Extraction of Heavy Metals from Soils Using Biodegradable Chelating

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Environ. Sci. Technol. 2004, 38, 937-944

Extraction of Heavy Metals from Soils Using Biodegradable Chelating Agents SUSAN TANDY, KARIN BOSSART, ROLAND MUELLER, JENS RITSCHEL, LUKAS HAUSER, RAINER SCHULIN, AND BERND NOWACK* Institute of Terrestrial Ecology (ITO ¨ ), Swiss Federal Institute of Technology Zu ¨ rich (ETH), Grabenstrasse 3, CH-8952 Schlieren, Switzerland

Metal pollution of soils is widespread across the globe, and the clean up of these soils is a difficult task. One possible remediation technique is ex-situ soil washing using chelating agents. Ethylenediaminetetraacetic acid (EDTA) is a very effective chelating agent for this purpose but has the disadvantage that it is quite persistent in the environment due to its low biodegradability. The aim of our work was to investigate the biodegradable chelating agents [S,S]-ethylenediaminedisuccinic acid (EDDS), iminodisuccinic acid (IDSA), methylglycine diacetic acid (MGDA), and nitrilotriacetic acid (NTA) as potential alternatives and compare them with EDTA for effectiveness. Kinetic experiments showed for all metals and soils that 24 h was the optimum extraction time. Longer times only gave minor additional benefits for heavy metal extraction but an unwanted increase in iron mobilization. For Cu at pH 7, the order of the extraction efficiency for equimolar ratios of chelating agent to metal was EDDS > NTA> IDSA > MGDA > EDTA and for Zn it was NTA > EDDS > EDTA >MGDA > IDSA. The comparatively low efficiency of EDTA resulted from competition between the heavy metals and co-extracted Ca. For Pb the order of extraction was EDTA > NTA >EDDS due to the much stronger complexation of Pb by EDTA compared to EDDS. At higher concentration of complexing agent, less difference between the agents was found and less pH dependence. There was an increase in heavy metal extraction with decreasing pH, but this was offset by an increase in Ca and Fe extraction. In sequential extractions EDDS extracted metals almost exclusively from the exchangeable, mobile, and Mn-oxide fractions. We conclude that the extraction with EDDS at pH 7 showed the best compromise between extraction efficiency for Cu, Zn, and Pb and loss of Ca and Fe from the soil.

Introduction Metal pollution of soils is widespread across the globe, and the clean up of these soils is a difficult task. Various in-situ and ex-situ remediation techniques have been employed, e.g., solidification, stabilization, flotation, soil washing, electroremediation, bioleaching, and phytoremediation (1). Soil washing includes the physical separation of the clay and * Corresponding author phone: +41 1 633 61 60; fax: +41 1 633 11 23; e-mail: [email protected]. 10.1021/es0348750 CCC: $27.50 Published on Web 12/23/2003

 2004 American Chemical Society

silt fraction containing the majority of the metals due to their high specific adsorption capacity as well as the extraction of metals by mineral acids or chelating agents. A particularly promising technique is ex-situ soil washing with chelating agents (2). The soil is removed from the site, treated in a closed reactor with the chelating agent, and returned to the site after separation of the extraction solution that now contains the extracted heavy metals. The advantage of the method is the high potential extraction efficiency and the specificity for heavy metals. To keep treatment costs low, it is necessary to achieve a cleanup so that the soil can be reused and it should be possible to recover and reuse the chelating agent for further extraction cycles. There are many factors to consider when comparing studies of chelating agent assisted soil washing in order to decide whether the chelating agent is suitable for field-scale decontamination of polluted sites. The ratio of chelating agent to toxic metals, pH, quantity of major cations extracted, and source of contamination (artificial or anthropogenic) are the most important ones. Most studies of chelant-assisted soil washing have found that a ratio >1 between chelant and toxic metal is required to give good toxic metal extraction (3-7). When the ratio is increased, so does the extracted fraction of metal until the extraction efficiency levels off (47). Many studies however do not give the chelant:metal ratio used. In some cases this ratio can be calculated from the concentration and volume of chelating agent and the mass of soil (8-14), but in others even this information is missing. One reason an excess of chelating agent is needed is that major cations in the soil such as Mn, Mg, Fe, and Ca along with toxic metals present in smaller amounts compete with the metals being studied for the chelating agent and are extracted too (7, 14, 15). This is important from the point of view both that it reduces the extraction of the target metals and that it extracts major cations which are important as plant nutrients and for maintaining soil structure in future reuse of the decontaminated soil. Another crucial factor to be considered in comparing studies on chelating agent extraction is the pH of the extraction solution. While extraction was investigated at various pH values in some studies (3, 5, 7, 14), some only stated the pH of the solution (10, 11, 13), while others did not consider pH at all (6, 8, 9, 12). In general, the lower the pH of the chelating agent solution, the greater the extraction efficiency of the toxic metals. The source of the metal and its form can also affect the extraction efficiency, especially for Pb(16). Furthermore, it has been found that significantly larger extraction efficiencies are obtained when chelating agents are applied to artificially contaminated soils than to soils with field contamination (7, 17). Added soluble metals quickly transfer to the exchangeable fraction and more slowly but still within the first hour to the carbonate and organic matter fractions (18). Inclusion in precipitates and diffusion into micropores takes place at much slower rates (19). In recently contaminated soils the metals are generally in a much more accessible form than in soils that have been contaminated years ago. This important factor needs to be considered when chelating agents are evaluated for field use. Many studies have compared EDTA to other chelating agents, acids, and surfactants and found it better suited than or equal to its competitors for the extraction of toxic metals from soils (3-5, 9, 11). Between 45% and 100% Pb, 54% and 100% Zn, and 47% and 98% Cu were extracted from various contaminated soils by EDTA (3-11, 14). The maximum extraction efficiencies of other chelating agents for Pb were VOL. 38, NO. 3, 2004 / ENVIRONMENTAL SCIENCE & TECHNOLOGY

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TABLE 1. Soil Properties

soil

pH

Dornach 1 7.0 Dornach 2 6.2 Rafz 5.5

sum of Ca Zn Cu Pb Ni heavy metals -1 -1 -1 -1 -1 CaCO3 (%) Corg (%) clay (%) silt (%) sand (%) (g kg ) (mg kg ) (mg kg ) (mg kg ) (mg kg ) (µmol g-1) 13.8 0.6 0.5

5.7 7.3 3.4

33 29 16

39 56 30

28 15 54

found to be the following: 63% for ADA (N-2-(acetamido)iminodiacetic acid), 60% for NTA, 58% for PDA (pyridine2,6-dicarboxylic acid), 11% for DPTA (diethylenetriamine pentaacetic acid), and 0% for citric acid (3-5, 9). For Zn maximum extraction efficiencies were reported as 41% with DPTA and 12% with citric acid (8, 9, 14), and for Cu they were 32% with DPTA and 11% with citric acid (9, 12, 14). Thus far, EDTA has been the most widely used chelating agent for these processes due to its high extraction efficiency. It is very effective in mobilizing metals, but unfortunately, due to its low biodegradability (20), it is also very persistent in the environment. This can cause a rather high risk of metal leaching to the groundwater (21). NTA, which is easily biodegradable, is under scrutiny due to possible adverse health effects (22). Recently the easily biodegradable chelating agent SS-EDDS (S,S-ethylenediaminedisuccinic acid) (23) has been proposed as a safe and environmentally benign replacement for EDTA in soil washing (24) and for chelantenhanced phytoremediation (25). The aim of this study was to investigate SS-EDDS and other chelating agents that can be potentially used in soil washing and that are less persistent in the environment than EDTA and to compare them with commonly used chelating agents for effectiveness. Extraction experiments were carried out under controlled conditions (pH and chelant: toxic metal ratio) using soil from contaminated sites requiring cleanup or other treatment according to Swiss legislation.

Materials and Methods Soils. The soil materials used in this study were taken from two polluted sites in Switzerland. Two soils were taken from an area in Dornach which has been heavily contaminated with Cu, Zn, and Cd for about a century by particulate emissions from an adjacent brass smelter (26). The two soils differed in their carbonate content: one was a Calcaric Regosol (Dornach 1) and the other a non-calcareous Regosol (Dornach 2) (27). Both soils had very similar total metal contents. The third soil, (Rafz) a Haplic Luvisol, originated from an agricultural field in Northern Switzerland, which had been contaminated with Zn, Pb, and Cd due to sewage sludge applications (27). The soil samples were taken from the top 20 cm, dried at 40 °C, and sieved to EDTA > MGDA > NTA > IDSA, and at pH 7 it was EDDS > NTA> IDSA > MGDA > EDTA. At pH 4 EDDS and IDSA did not enhance Zn extraction compared to the controls without chelating agent, due to their very weak complexes at this pH, but EDTA, NTA, and MGDA were able to mobilize Zn. At pH 7 the order of extraction efficiency was NTA > EDDS> EDTA > MGDA >IDSA. Pb was analyzed only for treatments of Rafz soil with EDTA, NTA, and EDDS. At both pH 4 and 7 the extraction efficiency was EDTA > NTA > EDDS, although at pH 7 the extraction efficiency of EDDS was much closer to that of NTA and EDTA than at pH 4 (Figure 3). At a chelant:metal ratio of 10, the pH dependence of the extraction and the differences between the compounds was much less pronounced (Figures 1-3). At this concentration EDTA was the most effective compound for Cu, Zn, and Pb over the whole pH range studied.

FIGURE 1. Extraction of Cu from the non-calcareous soil Dornach 2 as a function of pH with 0.4 mM (ratio chelant:metal 1) and 4 mM chelant (ratio chelant:metal 10). Conditions: 20 g L-1 soil, 0.01 M NaNO3. At the low chelant:metal ratio, neither EDDS or EDTA affected the Ca concentration in solution compared to the treatment without chelant (Figure 4). The Ca concentration was determined by ion exchange in the 0.01 M NaNO3 medium. At the high chelant:metal ratio of 10 significantly more Ca was extracted than in the absence of chelant. About 20% of the total Ca in the soil was extracted by EDTA and EDDS at pH 7 at the high ratio compared to about 10% at the low ratio. The Fe concentration was very low in the absence of chelant. Both EDTA and EDDS mobilized significant concentrations of Fe (Figure 4). The extracted iron was in the form of an Fe(III) complex because any extracted Fe(II) complex would have been immediately oxidized to the Fe(III) complex by oxygen (33). Whereas the EDTA extractable Fe dropped to very low levels at pH 7 and above, EDDS was still able to extract relatively high concentrations of Fe. At the high chelant:metal ratio EDTA extracted about 2% of the iron in the soil at pH 7 and EDDS only 0.25%. Fe extraction is important for chelate speciation, although it affected only a small fraction of the total iron in the soil. At pH 4 the order of Cu extraction roughly followed the decrease in the stability constants of the Cu complex but not at pH 7 (Table 2). The order of extraction efficiency of Cu and Zn at pH 7 was more closely related to the stability constant of the Ca complex. To understand this behavior, we performed speciation calculations. Reactions considered were complexation with Ca, Mg, Fe, Mn, Zn, and Cu. The VOL. 38, NO. 3, 2004 / ENVIRONMENTAL SCIENCE & TECHNOLOGY

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FIGURE 2. Extraction of Zn from soil Dornach 2 as a function of pH with 0.4 mM (ratio chelant:metal 1) and 4 mM chelant (ratio chelant: metal 10). Conditions: 20 g L-1 soil, 0.01 M NaNO3.

FIGURE 4. Concentration of Ca and Fe in the extraction solution from soil Dornach 2 with 0.4 (ratio chelant:metal 1) and 4 mM chelant. Conditions: 20 g L-1 soil, 0.01 M NaNO3.

FIGURE 3. Extraction of Pb from soil Rafz as a function of pH with 0.4 mM (ratio chelant:metal 1) and 4 mM EDTA, EDDS, and NTA (ratio chelant:metal 10). Conditions: 20 g L-1 soil, 0.01 M NaNO3.

FIGURE 5. Calculated speciation of EDTA and EDDS in the extraction solution from soil Dornach 2 at a chelant:metal ratio of 1.

results are presented in Figure 5. Two important differences are obvious: whereas CaEDTA resulted as the major species at pH 8, CaEDDS was found to be only a marginal species at that pH. Competition between heavy metals and Ca thus appears to be an important factor for extraction with EDTA but not with EDDS, and this results in a decrease in extraction efficiency for EDTA compared to EDDS. A very small fraction of EDTA and about 20-35% of EDDS were predicted to be in the free, uncomplexed form. Part of this fraction could also be complexed to metals not included in the calculations (e.g., Ni or Pb). The Fe complex is the major species for both EDTA and EDDS at low pH. Due to the Ca competition, FeEDTA decreases with increasing pH while FeEDDS is also a relevant species at neutral pH. These results contradict the statement of Vandevivere et al. (24) that Fe may be neglected when speciating EDDS in soil suspension. These authors

based their statement on a comparison of the log K values for heavy metals and Fe(III) but did not provide measurements of dissolved Fe. Our results show that Ca and Fe have to be taken into account in chelant-assisted extraction of heavy metals from soils. The competition between the two metals and the target pollutant metals for the available chelating agents is particular important at the low chelant: metal ratio of 1. The extraction of Pb at the low chelant:metal ratio seems to depend mainly on the stability constants of the Pb complexes (log K EDTA 17.9, EDDS 12.7, and NTA 11.3) apart from Ca competition in the case of EDTA at high pH (Figure 5). At low concentrations of chelating agent, Pb extraction showed a very strong dependence on pH (Figure 3). EDTA shows very strong extraction of Pb up to pH 6 due to its high log K value; after this however the extraction efficiency

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reduces by about 50% due to competition for EDTA from Ca. Although NTA has a lower log K value than EDDS, it extracts much greater amounts of Pb than EDDS at pHs below pH 7. This is due to the much higher conditional log K value of NTA at low pH values, which is due to the much smaller second pKa value of NTA (2.48) compared to EDDS (6.84) (34). NTA also suffers due to Ca competition at pHs above 7, although to a much smaller extent than EDTA, showing a reduction in extraction efficiency. Thus, at pH values below 7, EDDS was not suitable for Pb extraction, but above pH 7, it performed as well as EDTA and better than NTA due to the above-mentioned factors. At a chelant:metal ratio of 10, most of the EDTA and EDDS were present in uncomplexed form according to the speciation calculations. About 25% of EDTA was present as CaEDTA. Uncomplexed EDDS always accounted for about 90% of total EDDS. Thus, there was always enough free EDDS to extract further metals. For EDTA, this was only the case at the high ratio. However, metals are not only extracted by the free ligand but also by metal complexes. A metal in a chelate may not be at equilibrium and can be exchanged with another metal. CuEDTA, for example, was able to extract Zn efficiently from a river sediment, although the rate was only 20% of that of CaEDTA (35). Our results suggest that EDDS is a better metal extractant for Cu and Zn than EDTA at pH values above 6 at low chelant: metal ratios because it forms only a weak Ca complex. In addition, it has the advantage that it is readily biodegradable as classified by the modified Sturm test OECD 301B (36). Any residual EDDS that remains in the soil after extraction will rapidly be degraded and poses little risk with respect to leaching of metals to the groundwater. Chelates are only weakly adsorbed at neutral pH (37, 38) and are therefore easily leached. NTA extracted a similar amount of Zn as EDDS at neutral pH but significantly less Cu. IDSA and MGDA were less efficient at low concentrations but performed more or less as well as EDDS at high chelant:metal ratios. We conclude that the extraction with EDDS at pH 7 gave the best compromise between extraction efficiency for Cu, Zn, and Pb and loss of Ca and Fe from the soil. To extract polluting metals from soil with near-neutral pH, no acidification is necessary and unwanted extraction of major ions is minimal. Extraction Kinetics. The results presented so far refer to 24 h extraction time. Figure 6 shows the kinetics of Cu, Zn, and Pb extraction from the three soils using EDDS at pH 7 up to a reaction time of 48 h. Zn and Pb extraction exhibited a fast initial step followed by a much slower release of metals. Extraction of Cu was slower than that of Pb or Zn. The extraction kinetics of MGDA and EDTA were comparable to EDDS; only the extracted amounts were smaller (data not shown). EDDS extracted different amounts from the three soils, a fact that will be discussed later in more detail. IDSA first increased the dissolved Zn, but after 30 h the concentrations decreased again (Figure 7). The same trend could be seen for Cu. The hypothesis that this was caused by microbial degradation of the ligand and subsequent immobilization of the released metals was tested by conducting experiments in the presence of the biocide sodium azide. Indeed, no re-immobilization of solubilized Cu and Zn was observed with IDSA in the presence of the biocide. For the efficient use of IDSA and any other rapidly biodegradable chelating agent, therefore, the extraction time must be optimized in order to gain the maximum extraction before the start of biodegradation. At pH 4.5, Zn extraction by EDDS also showed a very rapid initial increase and then a gradual decrease in dissolved Zn concentration with time (Figure 7). This decrease, which was not observed at pH 7, also occurred in the presence of sodium azide. It cannot be explained, therefore, by biodeg-

FIGURE 6. Extraction kinetics of Cu, Zn, and Pb by EDDS from the three soils at pH 7. Conditions: chelant:metal ratio 1, 20 g L-1 soil, 0.01 M NaNO3. The “no chelant” data are for soil Dornach 1 (Cu and Zn) and for soil Rafz (Pb).

FIGURE 7. Extraction of Zn from soil Dornach 2 with EDDS at pH 4.5 and with IDSA at pH 7 in the absence and presence of the biocide sodium azide. Conditions: chelant:metal ratio 1, 20 g L-1 soil, 0.01 M NaNO3. radation. We suspect that it was caused by the dissolution of iron oxides by ZnEDDS. Metal-ligand complexes are able to dissolve oxides in a similar manner as the free ligands, although at a slower rate (39). The mobilization of Zn by free EDDS at pH 4 is very rapid, showing an exponential increase before the first sampling at 1 h (Figure 7). Subsequently, the newly formed ZnEDDS slowly dissolves Fe oxides. Equilibrium calculations show that about 70% of the EDDS in solution could have been present as Fe complex if the system would have been at equilibrium with hydrous ferric oxide. VOL. 38, NO. 3, 2004 / ENVIRONMENTAL SCIENCE & TECHNOLOGY

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TABLE 4. Percentages of Heavy Metals Extracted from Contaminated Soils by Applying Chelating Agents for 24 h at pH 7 at Various Ratios between Chelant and Metals

FIGURE 8. Extraction of Fe from Dornach soil 2 as a function of time at pH 4.5 and 7 with EDDS and EDTA. Conditions: chelant:metal ratio 1, 20 g L-1 soil, 0.01 M NaNO3. The observed FeEDDS concentration (as a percentage of the total chelating agent concentration) was 28% after 8 h and 36% after 24 h, only gradually increasing up to 48 h (Figure 8). Dissolution of iron oxides either by the free ligand or metal complexes therefore continued during the whole period of the experiment. The kinetic experiments showed for all metals and soils that 24 h was the optimum extraction time with the time period from 24 to 48 h only giving minor additional benefits. Comparison to Other Studies. The factor of primary importance in chelant-assisted metal extraction is the ratio of chelant to metal. The higher the ratio, the more uncomplexed ligand is present in the extraction solution and the faster and more complete is the extraction. This ratio is used in only a few investigations to guide the design of experiments. The concentration of the applied extractant solution, which is usually specified, is of little use if the metal concentrations of the soils and the solid:solution ratio are not known. To compare our results with those of other authors, we compiled data obtained under the following conditions: The pollution occurred in the field and not by fresh addition of metals in the laboratory, the pH during extraction was around 7, the extraction time was 24 h, and the ratio chelant:metal was at least 1. Table 4 summarizes the results of this compilation. Extraction of Zn spans a range from 17% to 63%, with most data ranging between 30% and 35%. Thus, Zn extraction was quite weak for most soils, and an increased chelant:metal ratio did not result in a significant increase in extraction efficiency. Cu extraction varied between 28% and 100%, with most data ranging between 40% and 80%, and Pb between 23% and 96%, with most data ranging between 30% and 89%. It is obvious that a higher ratio resulted in a more effective extraction of these two metals and that without consideration of this parameter, comparison of data from different studies has little meaning. The extraction efficiencies obtained with EDDS in this study for Cu are among the best observed so far at a low chelating agent:metal ratio. Zn extraction at pH 7 was about the same as that observed for EDTA in our study and comparable to other efficiencies reported in the literature for low chelating agent:metal ratios. Pb extraction by EDDS was lower than that for EDTA in most cases, but at the high ratio it exceeded some extraction efficiencies given in the literature for high EDTA concentrations. The use of a low ratio of EDDS at neutral pH is therefore very promising, and we can expect results comparable to or better than other yields reported in the literature. This is especially good considering that SS-EDDS was found to be readily biodegradable (40) and EDTA was not (20). Influence of Solid-Phase Speciation on Extraction Yield. Sequential extractions can give the information needed to explain different extraction efficiencies for different metals. 942

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chelant

ratio

Cu

Zn

EDTA EDTA EDTA EDTA EDTA EDTA EDTA EDTA EDTA EDTA EDTA EDTA EDTA EDTA EDTA EDTA EDTA EDTA EDTA EDTA

1.0 1.0 1.0 1.0 1.2 1.8 1.8 2 3.6 6 8.9 9 10 10 20 33 50 89 300 1000

29 36

17 32 46

NTA NTA NTA NTA NTA NTA

1.0 1.0 1.7 10 10 17

EDDS EDDS EDDS EDDS EDDS EDDS

1.0 1.0 1.0 4.9 10 10

37 36 60 56 44 55 36 84 80

35 35 37 38 60 43 32 13 48 63 54 14

60 57 100

37 50 54

46

39 58 33 50 63 40

41 66 61 53 61 28 67

19 34 48 32 29 60

Pb

29 50 65 80 83 41 91 88 95 70 33 37 96 88 23 65

16 50 67

ref this study this study this study 7 41 14 14 14 42 14 43 42 this study this study 14 44 44 14 14 14 this study this study 45 this study this study 45 this study this study this study 24 this study this study

Figure 9 shows the distribution of Cu and Zn fractions in soil Dornach 2 before and after the extraction with EDDS. Cu was mainly found in the “organic” fraction, while about 60% of the Zn was found in iron oxide and residual fractions. The applied EDDS solution extracted these metals mainly from the first four fractions, the exchangeable, mobile, Mn oxides, and organic fraction. Table 5 shows the relative reduction of the respective metal fractions by EDDS. For Cu and Zn, the first three fractions were reduced by 71%, 80%, and 75% on average, the organic Zn fraction by 36%, and the organic Cu fraction by 58%. The iron oxide and residual fractions were not or only marginally reduced. The much better extractability of Cu compared to Zn by chelating agents can therefore be explained by the presence of larger weakly bound fractions. The maximum extraction efficiency (pH 7) at high chelant: metal ratio for soil Dornach 2 was 84% for Cu and 48% for Zn (EDTA), while the first four fractions of the sequential extraction amounted to 79% for Cu and 41% for Zn. A similar result also holds for Pb in soil Rafz. The maximum extraction efficiency (pH 7) is 95% (EDTA), which is also the percentage of Pb in the first four fractions of the sequential extraction. Van Benschoten et al. (13) found similar results for seven soils from sites polluted with Pb. The Pb not removed was found in the Fe oxide, sulfide, and residual fractions. When looking at the extractants used in the sequential extraction, this result is not surprising since fraction 4 is determined by extraction with 25 mM EDTA (31, 32). Table 1 shows the properties of each soil. These properties influence the distribution of the metals between the different fractions as classified by sequential extraction. For example, soil Dornach 1 is a calcareous soil with a high pH, which is likely to make the metals less labile and less easily extractable. Soil Dornach 2 has little carbonate and a neutral pH, while the soil Rafz soil has a acidic pH with a low clay content,

TABLE 5. Reduction of Metal Fractions in Soils by Extraction with EDDS (in %), According to the Zeien and Bru1 mmer Scheme (31, 32) metal

soil

exchangeable

mobile

Mn oxide

organic

amorphous Fe

crystalline Fe

residual

Zn

Dornach 1 Dornach 2 Rafz

73 86 92

69 83 80

65 68 64

26 44 40

2 20 8

-5 1 -23

-32 -33 -25

Cu

Dornach 1 Dornach 2

63 40

84 86

87 91

46 69

12 33

-14 16

-32 -32

Pb

Rafz

16

37

36

17

0

-

-11

Literature Cited

FIGURE 9. Sequential extraction of Cu and Zn in soil Dornach 2 before and after extraction with EDDS. making the metals more easily available. This can be seen in the distribution of Zn in these three soils: 31% Zn in soil Dornach 1 is found in the first four fractions, while 41% is found in soil Dornach 2 and 57% in soil Rafz. The different extraction efficiencies for different metal obtained by the chelating agents are therefore simply due to the different distribution of the metal fractions. Extraction by chelating agents is therefore feasible for soils containing a large fraction of metals in the first four fractions of the sequential extraction procedure applied here but will not be an efficient treatment for soils containing a high percentage of metals in strongly bound fractions.

Acknowledgments We thank Werner Attinger and Anna Gru ¨ nwald for help with the soil sampling and the chemical analyses and Diederik Schowanek from Procter & Gamble for providing S,S-EDDS. This work was funded in part by the Federal Office for Education and Science within COST Action 837 and the Swiss National Science Foundation in the framework of the Swiss Priority Program Environment.

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Received for review August 7, 2003. Revised manuscript received November 3, 2003. Accepted November 4, 2003. ES0348750