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CHAPTER 6
ION-EXCHANGE
Chapter 6 Ion-Exchange
Department of Chemistry, S.P.U.
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6.1 INTRODUCTION
Ion-exchange may be defined as the reversible exchange of ions between the
substrate and surrounding medium. Ion-exchange resins are the polymers which are
capable of exchanging particular ions within the polymer with ions in a solution that
is passed through them. This ability is also seen in various natural systems such as
soils and living cells. The synthetic resins are used primarily for purifying water but
also for various other applications including separating out some elements. Ion-
exchange materials are insoluble substances containing loosely held ions which are
able to exchange with other ions in solutions which come in contact with them. These
exchanges take place without any physical alteration to the ion-exchange material.
Ion-exchange resins have the ability to absorb metal ions from large volumes
to smaller volumes in a concentrated form under appropriate conditions. Ion-exchange
technique can remove traces of ionic impurities from water/process liquors and gives
out a product of ultra pure quality in a simple, efficient and techno-economically
viable manner. The equipment required is generally compact and occupies small
space. Generally, the process is carried out at an ambient temperature and pressure
and in an intermittent manner if desired. In fact, no other technique removes traces of
ionic constituents from waste water/process liquors so rapidly, efficiently and at low
pressure as is done by the ion-exchange technique. So, today ion-exchange has
emerged out as a unit operation analogous to such classic operations as filtration,
distillation etc.
Ion-exchangers are widely used in analytical chemistry, hydrometallurgy,
antibiotic purification, separation of radio isotopes and find large scale application in
water treatment and pollution control, pharmaceutical industry, medicine, purification
of solvents and reagents and so on. [1-5]. It is also useful in many fields such as water
softening and deionization, sugar purification, extraction of uranium glycerol refining,
purification of formaldehyde and as catalysts [6-13].
The basic requirements, which are essential for any polymeric material to be useful as
an ion-exchange resins are:
(a) It must be sufficiently hydrophilic to permit diffusion of ions through the structure
at a finite and usable rate.
(b) It must contain sufficient number of accessible ion-exchangeable groups which do
not undergo degradation during use and
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(c) The swollen material must be denser than water.
The considerable interest has been developed in the synthesis of ion-exchange
resins having special properties and containing specific functional groups. These
resins are expected to work under crucial conditions of pH, temperature and at the
same time selective in adsorption of specific metal ions. The chelating ion-exchangers
are prepared by two methods. The first method is polymerization of monomers
containing an ion-exchangeable group and the second method is introduction of a
chelating functional group into the polymeric matrix.
The polymer containing 8-hydroxy quinoline and its derivatives has drawn
considerable attention due to tremendous application in ion-exchange field [14]. Kim
and co-workers [15] prepared 8-hydroxyquinoline–resorcinol (8-HQR) and 8-hyroxy
quinoline-resorcinol-salicylic acid (8-HQRS) resins by polycondensation and studied
ion-exchange capacity at different pH using Fe+3, Cu+2, Co+2, Pb+2 and Ni+2 metal ions
and found that the ion-exchange capacities of these resins were 4.1 and 5.9 meq.g-1
respectively. They also found that the maximum adsorption of these resins was
observed at pH 7.0 and the distribution coefficient of metal in these resin was
increasing with decreasing HCl concentration. Dhakite and co-workers [16] were
synthesized by the condensation of 8-hydroxyquinoline-5-sulphonic acid and biuret
with formaldehyde in the presence of hydrochloric acid as catalyst, proved to be
selective chelation ion exchange copolymer resins for certain metals. Chelation ion
exchange properties to these polymers were studied for Cu2+, Cd2+, Co2+ and Zn2+
ions. A batch equilibrium method was employed in the study of the selectivity of the
distribution of a given metal ions between the polymer sample and a solution
containing the metal ion. The study was carried out over a wide pH range and in a
media of various ions strengths. The polymer showed a higher selectivity for Cu2+
ions than for Cd2+, Co2+ and Zn2+ ions. Liu and Cheng [17] studied the interaction of
heavy metal ions and chelating ion-exchange resin containing 8-hydroxyquinoline
(8-HQ) moiety. The resin has good selectivity to absorb heavy metal ion including
Cu(II), Hg(II), Pb(II) and Mg(II) at pH 5.0. These authors suggested that the chelating
ion-exchange resin containing 8-HQ could be used to remove heavy metals from
water.
Patel and co-workers [18,19] synthesized the ion-exchange resins based on
N-phenyl maleimide and studied the effect of electrolyte strength, pH and shaking
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time on the adsorption of different metal ions. They reported that the synthesized
resins were highly selective for Cd+2 and Pb+2 ions. They also studied the separation
of Pb+2 from Ca+2, Fe+3 from Cr+3 and regenerability of these resins. Ahmad and
co-workers [20] prepared acrylic fibers, which is pretreated with hydrazine under
various concentrations to give crosslinked structure. The prepared crosslinked fiber
treated with hydroxylamine hydrochloride to develop ion-exchange fibers. The effects
of reaction conditions on physical properties, thermal characteristics, surface
morphology, ion adsorption quantity and reusability were investigated. The results
show that by increasing the reaction time, temperature and concentration of
hydroxylamine hydrochloride the ion adsorption capacity also increased. Onari [21]
synthesized ion-exchange resins having triazolylazophenol ion-exchangable group
and studied the effect of pH and shaking time on the adsorption of various metal ions
using batch equilibration method. He observed that the capacity of these polymers to
adsorb heavy metal ions reached its saturation in about 30 minutes and the polymer
can be practically employed for the removal of Ni+2 and Cu+2 ions from waste/water.
Lee and Hong [22] synthesized poly(hydroxamic acid) resins from poly(ethyl
acrylate-co-divinylbenzene) beads and their metal binding properties were determined
at specific pH. In acidic region, the chelating resin showed high adsorption capacity
for copper, iron, vanadium and uranium. Metal adsorption capacities varied according
to polymerization condition, i.e. crosslinking ratio and degree of dilution.
Gurnule and co-workers [23] synthesized terpolymer resins (4-HABF) by
condensation of 4-hydroxy acetophenone and biuret with formaldehyde in presence of
acid catalyst and using varied molar ratios of reacting monomers. Chelation ion-
exchange properties of this resin have been studied by employing batch equilibrium
method. It was employed to study selectivity of metal ion uptake over a wide pH
range and in media of various ionic strength. The overall rate of metal uptake follows
the order: Fe+3 > Cu+2 > Ni+2 > Co+2 = Zn+2 > Cd+2 > Pb+2 > Hg+2. The free radical
solution copolymerization of poly(hydroxyethyl methacrylate-co-acrylamide)
poly(HEMA-co-AAm) was studied by Maranbio and co-workers [24] in the range of
25 to 75% monomer feed ratios. They showed that poly(HEMA-co-AAm) can bind
metal ions: Cr(III), Co(II), Zn(II), Ni(II), Cu(II), Cd(II), Pb(II), Hg(II) and Fe(III) in
aqueous solution at pH 3.5 to 7.0 and the inorganic ion interaction with the
hydrophilic polymer was determined as a function of pH and filtration factor. Shah
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and co-workers [25] reported chelating ion-exchange resin containing 8-hydroxy
quinoline and separation of metal ions by selective adsorption in the resin column.
They measured the physicochemical properties like % moisture content, void volume
fraction, total exchange capacity, rate of exchange, thermal stability and effect of
metal ion concentration on exchange capacity. The quantitative separation of
Cu(II)-Ni(II) and Zn(II)-Ni(II) was accomplished by selective adsorption in column.
Ratna and co-workers [26] synthesized spherical beads from the copolymers
of styrene and methyl acrylate which were sulfonated using concentrated sulfuric
acid. They studied ion-exchange capacity of the sulfonated copolymer which
increases with time. The developed ion-exchange resin also demonstrated better
performance in demineralization of water as compared with the conventional
polystyrene based beads. Rivas and co-workers [27] synthesized water insoluble
functional copolymers and studied their metal ion uptake properties for silver(I),
copper(II), cadmium(II), zinc(II), lead(II), mercury(II), chromium(III) and
aluminium (III). Ozturk and Kose [28] studied the boron removal from aqueous
solutions using Dowex 2 x 8 anion exchange resin. The sorption behaviour of resin
was investigated as a function of pH, contact time and temperature, initial boron
concentration of solution, resin dosage and effect of other resins. The maximum
sorption value for boron was observed at pH 9.0. Kaliyappan and co-workers [29]
synthesized 8-(acryloyloxy) quinoline (8-AOQ) and polymerized it in methyl ethyl
ketone at 70oC using BPO initiator. They prepared polychelates by addition of
aqueous solution of Th(II) / Cd(II) / Zn(II) / Ni(II) and Mg(II) ions into the polymer in
aqueous NaOH. The IR spectra of these polychelates suggest that metals are
coordinated through oxygen of the ester carbonyl and the nitrogen atom. Roozemond
and co-workers [30] have carried out more systematic work in this direction for the
separation of metal ions from their mixtures. They reported that Cu+2 and Cd+2 metal
ions were successively eluted from the chelated resin quantitatively and the
regenerated resin could be reused many times.
Rivas and Villegas [31] synthesized crosslinked poly[3-(methacryloylamino)-
propyl]-dimethyl (3-sulfopropyl)ammonium hydroxide-co-2-acrylamidoglycolic acid
[PCMAAPDSA-co-AGCO] by radical polymerization and tested the synthesized
polymer as an absorbent under competitive and noncompetitive conditions for Cu(II),
Cd(II), Hg(II), Zn(II), Pb(II) and Cr(III) by batch and column equilibrium procedures.
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They reported that resin metal ion equilibrium was achieved before 1 hr. The resin
showed a maximum retention capacity value of 1.084 meq.g-1 for Hg(II) at pH 2.0.
The recovery of the resin was investigated at 20ºC under different concentrations of
HNO3 and HClO4. Shah and co-workers [32] synthesized chelating ion-exchange
resin from salicylic acid-formaldehyde-resorcinol (SFR) using DMF as solvent at
80ºC. The effect of pH, metal ion concentration and rate of exchange of metal ions
were also studied by employing batch equilibrium method. Masram and co-workers
[33] synthesized terpolymer resin from salicylic acid-hexamethylenediamine-
formaldehyde (SHMF) by the condensation of salicylic acid and
hexamethylenediamine with formaldehyde in the presence of hydrochloric acid
catalyst. Chelation ion-exchange properties of synthesized terpolymer have been
studied for Fe+3, Cu+2, Ni+2, Co+2, Zn+2, Cd+2 and Pb+2 ions employing batch
equilibrium method. It was employed to study the selectivity of metal ion uptake
involving the measurements of distribution of a given metal ion between the polymer
sample and a solution containing the metal ion. The study was carried out over wide
pH range and in the media of various ionic strengths. The terpolymer showed a higher
selectivity for Fe+3, Cu+2 and Ni+2 ions than for Co+2, Zn+2, Cd+2 and Pb+2 ions.
Rivas and co-workers [34] studied the effect of poly(acrylic acid) (PAA) in the
metal binding ability of Cu+2, Cd+2, Co+2, Pb+2, Zn+2, Ni+2 and Cr+3 to poly(sodium-2-
(N-acrylamido)-2-methyl-propanesulfonate) (PAMPS). At pH 3.0 and in presence of
PAA, the fraction of metal ions bound to the polymer decreases, as the sulfonate
molar fraction with respect to carboxylate groups decreases. A copolymer composed
by sodium-2-(N-acrylamido)-2-methyl-propanesulfonate and acrylic acid showed the
same binding ability than a mixture of PAMPS and PAA under the same relative
sulfonate/carboxylate composition. Lin and co-workers [35] studied the removal of
Cu+2 and Ni+2 from aqueous solutions using chelating exchange resin Amberlite IRC
748 by batch and fixed-bed ion-exchange processes. Rivas and co-workers [36]
synthesized water insoluble polymers containing multiligand groups. The uptake
metal ion properties were studied by batch equilibrium procedure for copper (II) and
uranyl ions.
The present chapter deals with the ion-exchange study of the synthesized
polymers. This study is carried out with the following aims:
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1. To determine the effect of pH of the aqueous medium on the metal adsorption
capacity of the polymers.
2. To determine the distribution of metal ions between polymeric and aqueous
phase.
3. To determine the rate of metal adsorption by polymers as a function of time.
4. To determine the effect of various electrolyte and their ionic strength on the
metal adsorption capacity of the polymers and
5. To evaluate the selectivity of different metal ions towards adsorption by the
polymers.
6.2 EXPERIMENTAL
A. Materials
Analytical grade ethylene diamine tetra acetic acid disodium salt (EDTA),
copper nitrate, cobalt nitrate, zinc nitrate, nickel nitrate and ferrous nitrate were used.
Double distilled water was used through out the study.
B. General Procedure:
The ion-exchange properties of poly(8-QMA) and copolymers of
poly(CMA-co-8-QMA) were investigated by batch equilibration method [37,38]. The
polymer samples were ground to fine powder and dried in a vacuum at 60oC for 24
hrs. The dried polymers were used for the ion-exchange study. Five metal ions Cu+2,
Ni+2, Zn+2, Co+2 and Fe+3 in the form of aqueous metal nitrate solution were used. The
ion-exchange study was carried out using three experimental variables: (i) pH of the
aqueous medium (ii) Electrolyte and its ionic strength and (iii) shaking time. Among
these three variables, two were kept constant and only one was varied at a time to
evaluate its effect on metal uptake capacity of the polymers. The details of
experimental procedure are as given below.
(a) Effect of pH of the aqueous medium on metal binding capacity:
The effect of pH on the metal binding capacity of the polymers was estimated
at room temperature in the presence of 1.0 M NaNO3 solution as an electrolyte.
The polymer sample (50 mg) was suspended in the electrolyte solution (1.0 M
NaNO3, 40 ml) and pH of the suspension was adjusted to required value by addition
of either 0.1 M HNO3 or 0.1 M NaOH solution. The conical flask with this content
was stoppered and placed on the mechanical stirrer for 24 hrs shaking, to allow the
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swelling of the polymer at room temperature. The metal ion solution (0.1 M metal
nitrate, 2 ml) was added to this and the pH of the content was adjusted to the required
value. The content was mechanically stirred for 24 hrs and then filtered and washed
with the distilled water. The filtrate was collected in a conical flask and the
unadsorbed metal was estimated by back titration with standard EDTA solution using
appropriate indicator. A separate blank experiment (without adding polymer sample)
was also carried out in the same manner. From the difference between a sample and
blank reading, the amount of metal adsorbed by the polymer was calculated and
expressed in terms of milliequivalent per gram of the polymer (meq.g-1).
The above experiment was performed using 0.1 M metal nitrate solutions of
Cu+2, Ni+2, Co+2, Zn+2 and Fe+3 in the presence of 1.0 M NaNO3 as an electrolyte at
the pH values of 3.0, 3.5, 4.0, 5.0, 5.5 and 6.0. For Fe+3 the experiments were carried
out at pH of 1.5, 2.0, 2.5, 3.0 and 3.5. The results of these experiments are presented
in Tables 6.1 to 6.5.
(b) Distribution ratios of metal ions as a function of pH:
The distribution of each metal ion (Cu+2, Ni+2, Co+2, Zn+2 and Fe+3) between
polymer and aqueous phase was estimated at different pH, using 1.0 M NaNO3
solution. 50 mg polymer was stirred in 1.0 M NaNO3 solution (40 ml) at required pH
value for 24 hrs. To the swelled polymer 0.1 M metal ion solution (2 ml) was added
and the pH was adjusted to the required value by addition of either 0.1 M HNO3 or
0.1 M NaOH. The content was mechanically stirred for 24 hrs. The experiments were
carried out from 3.0 to higher permissible pH for Cu+2, Ni+2, Co+2 and Zn+2. In case of
Fe+3 the study was carried out from pH 1.5 to 3.5.
After 24 hrs, the mixture was filtered, the filtrate and washing were collected.
Amount of the metal ion which remained in the aqueous phase was estimated by back
titration with standard EDTA solution using appropriate indicator. Similarly blank
experiment was carried out without adding polymer sample. The amount of metal
adsorbed by the polymer was calculated from the difference between sample and
blank reading. The original metal ion concentration is known and the metal ion
adsorbed by the polymer was estimated. The distribution ratio ‘KD’ is calculated from
the following equation.
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Amount of metal adsorbed on resin Volume of solution KD = X Amount of metal in solution Weight of resin
The results are presented in Tables 6.6 to 6.10.
(c) Effect of electrolyte and its ionic strength on metal uptake:
The effect of electrolyte and its ionic strength on metal uptake by polymers
was estimated at pH 5.5 for Cu+2, Ni+2, Co+2, Zn+2 and at pH 3.0 for Fe+3 using three
different electrolytes with four different concentrations of each.
The polymer sample (50 mg) was suspended in the electrolyte solution (40 ml)
of known concentration. The pH of the suspension was adjusted to the required value
by addition of either 0.1 M HNO3 or 0.1 M NaOH and the contents were
mechanically stirred for 24 hrs. To this, metal nitrate solution (0.1 M, 2 ml) was
added and the pH of the content was adjusted to the required value. The content was
mechanically stirred for 24 hrs and then filtered and washed with the distilled water.
The filtrate was collected in a conical flask and the unadsorbed metal was estimated
by back titration with standard EDTA solution using appropriate indicator. A separate
blank experiment (without adding polymer sample) was also carried out in the same
manner. From the difference between a sample and blank reading, the amount of
metal adsorbed by the polymer was calculated and expressed in terms of
milliequivalent per gram of the polymer (meq.g-1).
The above experiment was performed using 0.1 M metal nitrate solutions of
Cu+2, Ni+2, Co+2, Zn+2at pH 5.5 and of Fe+3 at pH 3.0 in the presence of three different
electrolytes (NaNO3, Na2SO4 and NaCl) each with four different concentrations (0.05,
0.1, 0.5 and 1.0 M). The results of these experiments are presented in Tables 6.11 to
6.13.
(d) Estimation of the rate of metal uptake as a function of time:
In order to estimate the time required to attain the state of equilibrium under
the prescribed experimental conditions, a series of experiments were conducted in
which the amount of metal ion adsorbed by the polymer was estimated at specific
time intervals.
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The polymer sample (50 mg) was mechanically stirred with 1.0 M NaNO3
solution (40 ml) at required pH value for 24 hrs to allow the swelling of the polymer.
Metal ion solution (0.1 M metal nitrate, 2 ml) was added to this and pH of the content
was adjusted to the required value by addition of either 0.1 M HNO3 or 0.1 M NaOH.
The contents were mechanically stirred for different time intervals (1.0, 2.0, 3.0, 4.0,
5.0, 6.0, 7.0 and 24 hrs). After the specific time interval, the particular suspension was
filtered and washed with the distilled water. The filtrate was collected and the
unadsorbed metal was estimated by titration with standard EDTA solution using
appropriate indicator. From the difference between the original amount of metal
added at the beginning of the experiment and the amount of unadsorbed metal, the
amount of metal adsorbed by the polymer after a specific time interval was calculated.
It was assumed that, under the prescribed experimental conditions, the system attains
the state of 100% equilibrium after 24 hrs.
Let us consider, ‘X’ mg of metal ion is adsorbed after 1.0 hr and ‘Y’ mg of
metal ion is adsorbed after 24 hrs, then the % of metal ion adsorbed after 1.0 hr will
be (X x 100) / Y. Using this expression, the amount of metal adsorbed by the polymer
after specific time intervals was calculated and expressed in terms of % metal ion
adsorbed. This experiment was performed using 0.1 M metal nitrate solutions of Cu+2,
Ni+2, Co+2, Zn+2 at pH 5.5 and Fe+3 at pH 3.0. The results are presented in Tables 6.14
to 6.18.
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Table 6.11: Effect of electrolyte concentration on metal ion adsorption capacity of poly(8-QMA) and poly(CMA-co-8-QMA)
Weight of polymer : 50 mg Electrolyte : NaNO3 solution (40 ml) pH of the medium : 5.5 (for Cu+2, Ni+2, Zn+2 and Co+2) and 3.0(for Fe+3)
Sample Electrolyte Metal ion uptake(meq.g-1)
Code concentration No. (Mol.lit-1 ) Cu+2 Ni+2 Co+2 Zn+2 Fe+3
2
0.05
0.10
0.50
1.00
-
0.08
0.16
0.35
-
0.05
0.18
0.37
-
0.04
0.14
0.08
0.66
0.30
0.14
0.08
0.20
0.14
0.10
0.08
3
0.05
0.10
0.50
1.00
0.08
0.12
0.38
0.64
0.10
0.18
0.48
0.52
-
0.05
0.28
0.52
0.64
0.60
0.31
0.18
0.72
0.68
0.24
0.20
4
0.05
0.10
0.50
1.00
0.50
0.54
0.76
0.92
0.52
0.58
0.84
0.88
0.32
0.38
0.56
0.90
1.02
0.96
0.62
0.32
1.08
1.00
0.70
0.32
5
0.05
0.10
0.50
1.00
0.62
0.66
0.82
1.18
0.68
0.72
0.92
1.02
0.44
0.48
0.60
1.20
1.28
1.24
0.90
0.41
1.40
1.36
0.92
0.38
6
0.05
0.10
0.50
1.00
0.78
0.82
1.08
1.48
0.86
0.92
1.32
1.38
0.72
0.76
0.98
1.58
1.40
1.32
0.98
0.48
1.52
1.46
1.02
0.56
7
0.05
0.10
0.50
1.00
1.76
1.92
2.68
3.04
1.82
2.04
2.80
3.12
0.82
0.98
1.46
2.92
3.20
2.92
1.06
0.80
3.36
2.94
1.96
1.02
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Table 6.12: Effect of electrolyte concentration on metal ion adsorption capacity of poly(8-QMA) and poly(CMA-co-8-QMA)
Weight of polymer : 50 mg Electrolyte : Na2SO4 solution (40 ml) pH of the medium : 5.5 (for Cu+2,Ni+2, Zn+2 and Co+2) and 3.0 (forFe+3)
Sample Electrolyte Metal ion uptake (meq.g-1)
Code concentration No. (Mol.lit-1 ) Cu+2 Ni+2 Co+2 Zn+2 Fe+3
2
0.05
0.10
0.50
1.00
0.10
0.08
0.04
-
0.08
0.08
0.06
-
0.06
0.06
0.04
-
0.06
0.06
0.04
-
0.08
0.08
0.04
-
3
0.05
0.10
0.50
1.00
0.22
0.20
0.08
0.04
0.20
0.16
0.06
0.06
0.16
0.10
0.08
0.04
0.16
0.12
0.06
0.04
0.10
0.08
0.04
0.04
4
0.05
0.10
0.50
1.00
0.72
0.66
0.48
0.40
0.66
0.60
0.32
0.30
0.36
0.30
0.20
0.16
0.22
0.18
0.10
0.06
0.20
0.18
0.08
0.06
5
0.05
0.10
0.50
1.00
1.06
0.98
0.74
0.70
1.02
0.96
0.68
0.62
0.90
0.86
0.52
0.48
0.68
0.64
0.50
0.46
0.60
0.54
0.38
0.32
6
0.05
0.10
0.50
1.00
1.54
1.48
1.14
1.10
1.42
1.34
1.08
1.02
1.40
1.38
1.06
1.00
1.22
1.20
0.96
0.90
1.10
1.04
0.90
0.86
7
0.05
0.10
0.50
1.00
3.22
2.98
2.42
2.08
2.94
2.52
2.04
1.78
2.72
1.88
0.90
0.86
2.02
1.78
0.82
0.66
1.96
1.70
0.62
0.56
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Table 6.13: Effect of electrolyte concentration on metal ion adsorption capacity of poly(8-QMA) and poly(CMA-co-8-QMA)
Weight of polymer : 50 mg Electrolyte : NaCl solution (40 ml) pH of the medium : 5.5 (for Cu+2, Ni+2, Zn+2 and Co+2)and 3.0 (for Fe+3)
Sample Electrolyte Metal ion uptake (meq.g-1 )
Code concentration No. (Mol.lit-1 ) Cu+2 Ni+2 Co+2 Zn+2 Fe+3
2
0.05
0.10
0.50
1.00
-
-
0.06
0.08
-
-
0.04
0.08
-
-
-
0.04
0.04
-
-
-
0.06
0.06
0.04
-
3
0.05
0.10
0.50
1.00
0.04
0.06
0.14
0.18
0.08
0.12
0.20
0.24
-
-
0.08
0.10
-
-
0.08
0.06
-
0.04
0.06
0.10
4
0.05
0.10
0.50
1.00
0.50
0.54
0.60
0.64
0.48
0.52
0.68
0.72
0.04
0.06
0.12
0.14
0.20
0.16
0.06
0.04
0.22
0.26
0.34
0.38
5
0.05
0.10
0.50
1.00
0.54
0.58
0.78
0.82
0.62
0.64
0.86
0.96
0.28
0.32
0.48
0.54
0.72
0.68
0.40
0.36
0.76
0.72
0.50
0.46
6
0.05
0.10
0.50
1.00
0.66
0.72
0.86
0.92
0.78
0.82
0.98
1.08
0.42
0.46
0.56
0.60
0.82
0.76
0.54
0.50
0.90
0.86
0.62
0.58
7
0.05
0.10
0.50
1.00
1.08
1.42
1.64
2.60
2.08
2.40
2.88
3.02
0.62
1.14
1.92
2.06
2.04
1.38
0.96
0.92
2.12
1.58
1.34
0.98
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6.3 RESULTS AND DISCUSSION
From the ion-exchange study with various metal ions under different
experimental conditions the behaviour of the synthesized polymers as chelating
ion-exchangers with respect to experimental variables is discussed as follows.
(a) Effect of pH on the metal binding capacity:
The metal binding capacity depends on the pH of the aqueous medium to a
great extent. The study of the influence of pH of the aqueous medium on the metal
uptake capacity of the polymers was carried out in the presence of a constant amount
of 1.0 M NaNO3 solution at various pH values between 3.0 to 6.0 for Cu+2, Ni+2, Zn+2
and Co+2 metal ions. The study was restricted up to pH 6.0 because at higher pH, due
to hydrolysis of metal salt metal hydroxides are formed which interfere with the ion-
exchange process of the respective metal ions. The ion-exchange study with Fe+3 in
the above mentioned pH range is quite difficult as it forms hydroxide even at pH 4.0.
Hence, its study was carried out separately at various pH values between 1.5 to 3.5
and these results are not compared with results of other metal ions. Tables 6.1 to 6.5
incorporate the results of the effect of pH on the metal binding capacity of the
polymers. It is observed from these results that the relative amount of the metal ion
adsorbed by the polymers increases with increasing pH of the medium. It is also
observed that particular metal ion is adsorbed selectively compared to others at certain
pH. The data clearly indicates that for all the polymers Ni+2 gets adsorbed selectively
to the highest extent and Zn+2 ion is adsorbed to the least extent over the entire pH
range studied. This suggests the possible use of these polymers for separation of Ni+2,
Cu+2 and Co+2 from Ni+2 – Zn+2, Cu+2 – Zn+2, and Co+2 – Zn+2 mixtures respectively.
The trend of metal adsorption by the polymers follows the order: Ni+2 > Cu+2 > Co+2 >
Zn+2. This clearly shows that almost all the polymers have highest affinity for Ni+2
and least for Zn+2. The lowest affinity of Zn+2 may be attributed to the very low
stability constants of complexes of Zn+2 with the ligands [39].
(b) Distribution ratios of metal ions as a function of pH:
The effect of pH on the amount of metal ion distributed between two phases
(in polymer and remained in solution) can be explained by the results shown in Tables
6.6 to 6.10. It is observed from the results that the value of the distribution coefficient
of each metal ions increases rapidly with an increase in the pH of the solution. It is
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also observed from the results that for all the polymers, the value of distribution
coefficient for divalent metal ions decreases in the following order:
Ni+2>Cu+2>Co+2>Zn+2
The data clearly shows that almost all polymers have higher affinity for Ni+2 and
lower affinity for Zn+2 and the amount of metal ions taken up by the polymers
increases with increasing pH of the medium at equilibrium.
(c) Effect of electrolyte and its concentration on the metal binding capacity:
The results of the effect of the nature and concentration of an electrolyte on
the amount of various metal ions adsorbed by the polymers from their solutions at
room temperature are shown in the Tables 6.11 to 6.13. Examination of these results
shows that the amount of metal ion adsorbed by a given amount of polymer is
affected considerably by the nature and concentration of the electrolyte present in the
solution. It is also observed from the results that the amount of Cu+2, Ni+2 and Co+2
ions adsorbed by the polymers increases with increasing concentration of NO3- and
Cl- ions, whereas that of Zn+2 ion, the adsorption decreases with increasing NO3- and
Cl- ions concentration. But in case of SO4-2 ion, the adsorption of Cu+2, Ni+2, Zn+2,
Co+2 and Fe+3 ions decreases with increasing concentration of SO4-2 ion. The
adsorption of Fe3+ ion decreases with the increasing concentration of NO3- and Cl-
ions. This may be explained in terms of the stability constants of the complexes of
Cu+2, Ni+2, Zn+2, Co+2 and Fe+3 cations with the NO3-, Cl- and SO4
-2 anions [40].
It may be inferred from the results that on an average, the metal ion adsorption
by the polymers is much better in the presence of 1.0 M NaNO3 solution. Moreover, it
is reported that nitrate and chloride ions have a tendency to form strong complexes
with many metal ions compare to the sulfate ions [37]. Therefore, the ion-exchange
study with respect to pH and shaking time was carried out in the presence of 1.0 M
NaNO3 solution.
(d) Rate of metal uptake as a function of time:
It was assumed in the present study, that under the prescribed experimental
conditions, the state of equilibrium is established within 24 hrs. Kunin and Barry [41]
also reported that equilibrium for adsorption of metal ion by ion-exchange resins are
attained in minutes or in hours. The results of the rate of metal uptake by the polymers
as a function of time are shown in the Tables 6.14 to 6.18. It is expressed in terms of
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% of the metal ions adsorbed by the polymers after regular time intervals with respect
to 100% adsorption after 24 hrs i.e. in the state of equilibrium. The rate of metal ion
adsorption by the polymers was determined for various metal ions to establish the
shortest time for which equilibrium could be attained so that while operating such
conditions could be maintained. The term “rate” refers to the speed of change in the
concentration of the metal ion in the aqueous solution, which is in contact with the
polymer. The examination of the data shows that amongst the five metal ions studied,
Zn+2 and Fe+3 ions required the shortest time of about 4.0 to 6.0 hrs, whereas Cu+2,
Ni+2 and Co+2 ions required 6.0 to 7.0 hrs to reach the state of equilibrium. It is also
observed from the results that the rate of metal adsorption by the polymers follows the
order of (Fe+3, Zn+2) > (Cu+2, Ni+2, Co+2). Due to this difference in the uptake rate of
metals, it may be possible to separate Zn+2 and Fe+3 ions from their mixtures with
Cu+2, Ni+2 and Co+2 ions using these polymers. Moreover, since the time required for
almost complete saturation of the adsorption capacity of the polymers is considerably
short, these polymers may be utilized for the extraction of heavy metal ions from the
aqueous solutions.
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