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Selectivity assessment of a sequential extraction procedure for metal mobility characterization using model phases
Talanta 52 (2000) 545 – 554 www.elsevier.com/locate/talanta
Selectivity assessment of a sequential extraction procedure for metal mobility characterization using model phases J.L. Go´mez Ariza *, I. Gira´ldez, D. Sa´nchez-Rodas, E. Morales Departamento de Quı´mica y Ciencia de los Materiales, Escuela Polite´cnica Superior, Uni6ersidad de Huel6a, 21819 Palos de la Frontera, Huel6a, 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
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 * Corresponding author. Tel.: +34-959-530246; fax: + 34959-350311. E-mail address: [email protected] (J.L. Go´mez Ariza)
metals associated with each phase of these matrices. 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] and sewage sludge [6]. This procedure was designed to differentiate between metals bound to exchangeable, carbonate, reducible (hydrous Fe/Mn oxides), oxidizable (sulfides and organic phases) and residual (mineral) fractions. However, the trace metals mobility characterized by sequential extraction procedures presents
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some controversial. The specificity and reproducibility of the procedures depend upon the chemical properties of the element and the chemical composition 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 concentration of extractant [11], and as a consequence the distribution of trace metals obtained by such procedures is operational [12]. Moreover, the metal mobility is influenced by inherent analytical problems such as incomplete selectivity [13,14] and readsorptions [15 – 17]. Two factors contribute to the extractant selectivity: 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 relative 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 procedure previously reported by the authors [11], model synthetic phases including calcite, amorphous 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, following similar experiments reported in the literature 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). Stock metal standard solutions were Merck Certificate AA standards (Merck). Milli-Q water (Millipore, Bedford, MA, USA) was used in all the experiments. Cleaning of plastic and glassware was carried out by soaking in 14% (v/v) HNO3 for 24 h and then rinsing with water.
Reference natural clays minerals such as kaolinite (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 studied: a commercial humic acid (Fluka, Everett, WA, USA), a certified reference material (TORT1, lobster hepatopancreas from the National Research 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 Martı´n-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, Australia). The model phases were characterized by X-ray diffraction (Phillips X-ray diffractometer, model 1130/90, Almelo, Netherlands, with Cu Ka radiation 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 phases Calcite 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 addition 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, lightly crushed, and stored in bottles. Metal-coprecipitated phases of calcite, amorphous iron oxides and hausmannite were prepared by spiking one of the precipitating reagents (calcium, 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, respectively) were washed thoroughly with double-distilled 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 identified 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 acid was 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 contents in 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 24 92°C using an end-over-end mechanical shaker, in order to allow the metals to adsorb onto their surfaces (materials prepared 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
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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 following operational fractions [11]: Fraction 1 (F1): 0.1 g (dry weight) of model phase was extracted with 8 ml 1 M MgCl2 solution initially at pH 7 in 50 ml polypropylene tube for 1 h. The samples were agitated on an endover-end mechanical shaker rotating at 40 rpm, at room temperature (2491°C). Fraction 2 (F2): the residue from F1 was extracted 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 temperature (2492°C). Fraction 3 (F3): the residue from F2 was extracted for 6 h at 96 9 1°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 extracted for 2 h at 85 9 1°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 859 1°C for another 3 h. The samples were occasionally agitated during the entire 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 temperature (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 to room temperature, boric acid was added in excess to neutralize residual HF. The solution was transferred 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 vessels 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 extracts spiked with the different metals and treated under the same experimental conditions were prepared to check possible adsorption processes onto the container walls. Metal concentrations (except As and Hg) in all the extracts were determined by air/acetylene FAAS. Quantification was achieved using matrixmatched 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 addition technique was used for quantification (three different 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 it could 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. Moreover, most of Cd (61%) was released in the fraction. 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 solubilized or adsorbed onto this colloidal iron and thus removed in this extract. When iron was determined, only 3% was found in the extract, a very low percentage considering that for Cd. For hausmannite, all trace metals were quantitatively 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 chemical degradation of the phase itself. Also, the low recoveries associated to fraction 4 using the spiked commercial humic acid may reflect that the operational method used to obtain the coprecipitated phase did not represent a reasonable model of their natural mixing. Therefore, additional experiments were performed with other organic ma-
Table 1 Concentrations of metal (mg kg−1)9 RSD (n=3) in the metal-coprecipitated phases, TORT-1 and natural humic acid Phases
As
Cd
Cr
Cu
Calcite Amorphous iron oxides Hausmannite Humic acid TORT-1 Natural humic acid
859 2 409 3 329 4 – 209 3 30092
878 93 620 92 54091 – 17 9 3 BDLa
142 91 260 92 500 92 – 109 3 81 9 3
1000 9 1 740 91 360 92 2200 91 360 92 200 93
a
DL, detection limit.
Hg 0.10 94 14.0 94 9.0 95 – 0.20 93 BDL
Ni 971 93 691 9 1 580 9 2 – 3.0 94 BDL
Pb
Zn
1000 9 1 720 9 2 660 9 2 959 3 109 3 1360 9 1
9209 2 6089 3 5109 1 1409 2 1209 3 29092
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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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550
3.2. Metal-sorbed phases The percentages of metal adsorption obtained for the preparation of the model phases are presented in Table 2. Although a low percentage was obtained for several metals and phases (for example, 9% for Hg on calcite), a sufficient amount of metal was retained by the solid allowing further studies. The phases were prepared under the hypothesis that sorbed metals would behave as exchangeable ones. Therefore, they should be extracted with the MgCl2 solution (fraction 1). However, most of the metals were not quantitatively removed at this stage from all the phases (Fig. 2). For calcite, the metals were not quantitatively 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 opposite behavior, being mainly released in the second fraction (about 87%) and in a minor extension in fraction 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 contained quantitative amounts of Cd, Ni and Zn sorbed on humic acid phase. However, the other metals required 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-coprecipitated phases and the reagents expected to attack them versus those experimentally determined to release the highest percentage of the metal. NaOAc/HOAc solution was designed to dissolve 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 with those obtained by Rapin and Fo¨rstner [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 the amorphous 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 amorphous iron oxide and hausmannite being released
Table 2 Percentage (%) 9 RSD (n=3) of sorbed metal in the metal-sorbed phases Phases
As
Cd
Cr
Cu
Hg
Ni
Pb
Zn
Calcite Amorphous iron oxides Hausmnnite Humic acid Illite Kaolinite
6891 1009 1 9992 30 94 7092 5894
28 9 3 100 91 66 9 3 92 9 2 85 91 22 93
50 9 2 45 9 3 94 9 1 82 9 2 100 9 1 50 9 2
98 92 99 91 99 91 9291 9892 9693
9 94 100 94 15 95 89 93 8 95 6 94
14 9 2 100 9 1 29 9 2 88 9 2 26 9 2 14 9 2
31 9 3 99 9 1 100 9 1 86 9 3 71 9 3 74 9 1
479 4 1009 1 989 2 90 9 2 549 1 839 3
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551
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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552
talline phase of iron oxides, as it has been established in literature [28]. Moreover, Kim and Fergusson [24] reported that Cd coprecipitated in the hausmannite was quantitatively released in fraction 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 Fergusson [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 overestimation of the significance of the metal bound to carbonates and an underestimation of the ‘exchangeable’ metal related to the extension of sorbed metals. Moreover, the sequential extraction 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 metal
Extract found to have the highest metal concentrationa Assumed mechanism
Calcite
HOAc/NaOAc
Amorphous iron oxides
NH2OH–HCl
M–HOAc/NaOAc Hg–MgCl2 M–NH2OH–HCl
Hausmannite
NH2OH–HCl
Humic acid
H2O2/NH4OAc
TORT-1
H2O2/NH4OAc
Natural humic acid
H2O2/NH4OAc
Cd–HOAc/NaOAc M–NH2OH–HCl Hg–H2O2/NH4OAc (Zn and Pb)–MgCl2 Cu–(MgCl2, NH2OH–HCl and H2O2/NH4OAc) As and Hg–MgCl2 (Zn and Ni)–HOAc/NaOAc Cd–(MgCl2 and HOAc/NaOAc) (Cu and Cr)–(MgCl2 and NH2OH–HCl) (As, Pb and Zn)–NH2OH–HCl Cu–(NH2OH–HCl and H2O2/NH4OAc) (Hg and Cr)–H2O2/NH4OAc
a
M, metals considered in this study.
Acid–base Reduction
Reduction Oxidation Oxidation
Oxidation
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553
Table 4 Comparison between the expected and experimentally determined reagents for sorbed individual phases Phase
Reagent expected to release the metal
Extract found to have the highest metal concentrationa Assumed mechanism
Calcite
MgCl2
(Cd and Hg)–MgCl2
Amorphous iron oxides
MgCl2
Hausmannite
MgCl2
M–HOAc/NaOAc Hg–(MgCl2, NH2OH–HCl,H2O2/NH4OAc and residual) Cd–(MgCl2, HOAc/NaOAc and NH2OH–HCl) (Zn, Cr, Cu and Ni)–(HOAc/NaOAc and NH2OH–HCl) (As and Pb)–NH2OH–HCl Hg–MgCl2
MgCl2
(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)–MgCl2
MgCl2
(As and Pb)–(MgCl2, HOAc/NaOAc and NH2OH–HCl) Hg–(NH2OH–HCl and residual) (Cu and Cr)–(NH2OH–HCl and H2O2/NH4OAc) (Cd and Hg)–MgCl2
MgCl2
(Zn, As and Ni)–(MgCl2 and HOAc/NaOAc) (Cu and Pb)–HOAc/NaOAc Cr–NH2OH–HCl (Cd, Zn, Hg and Ni)–MgCl2
Humic acid
Illite
Kaolinite
Ion exchangeable Ion exchangeable
Ion exchangeable
Ion exchangeable
Ion exchangeable
Ion 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, indicating that these metals occupied clay lattice sites. One possible explanation for the differential behavior 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 Kim and Fergusson [24] using Cd sorbed on hausmannite 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 mechanism with HOAc. However, Raksasataya et al. [23] compared both sequential extraction schemes obtaining the highest recovery for Pb in the reducing 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 initially 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 (using 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 performed at 95°C, whereas the extraction in the BCR scheme is performed at room temperature. Moreover, 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 certain amount 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, hausmannite and clay in the case of Cd and Hg, whereas the other metals were released with stronger reagents.
Acknowledgements The authors express their thanks to the DGICYT (Direccio´n General de Investigacio´n Cientı´fica y Te´cnica) for Grant No. PB95-0731, as well as the A.M.A. (Agencia de Medio Ambiente de Andalucı´a, Junta de Andalucı´a, Spain). The authors acknowledge assistance of the Geology Department of the University of Seville for the technical support in X-ray diffraction analysis. .
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Selectivity assessment of a sequential extraction procedure for metal mobility characterization using model phases J.L. Go´mez Ariza *, I. Gira´ldez, D. Sa´nchez-Rodas, E. Morales Departamento de Quı´mica y Ciencia de los Materiales, Escuela Polite´cnica Superior, Uni6ersidad de Huel6a, 21819 Palos de la Frontera, Huel6a, 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
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 * Corresponding author. Tel.: +34-959-530246; fax: + 34959-350311. E-mail address: [email protected] (J.L. Go´mez Ariza)
metals associated with each phase of these matrices. 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] and sewage sludge [6]. This procedure was designed to differentiate between metals bound to exchangeable, carbonate, reducible (hydrous Fe/Mn oxides), oxidizable (sulfides and organic phases) and residual (mineral) fractions. However, the trace metals mobility characterized by sequential extraction procedures presents
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some controversial. The specificity and reproducibility of the procedures depend upon the chemical properties of the element and the chemical composition 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 concentration of extractant [11], and as a consequence the distribution of trace metals obtained by such procedures is operational [12]. Moreover, the metal mobility is influenced by inherent analytical problems such as incomplete selectivity [13,14] and readsorptions [15 – 17]. Two factors contribute to the extractant selectivity: 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 relative 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 procedure previously reported by the authors [11], model synthetic phases including calcite, amorphous 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, following similar experiments reported in the literature 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). Stock metal standard solutions were Merck Certificate AA standards (Merck). Milli-Q water (Millipore, Bedford, MA, USA) was used in all the experiments. Cleaning of plastic and glassware was carried out by soaking in 14% (v/v) HNO3 for 24 h and then rinsing with water.
Reference natural clays minerals such as kaolinite (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 studied: a commercial humic acid (Fluka, Everett, WA, USA), a certified reference material (TORT1, lobster hepatopancreas from the National Research 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 Martı´n-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, Australia). The model phases were characterized by X-ray diffraction (Phillips X-ray diffractometer, model 1130/90, Almelo, Netherlands, with Cu Ka radiation 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 phases Calcite 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 addition 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, lightly crushed, and stored in bottles. Metal-coprecipitated phases of calcite, amorphous iron oxides and hausmannite were prepared by spiking one of the precipitating reagents (calcium, 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, respectively) were washed thoroughly with double-distilled 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 identified 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 acid was 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 contents in 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 24 92°C using an end-over-end mechanical shaker, in order to allow the metals to adsorb onto their surfaces (materials prepared 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
547
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 following operational fractions [11]: Fraction 1 (F1): 0.1 g (dry weight) of model phase was extracted with 8 ml 1 M MgCl2 solution initially at pH 7 in 50 ml polypropylene tube for 1 h. The samples were agitated on an endover-end mechanical shaker rotating at 40 rpm, at room temperature (2491°C). Fraction 2 (F2): the residue from F1 was extracted 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 temperature (2492°C). Fraction 3 (F3): the residue from F2 was extracted for 6 h at 96 9 1°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 extracted for 2 h at 85 9 1°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 859 1°C for another 3 h. The samples were occasionally agitated during the entire 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 temperature (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 to room temperature, boric acid was added in excess to neutralize residual HF. The solution was transferred 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 vessels 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 extracts spiked with the different metals and treated under the same experimental conditions were prepared to check possible adsorption processes onto the container walls. Metal concentrations (except As and Hg) in all the extracts were determined by air/acetylene FAAS. Quantification was achieved using matrixmatched 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 addition technique was used for quantification (three different 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 it could 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. Moreover, most of Cd (61%) was released in the fraction. 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 solubilized or adsorbed onto this colloidal iron and thus removed in this extract. When iron was determined, only 3% was found in the extract, a very low percentage considering that for Cd. For hausmannite, all trace metals were quantitatively 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 chemical degradation of the phase itself. Also, the low recoveries associated to fraction 4 using the spiked commercial humic acid may reflect that the operational method used to obtain the coprecipitated phase did not represent a reasonable model of their natural mixing. Therefore, additional experiments were performed with other organic ma-
Table 1 Concentrations of metal (mg kg−1)9 RSD (n=3) in the metal-coprecipitated phases, TORT-1 and natural humic acid Phases
As
Cd
Cr
Cu
Calcite Amorphous iron oxides Hausmannite Humic acid TORT-1 Natural humic acid
859 2 409 3 329 4 – 209 3 30092
878 93 620 92 54091 – 17 9 3 BDLa
142 91 260 92 500 92 – 109 3 81 9 3
1000 9 1 740 91 360 92 2200 91 360 92 200 93
a
DL, detection limit.
Hg 0.10 94 14.0 94 9.0 95 – 0.20 93 BDL
Ni 971 93 691 9 1 580 9 2 – 3.0 94 BDL
Pb
Zn
1000 9 1 720 9 2 660 9 2 959 3 109 3 1360 9 1
9209 2 6089 3 5109 1 1409 2 1209 3 29092
J.L. Go´mez Ariza et al. / Talanta 52 (2000) 545–554
549
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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550
3.2. Metal-sorbed phases The percentages of metal adsorption obtained for the preparation of the model phases are presented in Table 2. Although a low percentage was obtained for several metals and phases (for example, 9% for Hg on calcite), a sufficient amount of metal was retained by the solid allowing further studies. The phases were prepared under the hypothesis that sorbed metals would behave as exchangeable ones. Therefore, they should be extracted with the MgCl2 solution (fraction 1). However, most of the metals were not quantitatively removed at this stage from all the phases (Fig. 2). For calcite, the metals were not quantitatively 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 opposite behavior, being mainly released in the second fraction (about 87%) and in a minor extension in fraction 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 contained quantitative amounts of Cd, Ni and Zn sorbed on humic acid phase. However, the other metals required 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-coprecipitated phases and the reagents expected to attack them versus those experimentally determined to release the highest percentage of the metal. NaOAc/HOAc solution was designed to dissolve 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 with those obtained by Rapin and Fo¨rstner [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 the amorphous 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 amorphous iron oxide and hausmannite being released
Table 2 Percentage (%) 9 RSD (n=3) of sorbed metal in the metal-sorbed phases Phases
As
Cd
Cr
Cu
Hg
Ni
Pb
Zn
Calcite Amorphous iron oxides Hausmnnite Humic acid Illite Kaolinite
6891 1009 1 9992 30 94 7092 5894
28 9 3 100 91 66 9 3 92 9 2 85 91 22 93
50 9 2 45 9 3 94 9 1 82 9 2 100 9 1 50 9 2
98 92 99 91 99 91 9291 9892 9693
9 94 100 94 15 95 89 93 8 95 6 94
14 9 2 100 9 1 29 9 2 88 9 2 26 9 2 14 9 2
31 9 3 99 9 1 100 9 1 86 9 3 71 9 3 74 9 1
479 4 1009 1 989 2 90 9 2 549 1 839 3
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551
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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552
talline phase of iron oxides, as it has been established in literature [28]. Moreover, Kim and Fergusson [24] reported that Cd coprecipitated in the hausmannite was quantitatively released in fraction 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 Fergusson [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 overestimation of the significance of the metal bound to carbonates and an underestimation of the ‘exchangeable’ metal related to the extension of sorbed metals. Moreover, the sequential extraction 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 metal
Extract found to have the highest metal concentrationa Assumed mechanism
Calcite
HOAc/NaOAc
Amorphous iron oxides
NH2OH–HCl
M–HOAc/NaOAc Hg–MgCl2 M–NH2OH–HCl
Hausmannite
NH2OH–HCl
Humic acid
H2O2/NH4OAc
TORT-1
H2O2/NH4OAc
Natural humic acid
H2O2/NH4OAc
Cd–HOAc/NaOAc M–NH2OH–HCl Hg–H2O2/NH4OAc (Zn and Pb)–MgCl2 Cu–(MgCl2, NH2OH–HCl and H2O2/NH4OAc) As and Hg–MgCl2 (Zn and Ni)–HOAc/NaOAc Cd–(MgCl2 and HOAc/NaOAc) (Cu and Cr)–(MgCl2 and NH2OH–HCl) (As, Pb and Zn)–NH2OH–HCl Cu–(NH2OH–HCl and H2O2/NH4OAc) (Hg and Cr)–H2O2/NH4OAc
a
M, metals considered in this study.
Acid–base Reduction
Reduction Oxidation Oxidation
Oxidation
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553
Table 4 Comparison between the expected and experimentally determined reagents for sorbed individual phases Phase
Reagent expected to release the metal
Extract found to have the highest metal concentrationa Assumed mechanism
Calcite
MgCl2
(Cd and Hg)–MgCl2
Amorphous iron oxides
MgCl2
Hausmannite
MgCl2
M–HOAc/NaOAc Hg–(MgCl2, NH2OH–HCl,H2O2/NH4OAc and residual) Cd–(MgCl2, HOAc/NaOAc and NH2OH–HCl) (Zn, Cr, Cu and Ni)–(HOAc/NaOAc and NH2OH–HCl) (As and Pb)–NH2OH–HCl Hg–MgCl2
MgCl2
(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)–MgCl2
MgCl2
(As and Pb)–(MgCl2, HOAc/NaOAc and NH2OH–HCl) Hg–(NH2OH–HCl and residual) (Cu and Cr)–(NH2OH–HCl and H2O2/NH4OAc) (Cd and Hg)–MgCl2
MgCl2
(Zn, As and Ni)–(MgCl2 and HOAc/NaOAc) (Cu and Pb)–HOAc/NaOAc Cr–NH2OH–HCl (Cd, Zn, Hg and Ni)–MgCl2
Humic acid
Illite
Kaolinite
Ion exchangeable Ion exchangeable
Ion exchangeable
Ion exchangeable
Ion exchangeable
Ion 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, indicating that these metals occupied clay lattice sites. One possible explanation for the differential behavior 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 Kim and Fergusson [24] using Cd sorbed on hausmannite 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 mechanism with HOAc. However, Raksasataya et al. [23] compared both sequential extraction schemes obtaining the highest recovery for Pb in the reducing 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 initially 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 (using 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 performed at 95°C, whereas the extraction in the BCR scheme is performed at room temperature. Moreover, 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 certain amount 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, hausmannite and clay in the case of Cd and Hg, whereas the other metals were released with stronger reagents.
Acknowledgements The authors express their thanks to the DGICYT (Direccio´n General de Investigacio´n Cientı´fica y Te´cnica) for Grant No. PB95-0731, as well as the A.M.A. (Agencia de Medio Ambiente de Andalucı´a, Junta de Andalucı´a, Spain). The authors acknowledge assistance of the Geology Department of the University of Seville for the technical support in X-ray diffraction analysis. .
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