurs. It is then necessary to apply various techniques to suppress the
extraction of the undesired component.
2.1 Distribution Law
In the simplest extraction case, the distribution ratio is constant in accordance with
the classical Nernst distribution law, a solute will distribute itself between two essentially
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immiscible solvents so that at equilibrium the ratio of the concentrations of the solute in
the two phases at a particular temperature will be constant, provided the solute is not
involved in chemical interactions in either phase [74]. For such a solute, then
DAA
KAq
Orgd ==
][][
(2.5)
where “Kd” is termed as the distribution coefficient.
Deviations from the distribution law arise from two sources: (a) neglect of activity
corrections and (b) participations of the distributing solute in chemical interactions in
either or both of the two solvent phases. Although the distribution law, as described in
equation (2.5) is not thermodynamically rigorous, variation in Kd due to variation in
activity coefficients is likely to be under one order of magnitude for most extraction
systems of interest to analysts. Far more important are the changes in extraction
characteristics of solute because of chemical changes, which occur. Such changes do not
represent failure of the law. Rather, they add complexity to the distribution expressions,
which can be properly accounted for by using appropriate equilibrium expressions.
The distribution of acetic acid between benzene and water may serve as an
illustration of the effects of chemical interactions of the solute. The distribution of acetic
acid itself may be described as follows:
OrgAq COOHCHCOOHCH )()( 33 ⇔
Aq
OrgD COOHCH
COOHCHK
][][
3
3= (2.6)
However, acetic acid dissociates in aqueous phase
+− +↔ HCOOCHCOOHCH 33
WA COOHCH
COOCHHK
][]][[
3
3−+
= (2.7)
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and forms a dimmer in benzene
233 )(2 COOHCHCOOHCH ↔
23
23
][])[(
O
OP COOHCH
COOHCHK = (2.8)
The overall distribution of acetic acid is described by “D”, which is
][][])[(][
][][
33
2233
3
3−+
+==
COOCHCOOHCHCOOHCHCOOHCH
COOHCHCOOHCH
DW
OO
W
O (2.9)
Upon incorporation of the equilibrium expression in Eq. (2.6), (2.7) and (2.8) in Eq. (2.9)
there results
][1]][21[ 3
+++
=HK
COOHCHKKD
A
OPD (2.10)
This shows how the distribution of acetic acid varies as a function of pH and acetic acid
concentration.
2.2 Process of Extraction
From the above equations, it is clear that three essential aspects are involved in the
extraction of acetic acid:
A: Chemical interaction in the aqueous phase.
B: Distribution of extractable species.
C: Chemical interactions in the organic phase.
These three aspects are shared by almost all extraction systems and serve as the basis of a
useful organizational pattern.
2.2.1 Chemical interactions in the aqueous phase
A major point of differentiation between extraction of organic and inorganic
materials is the extent to which the formation of an uncharged extractable species
depends on chemical interactions in the aqueous phase. Most organic compounds are
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already uncharged and extractable. Such aqueous phase reactions if do occur might well
transform these to charged non-extractable species, e.g.
+− +↔+ OHRCOOOHRCOOH 32
−+
+↔+ OHHNROHRNH 322
In contrast, most of the inorganic compounds are dissociated, so that in order to
extract a species of interest into an organic solvent, reactions in the aqueous phase leading
to the formation of an uncharged, extractable complex must be utilized. For example, in
order to extract aluminum-III from an aqueous solution of aluminum nitrate, one must
bring about the reaction of the aluminum-III cation with a reagent such as 8-quinolinol to
form aluminum-8-quinolinate, which may be extracted into a variety of organic solvents
such as chloroform or benzene [75].
Therefore, the formation of an uncharged complex is very important in the
extraction of metals and other inorganic species that makes it convenient to classify such
extractions according to the nature of the complexes.
2.2.2 Distribution of extractable species
Although the ratio of solubilities of a solute in each of two solvents may not be
critically equated to the distribution coefficient of the solute between the two solvents
[76], the underlying factors affecting relative solubility and distribution are undoubtedly
similar. It is, therefore, useful to discuss solubility characteristics of various types of
substances and to note structural effects in both solvent and solute on the solubility.
In solutions where specific chemical forces are not active, the classical principle
of “like dissolves like” is of great help in predicting solubility. This principle may be
expressed in modern terms as Hildebrand’s theory of regular solutions from which, the
solubility is seen to increase as values of the solubility parameter “δ” of solute and
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solvent approach each other [76]. The solubility parameter, defined as the square root of
the heat of vaporization per milliliter, is a measure of cohesive energy density.
Comparison of solubility parameters should be of maximum assistance of dealing with
those organic extraction systems in which specific chemical or associative forces are
inoperative. Burrell has successfully used solubility parameters to rationalize the
solubility behavior of various polymers [77].
In systems, where hydrogen bonding may be present, particularly those involving
an aqueous phase, the solubility parameter is inadequate in predicting solubility. This
might be expected in as much as this concept is, strictly speaking, applicable only in
regular solutions. Collander has been able to observe regularities in distribution
characteristics in systems involving hydrogen bonding. On the basis of the determination
of Kd values for two hundred organic compounds in the ethyl ether-water system, he
noted that low Kd values were obtained for compounds having groups capable of
hydrogen bonding, such as alcohol, amines, carboxylic acids, and acid amides [78].
Increasing the molecular weight of the organic portion of the molecule would increase the
Kd value about two to four times for each additional methylene group in the homologous
series. The effect to the oxygen in the molecule seemed to be about the same for alcohols,
ketones, aldehydes and carboxylic acids. Increase in Kd resulting in replacing alcoholic or
carboxylic hydrogen with methyl group seemed to be little more than would be expected
upon the increase in molecular weight. Increase in Kd were observed with the introduction
of a halogen atom.
Pasquinelli has been able to correlate the mutual solubilities of a pair of liquids
with the electric and magnetic properties of the pure components [79]. The relation, that
has been used to predict solubilities with probable absolute error of about ± 3% for 100
pairs of liquids, involves the dipole moments, dielectric constant, specific magnetic
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susceptibility and molar volume. In as much as the prediction may be made for systems
involving hydrogen bonding, the relation may be more generally applicable than the
comparison of solubility parameters.
Solubility of metal salts in aqueous media can be explained on the basis of two
special properties of water. First, its high dielectric constant permits dissociation of ionic
species relatively easily. Even more important, the high basic character of water results in
the solvation of cations (and anions), which gives these ions a solvent sheath serving to
reduce electrostatic interaction and to make the ions more “solvent-like”. The role of the
complex forming extraction agent is largely to replace the coordinated water from around
the metal ion to give a species that is more likely to be soluble in organic solvents. The
solubility characteristics of metal chelates in organic solvents in general terms are not at
all unlike those of conventional organic compounds. For example, hydrocarbon
substituents will increase the solubility of chelates in organic solvents. Although the
neodymium chelate of cupferron-(I) is not soluble in chloroform where as the
corresponding neocupferron (II) is soluble in chloroform.
Polar substituents will of course reduce solubility in organic solvents. The chelates
of 8-quinolinol-5-sulphonic acid are not at all soluble in organic solvents but are quite
soluble in water.
N
N = O
O N
O
O
= N
(I) (II)
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Among the ion association complexes, the oxonium type is noteworthy, since in
most cases the solvent participates directly in complex formation. The ability of the
oxonium solvent to replace water from the coordination sphere of the metal would depend
upon the basicity of solvent, which in turn would reflect the electron density and steric
availability of the electron pair in the oxygen of the solvent molecule. Many ion
association extractions are aided by the use of salting-out agents, electrolytes used in high
concentrations to;
(a) Produce a mass action effect by adding a common ion,
(b) Reduce water activity greatly,
(c) Lower the dielectric constant so as to favor ion-pair formation.
The use of salting-out agents in organic extractions is also well known.
2.2.3 Chemical interactions in organic phase
Chemical interactions of the extracted species in the organic phase would
naturally lower its concentration in this phase and hence, improve extractability. If, in the
case of a carboxylic acid extraction, the organic solvents is one in which the acid
dimerizes, this would result in a higher D value than if the reaction does not occur. Ion
association complexes, being dipoles, tend to form higher aggregates in organic solvents
at higher concentrations. Where there is a polymerization reaction of any type, the value
of D will be found to vary with the concentration of the extracted material.
2.3 Extraction Systems
2.3.1 Types of inorganic extractable complexes
Most salts are strong electrolytes whose solubility in water can be attributed to the
high dielectric constant of water which greatly reduced the work of dissociation and
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solvating tendency of water since hydrated ions experience less inter ionic attraction and
resemble more closely the medium in which they are dispersed. In fact, for a metal to
form an extractable complex, it is necessary to remove some or all of the water molecules
associated with the metal ion.
Complexing of metal ions leading to the formation of uncharged species falls into
two main categories, one involving coordination and the other ion association.
2.3.1.1 Coordination complexes
A coordination complex, as the term implies, is formed by coordinate bonds in
which a previously unshared pair of electrons on donor atom or ion is now shared with an
acceptor atom or ion [80]. Three types of coordination complexes are of interest here:
2.3.1.1.1 Simple or monodentate complexes
In simple or monodentate complexes, central metal ion acting as acceptor having a
coordination number “n”, accepts ‘n’ pairs of electrons from ‘n’ individual donor groups,
e.g.
44 :4 GeClClGe →++
−+ →+ 43 :4 FeClClFe
++ →+ 2433
2 )(:4 NHCuNHCu
From the above examples, only the first one give the neutral, extractable complex.
2.3.1.1.2 Chelate or polydentate complexes
Chelate or polydentate complexes [81] with the central metal atom or ion having
coordination number n, combines with no more than n/2 molecules of a specie having at
least two donor atoms per molecule; these being so located as to permit the formation of a
relatively strain-free (i.e. , 5-6 membered) ring, e.g.
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1/3 Fe2+ +N N
N N
Fe/3
dipyridyl cationic
2+
1/2 Cu 2++ CH 3 C CCH
O HO
CH 3 3CH
OO
HC
CC3CH
Cu /2
acetylacetone uncharged
+ H +
0
Chelates have relatively large stability constants, so their formation greatly lowers the
concentration of hydrated metal ion. Those chelating agents such as acetylacetone,
cupferron, dithizone, and 8-quinolinol form uncharged, essentially covalent compounds,
which are readily soluble in organic solvents. Chelating agents such as dipyridyl or
ethylene diamine tetra acid (EDTA) which form charged chelates are useful as metal
masking agents.
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Table-2.1 Metal Extraction Systems Chelate Systems A. 4-Membered ring systems Reactive Grouping
1. Dialkyl dithiocarbamates (-)
N C S
2. Xanthates (-)
S C S B. 5-Membered ring systems
1. Benzoylphenylhydroxylamine (-)
O C N O
2. Cupferron (-)
O N N O
3. a-Dioximes (-)
N C C N
4. Dithizone (-)
N N C S
5. 8-Quinolinols (-)
N C C O
6. Toluene-3:4-dithiol (-) (-)
S C C S
7. Catechol (-) (-)
O C C O C. 6-Membered ring systems
1. β-Diketones and Hydroxycarbonyls
a) Acetylacetone b) Thenoyltrifluoracetone c) Morin d) Quinalizarine
(-)
O C C C O
2. Nitrosonaphthols (-)
O N C C O
3. Salicylaldoxime (-)
N C C C O
1. Pyridyl-azo-naphthol (PAN) (-)
N C N N C C O
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2.3.1.2 Ion association complexes
Ion association complexes are uncharged species formed by the association of
ions because of purely electrostatic attraction. The extent of such association increases
sharply as the dielectric constant of the solvent decreases below 40 to 50 [82]. This
condition not only exists in all of the commonly used organic solvents but also in highly
concentrated aqueous solutions of strong electrolytes [83]. Ion-pairs, which preferentially
dissolve in the organic phase, are those, which resemble the solvent. Ion association
complexes are capable of forming clusters larger than just pairs with increasing
concentration, particularly in organic solvents. In some cases, aggregates large enough to
be described as micelles are encountered.
Two categories of ion association complexes may be recognized. The first
includes those ion-pair formed from a reagent having large organic ion such as
tetraphenylarsonium ion, tribenzylammonium ion or perfluorobutyrate ion. These
reagents combine with a suitable metal-containing ion to give a large organic solvent-like
ion-pair. The second type of ion-pair is essentially like that of the first with the exception
that solvent molecules are directly involved in its formation. Thus in the extraction of
uranyl nitarate with isobutyl alcohol, the extractable complex is probably
UO2(BuOH)6.(NO3)2 in which the coordinated solvent molecules contribute both to the
size of cation and the resemblance of the complex to the solvent [84]. Most of the
solvents which participate directly in the formation of ion association complexes are
containing oxygen. The term oxonium complex is used here to describe such a complex,
since the solvent molecules from coordinate linkages to the metal atoms through their
oxygen atoms.
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Table-2.2 Ion Association Systems
A. Metals contained in cationic number of ion-pair
1. Alkylphosphoric Acids
2. Carboxylic Acids
3. Cationic chelates
a. Phenanthrolines
b. Polypyridyls
4. Nitrate
5. Trialkylphosphine oxide
B. Metal contained in anionic number of ion-pair
1. Halides (GaCl4-)
2. Thiocynate [Co (CNS)42-]
3. Oxyanions (MnO4- )
4. Anionic Chelates [Co (Nitroso R salt)33
In view of the important role played by complex formation in inorganic extraction
systems, it is appropriate to classify these systems in terms of the type of complex formed
[85]. Two broad categories are utilized in Table 2.1 and 2.2.
Chelate extraction systems include only those involving neutral chelates, since
charged chelates must pair with oppositely charged ions to form extractable species. It
will be noted that the chelate systems are ordered with respect to the size of the chelate
ring.
Differentiation of ion association extraction systems is based on the sign of the
charge of the metal-containing ion. In those systems in which the metal is part of the
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anion, a further classification on the basis of the nature of the cation is helpful. These
cations are usually varieties of “onium” ions such as oxonium, ammonium, arsonium,
etc., as mentioned in the Table 2.1.
Organic extractions may be classified in a manner similar to that utilized for metal
extractions that is on the basis of chemical interactions. However, aside from
conventional acid-base reactions which permit control of the extraction via pH, the
utilization of chemical reactions in organic extractions are not frequently encountered. In
view of the vast number of organic compounds and biological material that extract, a
simple method of classification of organic extractions based on the class of compounds to
which a particular solute belongs is often used [86, 87] and any general organic text may
be consulted for these classes.
2.4 Extraction Equilibria
A consideration of the extraction equilibria from a quantitative standpoint is
helpful in pointing out which experimental parameters play an important role in the
completeness as well as the selectivity of the extraction. Apart from the utility in predicting the course of extractions, such a quantitative
treatment opens the way for understanding the applicability of extractable substance.
Treatment of the extraction equilibria is illustrated below. Although a chelate extraction
was chosen to represent the inorganic type, the same general approach may be used for
ion association extractions [88].
2.4.1 Extraction of metal chelates
The equation describing the extraction of metal chelates may be derived by
considering the reactions occurring when an aqueous phase containing a metal ion is
contacted with an organic phase containing a chelate extractant. The steps leading to the
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extraction may be conveniently visualized as follows. The chelating agent distributes
between the two phases. Since the majority of chelating extractants exhibit an acid
dissociation, the symbol HR will serve as a general formula for the reagent.
orgaq HRHR ↔
aq
OrgD HR
HRK
R ][][
= (2.11)
The regent will dissociate in the aqueous phase
−+ +↔ RHHR
][]][[
HRRHK a
−+
= (2.12)
To give a chelating anion R-, which reacts with the metal ion and forms the extractable
chelate
nn MRnRM ↔+ −+
nnn
f RMMR
K]][[
][−+= (2.13)
Which, in turn distributes between the phases
)()( orgnaqn MRMR ↔
aqn
OrgnD MR
MRK
X ][][
= (2.14)
The distribution ratio “D” can be evaluated from these equilibrium expressions if the
metal chelate MRn, may be assumed to be the only metal-containing species in the organic
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phase and the metal ion, M+, essentially the only metal-containing species in the aqueous
phase. Thus
nAq
nOrg
nD
Dn
af
Aqn
Orgn
Aq
Org
HHR
K
KKKMMR
MM
DR
X
][][
][][
][][
++ ==≡ (2.15)
The validity of this equation was first verified by Kolthoff and Sandell [89] for
dithizone extractions and later by Furman et al. for cupferron extraction [90], extends to
many chelate extraction systems.
2.4.1.1 Effect of the reagent
Eq. (2.15) clearly indicates the importance of chelate stability (Kf) and the relative
solubility of the chelate in the organic phase (KDx). It is also seen, that an acidic reagent
having high Ka and relatively good solubility in water favours good extraction. Since
chelate stability increases as reagent acidity decreases, these effects must be considered
together [91]. Thus, if Ka values of a family of reagents increase faster than do the
corresponding Kf values, the Kf Ka value would be larger for the reagent forming less
stable chelates. This seems to be the case in the β-diketones, with TTA possessing this
type of advantage over acetylacetone.
2.4.1.2 Effect of reagent concentration and pH
It may be noted from equation (2.15) that the extractability of a metal with a given
reagent and organic solvent depends equally upon the organic phase concentration of the
reagent and upon the hydrogen ion concentration in the aqueous phase. A tenfold increase
in the reagent concentration will increase D as much as would a rise of one unit in pH. In
case, where metal hydrolysis is significant, i.e., where the reaction
+−−
+ +↔ HOHOHMOHM nx
nx
1122 )()()(
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is important, the assumption made in the derivation of equation (2.15) that the only metal-
containing species in the aqueous phase is +nM must be modified to include the hydrolysis
products. This results in an expression for D in which the average value of the exponent,
and hence the importance of the hydrogen ion concentration decreases, while the reagent
concentration factor remains unchanged [92].
In early extraction work, the reagent concentrations employed were little more
than needed to form the metal chelate [93]. An increase in the pH range of good
extraction was achieved by using somewhat higher reagent concentrations [94], with
employment of very high reagent concentrations a substantial reduction in the pH of
extraction can be achieved, permitting extraction from highly acidic solutions [95]. An
advantage in using high reagent concentrations that is not evident from Eq. (2.15) arises
when the metal involved commonly forms a hydrated, non-extractable chelate, which
with high reagent concentrations is transformed to one in which the coordinated water
molecules are replaced by those of the reagent [96].
2.4.1.3 Effect of metal ion concentration
As indicated by the absence of any metal ion concentration term in Eq.(2.15), the
distribution ratio is independent of initial metal concentration. Thus, both tracer and
macro amounts of metal may be expected to extract to the same extent under similar
equilibrium conditions, provided that the solubility of the chelate in the organic phase is
not exceeded.
2.4.1.4 Effect of the organic solvent
Quite a variety of organic solvents have been employed in metal chelate
extractions and by and large, the nature of the solvent has not been too critical factor in
determining the success of an extraction. The choice of benzene or chloroform for
example, seems to be dictated more by the desire to have the organic extract lighter or
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heavier than the aqueous phase than by the relative extraction efficiency of the solvents.
However, a closer examination reveals differences, which, may be put to practical use. As
may be seen from Eq. (2.15), the solvent affects two quantities, the distribution
coefficients of the reagent and of the chelate. A change in solvent would bring about the
following relative change in the value of D:
nDD
nDD
RX
RX
KK
KKDD
.
.*
**
= (2.16)
If it is assumed that the change in Kd value for a reagent and chelate are about the
same, then Eq. (2.16) predicts that a change to a solvent in which the reagent is more
soluble will result in lower “D” values for polyvalent metals (n >1). An interesting
confirmation of this may be found in the comparison of the use of carbon tetrachloride
and chloroform for dithizone extractions. Dithizone and its chelates are more soluble in
chloroform than in carbon tetrachloride and extractions with the former solvent require a
higher pH region than when the latter is used [97].
2.4.1.5 Selectivity in chelate extractions
The separation of two metals with a particular reagent-solvent system may be
evaluated with Eq. (2.15). The separation factor “γ” defined as the ratio of D values of the
metals in question, is seen to be
22
11
.
.
2
1
X
X
Df
Df
KK
KK
DD
==γ (17)
The ease of separation of two metals is seen to be depending not only on the
difference in the stability of their chelates but also on the relative solubility of these
chelates in the organic solvent. A sufficiently great difference in solubility may result in
an extraction sequence that differs from the stability sequence. For instance, although
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nickel-II and cobalt-II form more stable acetylacetonates than does zinc II [98], the latter
is extractable whereas the former two are not [95].
With regard to chelate stability, the order of stability of a number of metal ions
has been shown to be fairly independent of the nature of the chelating reagent employed.
Mellor and Maley [99] list the following stability sequence for bivalent metal ions:
Pd > Cu > Ni > Pb > Co > Zn > Cd > Fe > Mn > Mg
Despite the adherence of the behavior of many reagents to the “natural stability
sequence for metals” a number of interesting exceptions are noteworthy. One such
example involves the exceptionally high stability of the tris-phenanthroline-iron-II
complex [100,101]. The fact that, this complex is more stable than of the corresponding
nickel or cobalt chelats has been attributed to resonance stabilization.
Steric hindrance in a chelating agent can result increased selectivity. For example,
2,9-dimethylphenanthroline (neocuproine) (1)
N
CH 3
N
CH 3(1)
no longer gives the typical phenanthroline like complex with iron-II since the methyl
groups greatly hinder the attachment of three reagent molecules around the iron-II ion.
This hindrance is minimum in the tetrahedral geomety of two reagent molecules about a
univalent tetracoordinated ion such as copper. Steric hindrance of the 2-methyl groups to
chelate formation is the reason for the non-reactivity of 2-methyl-8-quinolinol with the
small aluminium ion [102]. This reagent offers a distinct advantage over 8-quinolinol in
the determination of many metals in the presence of aluminium [103]. It is also possible
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that reagents containing the mercapto functionality may exhibit a different metal stability
sequence than do those containing oxygen [104].
A more generally applicable approach to increasing selectivity in chelate
extractions than that of depending on “unusual” reagents may be based on the use of
competing complexing agents, called masking agents. These masking agents, illustrated
by cyanide ion or EDTA, form water-soluble complexes with some metals and thus alter
the extraction characteristics of these metals. The use of two competing reagents will
tend, in favorable cases, to exaggerate even small differences in the stability order to the
point where dramatic changes may be observed. For example, copper-II gives more stable
chelates with both 8-quinolinol and EDTA than does uranyl ion and hence in the presence
of EDTA only uranyl ion may be extracted with 8-quimolinol [105].
2.5 Kinetic Factors in Extraction
The previous discussion has been based on the assumption that the system had
achieved equilibrium. Although, in most cases, optimum extraction is obtained under
equilibrium conditions, occasionally the slow extraction rate of one or more components
may serve to improve selectivity. The rate of extraction depends on factors affecting (a)
the rate of formation of the extraction species and (b) the rate of transfer of the extractable
species.
As expected the rate of formation of ion association complexes which involve
essentially electrostatic forces is very rapid. Formation of metal chelates on the other
hand may sometimes take place at measurable rates. Slow extraction has been observed
with some of the chelates of dithizone [106] and thenoyltrifluoroacetone (HTTA) [107].
The presence of EDTA has been found to reduce the rate of extraction in a number of
instances [105,108]. In cases where the formation of metal chelate is the rate-determining
step, the extraction may be speeded by increasing the concentration of the reagent [109].
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The rate of transfer of the extractable species from one phase to the other is
relatively rapid when reasonable agitation is employed. Barry et al. have shown that
simple repeated inversion of the two phases is sufficient to give equilibrium in a relatively
short time even if the species concerned possessed a relatively high molecular weight
[110]. Unless the liquids are viscous, it may be expected that transfer rates are sufficiently
high to permit equilibration with a shaking time of several minutes.
2.6 Methods of Extraction
Three basic methods of liquid-liquid extraction are generally utilized in the
analytical laboratory. Batch extraction, the simplest and mostly used method, consists of
extracting the solute from one immiscible layer by shaking the two layers until
equilibrium is attained, after which the layers are allowed to settle before sampling. The
second type, continuous extraction, makes use of a continuous flow of immiscible solvent
through the solution or a continuous countercurrent flow of both phases. In the former
case the spent solvent may be stripped and recycled by distillation or fresh solvent may be
added continuously from a reservoir. Extraction by discontinuous countercurrent
distribution is the third general type and is used primarily for fractionation purposes. A
series of separatory funnels or contacting vessels are employed to achieve many
individual extractions rapidly and in sequence. The choice of method to be employed will
depend primarily upon the value of the distribution ratio of the solute of interest, as well
as on the separation factors of the interfering materials.
The basic principles in designing an extraction for laboratory use are relatively
simple and just a few basic types of apparatus are more than adequate for most needs. An
infinite number of modifications of these basic designs, however, have resulted from the
chemist’s desire to improve a particular apparatus for the specific problem.
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2.6.1 Batch extraction
The simplest extraction procedure possible and the technique most employed in
the laboratory for analytical separations involves the bringing of a given volume of
solution into contact with a given volume of solvent until equilibrium has been attained,
followed by separation of the liquid layers. If necessary, the procedure may be repeated
after the addition of fresh solvent. This batch extraction process provides rapid, simple,
and clean separations, and is more beneficial when the distribution ratio of the solute of
interest is larg. In such instances, a few extractions will effect quantitative separation.
Various methods for increasing the distribution ratio as well as the selectivity of an
extraction are discussed later.
The most commonly employed apparatus for performing a batch extraction is a
separatory funnel, since it is a relatively simple matter to add and withdraw the respective
liquid phases. When extracting from a heavier liquid to a lighter solvent, it is necessary to
remove the lower phase from the funnel after each extraction before removing the
extracting solvent as in the case of ethyl ether extractions from aqueous solutions.
When performing a batch extraction, it is important to follow a few simple steps
to separate the phases for sampling for subsequent processing or estimation. Most batch
extractors are separatory funnels taper off into a narrow bottom with a sealed stopcock.
Thus, it is a relatively easy task to separate the two phases on withdrawal for further
processing. It is, of course, essential to wait until the phases have completely settled after
agitation. Usually this will occur in a matter of minutes.
If only aliquots of the phases are to be used, it is necessary to notice any volume
changes of the phases due to mutual solubility of the solvents. The extraction and
sampling must be performed at a constant temperature, since both the distribution ratio
and the volumes of the solvents are influenced by temperature changes. A useful method
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of withdrawing the phases for sampling involves the use of three graduates. Most of the
heavier phase is withdrawn into the first graduate and then the remainder of the heavier
phase and a little of the lighter phase are withdrawn into the second graduate. The
remaining portion of the lighter phase is run into the third graduate, and the volumes of
the three are noted. The second graduate can now be discarded and aliquots of the other
two taken without danger of contamination of one by the other.
If droplets of aqueous phase are entrained in the organic extract, it is possible to
remove them by filtering the extract through a dry filter paper. The aqueous droplets will
be absorbed by the paper, which should be washed several times with fresh organic
solvent. Another method commonly used in extractions is the addition of a drying agent,
such as sodium sulphate, to the organic extract. Perhaps, the simplest technique for
removal of slight traces of the aqueous phase which may contain certain impurities is the
use of the backwash technique, described later.
2.6.2 Continuous extraction
Continuous extractions are particularly applicable when the distribution ratio is
relatively small, so that a large number of batch extractions would normally be necessary
for quantitative separation. Most continuous extraction devices operate on the same
general principle, which consists of distilling the extracting solvent from a boiler flask
and condensing it and passing it continuously through the solution being extracted. The
extracting liquid separates out and flows back into the receiving flask, where it is again
evaporated and recycled while the extracted solute remains in the receiving flask. When
the solvent cannot easily be distilled, a continuous supply of fresh solvent may be added
from a reservoir.
High efficiency in continuous extraction depends on the viscosity of the phases
and other factors affecting the rate of attaining equilibrium, the value of the distribution
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ratio and the relative volumes of the two phases and other factors. One practical method
of improving the efficiency is to ensure as high an area of contact as possible between the
two liquids. As the extracting solvent passes through the solution, fritted-glass discs,
small orifices, baffles and stirrers may be used to bring the two immiscible layers in
closer contact.
2.6.3 Countercurrent extractions
The separation through continuous countercurrent method is achieved by virtue of
the density difference between the fluids in contact. In vertical columns, the more dense
phase enters at the top and flows downwards while the less dense phase enters at the
bottom and flows upwards. Only one of the phases can be pumped through the column at
any desired flow rate, the maximum rate of the second will be limited by that of the
former and the physical properties of both. The method has the advantage for separating
materials for purification purposes and is extensively used in engineering problems.
2.7 Factors Influencing the Extraction Efficiency
Primary requirement of solvent extraction for separation /removal purposes is a
high distribution ratio of the solute of interest between the two liquid phases. Though,
continuous and countercurrent distribution techniques may be used for the cases where
low distribution ratios are present, it is generally desirable to attain as high a value as
possible for the development of simple analytical procedures. It is useful to employ a
number of different techniques for enhancing the distribution ratio. These depend on the
nature of the species being extracted and extraction system.
The attainment of selectivity in an extraction procedure is also very important.
Some of the factors, which affect the distribution of solute of intrest are given below.
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2.7.1 Choice of solvent
Use of a suitable solvent for effective separation is very important. Metal chelates
and many organic molecules, being essentially covalent compounds do not impose many
restrictions on the solvent and the general rules of solubility are of great use. In ion
association systems and particularly in oxonium type ions, the role of solvents is very
important. This is due to involvement of solvent in the formation of extractable species.
In addition to the consideration of the distribution of the solute in a particular
solvent system, the ease of recovery of the solute from the solvent is important for
subsequent analytical processing. Thus, the boiling point of the solvent or the ease of
stripping by chemical reagents is considered in the selection of a solvent where the choice
exists. Similarly, the degree of miscibility of the two phases, the relative specific
gravities, viscosity and tendency to form emulsions should be considered. With regard to
safety, the toxicity and flammability of the organic solvents must be considered.
Some times it is possible to achieve the desired characteristics of a solvent by
employing a mixed-solvent system. An example of this is the use of mixtures of alcohols
and ethers for the extraction of the thiocyanate complexes of metals. Another method of
varying the properties of the extracting solvent is to use organic diluents. Various organic
compounds such as kerosene and other hydrocarbons are employed to dilute tributyl
phosphate for extraction purposes.
2.7.2 Acidity of the aqueous phase
The extractability of metal complexes is greatly influenced by the acidity of the
aqueous phase, so it is necessary to assure optimum concentration of H+ ions for
maximum extraction.
In case of chelate extraction, it can be seen from Eq.(2.15) that provided the
chelating reagent concentration is maintained constant, the distribution of the metal in a
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given system is a function of pH. For this reason, curves of extractability versus pH at
constant reagent concentration are of great analytical significance.
Acidity greatly affects many of the oxonium type of extraction systems,
particularly when the metal is extracted as a complex acid. In many of the extractions
involving metal halide complexes, e.g., the distribution increases within certain limits
with increasing acid concentration. Thus, maximum extraction of iron as the chloride is
observed at 6M hydrochloric acid using ethyl ether as the solvent [111]. A decrease in
extraction at higher acid concentrations has been attributed to the high solubility of ethyl
ether in highly concentrated hydrochloric acid and extraction does not occur until there is
a much higher acid concentration with the less soluble isopropyl ether. No decrease is
observed even at 12 M hydrochloric acid when β: β’ – dichloroehtyl ether is used [112].
The addition of high concentrations of acid also enhances the distribution of metal
complexes as a result of the common ion effect resulting from the anion of the acid.
As the removal of the acidic or basic solute will tend to change the pH value, even
more important in multiple extractions (countercurrent distribution), the use of buffer
mixture in the aqueous phase aids in the attainment of reproducible constant distribution
ratios. Buffers should be chosen which do not interfere in the subsequent analysis [113].
2.7.3 Salting-out agents
A technique that has resulted in marked enhancement of extraction of metals,
particularly in the oxonium type of extraction systems, is the use of salting-out agents.
The addition of high concentrations of inorganic salts to the aqueous phase greatly
increases the distribution ratio of many metal complexes to the organic phase. This
salting-out effect may be explained in part by the pronounced effect of the added salt on
the activity of the distributing species, the common ion effect, as well as the strong ability
of these ions to bind water around them, thereby depleting the aqueous phase of water
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molecules for use as a solvent. High concentrations of inorganic salts are usually required
to produce the desired effect, the aqueous phase often being saturated with the added salt.
It is essential that the added salt` is not extracted to an appreciable extent with the desired
species in order to maintain the optimum effect and to permit direct use of the organic
extract without further separation. Sometimes the aqueous phase after extraction may be
of interest so that the presence of large amounts of added cations prevent further use of
this phase in the subsequent analytical steps unless the added salt can be easily removed
or destroyed, like ammonium salts.
In addition to enhancement of the extraction of the metal of interest using salting-
out agents, it is also possible to increase the extraction of impurities in the system. Thus,
it is necessary to choose an agent that produces a favorable separation factor between the
element of interest and the impurities. The magnitude of enhancement of extraction by the
added salt depends on the charge as well as the ionic size of the added cation for a given
anion. Thus, polyvalent cations provide better salting-out agents, and for a given charge,
the smaller the cationic size, the greater the effect on extraction. However, it must be
remembered that anomalies sometimes result from specific interaction effects. Aluminum
or calcium salts are strong salting-out agents, whereas ammonium salts are much weaker
but analytically more convenient.
Among the metal extraction systems that have benefitted through the use of
salting-out agents are the nitrate, halide, and thiocyanate systems. Other oxonium
extraction systems should be investigated so that useful analytical separations may result
where now only limited extractability of a substance occurs.
2.7.4 Oxidation state
A useful method of increasing the selectivity of metal extractions involves
modification of the oxidation states of the interfering ions present in solution, in order to
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prevent the formation of their extractable metal complexes. For example, the extraction of
iron from chloride solutions can be prevented by reduction to iron-II, which is not
extractble. Similarly, antimony-V may be reduced to the tervalent state to suppress its
extraction. Conversely, it is important in the preparation of a solution for extraction to
adjust the proper valence state of metal ion required for formation of the complex in order
to insure complete extraction of that element. Selectivity can also be achieved by
variation of the oxidation state of the co-extracted interfering ions during the stripping
operation.
2.7.5 pH
The attainment of selectivity in metal chelate extractions is greatly dependent
upon proper pH control. As has been mentioned earlier, the distribution of chelates in a
given system is a function of pH alone, provided the reagent concentration is maintained
constant. Increased selectivity can be achieved in the extraction of acidic or basic organic
substances by the addition of buffer salts to the aqueous phase to control the pH [113].
2.7.6 Masking
In the extraction procedures for metal pairs that are difficult to separate, masking
or sequestering agents are introduced to improve the separation factor. The masking agent
forms water-soluble complexes with the metals in competition with the extracting agent.
Masking agents form sufficiently strong complexes with interfering metals to prevent
their reactions with the extraction agents, either altogether or at least until the pH is much
higher than the value needed for quantitative extraction of the metal of interest. Very
often the metal of interest also forms a complex with masking agent, with the result that a
somewhat higher pH range is needed for the extraction The application of masking
agents, which include cyanide, tartarate, citrate, fluoride, and EDTA, is restricted largely
to metal chelate extraction systems, since in the highly acidic solutions encountered in
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many inorganic extraction systems most masking agents, being weak bases, do not
function effectively. Ethylenediaminetetraacetic acid, which is proving a most useful
masking agent, has been applied to dithizone, 8-quinolinol, carboxylic acids, acetyl
acetone, and diethyldithiocarbamate extractions.
2.7.7 Backwashing
An auxiliary technique used with batch extractions to effect quantitative
separations of elements is backwashing. The combined organic phases from several
extractions of the original aqueous phase contain practically all the desired elements and
possibly some of the impurities that have been extracted to a much smaller extent. This
combined organic phase when shaken with one or more small portions of a fresh aqueous
phase containing the optimum reagent /salting agent concentration, acidity, etc., will
result in a redistribution of the impurities in favour of the aqueous phase since their
distribution ratios are low. Under optimum conditions, most of the elements of interest
will remain in the organic layer, since their distribution ratio is high. This technique is
analogous in many respects to the re-precipitation step in a gravimetric precipitation
procedure. With the proper conditions, most of the impurities can be removed by this
backwashing operation, with negligible loss of the main component, thereby attaining a
selective separation.
2.8 Synergic Extraction
Synergism is defined as the combined action of two complexing reagents, which
is greater than the sum of the actions of the individual reagents used alone. A typical
example of the synergic extraction of Ce(III) with picrolonic acid (HPA) and benzo-15-
crown-5 (B15C5) system [114] is shown in Fig. 1, which reveals that no extraction was
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observed by the B15C5 while little extraction with HPA was observed but quantitative
extraction exhibits with the mixture.
Ce(III)
0
20
40
60
80
100
1 1.2 1.4 1.6 1.8 2
pH
Extra
ctio
n (%
)
Fig. 1 Extraction of Ce(III) B15C5(▲), HPA (■) and HPA+B15C5(♦)
In general, the enhancement of the extraction may be attributed to either
thermodynamical changes in the activities of the extractants or the composition of the
metal-bearing species in the organic phase, which is not the same as in the cases of
individual extractant systems. Synergic systems are usually mixture of cation exchange
extractant and coordination extractant and the synergistic effect is thought to operate by
an enhancement of the ease with which the coordination sphere of the metal ion can be
satisfied.
Two methods of accomplishing this have been proposed. In the first, the synergist,
“S” replaces coordinated water in an extracted metal complex, thus making the resulting
complex more organophilic. In the second, the original extractable complex is
coordinately unsaturated and the synergist “S” adds to the complex, thereby enhancing its
stability.
Synergism was first reported in the literature for the extraction of uranium (VI)
with various dialkylphosphoric acid and neutral phosphorus alkylesters [115]. Now the
synergism appears to be a common phenomenon in many mixed extractant systems. In
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addition to organophosphorus compounds, hetrocyclic bases, sulfoxides, carboxylic acids,
phenols and amines are also common synergists. The synergistic effect is extremely
useful in solvent extraction practices and has been used for the extraction of various alkali
and alkaline earth metals [116-119], transition metals [120-123] and rare earths [124-129]
using different combinations of reagents.
2.8.1 Methods used for the study of synergistic extraction
The study of the nature of species formed and the equilibrium constants involved
with the synergistic extraction of metal ions have mostly been evaluated by the slope
analysis method [52]. This method basically deals with the determination of distribution
coefficient values (Kd) of the metal ions by varying one of the parameters ie., pH,
concentration of one extractant (chelating agent) or the other (neutral donor), while
keeping the other parameters constant. Sekine and Dyrssen [130] have used a curve fitting
method for the calculation of synergistic equilibrium constants of different metal ions.
Taketatsu et. al., [131,132] have evaluated the equilibrium constant values of the rare
earths using HTTA – TOPO system in the presence of different anions by a
spectrophotometric method. Desreux et.al., [133] have calculated the stability constants
of the adduct formed between Eu(TTA)3 and 4-methyl-2-pantanone, TPPO, or 2-
methylpyridine from the analysis of the concentration dependence on the induced shift
yields by a proton nuclear magnetic resonance study. Due to the simplicity of the “slope
analysis method” it has been widely used to get a clear picture of the stoichiometry and
extraction constant of adducts formed in the synergistic extraction of metal ion, while the
other methods mentioned above have been used only rarely.
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2.8.1.1 Slope analysis method
Slope analysis method is applied to determine the number of ligand molecules in
simple extraction using single extractant as well as in synergistic extraction where a
neutral donor is also added to the extraction system.
2.8.1.1.1 Extraction with acidic ligand
As an example we can consider the equilibria of a metal ion, Mn+ with chelating
agent (e.g. HA), the extraction of Mn+ by HA alone can be represented by the following
reaction.
++ +→←+ nHMAnHAMorg
An
Korg
n
for which the equilibrium constant KA is
nOrg
n
norgn
A HAMHMA
K]][[
][][+
+
= (2.18)
Anorgn D
MMA
=+ ][][
norg
n
AA HAHDK
][][ +
= (2.19)
or
n
norg
AA HHA
KD][
][+= (2.20)
norg
nAA HAHDK ]log[]log[loglog −+= + (2.21)
orgA HAnHnD ]log[]log[log −+= +
]log[ +−= HpH
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orgA HAnnpHD ]log[log −−= (2.22)
orgAA HAnnpHKD ]log[loglog ++= (2.23)
At constant concentration of HA, this equation represents the equation of a line. If log DA is poltted a
of intercept of this line. At constant pH, the plot of log DA vs log [HA] will give a straight
line, having slope equal to total number of HA molecules participating in the complex
formation.
2.8.1.1.2 Synergistic Extraction
As an example we can consider the equilibria of a metal ion, Mn+ with chelating
agent (e.g. HA) and a neutral oxo-donor, S (TBP, TOPO, TPPO and DOSO etc.).
Considering the synergistic extraction
++ + →←++ aqorgnK
orgorgnaq nHmSAMmSnHAM nsyn .)( (2.24)
Where m=1 or 2 and nsynK is the mixed equilibrium constant.
It can be shown that
naq
Orgnorg
synsyn HSHA
KD][
][][ 2
11= (2.25)
naq
orgnorg
synsyn HSHA
KD][
][][ 2
22 += (2.26)
The equilibrium constants of such reactions refer only to concentration quotients
whose calculations are based on the assumption that the activity coefficient of the species
involved do not change significantly under the experimental conditions applied. If it is
assumed that M(A)n.2S are the only synergistic specie present in the organic phase, the
overall distribution coefficient D is given by
21 synsynA DDDD ++= (2.27)
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It can be shown by Eqs.2.20, 2.25, 2.26 and 2.27) that
orgnaq
norg
synnaq
norg
synorg
A SHHA
KHHA
KS
DD][
][][
][][
][ 21 ++ +=− (2.28)
It is clear from Eq. (2.28) that if orgHA][ and aqH ][ + are maintained constant, plot of
vsSDD orgA ][)( −− [-S]org would be straight line with intercept and slope equal to
)]/[}.([1
naq
norgsyn HHAK + and )]/[}.([
2
naq
norgsyn HHAK + respectively. The
organic phase equilibrium constants β1, β2 and K2 of the synergistic reactions
SAMSAM nn .)()( 1→←+ β (2.29)
)(2.)(2)( 2 SAMSAM nn →←+ β (2.30)
)(2.)(.)( 2 SAMSSAM nK
n →←+ (2.31)
can be easily obtained from the slope and intercept mentioned above. Further details of
these calculations can be seen from references [134] and [135].
In earlier publications [136,137,138,139,140] on the synergistic extraction of
metal ions (trivalent lanthanides), several authors have reported the formation of only
M(A)3.(S)2 type species in the organic phase. The conclusions have been drawn from the
observed second power dependence of the neutral donor (S) in the plots of log D vs log
[S]org (keeping other parameters constant). With the knowledge of the stepwise formation
of all the complex species, the above supposition would be highly misleading. However,
in most of the publications of seventies and later, the formation of the stepwise first and
second synergistic species have been reported.
2.8.1.2 Job’s Method
Job’s method or method of continuous variation is also applied to determine the
composition of extracted species in synergistic extraction system [62]. In this method,
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overall concentration of both the ligands used in the synergistic extraction is maintained
constant (e.g., 0.01 mol dm-3 ) while changing concentration of both the ligands. log D is
plotted vs. mol fraction of the ligands. Number of ligands of both the extractants is
determined from the mol fraction of the ligands where maximum extraction is achieved.
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CHAPTER – 3 LITERATURE REVIEW
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3. LITERATURE REVIEW
Chemical analysis has become an important need of modern age. Analytical
measurements play key role in many areas of every day life i.e. controlling the quality of
products and processes, monitoring health, enforcement of regulations and the protection
of environment etc. Analysis is frequently an essential ingredient in research and
development and innovation. Billions of analytical measurements are undertaken daily in
the world. Many analytical techniques and instrumental methods are used for estimating
trace and ultra-trace elements in different materials. After gravimetric and volumetric
analysis, colourimetric analysis was introduced for the measurements of trace metals. For
measurement or to detect metals at trace level, some organic reagents are required.
Organic compounds have a great role in inorganic analysis. During the past 60-70
years, several hundreds organic reagents described in the analytical chemistry, have been
used in the metals analyses. Many of those reported earlier have been completely replaced
by the new ones. In analytical practice, much more efficient reagents are being used now
adays for metals analyses. Dialkyldithiocarbamate, xanthates, dithizone and toluene-3, 4-
dithiol are the sulphur containing organic reagents and are being used in metals analyses
[141].
Yoe has given a list of organic compounds, which were used in more than twenty
different ways covering a great variety of purposes [142]. Welcher has surveyed the
analytical work published before 1947, listing over seven hundred organic reagents [143].
Organic reagents contain an acidic or basic group. Most of these organic reagents react
with metals by the formation of covalent or co-ordinate bonds or both. The extraction
procedures that have been developed at the laboratory scale use mainly the neutral,
anionic and acidic types of extractant.
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Organo-phosphorus compounds (TOPO, TBPO, TBP etc.) [40-42], alkyl
sulfoxides (DOSO, DBSO, etc.) [43, 44] and N, N, dialkylamides (DHHA, D2EHIBA)
[45-47] are classified as neutral extractants. The main feature of this class of extractants is
their use in the separation of light actinides such as U and Pu, which can exist in IV, V
and VI oxidation state in aqueous medium. As, most of the rare earths exist in + III state,
so these extractants have little application for the separation of rare earths. Some
macromolecules having cyclic structures such as crown ethers and calixarenes have also
been used for rare earth separation [48, 49]. The methods based on neutral extractants for
rare earth separation use high acid and salt concentration which causes waste problems in
nuclear industry.
The REEs are hydrolysable metals and amines have the potential to extract them
from basic media [50,51]. The extraction of lanthanides/actinides by amine extractants
suffers from many of the same limitations as the neutral organophosphorus extractants.
Roelandt has discussed briefly the application of the TRAMEX process (based on
trialkylamine or tetraalkylammonium salts) for the purification of 242Cm [38]. Primary
and quaternary amines are indicated to be useful for REEs extraction in alkaline medium.
Another class of extractants is termed as acidic extractants. β-diketones (HTTA,
HFAA etc.), 4-acyl pyrazolones, salicylic acid and organo-phosphoric acid or their thio-
derivatives are the extractants that belong to this category [52-55]. Some work has been
cited in the literature evaluating the extraction of the metal ions by sulfonic acids, but this
class of liquid cation exchangers exhibit little selectivity and they have not proven useful
for REEs separations. The separation factors for the REEs by HTTA/benzene system
varies from 1.18 to 9.1 [56].
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The solvent extraction of the lanthanide elements is a very broad subject and has
been reviewed by several authors. Interest has been shown in their group separation from
trivalent actinide elements, and in their mutual separation.
Sekine and Hasegawa have reviewed the work of several authors [144]. Extraction
of rare earths with various alcohols, with cyclohexanone, from nitrate solution with TBP,
with trioctylphosphine oxide (TOPO), with other alkyl phosphates and phosphine oxides
with mixture of TBP and TOPO, under various conditions have been investigated. The
extraction with TBP is not very effective. The distribution ratios of the heavy lanthanides
between undiluted TBP and 12.3M nitric acid were reported to be from 10 (Terbium) to
7.2 (Lutetium), and those of the lighter lanthanides even lower The higher the
concentration of nitric acid and the larger the atomic number, the better the extraction,
although it reaches a maximum in some systems and then falls off with elements of larger
atomic number. The extraction of trivalent lanthanides from hydrochloric acid with TBP
is also poor. Trivalent lanthanides can be effectively extracted from thioctyanate solution
and with TBP. In the case of TBP, the extraction equilibria for europium (and americium)
were studied in detail. These ions in perchlorate solutions are also extractable with
trialkylphophine oxide. The extraction of perchlorates decreases with decreasing ionic
radius, which is just opposite of the extraction of nitrates or thiocyanates. Extractions of
trivalent lanthanides in various solutions with primary to tertiary amines and with
quaternary ammonium salts have been reported. In general, the extraction with amines is
effective only from sulfuric or sulfate solution. Into xylene, with 5% triisooctylamine,
10% Amberlite LA-1, or 10% Primene JM-T, it is 1% or less when the aqueous phase is
nitric acid or hydrochloric acid. From sulfuric acid with 5% triisooctylamine or 10%
Amberlite LA-1 it is not effective either, but with Primene JM-T it is quantitative if the
aqueous phase contains less than 0.1 M sulfuric acid and the amine concentration is
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higher than 0.1%. Extractions of organic ligand complexes of lanthanides with amines are
sometimes useful. Extractions of lanthanides as ion-pairs with a dye or other reagents
were also reported. Extractions of trivalent lanthanides with dibutylphosphoric acid and
other monoalkyl and dialkylphosphoric acids and relatives have been frequently studied.
However, the extraction with di(2-ethyl-hexyl) phosphoric acid (DEHP) has been
investigated most systematically . The separation factor of adjacent lanthanides in the
trivalent state by extraction with DEHP is around 2.5, and the extraction is the highest at
pH 1 to 4 when the concentration of the reagent in the diluent is not very low. Extractions
of lanthanides with butyric naphthenic and other carboxylic acids are also effective under
certain conditions. Chelating extractions are another important group of reagents for the
trivalent lanthanides. β-Diketones such as acetylacetone which does not extract these ions
very well by a single extraction and TTA have been used very often, but there have also
been reports on the extraction with other β-diketones. Synergism in the extraction with
TTA and other β-diketones and the extraction of mixed chelates and ternary complexes
with β-diketones has been investigated. Studies on the extraction of some of the
lanthanides with β-isopropyltropolone, with oxine and its homologues and with PAN
have been reported
The phenomenon in which two extractants taken together extract a metal ion
species with a much higher efficiency as compared to the normal additive effect of these
extractants (separately) is called synergism. From the first observation of this
phenomenon of synergism by Blake et al. in 1958 [57], extensive work has been carried
out on the synergistic extraction of 4f block elements.
In the nuclear industry, synergistic extraction has been recommended for the
recovery of the precious metal ions. In the extraction of trivalent lanthanides by using the
various mixtures of extractants, it has been observed that the synergistic enhancement
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S. E. (S. E. = log [KD (1,2)/KD(1) + KD(2)) in many cases is very high. This is one of the
reasons for the great interest shown in the synergistic extraction of REEs.
Synergic extraction systems comprising of various acidic extractants HTTA, 1-
Phenyl-3-methyl-4-acyl-5-pyrazolone (HPMAP), Salicylic acid (SCA), 8-
Hydroxyquinoline (oxine), HDEHP etc.) and various neutral donors (TBP, TOPO,
DOSO, TOA, MIBK, etc) are mostly used for the extraction of REEs. Mathur has
published a review on the synergic extraction of trivalent actinides and lanthanides and
suggested that synergic systems in which adducts or mixed ligand complexes are formed
can be used for the optimum separation of lanthanides from each other [58].
It is evident from this literature that inspite of having so much research work done
in this field, no simple, efficient and economical method for rare earth separation is
available and need of such a method still exist.
The class of chelating extractants which have received the most attention in the
recent years have the basic structure of 4-acylpyrazolone [59-61]. Because of their
increased acidity (relative to β-diketones) and various synthetic modifications, which can
be made to the basic structure, these extractants possess some possibilities for the
improved separation for the f-block elements [54].
3.1 Use of Picrolonic acid (HPA) in Copmlexation / Extraction
Picrolonic acid (HPA) belongs to pyrazolone family of extractants with strong
acidity, having capability for the extraction/mutual separation of lanthanides from acidic
aqueous solutions. Literature shows that very little work has been done for the extraction
of lanthanides by using this reagent.
Aleksandrov and Aleksandrova have carried out the photometric deterimanation
of Sn(II) with picrolonic acid. Stannous ions are precipitated with picrolonic acid.
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Deterimanation of Sn(II) up to 125 mg L of solution can be carried out with an average
relative error of 1.58%. Myasoedov et al. carried out the extraction studies of americium
in different oxidation states from nitric acid solutions using picrolonic acid in MIBK as a
function of acidity of solution, concentration of extractant and time of extraction.
Composition of extracted chelate compounds of americium was determined. They
concluded that picrolonic acid and also its mixtures with sulfoxides can be used for the
extractive separation of trans-plutonium elements from nitric acid solutions. During
extraction from nitric acid solutions by a mixture of picrolonic acid with petroleum
sulfoxides (PSO) and dihexyl sulfoxide (DHSO) in xylene, one can isolate Am(V) from
mixture of actinides as it is not extracted appreciably in these conditions [63].
Nikolaev et al. studied the synergistic extraction of trans-plutonium elements
using a solution of picrolonic acid in sulfoxides. The compositions have been determined
for the compounds of americium formed in a mixture of picrolonic acid with dihexyl
sulfoxide (DHSO) in xylene, as AmA3.2DHSO, where A is picrolonic acid ion. Effect of
diluent was also studied. Maximum extraction constant was observed in cyclohexane
solution, but the use of cyclohexane was restricted due to low solubilities of the reagents
in it. Therefore, use of xylene, benzene or toluene was recommended [64].
The synergistic extraction of uranium was carried out by. Kuvatov et al. using a
mixture of sulfoxides and picrolonic acid. Composition of the extractable complex was
established as UO2A2.2S. A general mechanism of extraction was proposed. The
individual sulfoxides i.e., dihexyl sulfoxide (DHSO), diphenyl sulfoxide (DPSO) and
dicyclohexyl-sulfoxide (DCHSO) as well as petroleum sulfoxides (PSO) representing a
mixture of sulfoxides of natural origin, chiefly of cyclic structure were used. The
extraction constants were calculated. These constants exhibit an increasing trend with
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increasing basicity of the neutral ligand. The extraction involved the formation of a
sulfoxide-picrolonic acid adduct in the organic phase [62].
Osman et al. prepared the complexes of picrolonic acid with the divalent metals
(Mn, Fe, Co, Ni, Cu, and Zn) and trivalent lanthanides (La, Nd, Eu, Gd, and Er). They
characterized the compounds by the chemical / thermal analysis and IR. The electronic
spectra and magnetic spectra suggest the octahedral structure for the metal (II)
complexes. Neutral complexes of general formula M(PA)2.2H2O and M(PA)3.2H2O were
isolated for metal (II) and metal (III) ions respectively (PA = Picrolonic ion).
Coordination of the ligand with the metal ions occurs through the carbonyl and adjacent
nitro-group [145].
Lorenzotti et al. prepared and studied the complexes of Ni(II) with picrolonic
acid. The complexes were characterized by elemental analysis, magnetic susceptibility,
electronic, IR and NMR spectral methods. These complexes were generally insoluble due
to a polymeric structure probably involving a bridging of picrolonic anion [146].
Metal complexes of Mn, Fe, Co, Ni, Cu, and Zn were prepared and studied by
Lorenzotti et al. These were characterized as ML2.H2O, where M is metal and L is
picrolonic acid. The compounds were insoluble in common solvents except DMSO,
where solvation takes place. Some of the compounds were remarkably stable thermally
but some also deflagrate. Picrolonic acid acts as a possibly bidentate ligand. The
complexes were probably polymeric except in dimethyl sulfoxide DMSO [147].
Komarek et al. studied the estimation of Ca using atomic spectrophotometry. They
have extracted Ca using picrolonic acid in order to remove the interference of NH4+, PO4
3-
Cr, Ba, Mn, Li , Al and Ni. The picrolonic acid extraction procedure gave sensitivities of
0.07 and 0.05 ppm in C2H2-air and N2O-C2H2 flames respectively [148].
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Nina and Valerica have studied the complexes of Zn with picrolonic acid. Zn
reacts with picrolonic acid in the presence of 2, 2 Dipyridyl or 1-10 phenanthroline
forming ternary complexes with a component ratio of 1:2:2. The ternary complexes were
extracted in chloroform. Characterization of the complexes was investigated
spectrophotometrically. The molar absorptivities and the optimal concentration were
determined [149].
Ali has studied the extraction of Eu (III) and Tm (III) using picrolonic acid in
methyl isobutyl ketone. Composition of the adduct has been determined as M (PA)3 (M =
(Eu (III), Tm (III)) [65]. He also studied the extraction of Nd (III), Tb (III) and Lu (III)
with picrolonic acid in MIBK. Composition of complexes was found to be M(PA)3 [150].
Synergistic extraction of Ce (III), Eu (III) and Tm (III) was studied by Ali with a
mixture of picrolonic acid and tributyl phosphine oxide in chloroform. Composition of
synergistic adduct has been determined to be M (PA)3. 2TBPO (M = Ce (III)), Eu (III)
and Tm (III)) [151].
The literature cited indicates limited use of picrolonic acid for the determination
of a few divalent metals and for the complexation of trivalent lanthanide whereas only a
few references have been cited for the extraction studies of uranium and americium in
combination with sulfoxides and dipyridyl as neutral donors, as well as, as single
extrtactant in MIBK, which itself acts as neutral donor. The use of picrolonic acid in
combination with crown ethers as neutral donor has not been reported in literature so far.
3.2 Use of Crown Ethers in Extraction of REEs
Macrocyclic polyethers generally called as “Crown ethers” have gained attention
due to their special selectivity arising presumably from their ring-size comparable with
the ionic radii of certain alkali metals [152-154]. The crown ethers can be used as neutral
oxodonors for the synergistic extraction of various metal ions with chelating, acidic or
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neutral extractants and the information regarding such extraction systems have been
reviewed [6]. The synergistic extraction systems mostly used for the separation of alkali,
alkaline earth and divalent transition metals; comprises crown ethers and
thenoyltrifouoroacetone [158-163] or 4-acyl-pyrazolones [163-166]. The same extraction
system [167-175] as well as combinations of β-diketone (4, 4, 4-trifluoro-1-phenyl-1, 3-
butanedione) [176] and 4-acyl isoxazolone [177,178] with crown ethers have been used
for the extraction of lanthanides and actinides. Mixtures of crown ethers and
alkylcarboxylic acids [179,180] and organophosphoric acid [181] have also been
investigated for the extraction of various metal ions.
Aly et al. have studied the extraction of Eu3+, Tm3+ and Yb3+ using a mixture of
theonyltrifluoroacetone (HTTA) and 15-crown-5 (15C5) in chloroform from 0.1M ionic
strength acetate buffer. It was found that the extraction increases to a large extent when
the mixture is used. This enhancement is due to formation of adduct in the organic phase.
The complexes were characterized as Ln(TTA)3.2CE where CE is crown ether. No
enhancement in extraction was observed when 12-crown-4 (12C4) was used. This was
explained due to small cavity size of the 12C4 as compared to 15C5. The same system
(HTTA+ 15C5) was applied for the extraction of Pu4+, Am3+, Nd3+ and Er3+. Moderate
enhancement was observed in the extraction of Am3+ while no enhancement was
observed in the case of Pu4+. Effect of metal ion concentration was also studied and it was
found that concentration affect the synergistic factor (S.F) for Nd3+and Er3+[167].
The synergistic extraction studies of trivalent Am, Cm, Cf and Eu were carried
out by Mathur and Khopkar using a mixtures of 1-Phenyl-3-methyl-4-trifuoroacetyl
pyrazolone-5 (HPMTFP) and a crown ether dicylohexano-18-crown-6 (DCH18C6) or
monobenzo-15-crown-5 (B15C5) in chloroform. With (DCH18C6) the synergistic species
extracted were M (PMTFP)3 .(HPMTFP).(DCH18C6) where M = Am, Cm and Eu, and
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Cf (PMTFP)3.(DCH18C6), where as with B15C5 the species are M(PMTFP)3.n(B15C5),
n being 1 or 2 for all these metal ions. They have studied the effect of pH, HPMTFP
concentration and concentration of crown ethers. They have calculated synergistic
constants for the extraction of all the metal ions mentioned above [175]..
Pavithran et al. investigated the extraction behaviour of Nd(III), Eu (III) and
Tm(III) from perchlorate solution into chloroform with 1-phenyl-3-methyl-4-pivaloyl-5-
pyrazolone(HPMPP) in the presence and absence of various crown ethers i. e.,18-crown-6
(18C6), (DCH18C6) and dibenzo-18-Crown-6 (DB18C6). The complexes were
characterized as Ln(PMPP)3 with HPMPP alone and in the presence of CE as
Ln(PMPP)3.CE. They calculated the equilibrium constants of the extraction of complexes
and found to increase with decreasing ionic radii of these metal ions. The addition of CE
to metal chelate system not only enhances the extraction efficiency but also improves the
selectivity among these trivalent lanthanides [182].
Mathur and Choppin carried out the extraction studies of UO22+, Eu3+, La3+ and
Th4+ complexes of TTA with 12C4, 15C5, 18C6, DCH18C6 and DB18C6 in benzene and
chloroform. They studied the thermodynamic parameters, NMR spectra and the nature of
complexes. The complexes were characterized as Ln(TTA)3.CE, UO2(TTA)2.CE and
Th(TTA)4.CE. They have reported slope 1 for all the crown ethers except that of Eu-
TTA-B15C5 which is reported as 1.5±0.1 showing the equal amounts of extracted species
with 1 and 2 molecules of B15C5 [169].
Reddy et al. studied the synergistic extraction of trivalent lanthanides ( Nd, Eu and
Tm) using mixtures of 3-phenyl-4-benzoyl-5-isoxazolone (HPBI) and 18C6, 15C5,
B15C5 or DB18C6. The trivalent metal ions were extracted into chloroform as Ln(PBI)3
with HPBI alone and as Ln (PBI)3.CE in the presence of crown ethers . The equilibrium
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constants of the above species were found to increase monotonically with the decreasing
ionic radii of these trivalent lanthanides [183].
Dukov studied the synergistic solvent extraction of Pr, Gd, and Yb with mixtures
of 1-phenyl-3-methyl-4-benzoyl-pyrazol-5-one (HP) and benzo-15-crown-5 in CCl4, C6H6
and CHCl3. The composition of the extracted species were determined as LnP3.nS where
Ln =Pr, Gd, Yb and n = 1 or 2. Mixed complexes Ln P3.S have been found when C6H6
and CHCl3 were used as diluents and both LnP3.S and LnP3.2S when CCl4 was used[171].
Georgiev and Zakharieva carried out the extraction studies of Pr, Gd and Yb with
mixtures of heptanoic acid H3C(CH2)5COOH (HA) and crown ethers B15C5, DCH18C6
and 18C6 in CCl4 as a solvent from aqueous chloride medium at constant ionic strength
0.1mol dm-3. A synergistic effect in the extraction of Pr, Gd and Yb with B15C5 and
antisynergistic effects in the system containing the other two crown ethers DCH18C6 and
18C6 were observed. For Pr, formation of a mixed complex Pr(HA)3.B15C5 is found and
the corresponding equilibrium constants were reported, while Gd and Yb form the
complexes Gd(HA)3.nB15C5 and Yb(HA)3.nB15C5 respectively, where n < 1 [180].
The synergistic extraction of Co(II) and Ni(II) with 1-phenyl-3-methyl-4-benzoyl
pyrazol-5-one (HPMBP) in the presence of crown ethers 18C6 or DCH18C6 in toluene
from 1.0 mol dm-3 chloride medium was investigated by Lakkis et al. [184]. Slope
analysis of the distribution curves showed that the composition of extracted species
depends on the cations found in the aqueous solution. From a LiCl or (CH3)4NCl
solutions the extracted species were M (PMBP)2.CE (M = CO, Ni).
Meguro et al. have studied the extraction of Am3+ in benzene with HTTA and
crown ethers such as 15C5, 18C6, DCH18C6, 24C8 and DB24C8. Synergistic effect by
the CE was observed regardless of the kind of CE used. The extracted species was found
to be Am(TTA)3.CE in the organic phase [185].
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The synergic extraction of lanthanide ions with HTTA and crown ethers in 1, 2-
dichloroethane was studied by Yoshihiro et al. [186]. Characteristic ion pair extraction of
the lighter Ln (III) was observed with 1, 2-dichloroethane containing HTTA and 18C6 or
DCH18C6 in which the cationic complexes, Ln(TTA)2.CE+ was formed and extracted.
Remarkable increase of extractability and selectivity were attained in the synergistic ion
pair extraction of lighter Ln ions which could be elucidated on the basis of size fitting
effect in complex formation of the lighter Ln ion with CE.
Khalifa et al. investigated the synergistic extraction of Co(II) with HTTA and its
mixture with DB18C6 at different temperatures in nitrobenzene, toluene or their mixture
from perchlorate aqueous media of constant ionic strength(0.1 M; H+,NaClO4) buffered
with acetic acid- sodium acetate solutions [161]. Composition of the adduct extracted was
deduced as Co(TTA)2 and Co(TTA)2.DB18C6. The extraction constants of the chelate
(K2,0), the mixed species (K2,1) and the formation constant of the adduct (β2,1) were
evaluated for different diluents used at different temperatures. It was found that logK2,0
and logK2,1 were increased with the increase in the dielectric constant (E) of the diluents
whereas logβ2,1 was decreased with the increase in E. The thermodynamic constant of the
system were calculated.
Ensor and Shah studied the synergistic extraction of Ce, Pm, Eu, Tm, Am, Cm, Bk
and Cf using HTTA, a nitrogen containing cryptand (222BB) and crown ether (15C5) in
chloroform from an aqueous media containing 0.05M NaNO3 and 0.01M acetate buffer to
control the pH. Neither the 15C5 nor the 222BB showed any ability to extract the
trivalent metal ions by themselves alone under the experimental conditions used.
However, both the compounds showed synergistic activity when combined with HTTA.
222BB showed more enhancement in the extraction than 15C5 [168].
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Sachleben et al. carried out the solvent extraction studies of Li, Na, K, Rb and Cs
nitrate salts by solutions of crown ethers in 1,2-dichloroethane and 1-octanol. The crown
ethers used were 18C6, 21C7 and 24C8 bearing cyclohexano, benzo, t-alkylbenzo and for
no substituents. The extraction selectivities expressed as separation factor for Cs over Na
were examined in relation to crown ether structure. They reported that cyclohexano
substituted crown ethers extract cations more strongly than the corresponding benzo
substituted crwon ethers [156].
The synergistic extraction of trivalent actinides and lanthanides was studied by
Dale et al. using HTTA and an aza-crown ether, 4,13-didecyl-1, 7, 10, 16-tetraoxa-4, 13-
diaza cyclooctadecane (k22DD) [187]. The extraction of Am(III), Cm(III), Eu(III),
Ce(III) and Pm(III) from an aqueous acetate buffer system (pH 4.8) into
HTTA/K22DD/chloroform phase was studied at 25°C
. Distribution coefficients were measured as a function of pH and HTTA and K22DD
concentration. The synergistic adduct was characterized as M(TTA)3.K22DD. The results
showed that K22DD synergizes the extraction of each metal studied by a factor of 104-105
approximately. Slightly large stability constant were found for the trivalent actinides
relative to trivalent lanthanides.
Richard et al. have studied the extraction of cesium nitrate from a mixture of
alkali metal nitrates by calix(4)arene crown-6 ethers in 1,2-dichloroethane. Results
showed that smaller substituents (but larger than C2) at the phenolic position of calixarene
opposite the crown ether increases both the extraction efficiency and cesium selectivity.
Benzo subsitituents on the crown ethers tend to decrease the extraction, while increase the
cesium to sodium selectivity [188].
Dessouky et al. carried out the synergistic extraction studies of octahedral Co (II)
from nitrate medium by 8-hydroxy-quinoline (HOX) and DB18C6 in chloroform. The
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effect of stirring speed, temperature, specific interfacial area and interfacial tension on the
extraction showed, that the extraction process is bulk aqueous phase rather than at
interface [189].
Mathur studied the extraction of Am3+ with pyrazolones (HA = HPMBP or
HPMTFP) alone as well as their mixtures with B15C5 or DCH18C6 in chloroform. The
extracted complexes were stoichiometrically identified as Am(A)3.HA,
Am(A)3.HA.DCH18C6, Am(A)3HA.B15C5 and Am(A)3.B15C5 or Am(A)3.2B15C5
[190].
Lin-Mie et al. carried out the extraction of rare earth metals using crown ethers
such as 15C5, 12C4 and DB18C6 from aqueous solution containing picrate ion into
nitrobenzene solution. The rare earth metal ion Eu(III) was extracted as 2:1 crown-ion
sandwich complex with 12C4 but as 1:1 complex with both 15C5 and DB18C6. The
effect of picric acid concentration was also studied. The extracted species of Eu(NO3)3
with 15C5 and Db18C6 were characterized as Eu[(15C5)-(picrate)2.(NO3)] and
Eu[(DB18C6)-(picrate)2.(NO3)], respectively, but Eu[(12C4)2.(picrate)3)] were found
with 12C4. The extraction of the other rare earth ions showed that Tb3+, Eu3+, Gd3+, Nd3+
and Yb3+ can be easily extracted using 15C5, however, the extraction of Ce4+, Sm3+,
Dy3+and Lu3+ was difficult [191].
Dale et al. have studied the extraction of Ce(III), Pm(III), Eu(III) and Tm(III) by a
mixture of didodecylnaphthalensulfonic acid and 15C5. The extraction was carried out
from aqueous solution of pH 2 (0.5M, NaClO4+ HClO4) into toluene and it was found that
extraction of Ce(III), Pm(III) and Eu(III) was enhanced while extraction of Tm(III) was
unaffected [192].
Mohapatra and Manchanda studied the ion pair extraction behaviour of uranyl ion
from aqueous solution of pH 3 using B15C5, 18C6, DB18C6 and DB24C8 and picric acid
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in chloroform. The extracted species were characterized corresponding to
[UO2(CE)n]2+[pic-] where n = 1.5 for B15C5 and 1 for 18C6 as well as DB18C6. Adduct
of DB24C8 was not observed, as no extraction was found practically using this reagent.
Separation of lanthanides (Nd, Ce, La and Gd) from uranium was studied and separation
factors were calculated using B15C5, 18C6 and DB18C6. Highest separation factors for
these elements were found using B15C5 [193].
Shehata et al. have studied the synergistic extraction of trivalent Gd, Eu and Am
using 15C5 or 18C6 with HTTA in chloroform from perchlorate medium at pH 3.45. The
slope analyses results indicated a general formula of M(TTA)3.(CE)2 for the extracted
species. The extraction constants of the extracted species were also determined [194].
Vanura et al. have studied the extraction of Cs by nitrobenzene solution of
hydrogen bis-1, 2-dicarbolylcobaltate in the presence of 15C5 from a water- HNO3
system [195].
Yonezawa and Choppin invstigated the extraction of UO22+, Am3+ and Th4+ using
1-phenyl-3-methyl-4-benzoyl-5-pyrazolone and 12C4, 15C5, 18C6, DB18C6 and
DCH18C6 [196]. The extraction was carried out from 0.1M (NaClO4) solution into
toluene. The synergic equilibrium constants were calculated.
Pavithran and Reddy carried out the synergistic extraction of trivalent lanthanides
Nd(III), Eu(III) and Tm(III) from nitrate solution into chloroform with 3-phenyl-4(4-
fluorobenzoyl)-5-isoxazolone (HFBPI) in the presence and absence of various crown
ethers 18C6, DCH18C6, B18C6 and DB18C6. They reported that these lanthanide ions
were extracted into chloroform as Ln(FBPI)3 with HFBPI alone and as Ln(FBPI)3.CE in
the presence of crown ethers. The equilibrium constants for these extracted complexes
were found to increase monotonically with the decrease in ionic radii of these metal ions.
The addition of crown ether to metal chelate system significantly improves the extraction
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efficiency of these metal ions. The strength of trivalent lanthanides complexes with
various CEs follows the order DCH18C6>18C6>B18C6> DB18C6 [178].
Reddy et al. have investigated the synergistic extraction of the trivalent
lanthanides Nd, Eu and Tm with mixtures of 4, 4, 4-tri-fluoro-1-phenyl-1, 3-butan-dione
(HBTFA) and 18C6, DCH18C6 or DB18C6 in 1,2-dichloroethane from perchlorate
solution. The extracted species were characterized as Ln(BTFA)3.CE and heavier
lanthanide Tm(III) was extracted as Tm(BTFA)3.CE. The addition of crown ethers to
metal chelate system not only improves the extraction efficiency of these trivalent metal
ions but also improves the selectivities significantly among the lighter and middle
lanthanides [176].
Sahu et al. carried out investigation on the extraction of thorium (IV) and uranium
(VI) from nitric acid solution into chloroform using a mixture of 3-phenyl-4-benzoyl-5-
isoxazolone (FPBI) and DCH18C6, B18C6, DB18C6, or B15C5 [177]. These complexes
were extracted as Th(PBI)4 and UO2(PBI)2 with HPBI alone and as Th(PBI)4.CE and
UO2(PBI)2.CE in the presence of crown ethers. The equilibrium constants of the above
species were deduced by non-liear regression analysis. The addition of a CE to the metal
chelate system enhanced the extraction efficiency and also improved the selectivities
between thorium and uranium.
From the literature cited above and survey made from a lot of other published
work, it appears that crown ethers are being used extensively for the extraction of metal
ions as single extractant as well as, as neutral donors with many other complexing /
extracting agents. However, no reference has been cited showing the use of crown ethers
as neutral donor in combination with picrolonic acid for the extraction of lanthanides.
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CHAPTER – 4 EXPERIMENTAL
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4. EXPERIMENTAL
4.1 Apparatus
A pH meter model 605 from METROHM Ltd, Switzerland was used for the
measurement of pH of buffer solutions. Electric hot plate (George & Griffin) was used for
dissolution of samples. Nal(T1) scintillation detector coupled with the counting assembly
from Tennelec Inc., USA was used for gross gamma counting. The purity of
radionuclides was checked by p-type coaxial high purity germanium (HPGe) detector
(Eurisys Mesures) with 60% relative efficiency and 1.95 FWHM at 1332 keV γ-ray of
60Co. The detector is connected to an Ortec-570 amplifier and Trump PCI 8k ADC/MCA
card with Gamma Vision-32 ver.6 software. An electrical wrist action shaker from
George and Griffin, UK, was used for mixing aqueous and organic phases. A centrifuge
machine (Gallenkamp) was used for separating the aqueous and organic phases.
4.2 Materials
Picrolonic acid (HPA) was procured from Eastman Organic Chemicals, USA and
benzo-15-crown-5 (B15C5), 12-crown-4 (12C4) and 18-crown-6(18C6) were procured
from E. Merk, Germany. All other chemical used in this study were of Analar grade.
Solvents, acids and chemical reagents used in this study were acetylacetone, benzene,
chloroform, 1-butanol, 2-butanol, n-hexanol, 1-octanol, n-butylether, toluene,
dichloroethylether, cyclohexanone, di-isobutylketone, hydrochloric acid (HCl), nitric acid
(HNO3), perchloric acid (HClO4), sulfuric acid (H2SO4), sodium hydroxide (NaOH),
acetic acid (CH3 COOH), boric acid (H3BO3), potassium chloride (KCl) ammonium
hydroxide (NH4OH), sodium acetate (CH3COONa), tributylphosphate,
tributylphosphineoxide, triphenylphosphate, triphenylphosphineoxide, and were used as
received.
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For target preparation quartz ampoules (0.5×1.5 cm), polyethylene capsules
(1.25×2.5 cm) and aluminum capsules (2.2×4.6 cm) were used. For extraction of REEs
pyrex glass culture tubes (16x120 mm) with screw caps were used.
4.3 Buffer Solutions
The buffer solutions of pH 1.0 to 3.0 were prepared by mixing appropriate
quantities of potassium chloride (0.1 mol dm-3) and hydrochloric acid (0.1 mol dm-3)
using pH meter. The solutions of pH 4-6 were prepared by mixing 0.1 mol dm-3 solution
of acetic acid with 0.1 mol dm-3 sodium acetate solution. The solutions of pH 7-10 were
prepared by mixing 0.1 mol dm-3 boric acid solution with 0.1 mol dm-3 sodium hydroxide
solution. The stability of the buffer solutions was checked on alternate days.
4.4 Chemicals/reagents used to study anions and cations
effects
Sodium citrate(Na3C6H5O7.3H2O), ascorbic acid (C6H8O6), sodium thiosulphate
(Na2S2O3.5H2O), sodium oxalate (Na2C2O4), sodium tartrate (Na2C6H4O6.2H2O), sodium
acetate (CH3OONa.3H2O), sodium fluoride (NaF), potassium chloride (KCl), sodium
bromide (NaBr), sodium iodide (NaI), potassium thiocyanate (KSCN), potassium cyanide
(KCN), sodium carbonate, and sodium phosphate (Na3PO4) were used to observe the
anions effects. Cobalt chloride (CoCl2), copper sulphate (CuSO4), manganese chloride
(MnCl2), ferric nitrate [Fe(NO3)3], barium chloride (BaCl2), cadmium chloride (CdCl2),
strontium chloride (SrCl2), zirconium chloride (ZrCl2), lead nitrate [Pb(NO3)2], nikal
nitrate [Ni(NO3)2], chromium chloride (CrCl3), zinc chloride (ZnCl2) and magnesium
floride (MgCl2) were used to study effect of cations on the extraction of REEs using this
synergic system.
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4.5 Preparation of Radionuclides
All the radionuclides, i.e.141Ce, 147 Nd, 154Eu, 160Tb, 170Tm, 177Lu, 56Mn, 65Ni,
60Co, 59Fe, 64Cu, and 75Se were prepared by irradiating a known amount (~ 10 mg) of the
respective spec-pure metal or metal oxides (Johnson Matthey Chemical Ltd., England)
separately, in the Pakistan Research Reactor-1 (PARR-1) of Pakistan Institute of Nuclear
Science & Technology (PINSTECH) having an average thermal neutron flux of 5×1013 n
cm2 s-1. The irradiated material was given enough cooling time to allow the decay of the
short-lived radioisotopes formed (if any) during the thermal neutron irradiation process.
The time for thermal neutron irradiation and cooling was selected in such a way that the
major radioactivity was due to the radionuclide of interest. Each irradiated metal/metal
oxide was dissolved in 10 mL concentrated nitric acid in a 100 mL Pyrex glass beaker on
an electric hot plate, separately. The contents of the beaker were heated to near dryness
and then re-dissolved in 5.0 mL of 0.01 mol dm-3 nitric acid and the volume was made up
to 25 mL with de-ionized water and stored in pre-cleaned vials as stock solution. The
radioactivity of 50 µL of each stock solution was measured by using well type NaI(T1)
scintillation detector coupled with the counting assembly from Tennelec Inc., USA., and
further dilution was done using de-ionized water in such a way that the 50 µL of final
solution would have radioactivity of 25000-30000 counts per minute. This solution was
marked as radiotracer solution of particular radionuclide. The metal ion concentration of
appropriate amount of carrier solution (non-active solution of respective metal/metal
oxide was added to the radiotracer solution so that the 50 µL of each radiotracer in 2 mL
aqueous phase give constant metal ion concentration (~ 1.5×10-5 mol dm-3). 137Cs+ and
203Hg2+ solution were provided by isotope production division of this institute. Different
isotopes used in this study along with their half lives are given in Table 4.1.
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4.5.1 Calculation of the Activity of Radiotracer
Radioactivity of the radiotracer was calculated by the following equation.
Tt eeNA λλσφ −−−= )1( (4.1)
..wtAtfWA
N××
= o (4.2)
Where
λ = 0.693/t1/2
A = Disintegration of radiotracer in one second (dps).
σ = Thermal neutron absorption cross section of isotopes in barn (b=10-24cm).
f = % Abundance.
Ao = Avogadro number = 6.023×1023
φ = Neutrons flux of reactor = 5×1013 n cm-2 s-1
W = Weight of target material in gms
Wt. = Atomic wt. of target material
t = Time of irradiation
t1/2 = Half life of desired isotopes
Curi = 3.7×1010 dps
T= Cooling time
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Table 4.1 Radioisotopes prepared in PARR-1
Isotope Half life
Ce141 32.5d
Nd147 10.9h
Sm151 93 y
Eu154 8.8 y
Tb160 72.1d
Tm170 128.6d
Lu177 160.1d
56Mn 2.58h
59Fe 44.6d
60Co 5.27y
65Ni 2.52h
64Cu 12.7h
75Se 120d
137Cs
203Hg 46.4d
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4.6 Experimental Procedures
4.6.1 Extraction procedure A 2 mL portion of aqueous phase of known pH was taken in a glass culture tube
having a screw cap and added 50 µL solution of radiotracer solution of particular
radionuclide to it. Organic phase (2.0 mL) containing a known amount of picrolonic acid
or/and B15C5 in chloroform was added to the culture tube and mixed together using wrist
action electrical shaker for five minutes. After centrifugation for 3 minutes, one mL of
each phase was pippeted out in glass counting vials and assayed radiometrically using
well type Nal(Ti) scintillation counter. All the experiments were carried out in duplicate
and the average results were taken.
The extraction (%) and distribution ratio (D) of 154Eu+3 was deduced from counts
per minute (CPM) of the organic and aqueous phases by the following relationships:
backgroundphaseaqueous
backgroundphaseorganic
CPMCPMCPMCPM
D−
−= (4.3)
)/(
100%phaseorganicphaseaqueous VVD
DExtraction+
×= (4.4)
The same extraction procedure was applied using 141Ce+3, 147Nd+3, 160Tb3+,
170Tm+3 and 177Lu3+ radiotracers for the determination of their distribution ratios. The
estimated error in the distribution coefficient was about ± 4%.
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4.6.2 Effect of shaking time on extraction of REEs The equilibration time required for the extraction of lanthanide ions with a
mixture of HPA and B15C5 in chloroform was studied by shaking the aqueous and
organic phase. Two mL aqueous buffer solution of known pH containing ~1.5x10-5 mole
dm-3 solution of radiotracer of REEs(III) was taken in pyrex glass culture tubes. An equal
volume of chloroform containing 0.01mol dm-3 HPA and B15C5 was added, equilibrated
for a specific time and centrifuged for three minutes. One mL of aliquot of each phase
was taken and assayed radiometrically on Nuclear Chicago Model 8725 well type NaI(Tl)
scintillation counter for gross gamma counting. All the experiments were carried out in
duplicate and average results were taken for further use. The estimated error in the
distribution coefficient was about ± 4%.
4.6.3 Synergistic extraction with mixture of HPA and B15C5
The extraction of rare earth ions Ce(III) , Nd(III), Eu(III), Tb(III), Tm(III),
Lu(III), as representatives of the lanthanide series was studied using their radiotracers
141Ce3+, 147Nd3+, 154Eu3+, 160Tb3+, 170Tm3+ and 177Lu3+, respectively as per “Extraction
Procedure” (section 4.6.1). Equimolar (0.01 molL-1) solutions of HPA, B15C5 and their
mixture in chloroform were used for the extraction of these lanthanide ions from aqueous
solutions of known pH, separately. The synergism (Dsyn) produced was calculated using
the correlation:
Dsyn = Dmix/ (DHPA + DB15C5) (4.5)
DHPA, DB15C5 and Dmix represent the distribution coefficients using organic phase
containing HPA, B15C5 alone, and mixture of HPA and B15C5, respectively.
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4.6.4 Effect of REEs concentration
The procedure was similar to that of extraction. Only carrier of REEs having
different concentrations in the range of 6.5×10-5 to 3.3×10-3 mol L-1 was added before the
addition of radiotracer of desired rare earth metal. Rest of the procedure is same as given
under extraction in section 4.6.1.
4.6.5 Composition of extractable organometallic complex
Composition of the synergistic adduct responsible for the extraction of lanthanide
ions was investigated using the slope analysis and Job’s methods.
Slope Analysis method
Three types of experiments were performed with all the rare earths studied
separately. Number of conjugate base molecules of HPA, participating in extractable
complex formation to satisfy the primary valency of the central metal ion was determined
by studying the extraction of rare earth metal ions at different pH of aqueous phase. For
this purpose 2.0 mL aqueous buffer solution of known pH containing a fixed amount of
radiotracer of REE was taken in pyrex glass culture tubes and shaken for five minutes
with equal volume of chloroform having an equi-molar (0.01 mol dm-3) mixture of HPA
and B15C5. Centrifuged for three minutes and separated the layers. One mL from each
layer was taken and assayed radiometrically. Slope of the graph of distribution coefficient
(log D) vs. pH of aqueous phase gives the number of conjugate base (PA-) of HPA
participating in reaction to satisfy the primary valency of the rare earth metal ion.
To determine the total number of HPA molecules participating in the formation of
extractable species, extraction of REEs was studied at a fixed pH by varying the
concentration of HPA. For this purpose 2.0 mL aqueous buffer solution of known pH
containing a fixed amount of radiotracer of REE was taken in pyrex glass culture tubes
and shaken for five minutes with equal volume of chloroform having different
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concentration of HPA (e.g. 0.0001 to 0.005 mol dm-3) and a constant concentration (0.005
mol dm-3) of B15C5. Centrifuged for three minutes and separated the layers. One mL
from each layer was taken and assayed radiometrically. Distribution coefficients (log D)
were plotted against HPA concentration. Slope of this graph gives the total number of
molecules of HPA attached to central metal ion.
In order to find the number of molecules of B15C5, the same procedure was
repeated keeping the metal ion and HPA concentration constant and varying the
concentration of B15C5 (e.g 0.0001 to 0.01 mol dm-3). Distribution coefficients (log D)
were plotted against concentration of B15C5. Slope of this grap gives the number of
molecules of B15C5 attached to metal ion.
Job’s Method
In this method, overall concentration of both the ligands used in the synergistic
extraction is maintained constant (e.g., 0.01 mol dm-3 ) while changing concentration of
both the ligands. log D is plotted vs. mol fraction of the ligands. Number of ligands of
both the extractants is determined from the mol fraction of the ligands where maximum
extraction is achieved.
4.6.6 Effect of neutral ligands on the extraction of REEs
In order to study the effect of other neutral donors on the extraction of REEs using
HPA, extraction of Eu(III) was studied using TOPO, TPPO, TBP and TPP as neutral
donors. All the experiments were performed as per extraction procedure given in sections
4.6.1, 4.6.3 and 4.6.5.
4.6.7 Effect of Anions on the Extraction of REEs
Two mL aqueous buffer solution of pH 2 having a known concentration of
radiotracer of REE and anions as their sodium or potassium salts (as per list given in
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section 4.4) at a 100 fold higher concentration than the concentration of REE was shaken
with mixture of 0.01 mol dm-3 of HPA and B15C5 in chloroform for five minute.
Centrifuged for three minutes, separated the layers, one mL of liquid from each layer was
taken and was assayed radiometrically as per extraction procedure given in section 4.6.1.
4.6.8 Effect of cations on the extraction of REEs
Similar to anions effect, two mL aqueous buffer solution of pH 2 having a known
concentration of radio tracer of REE and cation at a ~100 fold higher concentration than
the concentration of RE metal ion was shaken with mixture of 0.01 mol dm-3 of HPA and
B15C5 in chloroform for five minute. Centrifuged for three minutes, separated the layers,
one mL of liquid from each layer was taken and was assayed radiometrically and rest of
the procedure was as per section 4.6.1.
4.6.9 Effect of solvents on the extraction of REEs
In order to study the effect of solvents on the extraction of REEs using HPA as
extractant, extraction of Eu(III) was studied in acetylacetone, benzene, cyclohexanone,
dichloroethylether, n-butylether, di-isobutylketone, 1-octanol, n-hexanol and toluene. All
the experiments were performed as per extraction procedure given in sections 4.6.1 and
4.6.5.
Composition of the synergistic adduct responsible for the extraction of lanthanide
ions was investigated using the slope analysis method. Two types of experiments were
performed with all the rare earths studied, separately.
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4.6.10 Extraction of other metal ions with HPA+B15C5
(Decontamination studies)
The selectivity of this synergic extraction system has been checked by the
extraction of various metal ions with (HPA+B15C5)/CHCl3 (0.01mol dm-3) at the
optimum conditions of extraction using the radiotracers of Cs+1, Hg+2, Fe+3, Mn+2, Co+2,
Ni+2, Se+4and Cu64.
Similar to anions and cations effect, two mL aqueous solution of buffer of pH 2
was taken, radiotracer of different metal (Cs+1, Hg+2, Fe+3, Mn+2, Co+2, Ni+2, Se+4, Cu64)
were added separately. An equal volume (2.0 mL) of 0.01 mol dm-3 (HPA+B15C5)
solution in chloroform was added. After shaking for five minutes, centrifuged for three
minutes, phases were separated and one mL was taken from each layer and assayed
radiometrically. Further procedure was carried as per section 4.6.1.
4.6.11 Back extraction of REEs
Twenty five mL buffer solution of pH 2 containing radiotracer of REEs was
shaken for five minutes with equal volume (25 mL) of 0.01 mol dm-3 (HPA+B15C5) in
chloroform and separated both the phases. Two mL of organic layer loaded with REEs
was taken and it was shaken for five minutes with equal volume of water and nitric acid
of different concentration (0.2-1.0 mol dm-3) separately and centrifuged for three minutes.
One mL from each layer was taken and assayed radiometrically. Similar experiments
were performed with hydrochloric acid and perchloric acid.
4.7 Acid dissociation equilibria of HPA
The acid dissociation equilibrium of HPA is given by the Eq. (4.6).
−+ +↔ PAHHPA
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][][][
HPAPAHKa
−+
= 4.6
Ka is the acid dissociation constant. This was determined by titrating HPA (0.01
mol dm-3) in a mixture of 1, 4-dioxane and water against sodium hydroxide in the same
solvent mixture. For this purpose, HPA solution was prepared by dissolving a known
amount of HPA in 1, 4-dioxane and diluting it with water. A series of solutions having
0.01 mol dm-3 HPA in 20%, 30%, 40% and 50% 1,4-dioxane in water wrer prepared.
Similarly, 0.01 mol dm-3 sodium hydroxide solution was prepared by dissolving a known
amount of NaOH in 20-50% 1,4-dioxane solutions separately.. 30 ml of HPA in 20% 1,4-
dioxane solution was taken in a conical flask and titrated with NaOH solution in 20%
dioxane solution, while measuring pH. Same titration was carried out using HPA
solutions in 30%, 40% and 50% dioxane, with corresponding NaOH solutions. pH of the
resulting solution after each addition of NaOH solution was measured , until the pH
became more than 12. pH was plotted against volume of NaOH solution used for each
concentration of dioxane solution and apparent pKa values were calculated for all the
solutions separately. The pKa (-log Ka) of HPA was determined by extrapolating the
linear plot of apparent pKa values obtained in 20%-50% υ/υ 1, 4-dioxane/ water solutions
against the 1, 4-dioxane concentration to the intercept.
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CHAPTER-6 RESULTS AND DISCUSSION
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5. RESULTS AND DISCUSSION
5.1 Extraction of REEs with HPA and Crown Ethers
In the preliminary studies, the synergistic extraction of Eu(III) (~ 1.5×10-5 mol
dm-3) was studied using equimolar solutions of crown ethers (0.01 mol dm-3 ) i.e. (12C4,
B15C5 and 18C6) as neutral donor and HPA in chloroform separately as well as in
combined from aqueous solutions of pH 1-2 having ionic strength 0.1 mol dm-3 (H+/K+,
Cl). Yellow colour of HPA appeared in aqueous phase beyond pH 2 indicating the
solubility of HPA in aqueous phase beyond pH 2 thus reducing its concentration in
organic phase. Therefore, extraction was not studied beyond pH 2. The results showed no
significant synergism in the extraction of metal ions by HPA with 12C4 and 18C6
contrary to B15C5 in the pH range studied. Therefore, further studies were carried out
using only B15C5 with HPA in chloroform. The various parameters of extraction such as
pH of the aqueous phase, effect of equilibration time etc. were optimized and results are
discussed below.
5.1.1 Effect of pH of aqueous phase
The extraction of Ce(III), Nd(III), Eu(III), Tb(III), Tm(III) and Lu(III) (~ 1.5×10-5
mol dm-3) separately with equimolar (0.01 mol dm-3) solutions of HPA, B15C5 and their
mixture in chloroform from buffer solutions of pH (1.0 - 2.0) having ionic strength of 0.1
mol dm-3 (H+/K+, Cl-) has been studied and results are shown in Figs. 2-4. The extraction
of these metal ions with HPA and B15C5 alone was negligible in this pH range, whereas
with the equimolar mixture of HPA and B15C5, extraction was quite high even at pH 1
and continued increasing with increase in pH and became quantitative (≥ 99%) at pH 2
showing a pronounced synergism
Dsyn = Dmix/(DHPA+DB15C5) 5.1.1
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in the range of 102 to 103. As the extraction of all the metal ions studied became
quantitative at pH 2, it was selected for all the further experimental work.
5.1.2 Effect of equilibration time
To optimize the equilibration time sufficient for the extraction of lanthanide ions,
extraction of Eu(III) (1.5×10-5 mol dm-3), as a representative of REEs, with 0.01 mol dm-3
mixture of HPA and B15C5 in chloroform was studied by shaking the aqueous and
organic phases for one minute to 10 minutes. The result showed that the equilibration was
achieved with in ≤ 3 minutes. However, five minutes was selected as optimum shaking
time for further studies.
5.1.3 Effect of metal ion concentration
The effect of metal ion concentration on the extraction of Nd(III), Eu(III) and
Tm(III) with 0.01 mol dm-3 HPA and B15C5 in chloroform. Results are shown in Fig. 5.
Extraction of Nd(III) in the range of 1.5×10-5 to 1.38×10-3 mol dm-3 was studied and
results showed that the extraction of Nd(III) was almost quantitative up to 9.7×10-4 mol
dm-3 after that it started decreasing and became 90.6% at 1.38×10-3 mol dm-3 . Extraction
of Eu(III) was studied in the range of 6.5×10-5 to 3.3×10-3 mol dm-3 and it was found that
the extraction of Eu(III) was almost quantitative up to 9.2×10-4 mol dm-3 after that it
started decreasing and became 92.5% at 3.3×10-3 mol dm-3. Extraction of Tm(III) in the
range of 1.5×10-5 to 5.92×10-3 mol dm-3 was studied and results showed that the
extraction of Tm(III) was almost quantitative up to 4.4×10-4 mol dm-3 after that it started
decreasing and became 94% at 5.92×10-4 mol dm-3 (highest concentration studied). This
can be attributed to the insufficient quantity of the extractants in the organic phase.
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5.1.4 Composition of synergic adduct
The composition of the synergistic adduct responsible for the extraction of Ce(III),
Nd(III), Eu(III), Tb(III), Tm(III) and Lu(III) into organic phase was investigated using the
slope analysis method as per section 4.6.5 and the results are presented in the Figs. 6-14
and 18.
5.1.4.1 Effect of pH variation
The Fig. 6-8 demonstrate the results of the plots of log D vs pH of the aqueous
solution for these six lanthanide ions, which gave the slope equal to three with coefficient
of correlation ≥ 0.995. The organic phase used was equimolar mixture of HPA and
B15C5 (0.005 mol dm-3) in chloroform. This suggests the presence of three conjugate
base ions (PA-) per adduct for each of the lanthanide ions investigated, which are required
to neutralize the charge of the central metal ion.
5.1.4.2 Effect of HPA concentration variation
The plots of log (D-DCE) vs log [HPA] at fixed concentration of B15C5 (0.005
mol dm-3) are given in Fig. 9-11. DCE represents the distribution coefficient using organic
phase containing 0.005 mol dm-3 of B15C5 alone. The plots gave the slope of three (i.e.
Ce: 2.95 ±0.08; Nd: 3.02±0.03; Eu: 2.97 ± 0.1; Tb: 3.02±0.02; Tm: 3.01 ± 0.08 and Lu:
2.99±0.04) with coefficients of correlation ≥ 0.998, thus indicating that only three HPA
molecule are present per adduct for each of these lanthanide ions and no HPA molecule
takes part as a neutral donor
5.1.4.3 Effect of crown ether concentration variation
Fig. 12-14 show the plots of log (D-DHPA) vs log[B15C5] at constant HPA
concentration (0.005 mol dm-3). DHPA represent the distribution coefficients using organic
phase containing 0.005 mol dm-3 of HPA alone. The plots of Fig. 12-14 present the
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straight lines of slopes, 1.49 ± 0.04, 1.52 ±0.04, 1.52 ± 0.03, 1.48±0.03, 1.61 ± 0.06 and
1.52±0.04 for Ce (III), Nd(III) , Eu (III), Tb(III), Tm (III) and Lu(III) respectively.
From the Fig. 6 to 14, the extraction reaction can be deduced as follow:
++ + →←++ HCnBPAMK
CnBHPAM nsyn 3515.)(5153 3,3 (5.1.2)
M and the expression under bar ( ) represents the rare earth metal ion and the
species in the organic phase, respectively. The value of n may be 1 or 2.
The values of the corresponding equilibrium constants, i.e. log Ksyn,1 and
log.Ksyn,2, for n one or two, respectively, can be deduced from the intercept and the slope
of the plots of (D-DHPA)/[B15C5] vs [B15C5] as shown in Figs. 15-17 . The plots of Figs.
15-17 are based on the Eq. 5.3 and the derivation of this equation can be seen on page 44.
]515[][][][][]515)[( 332.
331.
1CBHHPAKHHPAKCBDD synsynHPA
−+−+−+=− (5.1.3)
The values of log Ksyn,1 and log.Ksyn,2 are related with intercept and slope of the
plots of Figs.15-17 in simplified Eqs. 5.4 and 5.5, respectively which have been used for
the computation of these constants.
log Ksyn,1 = log (intercept) –3 log [HPA] –3pH (5.1.4)
log Ksyn,2 = log (slope) –3 log [HPA] –3pH (5.1.5)
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The computed values of logKsyn,1 and log Ksyn,2 using Eq. 5.1.4 and 5.1.5 are
presented in Table 5.1. The equilibrium constants thus calculated refer only to the
concentration quotients, calculated on the assumption that the activity coefficients of the
species involved do not change significantly under the experimental conditions. The
existence of two kinds of adducts (having one or two molecule of crown ether per adduct)
have been reported in the literature [167,170,172,175,180,183]. Synergistic adducts
containing one molecule of B15C5 with HPBI [183] and HA [180] while two molecules
of 15C5 with HTTA [167,177] for the extraction of various lanthanide ions in chloroform
has been reported. Mathur et al. have reported the synergistic adduct of the type Eu
(PMTFP)3.2B15C5 during the extraction of various lanthanides and actinides with
mixture of HPMTFP and crown ethers in chloroform [175]. We have observed a
synergistic adduct of M(PA)3.2TBPO with a synergistic mixture containing HPA and
TBPO in chloroform for the extraction of various lanthanide (III) ions from pH 2 aqueous
solution.
Table 1 Equilibrium constants of the synergistic extraction of lanthanide (III) ions with
(HPA + B15C5)/CHCl3
Metal ion Equilibrium constant Log Ksyn, 1 log Ksyn, 2
Ce (III) 4.12 ± 0.04 7.48 ± 0.03
Nd (III) 3.97 ± 0.07 7.59 ± 0.03
Eu (III) 4.47 ± 0.04 7.75 ± 0.03
Tb (III) 4.44 ± 0.01 7.86 ± 0.01
Tm (III) 3.81 ± 0.05 7.17 ± 0.04
Lu (III) 3.65 ± 0.04 7.00 ± 0.03
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It was assumed that M(PA)3.2B15C5 may be a sandwich type complex having one
crown ether molecule on either side of the metal chelate bound to the central metal only
through three oxygen atoms as suggested by Mathur on the basis of thermodynamic
studies [175].
The formation of an aqua complex for the rare earth metals with HPA in aqueous
phase has been cited in the literature [151]. Therefore, the formation of synergistic adduct
by the replacement of water molecules can be suggested and the reaction may be written
as:
++ +→←++ HOHnPAMOHnHPAM 3.)(3 2323 (5.1.6)
OHnCnBPAMCnBOHnPAM 2323 515.)(515.)( +←→+ (5.1.7)
The assumption of the formation of an aqua complex (M(PA)3 .nH2O) seems to be
supported by the fact that little extraction of these metal ions is observed with HPA alone
in chloroform at pH 2.0. Furthermore, the net total count rate [net total count rate =
(count rate of aqueous phase - background) + (count rate of organic phase - background)]
only amounted to 85 % and 80 % of 154Eu and 170Tm, respectively, at pH 2.0 suggesting
formation of aqua complex as shown in Eq. 5.6, which is not extractable in chloroform
and resides at the interface of aqueous-organic layer [114]. The formation of aqua
complexes for the rare earth metals (M(PA)3.nH2O) is also cited in the literature [151]. It
has been noticed that the adduct formation in the organic phase is a stepwise process [58].
So, it could be assumed that an adduct containing one crown ether molecule is formed by
replacement of water of hydration and, after that, depending on the concentration of
crown ether, the addition of second molecule of the crown ether is possible.
On the other hand, it can be suggested that in the organic phase HPA and B15C5
molecules are linked together forming a mixed adduct which reacts with metal ions, thus
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forming an extractable organic chelate. Kuvatov et al. have pointed out that HPA and
dihexylsufoxide (neutral donor) in organic phase are in a bound state and the degree of
interaction increases with decreasing polarity and dielectric permeability of the solvent
(o-xylene, chlorobenzene, benzene, and carbon tetrachloride) [62].
The values of the logKsyn,2 for Ce(III), Eu(III) and Tm(III) as shown in Table-1,
are somewhat higher than the values previously reported (6.53, 6.98 and 7.07) for the
extraction of these metal ions with HPA+TBPO/CHCl3 [35].
The importance of the cavity size of the crown ether relative to the cation radius is
emphasized by our finding that no significant synergism was observed for the extraction
of Eu(III) from pH 2 aqueous solution when 12C4 and 18C6 were used as neutral donor
with HPA in chloroform as the former has a relatively small cavity size ( 0.13nm) [168]
and the latter has too large size (0.26nm) [171].
5.1.5 The anions effect
The effect of various anions on the synergistic extraction of Ce(III), Nd(III,
Eu(III), Tb(III), Tm(III) and Lu(III) with (HPA+B15C5)/CHCl3 (0.01 mol dm-3) from pH
2 buffer solution having ionic strength 0.1 mol dm-3 (H+/K+, Cl-) at a ~100 fold higher
concentration than the concentration of the metal of interest, was studied and the results
are presented in Table 5.2. The anions were taken as their sodium salts except where
stated otherwise. The data show that among the anions tested oxalate has reduced the
extraction of Ce(III) and Tm(III) to ~92 %, Nd(III) and Lu(III) to 82%, fluoride and
thiosulphate that of Lu(III) to 93 and 94% respectively, carbonate that of Lu(III) to 96%,
cyanide that of Ce(III) to 85 % and tartrate and ascorbate that of Tm(III) to 95%.
However, all the other anions have not affected the extraction of these lanthanide ions. In
our earlier study, fluoride, oxalate, cyanide and citrate have masked the extraction of
these lanthanide (III) ions using (HPA+TBPO)/CHCl3. The present proposed synergistic
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mixture proved to be much less sensitive to the interference of these anions in the
extraction of these lanthanide ions.
5.1.6 The Cations Effect
Among the cations tested, Fe(II) has reduced the extraction of Tm(III) to 88 %
and Pb(II) to 95 %. Zn(II) has reduced the extraction of Ce(III), Nd(III), Eu(III), Tm(III)
and Lu(III) to 79 %, 93 %, 94%, 82 % and 91% respectively. Cu(II) has reduced the
extraction of Nd(III), Tb(III) and Lu(III) to 88%, 93% and 78% respectively. Fe(III) has
reduced the extraction ofTb(III) and Lu(III) to 96% and to 83% respectively. The
extraction of Nd(III) and Lu(III) is reduced to 96% and 93% respectively due to the
presence of Ni(II) ions. However, Zn(II) has reduced the extaction of Nd(II) and Lu(III)
to 93% and 91% respectively.The rest of the cations have no deleterious effect on the
extraction of the metal ions under study. The reason for the interference of these
transition metals may lie in the formation of organo-metallic complexes, which are also
co-extracted, with these rare earth metal ions [38]. As the concentrations of transition
metal ions are ~100 fold higher than Ce(III), Nd(III), Eu(III), Tb(III), Tm(III) and Lu(III),
the extraction of these rare earth metal ions may presumably be affected due to an
insufficient quantity of the extractant.
5.1.7 The Selectivity of Extraction System
Selectivity of this proposed synergic extraction system has been checked by
extraction of various metal ions with (HPA+B15C5)/CHCl3 (0.01 mol dm-3) at the
optimized conditions of extraction using the radiotracers such as 137Cs+(2.2 × 10-5 mol
dm-3), 56Mn2+(4.5×10-6 mol dm-3), 65Ni2+(2.2×10-4 mol dm-3), 60Co2+(3.4×10-5 mol dm-3),
59Fe3+(1.9×10-4) mol dm-3), 64Cu2+(2.4×10-4 mol dm-3), 75Se4+(3.3×10-4 mol dm-3) and
calculated the separation factor (Kd R.E / Kd M) for rare earth metal ions and results are
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84
presented in Table 4 . The data showed that Fe(III), Cu(II) and Se(IV) can be extracted
upto 65%, 70% and 95%, respectively. All other metals have low Kd values. The
separation factors for most of these metal ions lie in the range of 102-104 except Se(IV),
showing clean separation of rare earth metal ions under study from these metal ions
having the same concentration level.
Acid dissociation equilibria of HPA
The pKa of picrolonic acid was determined by using the procedure given in section
4.7. and results are plotted in Fig. 18. pKa (-log Ka) of HPA was determined by
extrapolating the linear plot of apparent pKa values obtained in 20%-50%υ/υ 1, 4-
dioxane/ water solutions against the 1, 4-dioxane concentration to the intercept, as shown
in Fig. 18. The pKa value thus obtained was 2.52 ± 0.01.
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85
Table 5.2 Effect of various anions on the extraction of lanthanide ions with 0.01 mol
dm-3 (HPA + B15C5)/CHCl3 from aqueous solution at pH 2
Extraction (%)
Aniona Ce(III) Nd(III) Eu(III) Tb (III) Tm (III) Lu(III)
Citrate 99 98 >99 99 99 98
Fluoride 98 97 >99 99 99 93
Oxalate 92 82 99 95 93 82
Bromide 99 99 >99 99 99 98
Phosphate 99 98 >99 99 99 99
Ascorbate 99 99 >99 98 95 98
Thiosulfate 99 99 >99 99 99 94
Acetate 99 99 >99 99 99 98
Thiocyanate 99 99 >99 99 99 98
Cyanide* 85 98 >99 99 99 97
Tartrate NS >99 >99 >99 95 98
Carbonate 99 98 >99 99 >99 96
a: Salt Concentration = 10µg/mL * Potassium salt used.
NS: Not studie
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86
Table 5.3 Extraction of lanthanide (III) ions in the presence of various cations with 0.01
mol L-1 (HPA + B15C5)/CHCL3 from pH 2 aqueous solution.
Extraction (%)
Cationa Ce(III)
Nd(III) Eu(III) Tb (III) Tm (III) Lu(III)
Mg (II) NS 98 98 99 99 97
Co (II) 99 98 99 99 99 97
Cu (II)* 98 88 99 93 99 78
Mn (II) 99 98 99 99 99 98
Fe (III) 98 91 98 97 98 83
Fe (II) 98 N.S. 96 N.S. N.S. N.S.
Ba (II) 99 98 99 99 99 99
Sr (II) 99 97 99 98 99 97
Pb (II) 92 98 N.S. 99 95 98
Ni (II) 98 96 99 97 98 93
Cr (III) 99 97 99 99 99 98
Zn(II) 79 93 94 98 82 91
a: Salt concentration=10µg/mL *: Sulphate salt used N. S. : Not Studied
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87
Tab
le 5
.4 E
xtra
ctio
n of
var
ious
met
al io
ns w
ith 0
.01
mol
L-1
(HPA
+ B
15C
5) /
CH
Cl 3
from
pH
2.0
aqu
eous
solu
tion.
Elem
ent
Con
c.
(mol
l-1)
Kd
Extra
ctio
(%
)
Se
para
tion
fact
ora
Ce3+
K
d = 6
54
Se
para
tion
fa
ctor
a N
d3+
Kd =
585
Se
para
tion
fa
ctor
a Eu
3+
Kd=
644
Se
para
tion
fa
ctor
a Tb
3+
Kd =
585
Se
para
tion
fa
ctor
a Tm
3+
Kd =
315
Se
para
tion
fact
ora
Lu3+
K
d=72
C
s1+
2.2 ×
10-5
0.01
3 1.
3 1.
2 ×
104
2.1 ×
104
5.0 ×
104
4.5 ×
104
2.4×
104
5.5 ×
103
Mn2+
4.
5 ×
10-6
0.01
6 1.
6 1.
1 ×
104
1.7 ×
104
4.1 ×
104
3.6 ×
104
2.01
04 4.
5 ×
104
Hg2+
3.
9 ×
1
0.05
5 5.
2 3.
2 ×
103
4.9 ×
104
1.2 ×
104
1.0 ×
104
5.7×
103
1.3 ×
103
Ni2+
2.
2 ×
10-4
0.06
5.
6
2.9 ×
103
4.5 ×
103
1.1 ×
104
9.7 ×
103
5.2×
103
1.2 ×
103
Co2+
3.
4 ×
10-5
0.09
8.
4 1.
9 ×
103
2.9 ×
103
7.1 ×
103
6.3 ×
103
3.4×
103
7.8 ×
102
Fe3+
1.
9 ×
10-4
1.87
65
.1
94.6
1.
4 ×
102
3.5×
102
3.1 ×
102
1.7×
102
38.5
Cu2+
2.
4 ×
10-4
2.3
69.7
76
.5
1.2 ×
102
2.8×
102
2.5 ×
102
1.4×
102
31.3
Se4+
3.
3 ×
10-4
23.5
95
.5
7.5
11.6
27
.8
24.9
13
.4
3.1
a: S
epar
tion
fact
or =
Kd R
.E /
Kd M
, R.E
= ra
re e
arth
ele
men
t
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88
Ce(III)
0
20
40
60
80
100
1 1.2 1.4 1.6 1.8 2
pH
Extra
ctio
n (%
)
Nd(III)
0
20
40
60
80
100
1 1.2 1.4 1.6 1.8 2
pH
Extra
ctio
n (%
)
Fig. 2 Extraction of Ce(III) and Nd(III) (1.5×10-5 mol dm-3) with 0.01 mol dm-3 B15C5(▲), HPA (■)and HPA+B15C5(♦) in chloroform.
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89
Eu(III)
0
20
40
60
80
100
1 1.2 1.4 1.6 1.8 2
pH
Extra
ctio
n (%
)
Tb(III)
0
20
40
60
80
100
1 1.2 1.4 1.6 1.8 2
pH
Extra
ctio
n (%
)
Fig. 3 Extraction of Eu(III) and Tb(III) (1.5×10-5 mol dm-3) with 0.01 mol dm-3
B15C5(▲), HPA (■)and HPA+B15C5(♦) in chloroform.
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90
Tm(III)
0
20
40
60
80
100
1 1.2 1.4 1.6 1.8 2
pH
Extra
ctio
n (%
)
Lu(III)
0
20
40
60
80
100
1 1.2 1.4 1.6 1.8 2
pH
Extra
ctio
n (%
)
Fig. 4 Extraction of Tm(III) and Lu(III) (1.5×10-5 mol dm-3) with 0.01 mol dm-3
B15C5(▲), HPA (■) and HPA+B15C5(♦) in chlororfor
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91
Nd(III)
90
92
94
96
98
100
-4 -3.5 -3 -2.5log[metal ion] (M)
Ext
ract
ion
(%)
Eu(III)
92
94
96
98
100
-4.5 -4 -3.5 -3 -2.5log[metal ion] (M)
Extra
ctio
n (%
)
Tm(III)
92
94
96
98
100
-4.5 -4 -3.5 -3log[metal ion] (M)
Ext
ract
ion
(%)
Fig. 5 Dependence of metal ion extraction on its concentration by (HPA+B15C5) from pH 2 buffer solution
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92
dy/dx(Ce)=2.99
-1.5
-0.5
0.5
1.5
2.5
1 1.2 1.4 1.6 1.8 2
pH
log
D
dy/dx(Nd)=3.02
-1.5
-0.5
0.5
1.5
2.5
1 1.2 1.4 1.6 1.8 2
pH
log
D
Fig. 6 log D as a function of pH for Ce(III) and Nd(III) (1.5×10-5 mol dm-3 ) with 0.005 mol dm-3 (HPA+B15C5)/CHCl3
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93
dy/dx(Eu)=2.92
-1.5
-0.5
0.5
1.5
2.5
1 1.2 1.4 1.6 1.8 2
pH
log
D
dy/dx(Tb)=2.98
-1.5
-0.5
0.5
1.5
2.5
1 1.2 1.4 1.6 1.8 2
pH
log
D
Fig. 7 log D as a function of pH for Eu(III) and Tb(III) (1.5×10-5 mol dm-3) with 0.005 mol dm-3 (HPA+B15C5)/CHCl3
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94
dy/dx(Tm)=3.02
-1.5
-0.5
0.5
1.5
2.5
1 1.2 1.4 1.6 1.8 2
pH
log
D
dy/dx(Lu)=2.94
-1.5
-0.5
0.5
1.5
2.5
1 1.2 1.4 1.6 1.8 2
pH
log
D
Fig. 8 log D as a function of pH for Tm(III) and Lu(III) (1.5×10-5 mol dm-3) with 0.005 mol dm-3 (HPA+B15C5)/CHCl3
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95
dy/dx(Ce)=2.99
-2
-1
0
1
2
-3.5 -3 -2.5 -2log [HPA]
log
(D-D
CE)
dy/dx(Nd)=3.02
-2
-1
0
1
2
-3.5 -3 -2.5 -2
log [HPA]
log
(D-D
CE)
Fig. 9 log-log plot of (D-DCE) related to Ce(III) and Nd(III) (1.5×10-5 mol dm-3 vs. [HPA]; [B15C5]= 0.005 mol dm-3, pH=2.0
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96
dy/dx(Eu)=2.98
-2
-1
0
1
2
-3.5 -3 -2.5 -2
log [HPA]
log
(D-D
CE)
dy/dx(Tb)=3.02
-2
-1
0
1
2
-3.5 -3 -2.5 -2
log [HPA]
log
(D-D
CE)
Fig. 10 log-log plot of (D-DCE) related to Eu(III) and Tb(III) (1.5×10-5 mol dm-3) vs. [HPA]; [B15C5]= 0.005 mol dm-3, pH=2.0
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97
dy/dx(Tm)=3.02
-2
-1
0
1
2
-3.5 -3 -2.5 -2log [HPA]
log
(D-D
CE)
dy/dx(Lu)=2.94
-2
-1
0
1
2
-3.5 -3 -2.5 -2log [HPA]
log
(D-D
CE)
Fig. 11 log-log plot of (D-DCE) related to Tm(III) and Lu(III) (1.5×10-5 mol dm-3) vs. [HPA]; [B15C5]= 0.005 mol dm-3, pH=2.0
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98
dy/dx(Ce) = 1.49
-1
0
1
2
-4 -3.5 -3 -2.5 -2log[B15C5]
log
(D-D
HPA
)
dy/dx(Nd)=1.52
-1.5
-0.5
0.5
1.5
2.5
-4 -3.5 -3 -2.5 -2log[B15C5]
log(
D-D
HPA
)
Fig.12 log-log plot of (D-DHPA) related to Ce(III) and Nd(III) (1.5×10-5 mol dm-3)
vs. [B15C5]; [HPA]= 0.005 mol dm-3, pH=2.0
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99
dy/d(Eu) = 1.52
-1
-0.5
0
0.5
1
1.5
2
-4 -3.5 -3 -2.5
log[B15C5]
log
(D-D
HP
A)
dy/dx(Tb)=1.48
-1
-0.5
0
0.5
1
1.5
2
-4 -3.5 -3 -2.5log[B15C5]
log(
D-D
HP
A)
Fig. 13 log-log plot of (D-DHPA) related to Eu(III) and Tb(III) (1.5×10-5 mol dm-3)
vs. [B15C5]; [HPA]= 0.005 mol dm-3, pH=2.0
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100
dy/dx(Tm) = 1.61
-1.5
-0.5
0.5
1.5
2.5
-4 -3.5 -3 -2.5 -2
log[B15C5]
log
(D-D
HP
A)
dy/dx(Lu)=1.52
-1.5
-0.5
0.5
1.5
2.5
-4 -3.5 -3 -2.5 -2
log[B15C5]
log(
D-D
HP
A)
Fig. 14 log-log plot of (D-DHPA) related to Tm(III) and Lu(III) (1.5×10-5 mol dm-3) vs. [B15C5]; [HPA]= 0.005 mol dm-3, pH=2.0
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101
y(Ce) = 4E+06x + 1680.1
0
1000
2000
3000
4000
5000
6000
0 0.0002 0.0004 0.0006 0.0008 0.001
[B15C5] (mol dm-3)
(D-D
HPA
)/[B
15C
5]
y(Nd) = 5E+06x + 1185.2
0
1000
2000
3000
4000
5000
6000
7000
0 0.0002 0.0004 0.0006 0.0008 0.001
[B15C5] (mol dm-3)
(D-D
HPA
)/[B
15C
5]
Fig. 15 (D-DHPA)/[B15C5] vs [B15C5] at [HPA]=0.005 mol dm-3; pH2.0
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102
y(Eu) = 7E+06x + 3789.8
0
2000
4000
6000
8000
10000
12000
0 0.0002 0.0004 0.0006 0.0008 0.001
[B15C5] (mol dm-3)
(D-D
HPA
)/[B
15C
5]
y(Tb) = 9E+06x + 3440.5
0
2000
4000
6000
8000
10000
12000
14000
0 0.0002 0.0004 0.0006 0.0008 0.001
[B15C5] (mol dm-3)
(D-D
HPA
)/[B
15C
5]
Fig. 16 (D-DHPA)/[B15C5] vs [B15C5] at [HPA]=0.005 mol dm-3; pH2.0
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y(Tm) = 2E+06x + 825.26
0
500
1000
1500
2000
2500
3000
0 0.0002 0.0004 0.0006 0.0008 0.001
[B15C5] (mol dm-3)
(D-D
HPA
)/[B
15C
5]
y(Lu)= 1E+06x + 565.13
0
400
800
1200
1600
2000
0 0.0002 0.0004 0.0006 0.0008 0.001
[B15C5] (mol dm-3)
(D-D
HPA
)/[B
15C
5]
Fig. 17 (D-DHPA)/[B15C5] vs [B15C5] at [HPA]=0.005 mol dm-3; pH2.0
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Fig. 18 Dependence of pKa of HPA on the concentration (v/v) of 1,4-dioxane in water.
y = 0.004x + 2.5246R2 = 0.9986
2.5
2.6
2.7
2.8
0 10 20 30 40 50
1,4-DIOXANE (%)
pKa
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5.2 Solvent Effect
In order to study the effect of solvents on the extraction of rare earth elements
using picrolonic acid as extractant, extraction of europium (Eu) as a representative of
REEs was studied in acetylacetone (ACAC), 1-octanol (ONL), hexanol (nHNL), 1-
butanol (nBNL), 2-butanol (iBNL), cyclohexanone (CHN), n-butyl ether (nBE) ,
dichloroethyl ether (DCEE), benzene, toluene, and diisobutylketone (DIBK). Extraction
of Eu(III) (~1.5×10-5 mol dm-3) was studied using picrolonic acid (0.01 mol dm-3) in the
above mentioned solvents as a function of pH from pH 1-2, separately. The results are
shown in Fig 19, where extraction of Eu(III) is plotted against the pH of aqueous phase.
This graph shows that, extraction of Eu (III) using picrolonic acid alone, increases with
increase in pH and becomes quantitative in ACAC, ONL, nHNL, CHN, nBE, DIBK, and
DCEE at pH 2 where as it is low in 1-BNL (<10%) and 2-BNL (<50%) and negligible in
benzene and toluene.
As the picrolonic acid alone can extract rare earths quantitatively in ACAC, ONL,
nHNL, CHN, nBE, DIBK, and DCEE, it can be concluded that synergism will be very
small in these solvents. Therefore, the effect of solvents was studied in order to
understand the nature and mechanism of extraction using picrolonic acid alone in all these
solvents.
Extraction of Eu(III) was very low in nBNL and iBNL and negligible in benzene,
and toluene using picrolonic acid alone, synergistic extraction of Eu(III) was studied in
these solvents using B15C5 as neutral donor except nBNL and iBNL and results of this
study are discussed separately in section 5.4
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0
20
40
60
80
100
1 1.2 1.4 1.6 1.8 2
pH
Extra
ctio
n (%
)
DCEEACACDIBKONLnBEnHNLCHN2-BNLnBNL
Fig. 19, Extraction of Eu(III) as a function of pH with HPA (0.01mol dm-3) in different solvents.
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5.2.1 Composition of the extracted adduct
In order to find the stoichiometric composition of the extracted adduct during the
extraction of Eu(III) in ACAC, ONL, nHNL, CHN, nBE, DIBK, and DCEE using HPA
as extractant, slope analysis technique was applied using Eq. 2.23, where log D is plotted
against pH and log[HPA] concentration, separately as per extraction procedure in sections
4.6.1 and 4.6.5
In order to find the stoichiometric ratio of metal ion and HPA from the pH studies,
extraction of Eu (III) at constant concentration (~ 1.5 × 10-5 mol. dm-3) was studied using
HPA (0.01 mol dm-3) in ACAC, ONL, nHNL, CHN, nBE, DIBK, and DCEE as a
function of pH of aqueous phase from pH 1-2 using the extraction procedure as given in
sections 4.6.1 and 4.6.5. The plots of log D vs pH are drawn and the results are shown in
Fig. 20. The slopes of these plots with the correlation coefficients for all the solvents are
shown in Table 5.5. These slopes are three or very close to three. This shows that, three
conjugate base molecules i.e. PA- of HPA are attached with each metal ion in the
extracted adduct.
Table 5.5 Slope with correlation coefficients, for the extractin of Eu(III) from different
solvents from Fig. 20
S.No. Solvent Slope Correlation Coefficient
1
2
3
4
5
6
7
Acetylacetone
Di-isobutyl ketone
n-butylether
Octanol
Dichloroethylether
n-hexanol
Cyclohexanone
2.93 ± 0.12
2.95 ± 0.07
2.93 ± 0.17
2.96 ± 0.06
2.78 ± 0.18
2.94 ± 0.08
2.93 ± 0.06
0.999 ± 0.103
0.999 ± 0.061
0.999 ± 0.148
0.992 ± 0.053
0.99 ± 0.151
0.999 ± 0.068
0.999 ± 0.055
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5.2.1.1 Effect of HPA concentration
Composition of adduct responsible for extraction was also studied by slope
analysis technique by varying the concentration of HPA (10-3 to 10-2 mol dm-3 ) in all the
solvents separately and studying the extraction of Eu(III) (1.5×10-5 mol dm-3) from
aqueous solution of pH-2. The plots of these experiments are depicted in Fig. 21, where
log D is plotted against log[HPA]. The slopes along with correlation coefficients for all
the solvents used are shown in Table 5.6. These values are also very close to three except
in DCEE which is close to four. It indicates the presence of three molecules of HPA in the
extracted adducts in all the solvents except DCEE where four molecules of HPA are
present. In DCEE one molecule of HPA is present as neutral donor.
Table 5.6 Slope with correlation coefficients, for the extractin of Eu(III) from different
solvents from Fig. 21
S.No. Solvent Slope Correlation Coefficient
1
2
3
4
5
6
7
Acetyl acetone,
Di-isobutyl ketone
n-butylether
Octanol
Dichloroethylether
n-hexanol
Cyclohexanone
2.94 ± 0.05
2.939 ± 0.07
2.78 ± 0.7
2.97 ± 0.08
4.06±0.06
2.95 ± 0.09
2.94 ± 0.09
0.996 ± 0.058
0.998 ± 0.037
0.997 ± 0.063
0.996 ± 0.069
0.995 ± 0.084
0.997 ± 0.075
0.999 ± 0.065
From the results of above experiments, the extraction mechanism can be deduced
as:
+−+ +→←++ mHnHPAPAEuHPAnmEu mm
Kex .)()( 33 5.2.1
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109
nm
mmm
ex HPAEuHnHPAPAEu
K ++
+−
=]][[
][])([3
3
5.2.2
m=3 and n=0 / 1 as the case may be.
The bar represents the organic phase.
In all the solvents studied as diluents, the extracted species is characterized as
Eu(PA)3 while in case of DCEE, the composition of the species is suggested as
Eu(PA)3.HPA.
Two types of extraction mechanisms may be proposed
1- Water molecules present in the aqua complex “Eu(PA)3.nH2O” may be replaced
by oxygenated solvent molecules directly.
2- The interaction of HPA with solvent molecules results in a mixed adduct which
further reacts with Eu(III) in aqueous phase to extract it in organic phase.
As indicated by Osman, that in aqueous medium Eu(III) forms an aqua complex
with HPA like M(PA)3.nH2O [45]. In our studies, formation of aqua complexes of Eu of
type Eu(PA)3.nH2O which are not extractable in organic solvents has already been
discussed in section 5.4.3. The formation of aqua complexes of Eu of type M(PA)3.nH2O
and role of organic solvent as neutral donor has been discussed by Ali in his study on the
extraction of Nd(III), Eu(III), Tb(III), Tm(III) and Lu(III) using HPA from an aqueous
solution of pH 2 in MIBK. He suggested that, in MIBK, water molecules of aqua
complex may be replaced by MIBK thus making adduct extractable into organic phase
[150, 151].
It has been reported by Kuvatov et al. that in the organic phase, HPA and di-
hexylsulfoxide (DHSO), a neutral donor, are in a bound state and form a mixed adduct
which further reacts with metal ion [62].
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110
On the basis of the results of these experiments, we can suggest that Eu(III) is
extracted as Eu(PA)3 in ACAC, ONL, nHNL, CHN, nBE and DIBK and HPA is not
acting as neutral donor. Water molecules of Eu(PA)3.nH2O complex are replaced by
solvent making it extractable. However, from the slope of Fig.21 for DCEE, the extracted
species may be as Eu(PA)3.HPA. In this case one molecule of HPA is present as neutral
donor. This may be due to less electron donating ability of O in DCEE due to the
presence of two chlorine atoms in the molecule which have more electron withdrawing
effect and thus reducing the electron density on the O atom of DCEE. Sadanobu and
Qiangbin have studied the extraction of lanthanides using BPHA (HL) in various solvents
and they have reported the formation of LnL3.(HL) in chloroform [199]. Their findings
support the composition of our proposed extracted species Eu(PA)3.HPA in DCEE
Extraction constant were also calculated. These are given below in Table-5.7.
Table 5.7 Extractin constants for Eu(III) extraction in different solvents.
S. No Solvent Log kex
1
2
3
4
5
6
7
Acetyl acetone
Di-isobutylketone
n-butylether
Dichloroethylether
Octanol
n-hexanol
Cyclohexanone
3.14±0.13
2.96±0.11
2.94±0.09
2.23±0.09
2.22±0.07
1.95±0.08
1.91±0.06
On the basis of log Kex, the solvents can be arranged with respect to thsir
extractability in the order ACAC > DIBK > Nbe > DCEE > ONL > nHNL > CHN.
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111
ACAC and DIBK are better extractants as compared to cyclohexanone. Extraction
constants (log kex) for Eu in ACAC is greater than that of DIBK and CHN which may be
due to the presence of two oxygen atoms in the ACAC molecule. Extraction constant for
Eu is higher in DIBK as compared to CHN. This may be due to cyclic structure of CHN,
since the aliphatic solvents are considered better extractant [201].
Similarly, in case of alcohols, log kex for Eu(III) is higher in ONL than nHNL.
This may be due to longer carbon chain in ONL than in nHNL and hence more electron
donating ability of O in ONL as compared to nHNL.
In case of ethers used as diluents, log kex is higher in nBE than DCEE and hence
nBE seems to be a better solvent for extraction of Eu(III) using HPA as extractant. This
may be due to less electron donating ability of O in DCEE due to the presence of two
chlorine atoms in the molecule which have more electron withdrawing effect and thus
reducing the electron density on the O atom of DCEE.
The role of solvent in the extraction of rare earth elements using a single
extractant has not been discussed to a greater extant in the literature. However, some
workers have studied the role of solvents in the extraction of REEs and various other
metal ions and can be discussed shortly to compare the extraction mechanism proposed in
our studies.
Healy et al. have studied the extraction of Am(III) and Pm(III) using HTTA as
extractant and TOPO, TPPO, TBP, TPP, ethyl hexyl alcohol (EHA) and MIBK as neutral
donors in cyclohexane and benzene as solvents. They reported the formation of
Am(TTA)3.(EHA)2, Pm(TTA)3.(EHA)2, Am(TTA)3.(MIBK)2 and Pm(TTA)3.(MIBK)2
They have concluded that, this synergistic system increases the partition coefficient up to
400 times greater than for either HTTA or the neutral donor (EHA, MIBK) alone [200].
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112
This report also supports the proposed composition of the extracted species in the present
work.
Low extraction of Eu(III) in CHN as compared to DIBK can be supported to some
extant by the conclusion drawn by Akiba, who pointed out that aliphatic diluents are
better solvents for the extraction of metal ions [201].
It is very difficult to correlate extraction constants (log kex) given in Table-5.7
with any single physical property of the solvents. Yang et al. have studied the extraction
of U(VI) with petroleum sulfooxide (PSO) in seven solvents and concluded that no
correlation can be drawn with any single physical parameter of the diluents [202] which is
in accordance with the findings of the present study.
-1.5
-1
-0.5
0
0.5
1
1.5
2
2.5
3
3.5
1 1.2 1.4 1.6 1.8 2
pH
log
D
ACACDIBKnBEDCEEONLnHNLCHN
Fig. 20 log D as a function of pH for Eu(III) (~1.5×10-5 mol dm-3)with 0.01mol dm-3 (HPA) ACAC, DIBK, ONL, Nbe, CHN, nHNL and DCEE
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113
-3
-2
-1
0
1
2
3
-3 -2.8 -2.6 -2.4 -2.2 -2log [HPA]
log
D
ACACDIBKnBEDCEEONLnHNLCHN
Fig. 21 log – log plot of D related to Eu(III) (~1.5×10-5 mol dm-3) vs. HPA concentration in ACAC, DIBK, ONL, nBE, CHN, nHNL and DCEE
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114
5.3 Extraction of Rare Earth Elements in Different Solvents
During our study on role of various diluents on the extraction of Eu(III) using
HPA as extractant, it was found that alcohols, ketones and ethers were good solvents.
Among the three categories of solvents, namely, alcohols, ketones and ethers, ONL, CHN
and DCEE were chosen as representative diluents of each category, respectively. Further
to investigate the extraction behaviour of these solvents along the lanthanide series,
extraction studies of Ce, Tb and Lu were carried in ONL and CHN while that of Tb and
Lu in DCEE.
5.3.1 Extraction of Ce(III), Tb(III) and Lu(III) in Octanol
To study the extraction of REEs with in the series, extraction of Ce(III), Tb(III)
and Lu(III) as a representative of REEs was carried out in ONL. Extraction of Ce(III)
(~1.52×105 mol dm3), Tb(III) (~1.48×105 mol dm3) and Lu(III) (~1.54×105 mol
dm3) was studied from aqueous solutions of pH 1-2 using HPA (0.01 mol dm-3) in ONL.
The results are plotted in Fig. 22 which shows that the extraction increases with the
increase in pH and becomes quantitative (>98%) at pH 2 for Ce(III) and Tb(III) where
as it is low for Lu(III) (78%). The extraction trend among these elements appeared to be
Ce(III) >Tb(III) >Lu(III). This shows that extraction of rare earth elements using HPA in
octanol decreases with the decrease in ionic radii.
Composition of the extracted species was determined using slope analysis. For
this purpose, extraction of Ce(III), Tb(III) and Lu(III) was studied using HPA (0.01 mol
dm-3) from aqueous solutions of pH 1-2 as per extraction procedure ginen in section 4.6.1
and 4.6.5. Log D was plotted against pH of aqueous phase for all the three elements and
the results are shown in Fig. 23. The slope of these graphs gives the number of conjugate
base (PA-) of HPA molecules attached to RE metal ion. These plots show slopes 2.97,
2.93 and 2.92 with coefficients of correlation having values 0.99, 0.995 and 0.992 for Ce,
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115
Tb and Lu, respectively. This shows that three molecules of conjugate base of HPA are
present per molecule of extracted species.
Presence of three molecules of HPA was further verified by studying slope
analysis by varying the HPA from 0.001 to 0.01 mol dm-3 concentration at pH 2 as per
extraction procedure ginen in section 4.6.9. Log D is plotted against log [HPA] and the
results are presented in Fig.24. The slope of these plots gives the number of molecules of
HPA attached to metal ion and from the plots slopes of 2.96, 2.94 and 2.97 with
coefficients of correlation having values 0.997, 0.997 and 0.996 were observed for Ce, Tb
and Lu, respectively. This also confirms the presence of three conjugate base molecules
of HPA per extracted adduct. On the basis of these results, composition of the extracted
adduct can be suggested as M(PA)3 where M = Ce(III), Tb(III) and Lu(III).
Extraction constants (log Kex) were also calculated. These were found to be2.20,
1.77 and 0.86 for Ce(III), Tb(III) and Lu(III), respectively. This shows that extraction
decreases as the ionic radii of the REEs decrease. Similar trend has been reported by Ali
in his studies on the extraction of Nd(III), Tb(III) and Lu(III) [150] and Eu(III), Tm(III)
[151] using HPA in MIBK. As far as the mechanism of extraction is concerned, similar
extraction mechanism can be proposed as has already been discussed in section 5.2.1.1 on
the studies of solvent effect.
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116
0
20
40
60
80
100
1 1.2 1.4 1.6 1.8 2
pH
Extr
actio
n (%
)
Fig.22 Extraction of Ce(♦), Tb(■) and Lu(▲) as a function of pH with HPA 0.01 mol dm-3 in octanol
dy/dx(Ce) = 2.97
dy/dx(Lu) = 2.92
dy/dx(Tb) = 2.93
-3
-2
-1
0
1
2
3
1 1.2 1.4 1.6 1.8 2
pH
log
D
Fig.23 log D as a function of pH for Ce(♦), Tb(■) and Lu(▲) with (HPA) 0.01 mol dm-3 in octanol
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117
dy/dx(Ce) = 2.96
dy/dx(Lu) = 2.97
dy/dx(Tb) = 2.94
-3
-2
-1
0
1
2
-3 -2.8 -2.6 -2.4 -2.2 -2
log[HPA]
log
D
Fig.24 log – log plot of D related to Ce(♦), Tb(■) and Lu(▲) vs. HPA concentration in octanol
5.3.2. Extraction of Ce, Tb and Lu in Cyclohexanone
Among the ketones such as ACAC, DIBK and CHN, studied earlier in section
5.2 for the extraction of Eu(III) using HPA as chelating agent , CHN has been chosen
for further studies about the extraction trend along the REEs series. Extraction of Ce(III)
(~1.52×105 mol dm3), Tb(III) (~1.48×105 mol dm3) and Lu(III) (~1.54×105 mol
dm3) was studied from aqueous solution of pH 1-2 using HPA (0.01 mol dm-3) in CHN
as per extraction procedure given in section 4.6.1 and 4.6.5 and the results are plotted in
Fig.25. This Fig. shows that extraction increased with increase in pH and became
quantitative (>98%) at pH 2 for Ce and Tb where as, it was low for Lu (46%).
Extraction was not studied beyond pH 2 due to the increased solubility of HPA in
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118
aqueous phase above this pH, as indicated by the presence of yellow colour of HPA in
aqueous phase. Highest extraction was observed for Ce(III), less for Tb(III) and least
for Lu(III). This shows that extraction of REEs using HPA in CHN decreases with the
decrease in ionic radii.
0
20
40
60
80
100
1 1.2 1.4 1.6 1.8 2
pH
Extr
actio
n ( %
)
Fig.25 Extraction of Ce(♦), Tb(■) and Lu(▲) as a function of pH with HPA (0.01mol dm-3) in cyclohexanone
Composition of the extracted species was determined using slope analysis. For
this purpose, extraction of Ce(III), Tb(III) and Lu(III) was studied from aqueous solutions
of pH 1-2 separately using HPA (0.01 mol dm-3) as per extraction procedure given in
section 4.6.5 to study the effect of pH. log D was plotted as a function of pH of aqueous
phase for all the three elements. The results are shown in Fig.26. These plots show slope
2.98, 2.93 and 2.91 with coefficients of correlation having values 0.996, 0.996 and 0.998
for Ce(III), Tb(III), and Lu(III), respectively. This shows that, three molecules of
conjugate base of HPA are attached per molecule of extracted species.
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dy/dx(Ce)= 2.98dy/dx (Tb)= 2.93dy/dx(Lu) = 2.91
-3
-2
-1
0
1
2
1 1.2 1.4 1.6 1.8 2
pH
log
D
Fig.26 log D as a function of pH for Ce(♦), Tb(■) and Lu(▲) with (HPA) 0.01mol dm-3
in cyclohexanone Presence of three molecules of HPA in extracted species for each metal ions was
further verified by varying the HPA concentration from 0.001 mol dm-3 to 0.01 mol dm-3 at
pH 2. The extraction was carried as per extraction procedure given in section 4.6.9 to study
the effect of HPA concentration. Log D is plotted against log [HPA] and the results are
shown in Fig.27. These plots also show slope 3.22, 3.12 and 3.03 with coefficients of
correlation having values 0.996, 0.99 and 0.997 for Ce(III), Tb(III) and Lu(III). This also
confirms the presence of three conjugate base molecules of HPA per extracted adduct. On
the basis of the results of these experiments, composition of the extracted species can be
suggested as M(PA)3.
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dy/dx(Tb) = 3.13
dy/dx(Lu) = 3.03
dy/dx(Ce) = 3.23
-4
-3
-2
-1
0
1
2
-3 -2.8 -2.6 -2.4 -2.2 -2
log[HPA]
log
D
Fig.27 log – log plot of D related to Ce(♦), Tb(■) and Lu(▲) vs. varying concentration of HPA in cyclohexanone
Extraction constants (log Kex) were also calculated. These were found to be1.78,
1.32 and 0.41 for Ce, Tb and Lu, respectively. This shows that rate of extraction
decreased with the dexrease in ionic radii of the REE, which is the same trend as
observed earlier wih HPA/ONL and in accordance with the literature [150,151]. As far as
the mechanism of extraction is concerned, similar extraction mechanism can be proposed
as has already been discussed in section 5.2.1.1 on the studies of solvent effect.
5.3.3 Extraction of Tb and Lu in DCEE
During our study on the effect of solvents on the extraction of rare earth elements,
ethers (nBE and DCEE) were found as good solvents for the extraction Eu(III) using HPA
as extractant. Further to study the trend of extraction of REEs with in the period,
extraction of Tb(III) (~1.48×105 mol dm3) and Lu(III) (~1.54×105 mol dm3) was
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carried out in DCEE. Extraction of these elements was studied from aqueous solutions of
pH 1-2 using HPA (0.01 mol dm-3). The extraction was carried as per extraction
procedure given in section 4.6.1 and 4.6.5. The results are plotted in Fig.28 where
extraction is plotted against pH. This Fig. shows that extraction increases with increase in
pH and becomes quantitative at pH 2 for Tb(III) where as it is low for Lu(III) (60%). The
order of extraction is Tb(III) >Lu(III). This shows that extraction using HPA in DCEE
decreases with the decrease in ionic radii.
0
20
40
60
80
100
1 1.2 1.4 1.6 1.8 2
pH
Ext
ract
ion
( % )
Fig. 28 Extraction of Tb(■) and Lu(▲) as a function of varying pH with HPA
(0.01mol L-1) in DCEE
Composition of the extracted species was determined using slope analysis. For
this purpose, extraction of Tb(III) and Lu(III) was studied separately using HPA (0.01
mol dm-3) from aqueous solutions of pH 1-2 as per extraction procedure given in section
4.6.1 and 4.6.5. The results are shown in Fig.29. These plots show as slope of 2.80 and
2.87 for Tb and Lu with coefficients of correlation having values 0.999 and 0.998 for Tb
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122
and Lu, respectively. This shows that three molecules of conjugate base of HPA are
attached per molecule of extracted species.
dy/dx (Tb)= 2.80
dy/dx (Lu)= 2.87
-2.5
-1.5
-0.5
0.5
1.5
2.5
1 1.2 1.4 1.6 1.8 2pH
log
D
Fig. 29 log D as a function of pH for Tb(■) and Lu(▲) with 0.01mol L-1 (HPA) in DCEE
To determine the number of HPA molecules taking part in the complex formation,
extraction of Tb(III) and Lu(III) was studied by varying the HPA concentration from
0.001 mol dm-3 to 0.01 mol dm-3 at pH 2. The extraction was carried out as per extraction
procedure given in section 4.6.1 and 4.6.5 to study the effect of HPA concentration. Log
D is plotted against log [HPA]. The results are shown in Fig.30. These plots show slope
3.86 and 3.89 for Tb and Lu with coefficients of correlation having values 0.995 and
0.999. This slope is close to four which indicates involvement of four molecules of HPA
per extracted adduct. In this case one molecule of HPA is present as a neutral donor. This
may be due to less electron donating ability of oxygen in DCEE due to the presence of
two chlorine atoms in the molecule which have more electron withdrawing effect and
thus reducing the electron density on the oxygen atom of DCEE. On the basis of these
experiments, composition of the extracted adduct can be suggested as M(PA)3.HPA.
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Sadanobu and Qiangbin have studied the extraction of lanthanides using BPHA (HL) in
various solvents and they have reported the formation of LnL3.(HL) in chloroform [199].
Their findings support the composition of our proposed extracted species M(PA)3.HPA in
DCEE.
dy/dx(Tb) = 3.86 dy/dx(Lu) = 3.89
-3.5
-3
-2.5
-2
-1.5
-1
-0.5
0
0.5
1
1.5
-3.5 -3 -2.5 -2
log[HPA]
log
D
Fig. 30 log – log plot of D related to Tb(■) and Lu(▲) vs. HPA concentration in DCEE Extraction constants (log Kex) were also calculated. These were found to be 2.16
and 1.11 for Tb and Lu, respectively. This shows that extraction of REEs decrease as the
ionic radii of the REEs decreases. Similar trend has been reported by Ali in his studies on
the extraction of lanthanides using HPA in MIBK [151]. As far as the mechanism of
extraction is concerned, similar extraction mechanism can be proposed as has already
been discussed in section 5.2.1.1on solvent effect.
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5.4 Synergistic Extraction of Eu(III) in Benzene and
Toluene
During our study on the role of solvent in the extraction of Eu(III) using
picrolonic acid alone as extractant, no significant extraction of Eu(III) was observed in
benzene and toluene. Therefore, synergistic extraction of Eu(III) (~1.5×10-5 mol dm-3)
using picrolonic acid and B15C5 as neutral donor was studied in these solvents. All the
experiments were carried in the same way as described earlier in sections 4.6.1, and 4.6.5
in chloroform as solvent.
5.4.1 Effect of pH of aqueous phase
The extraction of Eu(III) (~1.5×10-5 mol dm-3) with equimolar (0.01 mol dm-3)
solutions of HPA & B15C5 separately and with their mixture in benzene or toluene from
pH buffer solutions (1.0 – 2.0) having ionic strength of 0.1 mol dm-3 (H+ / K+ , Cl-) has
been studied and results are shown in Figs. 31 and 32. The extraction of Eu(III) with
B15C5 and HPA alone was negligible in this pH range. Whereas, with the mixture of
HPA and B15C5, extraction was quantitative ( ≥ 98 %) at pH 1 in benzene and quite high
( ≥ 95 %) in toluene showing a pronounced synergism [Dsyn = Dmix / (DHPA + DB15C5) ]
of 6.3×103 and 1.4×103 in benzene and toluene respectively. As the extraction of Eu(III)
became quantitative at pH 1 in both the solvents, therefore pH 1 was selected for all the
further experimental work.
5.4.2 Composition of synergic adduct
The composition of the synergistic adduct responsible for the extraction of Eu(III)
into organic phase was investigated using the slope analysis method and the results are
presented in the Fig. 33-35
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5.4.2.1 Effect of pH variation
The Fig. 33 demonstrates the results of the plots of log D vs. pH of the aqueous
solution of Eu(III) which gave the slopes as 3.27, 3.25 with coefficient of correlation
0.997 and 0.998 for benzene and toluene respectively. The organic phase used was equi
molar mixture of HPA and B15C5 (0.005 mol dm-3) in both the solvents. These slopes
which are close to three indicate the presence of three conjugate base molecules ( PA-)
per adduct for each Eu(III) ion under investigation.
5.4.2.2 Effect of HPA concentration variation
The plots of log [D-DB15C5] vs. log [HPA] (0.001 -0.01 mol dm-3) at fixed
concentration of B15C5 in (0.01 mol dm-3) in benzene and toluene are given in Fig 34.
DB15C5 is distribution coefficient of Eu(III) using B15C5 alone. The plots gave the slope
of three (3.14 and 3.00), with coefficients of correlation 0.992 and 0.995 for benzene and
toluene, respectively, indicating that only three HPA molecule are involved in the
extraction of Eu(III).
5.4.2.3 Effect of CE concentration variation
Fig. 35 shows the plots of log [D-DHPA] vs. log [B15C5] at constant HPA
concentration (0.005 mol dm3) for toluene and (0.01 mol dm3) for benzene. DHPA is
distribution coefficient of Eu(III) using HPA alone The plots of Fig. 35 present the
straight lines having slopes 2.12 and 1.87 with correlation coefficient 0.999 and 0.99, for
benzene and toluene, respectively.
From the Fig. 34 and 35, the extraction reaction can be deduced as follow:
++ +→←++ HOHPAEuOHHPAEu 32.)(23 2323 (5.4.1)
OHCBPAEuCBOHPAEu 2323 25152.)(51522.)( +→←+ (5.4.2)
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126
Where M and the expression under bar ( ) represent the rare earth metal ion and the
species in the organic phase respectively.
The formation of aqua complex (Eu (PA)3.2H2O is supported by the fact that low
extraction of Eu(III) is observed in chloroform at pH 2 and has already been discussed in
section 5.4.3 .
The overall extraction reaction by the mixture of HPA and B15C5 in both the
solvents can be suggested by the following reaction
++ +→←++ HCBPAEuCBHPAEu 35152.)(51523 33 (5.4.3)
233 ]515[][][ CBHPAHKK d+= (5.4.4)
On the basis of the results of these experiments, we can propose the extracted
species as Eu(PA)3.2B15C5.
The values of the corresponding extraction constants i.e. logKex for the extraction
of Eu(III) for both the solvents were found to be 8.85 and 8.31 using Eq.(4) for benzene
and toluene respectively. These extraction constants show that extraction of Eu(III) was
more favourable in benzene than in toluene under these experimental conditions
Synergistic adducts containing two molecules of 15C5 with HTTA for the
extraction of various lanthanide ions in chloroform has been reported [167, 177] and
support our proposed composition. Mathur et al. have reported the synergistic adduct of
the type Eu(PMTFP)3.2B15C5 during the extraction of various lanthanides and actinides
with a mixture of (HPMTFP) and crown ethers in chloroform [175]. Similarly the
synergistic adduct of M(PA)3.2TBPO with a mixture of HPA and TBPO in chloroform
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127
for the extraction of various lanthanide (III) ions from pH 2 aqueous solution has been
reported in literature [150].
We assume that M(PA)3.2B15C5 may be a sandwich type complex having one
crown ether molecule on either side of the metal chelate bound to the central metal only
through three oxygen atoms as suggested by Mathur on the basis of thermodynamic
studies [175].
It has been noticed that the adduct formation in the organic phase is a stepwise
process [58]. So, it could be assumed that an adduct containing one crown ether molecule
is formed by replacement of water of hydration and after that, addition of second
molecule of the crown ether is possible.
On the other hand, it can be suggested that in the organic phase HPA and B15C5
molecule are linked together forming a mixed adduct which reacts with metal ions, thus
forming an extractable organic chelate. Kuvatov at al. have pointed out that HPA and
DHSO (neutral donor) in the organic phase are in a bound state [62]. All these reports
support the composition of the extracted species and mechanism of extraction proposed in
the present studies.
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128
0
20
40
60
80
100
1 1.2 1.4 1.6 1.8
pH
Extra
ctio
n (%
)
Fig. 31 Extraction of Eu (III) with (0.01mol dm-3) HPA(♦), B15C5 (■) and HPA+ B15C5 ( ▲) in Benzene
0
20
40
60
80
100
1 1.2 1.4 1.6 1.8
pH
Extra
ctio
n (%
)
Fig. 32 Extraction of Eu (III) with (0.01mol dm-3) HPA(♦), B15C5 (■) and
HPA+ B15C5 ( ▲) in Toluene
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129
dy/dx(B) = 3.27
dy/dx(T) = 3.25
-1
0
1
2
3
4
1 1.2 1.4 1.6 1.8 2
pH
log
D
Fig. 33 Effect of pH on the extraction of Eu (III) with HPA+B15C5 (0.01 mol dm-3) in Benzene (♦)and Toluene (■)
dy/dx(B) = 3.14
dy/dx(T) = 3.00
-2.5
-1.5
-0.5
0.5
1.5
-3.1 -2.9 -2.7 -2.5 -2.3 -2.1
log[HPA]
log
[D-D
CE]
Fig. 34 log – log plot of [D-DB15C5] related to Eu(III)(~1.5×10-5 mol dm-3) vs. HPA Concentration in Benzene (♦)and Toluene (■) at constant concentration of B15C5 (0.01 mol dm-3 )
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dy/dx(B) = 2.127
dy/dx(T) = 1.87
-3.5
-2.5
-1.5
-0.5
0.5
1.5
-3.5 -3 -2.5 -2
log [B15C5]
log
[D-D
HPA
]
Fig. 35, log – log plot of [D-DHPA] related to Eu(III)(~1.5×10-5 mol dm-3) vs. B15C5 concentration in Benzene (♦)and Toluene (■) at constant concentration of HPA (0.01 mol dm-3)
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5.5 Effect of Neutral donors In order to study the effect of neutral donors on the extraction of rare earth
elements, extraction of Eu(III) (~1.5×10-5 mol dm-3) was studied with HPA using TOPO,
TPPO, TBP and TPP, as neutral donors in chloroform. Extraction was studied using
0.01mol dm-3 of HPA, TBP, TPPO, TPP and TOPO as single extractant and equimolar
mixture of each neutral donor with HPA in chloroform separately from aqueous buffer
solutions of pH 1-2. The results are shown in figs. 36-39 where extraction is plotted
against pH of the aqueous phase. It is clear from these figures that HPA as well as all the
four neutral donors separately, do not extract Eu from aqueous solution. However, using
the mixture of HPA and neutral donors, extraction is quite high even at pH 1 and it
increases with increase in pH and becomes quantitative (≥98 %) at pH 2 with TBP, TPPO
and TOPO using as neutral donors. Extraction of Eu with the mixture of HPA and TPP
was (~86%) at pH 2 and yellow colour of HPA appeard in aqueous phase beyond pH 2.
Therefore, higher pH of aqueous phase was not studied and pH 2 was selected for further
studies related to the extraction of Eu(III) with these synergic extraction systems. It is
evident from Figs. 36-39, that quantitative extraction of Eu was observed at pH 1.2 for the
mixture of HPA with TOPO and TPPO and at pH 1.6 for HPA and TBP mixture.
Therefore, synergism was calculated at these pH values and was found to be 1.49×104,
1.12×104 and 1.35×104 for the extraction of Eu using TOPO, TPPO and TBP
respectively, as neutral donors with HPA. Synergism for HPA and TPP mixture was
calculated at pH 2 and was found to be 53.46.
5.5.1 Composition of the synergistic adducts
Composition of the extracted complexes of Eu with HPA and TOPO, TPPO &
TBP as neutral donors was studied using slope analysis method and Job’s method was
was also applied to Eu-HPA-TPPO system. The results are shown in Fig. 40-43.
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5.5.1.1 Effect of pH
The effect of pH of the aqueous phase on the extraction of Eu(III) (~1.5×10-5 mol
dm-3 ) was studied using an equimolar concentration (0.01 mol. dm-3) of a mixture of
HPA with TBP. In case of synergic mixture of HPA with TOPO and TPPO, concentration
of HPA was 0.01mol dm-3 while that of TOPO and TPPO was 0.002 mol dm-3 in
chloroform. The results are shown in Fig. 40, where log D is plotted against pH. These
plots have a slope of 2.97, 3.08 and 2.88 with correlation coefficients 0.997, 0.996 and
0.999 for TOPO, TPPO and TBP respectively. This shows that three molecules of
conjugate base (PA-) of HPA are present per extracted adduct using all the three neutral
donors in chloroform.
5.5.1.2 Effect of HPA concentratiom
In order to study the effect of HPA concentration on the extraction of Eu(III),
studies were carried out using a mixture of HPA with TOPO, TPPO and TBP as neutral
donors separately at constant concentration of TOPO, TPPO (0.002 mol dm-3 ) and TBP
(0.01 mol dm-3 ) while varying the concentration of HPA (0.001- 0.01 mol dm-3 ) at pH 2.
The results of this study are plotted in Fig. 41 where log(D-Ds) is plotted against log
[HPA]. DS is the distribution coefficient for neutral donor alone used as extractant. All the
three plots have a slope of 2.95, 3.03 and 3.05 with the coefficient of correlation having
values of 0.992, 0.99 and 0.995 for TOPO, TPPO and TBP respectively. The values of
three slopes which are approaching to 3 also suggest the presence of three conjugate base
[PA-] of HPA molecume per adduct and no HPA molecule is acting as neutral donor.
5.5.1.3 Effect of concentration of neutral donors
In order to study the effect of concentration of neutral donors on the extraction of
Eu(III), extractions were carried out keeping the concentration of HPA constant (0.01
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133
mol dm-3) and varying the concentration of TOPO, TPPO (0.0001- 0.001 mol dm-3 ) and
TBP (0.001- 0.01 mol dm-3) at pH 2. The results are plotted in Fig.42 where log[D-DHPA]
is plotted vs. log[S] (S=TOPO, TPPO and TBP). DHPA is the distribution coefficient for
HPA alone used as extractant. This graph shows a slope of 2.09, 2.05 and 0.95 for
TOPO, TPPO and TBP respectively. This shows that two molecules of TOPO and TPPO
are present per adduct extracted while in case of TBP only one molecule of TBP is
present. On the basis of the data of these experiments, composition of the extracted
species can be suggested as Eu(PA)3.2TOPO, Eu(PA)3.2TPPO and Eu(PA)3.TBP
Composition of adduct responsible for extraction of Eu(III) using HPA and TPPO
was further investigated using Job’s method ( method of continuous variation) [62, 197]
where overall concentration of HPA and TPPO is maintained constant at 0.01 mol dm-3
while changing concentration of both the ligands. The log D vs. mol fraction of HPA has
been plotted in Fig 43. The results showed that maximum extraction of Eu(III) was
observed at a ratio of HPA : TPPO as 0.006 : 0.004 mol dm-3, respectively, depicting the
composition of the complexes as M(PA)3.2TPPO which is in accordance with the earlier
observations using slope analysis method in the preceeding section.
The extraction mechanism which can be proposed on the basis of above
mentioned results is as follows
++ +↔++ HOnHPAMOnHHPAEu 3.)(3 2323 (5.5.1)
OnHnSPAEunSOnHPAEu 2323 .)(.)( +↔+ (5.5.2)
Where S is TOPO, TPPO and TBP and value of n is 2 for TOPO, TPPO and 1 for TBP.
Expression under bar ( ) represent the organic phase.
The formation of aqua complexes of rare earths is also cited in literature [145].
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Kuvatov et al., studied the extraction of Am(III) using HPA and dihexylsulfoxide
(DHSO) (neutral donor) and pointed out that in the organic phase HPA and DHSO are
in a bound state [62]. Considering their finding about the mutual bound state of both the
ligands, it can be suggested that in the organic phase HPA and each neutral donor (TOPO,
TPPO or TBP) are linked together forming a mixed adduct which further reacts with
metal ion forming an extractable metal chelate. The extraction mechanism for the
extraction of Eu(III) by the mixture of HPA and all the three neutral donors (TOPO,
TPPO and TBP) separately in chloroform can be expressed by the following expression.
nSmHPAnSmHPA ..→+ (5.5.3)
++ + →←+ mHnSPAEumHPAnSEu mKMix .)(3 (5.5.4)
or simply it can be written as
++ + →←++ HnSPAMnSHPAM MixK 3.)(3 3
3 (5.5.5) and
ndMix SHPAHkK
][][][
3
3+
= (5.5.6)
where S is TOPO, TPPO and TBP and value of n may be 1 or 2.
On the basis of the results of all these experiments, composition of the extracted
adduct can be suggested as M(PA)3.2TOPO, M(PA)3.2TPPO and M(PA)3.TBP.
The equilibrium constants (logKMix) for Eu(III) were calculated to be 4.54, 5.69
and 6.17 using TBP, TPPO and TOPO respectively. These extraction constants show that
highest extraction rate is observed with TOPO, followed by TPPO and least with TBP.
Therefore, the order of extraction becomes as TOPO > TPPO > TBP.
The formation of two kinds of adducts having one or two molecules of neutral
donor per adduct have been reported in the literature. During the synergistic extraction
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studies of trivalent Pm, Tm, Am and Cm from aqueous chloride medium with mixtures of
HTTA and TOPO, TBP, DBB, TPPO or TPP in benzene, Healy has reported very high
extraction of these metal ions with TOPO and TPPO. They reported that the rate of
extraction increases in the order from neutral alkyl phosphates, through phosphonates to
the phosphine oxides. The extraction follows the order of basicity of the neutral oxo-
donors used. During this study, the synergistic extracted species into the organic phase
were characterized as M(TTA)3.2S for Am(III), Cm(III) and Pm(III) and M(TTA)3.S and
M(TTA)3.2S for Tm(III) [119, 134]. These reports support extraction mechanism and
order of extraction (TOPO > TPPO > TBP) which has been observed in our studies on the
extraction of Eu(III) using picrolonic acid along with TOPO, OPPO and TBP in
chloroform.
Formation of the synergistic species Nd (TTA)3.(TOPO)2 has also been reported
by Healy and Ferraro. They have confirmed the formation of such complexes using
visible and I.R. spectra [204,205]. Newman has also studied the synergistic extraction of
trivalent Am, Cm, Pm and Tm with a mixture of HTTA and TBP (or TOPO) in benzene
and cyclohexane as the diluents and reported similar results [206].
Sekine and Dyressen have carried out the extraction studies of La, Eu, Lu and Am
in carbon tetrachloride using a mixtures of HTTA + S (S = TBP, TOPO, DBSO, Hexone
and several other neutral donors) and reported the formation of species M (TTA)3.(S) and
M(TTA)3.2S [207, 208]. Cary and Banks have reported the formation of the species Eu
(TTA)3.2S when S is TOPO [209]. Mathur et al. have studied the extraction of Eu(III)
using HTTA chelate with DPhSO, TBP and TOPO in benzene and have reported the
formation of Eu(TTA)3.S and Eu(TTA)3.2S adduct species of considerable stability.
Synergistic equilibrium constants follow the order DPhSO< TBP<TOPO [210]. Aly et al.
have studied extraction of Sm (III) using a mixtures of HTTA + TPPO or TOPO in
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benzene and have characterized the species to be Sm(TTA)3.2S [211]. Synergistic
extraction of trivalent lanthanides (all except La and Eu) was carried out by Frabu et al.,
using a mixture of HTTA and TBP in carbon tetrachloride as diluent. The extraction
constants for the species M(TTA)3.S and M(TTA)3.2S have been calculated [212]. Akiba
et al., have extensively studied the extraction of Eu(III) – HTTA – TOPO system and
reported similar results [215].
All these studies support the formation of extracted species Eu(PA)3.nS and
mechanism proposed in our studies on the extraction of Eu(III) in chloroform.
0
20
40
60
80
100
1 1.2 1.4 1.6 1.8 2
pH
Extra
ctio
n (%
)
Fig. 36 Extraction of Eu (III) with (0.01mol L-1) HPA (■), TOPO (♦) and HPA+TOPO (▲) in chloroform
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137
0
20
40
60
80
100
1 1.2 1.4 1.6 1.8 2
pH
Extra
ctio
n (%
)
Fig. 37 Extraction of Eu (III) with (0.01mol L-1) HPA (■), TPPO (♦) and HPA+TPPO (▲) in chloroform
0
20
40
60
80
100
1 1.2 1.4 1.6 1.8 2
pH
Extra
ctio
n (%
)
Fig. 38 Extraction of Eu (III) with (0.01mol L-1) HPA (■), TBP (♦) an HPA+TBP(▲) in chloroform
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138
0
20
40
60
80
100
1 1.2 1.4 1.6 1.8 2
pH
Extra
ctio
n (%
)
Fig. 39 Extraction of Eu (III) with (0.01mol L-1) HPA (■), TPP (♦) an HPA+TPP (▲) in chloroform
dy/dx(TOPO) = 2.97
dy/dx(TPPO) = 3.08
dy/dx(TBP) = 2.96
-2
-1
0
1
2
3
4
1 1.2 1.4 1.6 1.8 2
pH
log
D
Fig.40 Effect of pH on the extraction of Eu (III) with (HPA+S) in chloroform, HPA= 0.01 mol dm-3, S = 0.002 mol dm-3 (S= TOPO ♦ and TPPO ■) and (HPA+TBP ▲) = 0.01 mol dm-3
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dy/dx(TPPO) = 3.02
dy/dx(TOPO) = 2.97
dy/dx (TBP) = 3.01
-1
0
1
2
3
-3 -2.8 -2.6 -2.4 -2.2 -2
log[HPA]
log
D
Fig. 41 log – log plot of [D-DS] related to Eu(III)(~1.5×10-5 mol dm-3) vs. HPA concentration in chloroform at constant concentration (0.002 mol dm-3) TOPO(♦), TPPO (■) and (0.01 mol dm-3) TBP (▲)
dy/dx(TOPO) = 2.05
d/dxy(TPPO) = 2.09
dy/dxy (TBP) = 0.95
-1.5
-1
-0.5
0
0.5
1
1.5
-4 -3.5 -3 -2.5 -2
log[S]
log[
D-D H
PA]
Fig. 42 log – log plot of [D-DHPA] related to Eu(III)(~1.5×10-5 mol dm-3) vs. [S], (S=TOPO ♦, TPPO ■ and TBP ▲) concentration into chloroform at a constant concentration of HPA (0.01 mol dm-3)
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0
0.5
1
1.5
2
2.5
3
0 0.002 0.004 0.006 0.008 0.01
Conc. of HPA (mol dm-3)
D
Fig. 43 Extraction of Eu(III) with isomolar mixture of HPA and TPPO
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5.6 Conclusion
During this study, synergic extraction of rare earth elements was carried out using
picrolonic acid with various neutral donors in different solvents. Extraction of rare earth
elements (Ce(III), Nd(III), Eu(III), Tb(III), Tm(III) and Lu(III) ) was studied using
picrolonic acid and crown ethers (12C4, B15C5 and 18C6) in chloroform from aqueous
solution of pH 1-2. It was found that quantitative extraction can be achieved with in three
minutes from aqueous solution of pH 2 in chloroform using B15C5, as neutral oxo-donor
where as 12C4 and 18C6 did not show significant synergistic effect indicating that ring
size of these crown ethers also played a very important role in addition to their simple
neutral donor effect. This synergic extraction system showed a good tolerance to a
number of anions (citrate, ascorbate, thiosulphate, tartarate, acetate, fluoride, chloride,
bromide, iodide, thiocyanate, cyanide, carbonate, nitrate and phosphate) and cations
(cobalt, copper, manganese, ferric, barium, cadmium, strontium zirconium, lead, nickel,
chromium, zinc and magnesium) as the extraction of rare earth elements is not affected
by their presence in the aqueous phase. High metal concentration can be quantitatively
extracted up to 10-3 mol dm-3 showing its good loading capacity. However, the solubility
of picrolonic acid in aqueous phase became apparent at higher pH, therefore, it can not be
considered suitable for the extraction of metal ions from aqueous phase of pH >3.
During synergic extraction of Eu(III) using picrolonic acid and benzo-15-crown-5
as extraction system in benzene, butanol and toluene, no synergism was observed in
butanol and quantitative extraction was observed in benzene and toluene with in five
minutes, from aqueous solution of pH1. In this way benzene and toluene proved to be
better solvents for the extraction of rare earth elements using picrolonic acid and benzo-
15-crown-5 as synergic extraction system.
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During the study on the use of other neutral donors such as TOPO, TPPO, TBP
and TPP with picrolonic acid, it was found that quantitative extraction of lanthanides can
be achieved with all the neutral donors except TPP from aqueous solution up to pH 2.
On the study of the effect of solvents, using picrolonic acid as extractant, it was
found that picrolonic acid alone can extract rare earth elements quantitatively from
aqueous solutions at pH2 in ACAC, DIBK, nBE, DCEE, ONL HNL and CHN with in
five minutes. By comparing the extraction of Eu(III) with HPA in these solvents, it
appeared that those solvents having the oxygen proved to be good diluents due to their
ability to coordinate as neutral donor in the formation of the metal complex responsible
for metal extraction. The order of their extraction ability follow the trend as
ketones>ethers>alcohols.
From this study, it is concluded that synergic extraction system comprising
picrolonic acid and any one of the neutral donors i.e., B15C5, TOPO, TPPO and TBP, in
any one of the diluents, chloroform, benzene and toluene; and HPA alone in any
oxygenated diluents can be used for the rapid and quantitative extraction of rare earth
elements from the aqueous phase of pH < 2 where these elements have least possibility of
hydrolysis..
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CHAPTER – 6 REFERENCES
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