Analytica Chimica Acta 414 (2000) 151–164
Metal sequential extraction procedure optimized for heavily polluted and iron oxide rich sediments J.L. Gómez Ariza∗ , I. Giráldez, D. Sánchez-Rodas, E. Morales Departamento de Qu´ımica y Ciencia de los Materiales, Escuela Politécnica Superior, Universidad de Huelva, 21819 La Rábida, Huelva, Spain Received 28 September 1999; received in revised form 15 February 2000; accepted 15 February 2000
Abstract A sequential extraction procedure has been optimized in relation with the characteristics of the sediments found in the Odiel Marshes Natural Park. They presented high percentages of iron oxides (up to 17%) and a total metal content, in the range of hundreds of g g−1 and higher, for As, Cu, Pb and Zn. The metals studied included As, Cr, Cd, Cu, Fe, Hg, Mn, Ni, Pb and Zn. NH4 OAc was superior to MgCl2 and NaOAc in relation to the metal determination using AAS. However, it was not possible to distinguish between exchangeable and bound-to-carbonate metals. For the evaluation of the sum of the metal concentration associated with both these phases, an optimal volume of 35 ml and a concentration of 1 M was selected for the extractant NH4 OAc. No readsorption problems were observed for any metal except for Hg. Several repetitive extractions with NH2 OH·HCl and H2 O2 –NH4 OAc were necessary for the complete extraction of metals bound to oxides and organic matter, respectively. Percentages of iron oxide coated with organic matter were found insignificant for the low organic matter content sediments considered in the study. Finally, a comparison between the optimized and a low selectivity (LS) sequential extraction scheme for metal mobility assessment showed an important overestimation of As, Cr and Ni released by oxidant mechanism and an underestimation of As, Cd, Cr and Ni by ion exchangeable+acid–base and reductant mechanism using the LS scheme. © 2000 Elsevier Science B.V. All rights reserved. Keywords: Heavy metals; Oxic sediments; Sequential extraction; Readsorption
1. Introduction The mobility of metals in environmental solid samples is generally evaluated by using sequential extraction schemes (SESs). A large number of procedures have been developed in which samples were treated with a succession of reagents to liberate metals with ∗ Corresponding author. Tel.: +34-959-530246; fax: +34-959-350311. E-mail address:
[email protected] (J.L. G´omez Ariza)
different affinities for the matrix. The most widely applied SES has been that proposed by Tessier et al. [1]. The analytical results of this and others schemes are usually affected by selectivity and readsorption problems. The problem of the incomplete selectivity of reagents used in SESs to dissolve one particular phase arises in the additional attack to other phases [2–4]. Moreover, they may be insufficiently efficient to completely dissolve the phase [5]. To avoid these problems, modifications in the experimental conditions,
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such as the extraction time, the extractant-sample ratio, the reagent concentration, the extraction temperature, the use of successive extractions with the same reagent, etc., have been proposed [6,7]. The problem of post-extraction readsorption has been clearly demonstrated [7–11]. Several authors [7] have observed readsorptions for As and Cu when 0.04 M NH2 OH·HCl, prescribed in the procedure of Tessier, was used to release the metal under reducing conditions from iron oxide rich sediments (>13% of Fe2 O3 ), and a 10-times higher concentration of NH2 OH–HCl was proposed for these sediments. Moreover, these authors [11] have also studied readsorption and redistribution processes of several trace elements that occur during the application of a modified Tessier SES on oxic iron rich sediments, observing readsorptions for Cu, Hg and As, when both 1 M MgCl2 and 1 M NaOAc (HOAc, pH 5) were used as extractants. The importance of these processes was influenced by the geochemical characteristics of the sediments. They suggested that modifications to the SES must be established to obtain a reliable metal distribution pattern in the sediments, mainly for the most labile fractions. A wide range of neutral solutions of salts (NH4 OAc, NH4 NO3 , CaCl2 , MgCl2 , NaNO3 or KNO3 ) has been used to assess exchangeable metal content in sediments using SESs. One molar NH4 OAc solution at pH 7 was perhaps the preferred reagent as its relatively high concentration and the metal complexing power of acetate prevented readsorption or precipitation of the released metal ions. Moreover, the main advantage of solutions of ammonium salts over alkali and alkaline earth salts lies in the interference effects that can result from the use of relatively strong solutions of alkali and alkali metals in both flame and electrothermal atomic absorption spectrometry [12,13]. In the present study, a previously described low selectivity (LS)-SES [7] has been optimized to correct readsorption problems observed with both MgCl2 and NaOAc/HOAc and incomplete extractions under oxiding and reducing conditions. The optimization has been performed on seven sediments from Odiel Marshes Natural Park (located at the Atlantic coast of southern Spain) along with a candidate to serve as the reference material. The metals studied were As, Cd, Cr, Cu, Hg, Fe, Mn, Ni, Pb and Zn.
2. Experimental 2.1. Area of study and sampling Surface sediments (5 cm depth) were collected from the intertidal area at seven points in Odiel estuary and marshes (Huelva, southwest Spain) at low tide. This area receives the inputs of two rivers. These are the Odiel and Tinto Rivers, which drain the region of the world’s oldest continuously operating mine [14], resulting in significant amounts of cadmium, copper, iron, manganese, lead and zinc being introduced to the estuary. A second source of heavy metal pollution in the Odiel estuary, mainly arsenic, mercury and titanium, is constituted by several processing plants. 1 A plastic spatula was used to transfer sediment to acid-rinsed polypropylene bottles to transport to the laboratory. Although the use of wet sediments has been recommended, mainly for the anoxic ones [15], samples were air-dried in order to obtain a more homogeneous material. Then, they were mildly ground with a wooden roller to pass through a 2 mm sieve, homogenized and stored at 4◦ C in polypropylene until analysis. A sediment which has been used as candidate to reference material (river sediment S12, Standards, Measurements and Testing Programme, S&T) [16] from Besós River (Catalonia, Spain) was simultaneously studied to compare results between samples from different origins. Data for total organic carbon (OM) and carbonate contents, particle-size distribution and chemical analysis for the studied sediments have been previously reported [11]. The sediments were quite similar to each other in terms of overall mineralogical composition, mainly consisting of kaolinite, illite, anhdrite, Na-feldspar, K-feldspar, quartz and amorphous iron oxides. In general, they presented low carbonate (ranged between 0.2 and 6.6%) and OM (0.9–7.6%) contents, except for S12 with percentages of 20 and 9.6% for carbonate and OM, respectively. They were classified depending on the particle-size distribution in sand (samples M1 and M5), sand-silty (sample M4), 1 A.M.A. (Agencia de Medio Ambiente de Andaluc´ıa, Spain). Plan de Polic´ıa de Aguas del litoral Andaluz, A.M.A., Sevilla, Spain, 1994.
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clay (sample M2) and clay-silty (samples M3, M6 and M7). The samples classified as sand and sand-silty were characterized by low iron oxides content and low OM content, whereas the clay and clay-silty samples were characterized by higher iron oxide content and higher OM content. 2.2. Reagents, model phases and apparatus All the reagents were analytical grade or Suprapur quality (Merck, Darmstadt, Germany). Stock standard solutions were Merck Certificate AA standards (Merck). Milli-Q (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. Calcium carbonate (Fluka, Ronkonkoma, NY, USA) and clam shells of Venerupis decussata were used as model phases of carbonates. Shells were air-dried and mildly ground to pass through a 0.1 mm sieve. 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 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. X-ray powder diffraction (XRD) patterns were obtained for each model phase. The d-spacings and relative intensities compared well with those listed for the crystalline forms of calcite and hausmannite in the literature [17]. Amorphous iron oxide was identified by the absence of any diffraction peaks. An atomic absorption spectrophotometer, Perkin– Elmer AAS (model 3100, Ontario, Canada) and
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Pye-Unicam AAS (model SP9, Cambridge, UK), with double beam were used for flame measurements. An atomic spectrophotometer, GBC AAS (model 904, Arlington Heights, IL, USA) with double beam and D2 lamp background correction, coupled with a graphite furnace GBC Model GF 3000 was also used. Sample introduction was carried out using the GBC PAL-3000 autosampler. Hollow cathode lamps were used as radiation sources (Photron, Victoria, Australia). Centrifugation was performed with a Sigma centrifuge (model 4-10, Osteroder am Harz, Germany). 2.3. Analytical procedures Table 1 summarizes the high selectivity (HS)-SES optimized in this study, which was conducted in centrifuge tubes (polyethylene, 50 ml) stoppered with a screw cap. Between each successive extraction, separation was effected by centrifugation at 10,000 rpm for 10 min. The 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. Concentrations of Fe, Mn, Zn in all the extracts were determined by air/acetylene FAAS. Quantification was achieved using matrix-matched standards. Solution concentrations of Cd, Cr, Ni, Pb and Cu were determined by either FAAS or GFAAS. Pyrolytically coated graphite tubes were used for Cr and Ni determinations and L’vov platform pyrolytically coated graphite tubes for Cu, Cd and Pb. NH4 H2 PO4 was used as matrix modifier for Cd and Pb determinations. Arsenic was determined by flow injection-hydride generation (FI-HG)-FAAS in aliquots of 2 ml treated with 0.5 ml of 50% (w/v) KI overnight to reduce As(V)
Table 1 Optimised high selectivity sequential extraction scheme (HS-SES) for partitioning sediment samples (0.5 g sample) Fraction
Procedure
Acid (F1) Reductant (F21 –F28 ) Oxidant (F31 –F38 )
1 M NH4 OAc (35 ml), pH 5, 5 h. ca. 20◦ C, continuous agitation (end-over-end, 40 rpm) 0.4 M NH2 OH·HCl in 25% acetic acid (20 ml), 6 h, 96◦ C, manual shaking every 30 min 0.02 M HNO3 (3 ml)+30% H2 O2 (5 ml), pH 2, 2 h, 85◦ C, manual shaking every 30 min; further 30% H2 O2 (3 ml), pH 2, 3 h, 85◦ C, manual shaking every 30 min; then 3.2 M NH4 OAc in 20% HNO3 (5 ml). 0.5 h, ca. 20◦ C, continuous agitation (end-over-end, 40 rpm) 10/3/1 HF/HNO3 /HCl (14 ml), 6 min, 750 W
Residue (F4)
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to As(III) and mercury by a home-made cold vapour technique. In both the cases, NaBH4 was used as the reductant. Quantification in GF-AAS and mercury and arsenic analysis was achieved by standard addition at three different levels (making duplicates for the different levels). 2.4. The use of NH4 OAc This reagent was used to extract both exchangeable and bound-to-carbonate metals, depending on the pH value 7 and 5, respectively. Optimization of the use of these extractants consisted on repetitive extractions to determine the maximum percentage of metals extracted using each reagent and their selectivity using model phases. Later, the volume of NH4 OAc (at pH 5) to sediment ratio, the reagent concentration and the metal readsorption were studied.
for 5 h with 8 ml 1 M NH4 OAc solution adjusted to pH 5.0 with HNO3 . In both the cases, the samples were agitated on an end-over-end mechanical shaker rotating at 40 rpm at room temperature (24±2◦ C). 2.4.2.2. Experiments with oxide phases. Model phase 0.075 g was extracted for 5 h with 8 ml 1 M NH4 OAc solution adjusted to pH 5.0 with HNO3 . The samples were agitated on an end-over-end shaker at room temperature (24±2◦ C). The residue was then extracted for 6 h at 96±1◦ C with 20 ml 0.4 M NH2 OH·HCl in 25% (v/v) HOAc. The samples were periodically agitated during the course of this step. 2.4.3. Optimization of the volume of NH4 OAc (at pH 5) to sediment ratio and its concentration Several volumes and concentrations of NH4 OAc were tested for the metal extraction in fraction 1 following the procedure of Table 1.
2.4.1. Repetitive extractions 2.4.1.1. Fraction 1 (F1). Sediment sample 0.5 g (dry weight) was extracted in 50 ml polypropylene tube for 1 h with 8 ml 1 M NH4 OAc solution initially at pH 7. The samples were agitated on an end-over-end mechanical shaker rotating at 40 rpm, at room temperature (24±1◦ C). The supernatant (F1a) was separated from the residue by centrifugation at 10,000 rpm and analyzed for metals. The process was repeated four times, obtaining the supernatants F1a–F1d. 2.4.1.2. Fraction 2 (F2). The residue from F1 was extracted for 5 h with 8 ml 1 M NH4 OAc solution adjusted to pH 5.0 with HNO3 . The samples were agitated on an end-over-end shaker at room temperature (24±2◦ C). This extraction was repeated four times (supernatants F2a–F2d). 2.4.2. Study of selectivity of NH4 OAc This study was performed to evaluate the dissolving effect of NH4 OAc on the carbonate and the oxide phases at pH values of 7 and 5, respectively. 2.4.2.1. Experiments with carbonate phases. Model phase 0.04 g (dry weight) was extracted with 8 ml 1 M NH4 OAc solution initially at pH 7 in 50 ml polypropylene tube for 1 h. The resulting residue was extracted
2.4.4. Readsorption study of metals during the sediment extraction with 35 ml of 1 M NH4 OAc These experiments were designed to evaluate the importance of post-extraction readsorptions of several trace elements (arsenic, cadmium, copper, chromium, zinc, lead and nickel in a set of experiments and mercury in other independent set) with 35 ml of 1 M NH4 OAc (pH=5). They were performed using the standard addition technique [11]. The sediments were subjected to one of the two following treatments with NH4 OAc: 1. Leaching with non-spiked extractant to determine the natural concentration of metals in the ‘acidic medium’ operational defined fraction (F1, Table 1). The extractions served to establish the amount of each trace metal that had to be added as standard addition; 2. Extraction with metals spiked extractant, using a 100% of the amount of these elements evaluated in (1). The metals were added as small volumes (50–200 l) from stock solutions to the extraction reagent. Then pH was readjusted to the original value and added to the sediment immediately. Finally, readsorption percentages were calculated as follows: % of readsorption =
(A + B − C) × 1000 B
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where B is the concentration (g ml−1 ) of the element added to the extractant in the treatment (2), and A and C are the measured concentrations (g ml−1 ) of the element in the extracts obtained in treatments (1) and (2), respectively.
were tested for homogeneity of variance using the Barlett and Levene test. An α-value of 0.05 was adopted as the critical level for all statistical testing giving a 95% confidence level (CSS:STATISTICATM )
2.5. Optimization of the number of repetitive extractions using NH2 OH·HCl
3. Results
The residue from F1 (Table 1) was extracted for 6 h at 96±1◦ C with 20 ml of 0.4 M NH2 OH·HCl in 25% (v/v) HOAc. The samples were periodically agitated during the course of this step. The residue was extracted using the same procedure up to a total of nine times (supernatants F21 –F29 ). 2.6. Optimization of the number of repetitive extractions using H2 O2 –NH4 OAc The residue from F29 (Table 1) was extracted for 2 h at 85±1◦ C with 3 ml 0.02 M HNO3 and 5 ml 30% H2 O2 , adjusted to pH 2.0 with HNO3 . After 2 h, an additional 3 ml 30% H2 O2 (pH 2.0 with HNO3 ) was added and extraction continued at 85±1◦ C for another 3 h. The samples were occasionally agitated during the entire procedure and then cooled to room temperature. Five millilitres of 3.2 M NH4 OAc 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 (24±1◦ C) on an end-over-end shaker. The residue was extracted using the same procedure up to a total of eight times (supernatants F31 –F38 ). 2.7. Evaluation of metals released with NH2 OH·HCl after H2 O2 –NH4 OAc treatment The residue from F38 (Table 1) was extracted for 6 h at 96±1◦ C with 20 ml of 0.4 M NH2 OH·HCl in 25% (v/v) HOAc. The samples were periodically agitated during the course of this step. The residue was extracted again using the same procedure (supernatants F41 and F42 ). 2.8. Statistical analysis The data were analyzed statistically using analysis of variance (ANOVA). Prior to analysis, all the data
3.1. Optimization of NH4 OAc as reagent for metal leaching under ion exchange and acid–base mechanisms Fig. 1 shows the metal concentrations released by repetitive extractions with 1 M NH4 OAc at pH 7. RSD for results obtained with this reagent were always lower than 13%. The variation of the Ca released in each successive step depended on the sediment considered following either an exponential (samples M2, M3, M5, M6, M7 and S12) or a linear (M4) decrease, except for M1 in which Ca was released in similar amounts for each extraction. No significant differences were found in the released Ca to metals ratio with the number of extractions for M2 and S12 (ANOVA, p>0.16), whereas the opposite behaviour was found for the other samples (ANOVA, p0.05). However, the amount released of Fe for samples with the highest content of iron oxides (M2 and M6) did not reach a maximum (ANOVA, p0.07) except for Fe (ANOVA, p0.999). This metal was not considered in further experiments. 3.4. Optimization of the number of repetitive extractions using NH2 OH·HCl Fig. 3 shows the metal amounts released in the sediments using nine repetitive extractions. Released metals followed a first-order exponential decay trend (explained variance >99.4%). The number of repetitive extractions needed for releasing a 95% of metal by the reductant mechanism depended on both metal and sample. A higher number of extractions was needed
for samples with both higher OM and iron oxide contents, mainly for Fe, Cr and Ni. Considering the most unfavourable case, eight repetitive extractions were needed. RSD for results obtained with this reagent was always lower than 10%. 3.5. Optimization of the number of repetitive extractions using H2 O2 –NH4 OAc Fig. 4 shows the metal amounts released in the sediments using eight repetitive extractions. Released metals followed a first-order exponential decay trend (explained variance >94%). The number of repetitive extractions needed for releasing a 95% of metal by oxiding mechanism depended on both metal and sample. A higher number of extractions was needed for samples with higher OM, mainly for Fe, Cr and Ni. Considering the most unfavourable case, eight repetitive extractions were needed. RSD for results obtained with this reagent was always lower than 10%.
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Table 2 Metal readsorption (%) from 1 M NH4 OAc (pH 5 with HNO3 )a Sample
1
2
Arsenic Added amount (g) Readsorption (%)
5.03 7.40
Cadmium Added amount (g) Readsorption (%)
0.583 5.6
3 56.0 1.9
4 23.2 6.5
1.17 2.6
2.84 0
5
6
7
S12
5.03 1.2
5.03 4.8
5.03 5.8
7.71 8.3
5.03 3.8
0.219 2.9
0.219 0
0.219 0
1.17 7.4
0.081 1.4
Chromium Added amount (ng) Readsorption (%)
657 6.2
359 5.9
657 1.4
68.7 5
163 7.8
163 13.9
Copper Added amount (g) Readsorption (%)
92.1 0
350 8.3
92.1 5.1
49.6 2.6
92.1 1.8
92.1 14
Lead Added amount (g) Readsorption (%)
56.9 8.10
10.8 14.3
35.5 10.3
35.5 5.8
10.8 6.1
19.1 15.4
19.1 17.3
19.1 12.6
Mercury Added amount (ng) Readsorption (%)
27.4 36.7
27.4 78.3
27.4 61.5
27.4 29.5
27.4 26.1
27.4 82.4
27.4 73.5
27.4 93.4
Nickel Added amount (ng) Readsorption (%)
587 1.2
354 1.4
354 3.8
354 4.6
587 3.4
587 0
Zinc Added amount (g) Readsorption (%)
178 0
481 5.1
331 3.2
178 6.8
273 4.4
178 6.8
a
1743 0 331 7.3
163 12.1 350 9
1.12×104 3.8 3.24 23
8.66×103 5.6 273 5.7
Relative standard deviation were lower than 5% in trace metal spiked solutions amount added and lower than 15% in readsorption results.
3.6. Evaluation of metals released with NH2 OH·HCl after H2 O2 –NH4 OAc Table 3 shows the results obtained using NH2 OH·HCl after eight extractions with H2 O2 –NH4 OAc. As, Cd, Cr and Mn were below detection limit. The amount of metal released was higher than that in the last extraction with both H2 O2 –NH4 OAc and NH2 OH·HCl, which indicated that the metal was released from iron oxides occluded with OM. However, the percentage of metal released was lower than 1% of the total metal content in the sediments, except for both Ni in all the samples and Cu in S12. 3.7. Results obtained using the high selectivity sequential extraction scheme Results obtained from the use of the optimized HS-SES on seven samples collected at Odiel Marshes and the candidate to reference material S12 are shown
in Fig. 5. The highest residual contents were found in sandy samples, such as M2, M21 and M53, with average percentages of 23% of the total metal content. The lowest residual content was found in sample S12, clay in character with high contents of both carbonate and OM and low content in iron oxides. A different distribution pattern was observed for the different metals. The most residual character was found for Cr and Ni (average percentage of 37%) followed by Fe and Mn (30 and 27%, respectively), Pb (14%) and As, Cu, Zn and Cd (7, 6, 4 and 2%, respectively). However, most of the metals were released under reducing conditions, mainly As, Fe, Pb, Cr and Ni with average percentages of 85, 74, 66, 57 and 59%, respectively. Otherwise, Zn, Mn and Cu were distributed between phases leachable under acid and reducing conditions, with average percentages of 89, 73 and 72%, respectively. Finally, Cd was released under acidic conditions (65%).
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Fig. 3. Metal concentration released by nine repetitive extractions with 20 ml of 0.4 M NH2 OH·HCl in 25% (v/v) for 6 h at 96±1◦ C. Error bars represent standard deviations.
4. Discussion The SES proposed by Tessier et al. [1] recommended 1 M MgCl2 and 1 M NaOAc (pH=5, HOAc) solutions for the selective extraction of metals by ion exchange and acid–base mechanisms, respectively. However, due to metal readsorption problems in the oxic sediments of the Odiel Marshes Natural Park (southwest Spain) [11], other extractant must be studied. NH4 OAc presents a high complexing power due to the acetate ion which may prevent readsorption or precipitation of the released metal ions from sediments. In addition, the solutions of ammonium salts present lower interference effects than the alkali and alkali metals solutions in both flame and electrothermal atomic absorption spectrometry [12,13]. The results obtained by repetitive extractions in the sediments and model phases (calcite and clam shell) with 1 M NH4 OAc at pH 7 suggested that the carbon-
ates were partially dissolved, and agreed with those obtained by other authors using riverine sediments [18]. Therefore, this reagent was not suitable to distinguish between exchangeable and bound-to-carbonate metals. NH4 OAc was also studied as extractant for the metal bound to the carbonate phase. In this case, a pH 5 was selected, as recommended in the literature [1]. However, we observed that the Ca-metal ratio for Fe, Cd, Cr and Zn decreased when several repetitive extractions were performed, which may indicate that the iron oxides were partially dissolved or that metals sorbed on other phases may be partially released at pH 5 during this acid extractable step, depending on their binding strengths, as reported in the literature [5]. However, only a low percentage of iron oxides was really dissolved with NH4 OAc, as it has been demonstrated with both natural sediments and model phases, and could only be observed because of the iron ox-
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Fig. 4. Metal concentration released by eight repetitive extractions with 3 ml 0.02 M HNO3 and 5 ml of 30% H2 O2 (pH 2.0) for 2 h at 85±1◦ C followed by 3 ml of 30% H2 O2 (pH 2.0) for 3 h at 85±1◦ C and 5 ml of 3.2 M NH4 OAc in 20% (v/v) HNO3 for 30 min at room temperature. Error bars represent standard deviations.
ide rich characteristics of the studied sediments. This may mainly affect the evaluation of exchangeable and acid-leachable metals, but not the evaluation of metal leachable under reducing conditions. In summary, only one fraction consisting of both exchangeable and bound-to-carbonate metals was selected for the optimized HS-SES using NH4 OAc at pH 5, and this step was only performed once since an exponential decay in the released metal was obtained when several repetitive extractions were carried
out as otherwise, metals bound to iron oxides may be released. A combined evaluation of metal released under an exchange+acid–base mechanism has also been proposed by T&M (formerly BCR) [19]. Previous studies on different extractant volume-tosample mass ratios have shown that this parameter was also critical to get a quantitative extraction of metals from sediments [7,20]. Therefore, the next step was the optimization of both volume and concentration of NH4 OAc, 35 ml of 1 M NH4 OAc solution being
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Table 3 Metal concentration (mg kg−1 ) ±standard deviation from the reductant fraction after the oxidant fraction Sample Copper 1 2 Iron 1 2 Lead 1 2 Nickel 1 2 Zinc 1 2
1
2
3
4
5
6
7
S12
0.15±0.01 0.09±0.01
2.9±0.4 0.59±0.08
2.5±0.3 0.54±0.06
0.18±0.03
