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Talanta 52 (2000) 545–554

Selectivity assessment of a sequential extraction procedurefor metal mobility characterization using model phases

J.L. Gomez Ariza *, I. Giraldez, D. Sanchez-Rodas, E. Morales

Departamento de Quımica y Ciencia de los Materiales, Escuela Politecnica Superior, Uni 6ersidad de Huel 6a,

21819 Palos de la Frontera, Huel 6a, Spain

Received 6 August 1999; received in revised form 13 March 2000; accepted 8 April 2000

Abstract

This study considers the selectivity of the extractants used in a sequential extraction scheme for metals mobility

assessment by analyzing individual mineral phases previously coprecipitated or sorbed with trace metals. The scheme

evaluated was a modification of the Tessier et al. [A. Tessier, P.G.C. Campbell, M. Bisson, Anal. Chem. 51 (1979)

844] sequential procedure proposed by the authors. The phases studied were calcite, amorphous iron oxide,

hausmannite, humic acid, kaolinite and illite. Selective extractions were obtained for As, Cr, Cu, Ni, Pb and Zn in

metal-coprecipitated phases whereas NH2OH–HCl was not selective for the extraction of Hg and Cd coprecipitated

in hausmannite and amorphous iron oxide, respectively. Otherwise, Cd, Hg, Ni and Zn sorbed on the different phases

were released with MgCl2 and NaOAc/HOAc, but stronger reagents were needed to release As, Cr, Cu and Pb.

© 2000 Elsevier Science B.V. All rights reserved.

Keywords: Sequential extraction; Trace metals; Model phases

www.elsevier.com /locate/talanta

1. Introduction

The sediments consist of different geochemical

phases such as carbonates, iron and manganese

oxides, humic and fulvic acids and clay minerals

which behave as reservoirs of trace metals in the

environment. The mobility of trace metals

strongly depends on their specific chemical forms

and ways of binding to the sediments, being of

interest to quantitatively determine the trace

metals associated with each phase of these ma-

trices. Several sequential extraction schemes [1–3]

have been developed to evaluate the mobility of

trace metals in environmental samples and the

sequential extraction scheme of Tessier et al. [1]

has been the most widely used and extensively

applied to aquatic sediments [4], soils [5] andsewage sludge [6]. This procedure was designed to

differentiate between metals bound to exchange-

able, carbonate, reducible (hydrous Fe/Mn ox-

ides), oxidizable (sulfides and organic phases) and

residual (mineral) fractions.

However, the trace metals mobility character-

ized by sequential extraction procedures presents

* Corresponding author. Tel.: +34-959-530246; fax: +34-

959-350311.

E -mail address: [email protected] (J.L. Gomez Ariza)

0039-9140/00/$ - see front matter © 2000 Elsevier Science B.V. All rights reserved.

PII: S 0 0 3 9 - 9 1 4 0 ( 0 0 ) 0 0 4 1 0 - 0

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J .L. Gomez Ariza et al . / Talanta 52 (2000) 545–554 546

some controversial. The specificity and reproduci-

bility of the procedures depend upon the chemical

properties of the element and the chemical com-

position of the samples. Therefore, the results are

influenced by experimental factors such as the

choice of reagents [7,8], the time of extraction [9],

the extractant to sediment ratio [10] and the con-

centration of extractant [11], and as a conse-

quence the distribution of trace metals obtainedby such procedures is operational [12]. Moreover,

the metal mobility is influenced by inherent ana-

lytical problems such as incomplete selectivity

[13,14] and readsorptions [15–17].

Two factors contribute to the extractant selec-

tivity: first, an extractant designed to dissolve one

particular phase may also attack other phases

[8,14,18,19], and the second, an extractant may

release a particular metal depending on the rela-

tive binding strengths of each phase for the trace

metal and the number of available binding sites of each component [3,20].

In order to evaluate the selectivity of several

extractants during a sequential extraction proce-

dure previously reported by the authors [11],

model synthetic phases including calcite, amor-

phous iron oxide, hausmannite, humic acid,

kaolinite and illite, used as separated entities and

spiked with Cd(II), Cr(VI), Cu(II), Hg(II), Ni(II),

Pb(II), Zn(II) and As(V) (as nitrate salts and

di-sodium hydrogen arsenate) were studied, fol-

lowing similar experiments reported in the litera-ture for single [3,20,21] and sequential extractions

[14,22–25], that used those matrix as model

phases.

2. Experimental

2 .1. Reagents and apparatus

All reagents were analytical grade or Suprapur

quality (Merck, Darmstadt, Germany). Stockmetal standard solutions were Merck Certificate

AA standards (Merck). Milli-Q water (Millipore,

Bedford, MA, USA) was used in all the experi-

ments. Cleaning of plastic and glassware was car-

ried out by soaking in 14% (v/v) HNO3 for 24 h

and then rinsing with water.

Reference natural clays minerals such as kaolin-

ite (Washington County, Georgia; Clay Minerals

Society) and illite (Cambrian Shane Silver Hill,

Montana; Clay Minerals Society) were pulverized

and screened through a 45 mm sieve before use.

Three types of organic matter phases were stud-

ied: a commercial humic acid (Fluka, Everett,

WA, USA), a certified reference material (TORT-

1, lobster hepatopancreas from the National Re-search Council, Canada) and a humic acid extract

from a sediment collected at the Odiel River

(Huelva, Spain). The humic acid was extracted

from the sediment following the method of Mar-

tın-Martinez et al. [26].

An Atomic Absorption Spectrophotometer,

Perkin–Elmer AAS (model 3100, Ontario,

Canada) with double beam was used for flame

measurements. Hollow cathode lamps were used

as radiation sources (Photron, Victoria, Aus-

tralia).The model phases were characterized by X-ray

diffraction (Phillips X-ray diffractometer, model

1130/90, Almelo, Netherlands, with Cu Ka

radia-

tion and a nickel filter).

Centrifugation was performed with a Sigma

centrifuge (model 4-10, Osteroder am Harz,

Germany).

2 .2 . Analytical procedures

2 .2 .1. Synthesis and characterization of phasesCalcite was prepared by dropwise addition of

0.5 M Na2CO3 aqueous solution to a 2.5 M Ca2+

solution to obtain a final pH value of 8. Iron and

manganese hydroxides were precipitated by addi-

tion of 1 M NaOH to 2.5 M Fe(III) or 3.7 M

Mn(II) aqueous solutions, respectively, to obtain

a final pH value of 9. Precipitates of ‘clean’

calcite, iron and manganese hydroxides were

washed thoroughly with double-distilled water by

decantation, and then dried at room temperature

in a desiccator with silica gel for 10 days, lightlycrushed, and stored in bottles.

Metal-coprecipitated phases of calcite, amor-

phous iron oxides and hausmannite were prepared

by spiking one of the precipitating reagents (cal-

cium, ferric and manganese solutions) with 5000

mg of Cr, Cd, Ni, Pb, Cu and Zn, 500 mg of As

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and 250 mg of Hg. The spiked phases obtained

(calcite, iron and manganese hydroxides, respec-

tively) were washed thoroughly with double-dis-

tilled water and dried in a desiccator with silica

gel for 10 days, then pulverized and screened

through a 45 mm sieve and stored prior to use.

Previously, the absence of a significant amount of

metals in the precipitating reagents was tested.

X-ray powder diffraction (XRD) patterns of both clean and metal-coprecipitated calcite and

manganese hydroxides (Hausmannite, Mn3O4)

were obtained and the d-spacings and relative

intensities compared well with those listed for the

crystalline forms of calcite and hausmannite in the

literature [27]. Amorphous iron oxide was iden-

tified by the absence of any diffraction peaks.

Identifications were made by means of reference

spectra from the Joint Committee on Powder

Diffraction Standards [27].

Metal-coprecipitated commercial humic acidwas prepared by dissolving the humic acid with 1

M NaOH solution to a final pH value of 14. The

solution was spiked with 10 000 mg of Cu and Pb,

5000 mg of Zn and then the humic acid was

precipitated with 1 M HCl to a final pH value of

1. The solid was separated by centrifugation at

10 000 rpm, washed thoroughly with water and

dried in a desiccator with silica gel for 10 days.

Finally, they were pulverized and screened

through a 45 mm sieve and stored prior to use.

Analysis of coprecipitated-trace metal contentsin the synthesized phases were carried out by

FAAS after digestion with a 10/3/2 mixture of

HF/HNO3/HClO4.

Portions of 3.00 g of clean calcite, amorphous

iron oxide, humic acid and clays were added to 50

ml solutions, each containing 102, 113, 106, 109,

11.1, 142, 35.6 and 65 mg l−1 of As, Cd, Cu, Cr,

Hg, Ni, Pb and Zn, respectively, at pH 5.7. They

were equilibrated for 24 h at 2492°C using an

end-over-end mechanical shaker, in order to allow

the metals to adsorb onto their surfaces (materialsprepared in this manner will be referred to as

‘metal-sorbed phases’). Then, the samples were

centrifuged at 8200 rpm for 15 min, freeze-dried,

pulverized and homogenized. The supernatant

was collected and the metal remaining in solution

determined by FAAS. The metal sorbed in the

phases was estimated as the difference between

this concentration and that in the control spiked

solution under the same conditions.

2 .2 .2 . Sequential extraction scheme and analysis

The sequential extraction scheme used in this

study partitioned the trace elements into the fol-

lowing operational fractions [11]:

Fraction 1 (F1): 0.1 g (dry weight) of modelphase was extracted with 8 ml 1 M MgCl2 solu-

tion initially at pH 7 in 50 ml polypropylene tube

for 1 h. The samples were agitated on an end-

over-end mechanical shaker rotating at 40 rpm, at

room temperature (2491°C).

Fraction 2 (F2): the residue from F1 was ex-

tracted for 5 h with 8 ml 1 M NaOAc solution

adjusted to pH 5.0 with HOAc. The samples were

agitated on an end-over-end shaker at room tem-

perature (2492°C).

Fraction 3 (F3): the residue from F2 was ex-tracted for 6 h at 9691°C with 20 ml 0.4 M

NH2OH–HCl in 25% (v/v) HOAc. The samples

were periodically agitated during the course of

this step.

Fraction 4 (F4): the residue from F3 was ex-

tracted for 2 h at 8591°C with 3 ml 0.02 M

HNO3 and 5 ml 30% H2O2, adjusted to pH 2.0

with HNO3. After 2 h, an additional 3 ml 30%

H2O2 (pH 2.0 with HNO3) was added and the

extraction continued at 8591°C for another 3 h.

The samples were occasionally agitated during theentire procedure and then cooled. 5 ml of 3.2 M

NH4OAc in 20% (v/v) HNO3 was added. The

samples were diluted to 20 ml with water and

continuously agitated for 30 min at room temper-

ature (2491°C) on an end-over-end shaker.

Residue (F5): the residue from F4 was digested

with 14 ml of 10:3:1 HF/HNO3/HCl mixture in

Teflon bombs in a commercial microwave system

(model Sanyo, 6 min at 750 W) with a rotating

tray. The digestion was performed using 4 bombs

simultaneously. After the mixture was cooled toroom temperature, boric acid was added in excess

to neutralize residual HF. The solution was trans-

ferred to 25 ml volumetric flasks and diluted with

water.

After each extraction step, the model phases

were centrifuged for 10 min at 10 000 rpm. The

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supernatant was decanted with a Pasteur pipette

and stored at 4°C in stoppered polyethylene ves-

sels until analysis, whereas the residue was washed

with 8 ml of water. After centrifugation for 10

min, this second supernatant was discharged.

Control samples containing the different ex-

tracts spiked with the different metals and treated

under the same experimental conditions were pre-

pared to check possible adsorption processes ontothe container walls.

Metal concentrations (except As and Hg) in all

the extracts were determined by air/acetylene

FAAS. Quantification was achieved using matrix-

matched standards. Sample aliquots for arsenic

determinations were treated with 0.5 ml of 50%

(w/v) KI overnight to reduce As(V) to As(III) and

determined by hydride generation (Perkin–Elmer

MHS-10). The Hg analysis was carried out by

means cold vapor (CV)-AAS. The standard addi-

tion technique was used for quantification (threedifferent levels making duplicates for each level).

3. Results

3 .1. Metal -coprecipitated phases

The content of metals in the coprecipitated

phases is shown in Table 1, and the concentration

of trace metals in each fraction of the sequential

extraction scheme is presented in Fig. 1. As itcould be expected, the trace metals were mostly

released from calcite in fraction 2. Quantitative

recoveries were obtained for Cu, Pb and Cd,

whereas the 93, 85 and 82% yields were found for

Ni, Cr and As, respectively, in this fraction. Al-

though most of As, Cr, Cu, Ni, Pb and Zn

coprecipitated with amorphous iron oxides were

released in fraction 3, a significant percentage as

high as 32% was obtained in fraction 2. More-

over, most of Cd (61%) was released in the frac-

tion. The extract of this second fraction had a

light orange color possibly indicative of dissolved

or colloidal iron. Centrifugation did not remove

this color. Therefore, some metals may be solubi-lized or adsorbed onto this colloidal iron and thus

removed in this extract. When iron was deter-

mined, only 3% was found in the extract, a very

low percentage considering that for Cd.

For hausmannite, all trace metals were quanti-

tatively released in fraction 3, also as expected. In

contrast, Hg was released in fraction 4 as well as

occurred in the control sample. This fact indicated

that this metal was adsorbed in the extraction

tube under reducing conditions and the procedure

was not suitable for this metal.In contrast with the above results, most of the

metals were not recovered in fraction 4 when the

organic matter phases were considered. As an

example, Pb and Zn were quantitatively released

in fraction 1, whereas Cu was distributed among

fractions 1 (46%), 3 (24%) and 4 (24%). Early

recovery of organically bound metal suggested

that the metal release may be determined by the

relative binding capacities, rather than the chemi-

cal degradation of the phase itself. Also, the low

recoveries associated to fraction 4 using thespiked commercial humic acid may reflect that the

operational method used to obtain the coprecipi-

tated phase did not represent a reasonable model

of their natural mixing. Therefore, additional ex-

periments were performed with other organic ma-

Table 1

Concentrations of metal (mg kg−1)9RSD (n=3) in the metal-coprecipitated phases, TORT-1 and natural humic acid

Cd Cu NiHgAsPhases ZnPbCr

8592 87893 14291Calcite 100091 0.1094 97193 100091 92092Amorphous iron oxides 60893720926919114.0947409126092620924093

580929.0953609250092 66092540913294Hausmannite 51091

Humic acid – – – 220091 – – 9593 14092

2093 1793 1093 36092 0.2093 3.094 1093 12093TORT-1

BDLa 8193 20093 BDL BDLNatural humic acid 13609130092 29092

a DL, detection limit.

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Fig. 1. Metal concentrations (mg kg−1) in the coprecipitated phases as determined by the sequential extraction.

trices such as a certified reference material

(TORT-1) and the humic acid extracted from the

sediment collected at the Odiel River. The

metals were mainly released from TORT-1 in

fractions 1, 2 and 3. However, a quantitative

recovery for Cr and percentages of 68 and 38%

for Hg and Cu, respectively, were obtained in

fraction 4.

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3 .2 . Metal -sorbed phases

The percentages of metal adsorption obtained

for the preparation of the model phases are pre-

sented in Table 2. Although a low percentage was

obtained for several metals and phases (for exam-

ple, 9% for Hg on calcite), a sufficient amount of

metal was retained by the solid allowing further

studies. The phases were prepared under the hy-pothesis that sorbed metals would behave as ex-

changeable ones. Therefore, they should be

extracted with the MgCl2 solution (fraction 1).

However, most of the metals were not quantita-

tively removed at this stage from all the phases

(Fig. 2). For calcite, the metals were not quantita-

tively released in fraction 1, and only 64% of Cd

was recovered with MgCl2 and 31% of this metal

in fraction 2. The other metals presented an oppo-

site behavior, being mainly released in the second

fraction (about 87%) and in a minor extension infraction 1 (about 9%). For the iron oxide phase,

only 48 and 26% of Hg and Cd were released with

MgCl2, respectively. Most of As and Pb were

removed in fraction 3 (85 and 89%, respectively),

whereas the other metals were released in fraction

2 (52–79%). In the manganese oxide phase, Hg

was quantitatively released in fraction 1, and 63

and 40% of Cd and Ni were removed with MgCl2,

respectively. However, less than about 6% of the

other metals (Cu, Cr and Zn) were released with

this reagent. Finally, the MgCl2 extract containedquantitative amounts of Cd, Ni and Zn sorbed on

humic acid phase. However, the other metals re-

quired a stronger reagent for releasing.

Both illite and kaolinite presented a similar

behavior, Cd, Hg and Ni being predominantly

released in fraction 1, although no quantitative

recoveries were obtained using MgCl2. Other

metals, including As, Cu, Pb, and Zn, necessitated

a lower pH to be released from these clays.

4. Discussion

4 .1. Metal -coprecipitated phases

Table 3 shows a comparison between metal-co-

precipitated phases and the reagents expected to

attack them versus those experimentally deter-

mined to release the highest percentage of the

metal.

NaOAc/HOAc solution was designed to dis-

solve carbonates and release the associated trace

metals into solution. In this work, it was observed

that all the metals considered when coprecipitated

in calcite were released in fraction 2, with the

exception of mercury. These results agreed withthose obtained by Rapin and Forstner [22] and

Kim and Fergusson [24] who found that Cd and

Pb coprecipitated in carbonates were released

through an acid– base mechanism. However,

Xiao-Quan and Bin [28] found that Cu, Ni, Zn

and Pb in the natural mineral calcite were not

released in fraction 2 and was attributed to the

presence of a residual dolomite which was not

possible brought into solution.

NH2OH–HCl solution in HOAc at 96°C was

designed to dissolve the manganese oxide and theamorphous iron oxide, releasing the trace metals

coprecipitated in those phases. Results in Table 3

show that all the metals coprecipitated in both

phases were released by a reducing mechanism,

except for Cd and Hg coprecipitated in amor-

phous iron oxide and hausmannite being released

Table 2

Percentage (%)9RSD (n=3) of sorbed metal in the metal-sorbed phases

HgCu Pb ZnCdAsPhases Cr Ni

6891 2893 5092Calcite 9892 994 1492 3193 4794

Amorphous iron oxides 1009199911009110094999145931009110091

2992159599919491 1009166939992Hausmnnite 9892

Humic acid 3094 9292 8292 9291 8993 8892 8693 9092

8591 10091 9892 895 2692 7193 5491Illite 7092

2293 5092 9693 694 1492Kaolinite 74915894 8393

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Fig. 2. Metal concentrations (mg kg−1) in the sorbed phases as determined by the sequential extraction.

with an acid –base mechanism and an oxidation

mechanism, respectively. This result agreed with

those obtained by Gruebel et al. [21] studying As

on amorphous iron oxide and Xiao-Quan and Bin

[28] studying Cu, Ni, Zn, Pb and Cr on a natural

mineral of manganese oxide (pyrolusite). How-

ever, when Xiao-Quan et al. [28] used an iron

oxide phase, those metals were released in fraction

5. They used crystalline iron oxides (hematite)

instead of the amorphous iron oxides used in our

study and this fact confirms that NH2OH–HCl

dissolved the amorphous phase but no the crys-

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talline phase of iron oxides, as it has been estab-

lished in literature [28]. Moreover, Kim and Fer-

gusson [24] reported that Cd coprecipitated in the

hausmannite was quantitatively released in frac-

tion 3, whereas 77% of the Cd coprecipitated in

the crystalline goethite (iron oxide) was liberated

in fraction 5. These authors attributed the 23% of

the Cd in fraction 3 to the presence of some

amorphous iron oxides.H2O2 was used as a reagent to release the trace

metals bound to organic matter. However, the

metals coprecipited in both humic acid and

TORT-1 were released in previous fractions,

which indicated that they were not forming stable

complexes. This is in opposite to the findings of

Xiao-Quan and Bin [28] who reported that

amounts of Cu, Zn and Cd in natural humic acid

were released in fraction 4. A different nature of

the organic phase between both studies could

explain these differences.

4 .2 . Metal -sorbed phases

Table 4 shows a comparison between phases

sorbed with metals and the reagent expected to

attack them versus those experimentally deter-

mined to release the highest percentage of the

metal. The specificity of reagents was erratic and

therefore a certain amount of confusion is likely

to arise from the classification system used in the

sequential extraction. MgCl2 was used to remove

the ‘exchangeable’ metals and should release the

sorbed metal on the different model phases. Only

Hg and Cd were mainly released with MgCl2.

This result agreed with those of Kim and Fergus-son [24] using calcite and illite and Cd. However,

these authors found that only a low percentage of

this metal (22 and 40%) was recovered in fraction

1 from hausmannite and humic acid, respectively.

Sometimes, a more acid extractant (NaOAc/

HOAc, pH 5) was needed to release the sorbed

metal. It was the case of Cu, Ni and Zn sorbed on

illite. This behavior could easily result in an over-

estimation of the significance of the metal bound

to carbonates and an underestimation of the ‘ex-

changeable’ metal related to the extension of sorbed metals. Moreover, the sequential extrac-

tion scheme proposed by BCR [2] does not use

MgCl2 or other ‘exchangeable’ phases and the

first fraction consists of an acid treatment. In

contrast to our results, Kheboian and Bauer [14]

observed that Cu was easily released in fraction 1.

Table 3

Comparison between the expected and experimentally determined reagents for coprecipitated individual phases

Phase Reagent expected to release the AssumedExtract found to have the highest metal concentrationa

mechanismmetal

M–HOAc/NaOAcCalcite Acid–baseHOAc/NaOAc

Hg–MgCl2NH2OH–HCl ReductionAmorphous M–NH2OH–HCl

iron oxides

Cd–HOAc/NaOAc

M–NH2OH–HClHausmannite ReductionNH2OH–HCl

Hg–H2O2/NH4OAc

Humic acid OxidationH2O2/NH4OAc (Zn and Pb)–MgCl2Cu–(MgCl2, NH2OH–HCl and H2O2/NH4OAc)

TORT-1 OxidationH2O2/NH4OAc As and Hg–MgCl2(Zn and Ni)–HOAc/NaOAc

Cd–(MgCl2 and HOAc/NaOAc)(Cu and Cr)–(MgCl2 and NH2OH–HCl)

OxidationH2O2/NH4OAcNatural humic (As, Pb and Zn)–NH2OH–HCl

acid

Cu–(NH2OH–HCl and H2O2/NH4OAc)

(Hg and Cr)–H2O2/NH4OAc

a M, metals considered in this study.

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Table 4

Comparison between the expected and experimentally determined reagents for sorbed individual phases

Reagent expected to release the Extract found to have the highest metal concentrationaPhase Assumed

mechanismmetal

Calcite MgCl2 (Cd and Hg)–MgCl2 Ion

exchangeable

M–HOAc/NaOAc

Amorphous MgCl2 Hg–(MgCl2, NH2OH–HCl,H2O2/NH4OAc and Ion

exchangeableresidual)iron oxidesCd–(MgCl2, HOAc/NaOAc and NH2OH–HCl)

(Zn, Cr, Cu and Ni)–(HOAc/NaOAc and

NH2OH–HCl)

(As and Pb)–NH2OH–HCl

Hg–MgCl2MgCl2 IonHausmannite

exchangeable

(Cd and Ni)–(MgCl2 and HOAc/NaOAc)

(As, Cr and Cu)–(HOAc/NaOAc and NH2OH–HCl)

(Pb and Zn)–NH2OH–HCl

(Zn, Cd and Ni)–MgCl2MgCl2 IonHumic acid

exchangeable

(As and Pb)–(MgCl2, HOAc/NaOAc and

NH2OH–HCl)Hg–(NH2OH–HCl and residual)

(Cu and Cr)–(NH2OH–HCl and H2O2/NH4OAc)

MgCl2Illite (Cd and Hg)–MgCl2 Ion

exchangeable

(Zn, As and Ni)–(MgCl2 and HOAc/NaOAc)

(Cu and Pb)–HOAc/NaOAc

Cr–NH2OH–HCl

MgCl2 (Cd, Zn, Hg and Ni)–MgCl2 IonKaolinite

exchangeable

(As, Cr, Cu and Pb)–HOAc/NaOAc

a M, metals considered in this study.

However these authors found that Ni and Zn

were distributed in fractions 2, 3, 4 and 5, indicat-

ing that these metals occupied clay lattice sites.

One possible explanation for the differential be-

havior of Cu may be the lower contact time

(20–30 min) between the metal solution and the

solid phase used in their experiment.

Several metals sorbed on hausmannite and

amorphous iron hydroxide, such as As, Cr, Cu,

Ni, Pb and Zn were released under reducing

conditions. Similar results were obtained by Kimand Fergusson [24] using Cd sorbed on hausman-

nite and goethite, Raksasataya et al. [23] using Pb

sorbed on hausmannite and goethite and Whalley

and Grant [25] using Cu, Ni and Zn sorbed on

manganese dioxide and the sequential extraction

scheme proposed by BCR. In contrast to our

results Whalley and Grant [25] released the metals

sorbed on ferrihydrite using an acid–base mecha-

nism with HOAc. However, Raksasataya et al.

[23] compared both sequential extraction schemes

obtaining the highest recovery for Pb in the reduc-

ing step for Tessier and in the acid step for BCR

scheme. It was attributed to the dissolution of

iron amorphous iron oxides films under the ini-

tially acetic acid conditions (pH 3) of fraction 1 in

the BCR scheme, but not until the later fraction 3

in the Tessier et al. scheme.Lead sorbed on humic acid was released in

fractions 1, 2 and 3, according to the results of

Raksasataya et al. [23]. However, results obtained

for Cd, Cu and Ni in our study differed from

those of Whalley and Grant [25] who found (us-

ing the BCR scheme) releasable Ni and Cu under

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reducing and oxidizing conditions, respectively,

whereas our results showed that Ni was

released under ion exchange conditions and Cu

under reducing and oxidizing conditions.

A possible difference between both Tessier and

BCR schemes affecting the Cu extraction from

humic acid could be the temperature used in the

reducing fraction. In the scheme described by

Tessier et al., the extraction is performedat 95°C, whereas the extraction in the BCR

scheme is performed at room temperature. More-

over, Cd was released in the first fraction in our

study, but Kim and Fergusson [24] using similar

extractants recovered the metal both in fractions 1

and 3.

5. Conclusions

Based on the preceding discussions, a certainamount of confusion is likely to arise from the

classification system used in the sequential

extraction. Reagent selectivity was suitable

for the coprecipitated phases (calcite, amorphous

iron oxide and hausmannite) and all metals

considered in this study, with the exception

of Cd in the amorphous iron oxides phase and for

Hg in calcite and hausmannite phases.

In the sorbed phases, extractant selectivity was

high for calcite, amorphous iron oxides, hausman-

nite and clay in the case of Cd and Hg, whereasthe other metals were released with stronger

reagents.

Acknowledgements

The authors express their thanks to the DGI-

CYT (Direccion General de Investigacion Cien-

tıfica y Tecnica) for Grant No. PB95-0731, as well

as the A.M.A. (Agencia de Medio Ambiente de

Andalucıa, Junta de Andalucıa, Spain).The authors acknowledge assistance of the Geol-

ogy Department of the University of Seville

for the technical support in X-ray diffraction

analysis.

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