Chemosphere 51 (2003) 845–853 www.elsevier.com/locate/chemosphere
Factors aﬀecting EDTA extraction of lead from lead-contaminated soils Chulsung Kim
, Yongwoo Lee b, Say Kee Ong
Rainy River Community College, Department of Chemistry, Environmental Science Program, International Falls, MN 56649, USA b Research and Development Center, Samsung Engineering Company Co., Ltd., Seoul, South Korea c Department of Civil and Construction Engineering, Iowa State University, Ames, IA 50011, USA Received 15 August 2002; received in revised form 29 January 2003; accepted 29 January 2003
Abstract The eﬀects of solution:soil ratio, major cations present in soils, and the ethylenediaminetetraacetic acid (EDTA):lead stoichiometric ratio on the extraction of lead using EDTA were studied for three diﬀerent Superfund site soils, one riﬂe range soil, and one artiﬁcially lead-contaminated soil. Extraction of lead from the lead-contaminated soils was not aﬀected by a solution:soil ratio as low as 3:1 but instead was dependent on the quantity of EDTA present. Results of the experiments showed that the extraction eﬃciencies were diﬀerent for each soil. If suﬃciently large amount of EDTA was applied (EDTA–Pb stoichiometric ratio greater than 10), most of the lead were extracted for all soils tested except for a Superfund site soil from a lead mining area. The diﬀerences in extraction eﬃciencies may be due to the major cations present in soils which may compete with lead for active sites on EDTA. For example, iron ions most probably competed strongly with lead for EDTA ligand sites for pH less than 6. In addition, copper and zinc may potentially compete with lead for EDTA ligand sites. Experimental results showed that addition of EDTA to the soil resulted in a very large increase in metals solubility. The total molar concentrations of major cations extracted were as much as 20 times the added molar concentration of EDTA. For some of the soils tested, lead may have been occluded in the iron oxides present in the soil which may aﬀect lead extraction. While major cations present in the soil may be one of the factors aﬀecting lead extraction eﬃciency, the type of lead species present also play a role. Ó 2003 Elsevier Science Ltd. All rights reserved. Keywords: Lead; Contaminated soil; Extraction; EDTA; Cations
1. Introduction Discharge and disposal of waste products contaminated with heavy metals have resulted in the contamination of valuable land resources and groundwater. Because heavy metals do not degrade and are toxic to biological systems, heavy metals will continue to be an environmental concern for a long time unless they are
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taken out from the ecosystem. Lead (Pb) is one of the many concerned heavy metals. The major sources of lead contamination of soil include lead mining and smelting activities, disposal of lead-based paints, and lead battery reclamation. At areas closed to riﬂe ranges, lead contamination may result from the pellets and bullets from ﬁrearms. The concentration of lead in uncontaminated soil is between 10 and 200 mg/kg (Lindsay, 1979). Urban soils show higher lead concentration than rural soils mainly because of motor vehicle emissions from using leaded gasoline (Davies, 1988). Elliott and Brown (1989) found that the lead concentration in the soils near an automobile battery recycling facility
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may be as high as 21% (w/w). Austin et al. (1993) reported that the lead concentration in the soil at an old smelter site may be as high as 30 000 mg/kg. Various techniques have been introduced to remediate metal-contaminated soils. One of these techniques is to separate the metals from soil by using chelating agents to form soluble metal–chelate complexes. Chelating agents such as ethylenediaminetetraacetic acid (EDTA) have been shown to form strong metal–ligand coordination compounds and are highly eﬀective in remediating lead-contaminated soils (Norvell, 1984; Elliott and Brown, 1989; Elliott et al., 1989; Brown and Elliott, 1992; Peters and Shem, 1992; Cline et al., 1993; Kim and Ong, 1998; Kim and Ong, 2000). Ideally, the minimum EDTA molar amount needed to extract lead from contaminated soil should be the same as the molar amount of lead in the soil. However, EDTA is a nonspeciﬁc chelating agent and it reacts with other metals present in soil. Fig. 1 shows the change in conditional stability constants with pH for various metal–EDTA complexes by assuming equal molar of metals present. As shown in Fig. 1, ferric ions may have a stronger afﬁnity for EDTA depending on the pH of the soil or solution. In some soils, the molar amount of metals such as ferric and calcium ions may be larger than that of lead resulting in the formation of metal–EDTA complexes
Fig. 1. Eﬀect of pH on conditional stability constants of metal– EDTA complexes (adapted from Kim and Ong, 1999). (The general conditional stability equation is given by Ps ¼ ðCT;M Þ ðCT;A Þ where Ps is the conditional stability constant, CT;M and CT;A are the total concentrations of the metal and the anion in all forms.)
rather than Pb–EDTA complexes. Therefore, in most studies, EDTA molar concentrations higher than the molar concentration of lead in soil are used to achieve maximum lead extraction from lead-contaminated soils. Even though EDTA has been shown to be a suitable chelating agent for the remediation of lead-contaminated soils, not much information is available on the impact of the other metals present in the soil on the extraction of target metals such as lead with EDTA. The objectives of this study were to investigate the eﬀects of major cations present in soils such as iron, aluminum, calcium, zinc, copper, magnesium, and manganese on the extraction eﬃciency of lead using EDTA. Several lead-contaminated soils from Superfund sites were used and variables such as solution:soil ratio and EDTA–Pb stoichiometric ratio were also investigated.
2. Materials and method 2.1. Sample characterization and preparation Soil samples from lead-contaminated sites were used for this research. Three soil samples (identiﬁed as Soils A, B and C) were collected from Superfund sites in New Mexico. One soil sample (Soil D) was taken from a riﬂe range in Florida while Soil E was an oxidized glacial till from Iowa that was artiﬁcially contaminated with lead. Soil A came from a lead smelter area while Soil B came from an abandoned lead mining area. Soil C was obtained from a former battery recycling and smelter facility area. All the soil samples were air dried, screened through Number 25 (0.707 mm) sieve to remove large particles including lead pellets and organic debris before they were placed in a plastic containers with screw caps. The oxidized glacial till was contaminated with lead by mixing batches of 200 g of till with lead nitrate solution. The lead-contaminated soil was air dried, ground and sieved with a Number 25 sieve. The target lead concentration of the artiﬁcially contaminated soil was 2500 mg/kg. Each soil sample was characterized by measuring the soil pH, speciﬁc surface area, cation exchange capacity (CEC) and soil organic carbon (SOC). Soil pH was measured using an Accumet model 25 pH/Ion meter with a glass pH-indicating electrode and a calomel reference electrode. Soil–water ratio used for pH measurement was 1:2. The speciﬁc surface area of the each soil sample was measured using the ethylene glycol monoether (EGME) method (Carter et al., 1966) after the soil was treated with H2 O2 to remove organic matter (Kunze and Dixon, 1986). The CEC of each soil was determined by saturating the exchangeable sites with sodium followed by substitution with magnesium ions (Polemio and Rhoades, 1977). SOC was measured using
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using Nanopure water at diﬀerent pH values, adjusted with diluted HNO3 or NaOH solution, were conducted to measure the solubility of the cations in water. All experiments were conducted in duplicate under room temperature. The solution:soil mass ratio on lead extraction eﬃciencies were assessed by using two soils, Soil D and Soil E (riﬂe range and artiﬁcially contaminated). The volumes of EDTA solution used were 3, 5, and 10 ml giving solution:soil mass ratios of 3, 5, and 10, respectively while the mass of soil used was 1 g. To study the eﬀects of EDTA–Pb stoichiometric ratio, applied EDTA concentration varied from 0.0001 to 0.2 M giving an EDTA–Pb stoichiometric ratio of 0.1 to 100. All ﬁve soils were used to study the impact of EDTA–Pb stoichiometric ratio on lead extraction eﬃciency. The solution:soil ratio used was 10:1. The extraction time was 24 hours for both sets of the above experiments. For the study on the eﬀects of major cations on lead extraction eﬃciency, the concentrations of EDTA used were 0.005, 0.002 and 0.003 M for Soil C (battery recycling), Soil D (riﬂe range) and Soil A (lead smelter) giving an EDTA– Pb stoichiometric ratio of 0.78, 1.0 and 1.0, respectively. In this set of experiments, the extraction periods were 24 hours and 7 days and the solution:soil ratio was 10:1.
the method suggested by Nelson and Sommers (1982). Cations present in the soil such as lead, iron, aluminum, magnesium, manganese, calcium, copper and zinc were determined using Smith Hieftje 12 atomic absorption spectrophotometer after the soil samples were acid digested by boiling with concentrated HNO3 . The total mass of available cations present in soil was assumed to be equal to the sum of the acid extracted cations (lead, iron, aluminum, magnesium, manganese, calcium, copper and zinc) which were assumed to be the dominant cations in soil. For iron ions, oxalate extractable iron concentration was also measured. The oxalate extractable iron generally reﬂects the amorphous iron present in the soil (McKeague and Day, 1966). 2.2. Extraction procedure Three sets of extraction experiments were conducted to assess the eﬀects of (i) solution:soil ratio, (ii) stoichiometric molar ratios of EDTA–Pb, and (iii) major cations on lead extraction eﬃciencies. A typical extraction procedure consists of placing 1 g of soil with a measured volume of EDTA solution in a 50 ml polypropylene centrifuge tube. Diluted HNO3 or NaOH solution was added, if needed, for pH adjustment. The slurry was then shaken with a Burrell wrist-action shaker for up to 15 days. Kinetic experiments previously conducted showed that 24 hours were suﬃcient to extract lead from the soil matrix (Kim and Ong, 2000). Studies by other researchers also indicate that 24 hours were more than suﬃcient to reach steady state conditions (Elliott and Brown, 1989; Peters and Shem, 1992; Cline et al., 1993). However, as shown later for one or two soils, the extraction period needs to be extended. The samples were then centrifuged for 30 min at 3000 rpm followed by ﬁltration using 0.45 lm membrane ﬁlter paper to remove particulates in the solution and the pH of the ﬁltrate was measured. An adequate volume of the ﬁltrate was set aside and preserved with 5% (v/v) nitric acid in a 100 ml volumetric ﬂask. The concentrations of major cations were analyzed using an atomic absorption spectrophotometer. Separate experiments
3. Results and discussions Selected physical–chemical properties and the major cation concentrations in the ﬁve soils are presented in Table 1. All the soil samples except for Soil B (lead mine) had a soil pH higher than 8. Soil B (lead mine) was acidic with a pH of 2.7. In addition, Soil B (lead mine) had a high amorphous iron content (31 700 mg/ kg) and calcium content (103 900 mg/kg) while the other soils had relatively low amorphous iron content (between 300 and 500 mg/kg) and a calcium content of 7000–20 000 mg/kg. Soil D (riﬂe range) is a sandy soil with a low speciﬁc surface area. Lead extraction eﬃciencies for diﬀerent solution:soil ratios and EDTA–Pb stoichiometric ratios are present in
Table 1 Soil properties and major cations concentrations for the soil samples Sample
Speciﬁc surface area (m2 /g)
CEC (meq/ 100 g)
% Organic carbon
Artiﬁciala Soil A Soil B Soil C Soil D
2413 4180 1247 13 260 6238
544 31 720 316 328
384 91 2820 21
– 25 940 707 14 330 2440
– 19 580 103 900 12 410 7450
– 6780 115 5270 436
– 129 557 30 279
– 1860 1078 86 70
9.7 37.3 4.4 15.4 0.7
11.6 17.8 4.3 17.5 5.8
0.75 2.26 0.10 2.52 0.18
8.16 8.55 2.68 8.13 8.47
Major metal cations were not measured.
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Fig. 2. Stoichiometric and volume ratio eﬀects on lead extraction for Soil D (riﬂe range) and Soil E (artiﬁcially contaminated) (24-hour extraction period without pH adjustment).
Fig. 3. Impact of EDTA:lead stoichiometric ratio on lead extraction (10 ml of EDTA solution to 1 g of soil) (24-hour extraction period without pH adjustment).
Fig. 2 for Soil D (riﬂe range) and Soil E (artiﬁcially contaminated). Extraction time for these experiments was 24 hours. The ﬁgure shows that the extraction with solution:soil ratio as low as 3:1 on a mass basis were similar to that of a solution:soil ratio of 10:1 for both contaminated soils. The extraction eﬃciencies were found to be dependent on the quantity of EDTA present. Since the wastewater generated from the extraction process should be treated before disposal, reducing the volume of wastewater would reduce the treatment costs and provide additional savings for the application of soil washing technology. It is interesting to note that the extraction eﬃciency for Soil D (riﬂe range) was gradual for higher EDTA–Pb stoichiometric ratio while for Soil E (artiﬁcially contaminated), the change in extraction eﬃciencies seemed to be quite steep for a small change in EDTA–Pb stoichiometric ratio. As shown by the results, caution should be used when studying metal extraction eﬃciency with artiﬁcially prepared soil since metal extraction by EDTA tends to be much easier than actual lead-contaminated soils even though artiﬁcially contaminated soil may facilitate consistency in the soil samples. Fig. 3 shows that the results of lead extraction from lead-contaminated soils and the artiﬁcially contaminated soil for EDTA–Pb stoichiometric ratios of between 0.1 and 100 over a 24-hour extraction period without pH adjustment. This ﬁgure demonstrates that lead extraction eﬃciency for each soil was diﬀerent and that lead extraction was a function of the stoichiometric ratio of the applied EDTA concentration to the total
lead concentration in the soil sample. However, if sufﬁciently large amount of EDTA was applied, all the lead may be extracted for certain soils. Fig. 3 shows that for a unit stoichiometric ratio of EDTA to total lead concentration, approximately 75% of lead were extracted from artiﬁcially contaminated soil were extracted, 55% of lead from Soil C (battery recycling), 40% from Soil D (riﬂe range), 10% from Soil A (lead smelter) while none of the lead was extracted from Soil B (lead mine). The diﬀerent lead extraction eﬃciencies for diﬀerent soils at unit stoichiometric ratio may be due to the diﬀerent soil and solution properties such lead species present in the soil sample and the cations present in the soil. EDTA is a non-speciﬁc chelating agent and therefore may react with metal ions other than lead. Because each metal ion has diﬀerent reactivity with EDTA, the competition between lead ion and other metal ions is dependent on the dissolved concentration of the speciﬁc metal ion, dissolved anions, pH and the stability constant between the speciﬁc metal ion and EDTA. As shown in Fig. 3, EDTA concentrations above unit stoichiometric requirement was needed for most soils to maximize lead extraction. For example, an EDTA–Pb stoichiometric ratio of at least 7 is needed to achieve the maximum lead extraction eﬃciency for Soil E (artiﬁcially contaminated) while a EDTA–Pb stoichiometric ratio of at least 20 was needed for the Soil A (lead smelter). Of the four actual lead-contaminated soils, lead extraction from Soil C (battery recycling) seemed to be easier than the other three lead-contaminated soils. In the case of Soil B (lead mine), lead was not extracted at
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all even for very high EDTA–Pb stoichiometric ratio (up to 30) over a 24-hour extraction period. The possible reasons for this lack of lead extraction are that the lead species present in the soil have very low solubility and that the lead was occluded within the diﬀerent oxides in the soil matrix. It is also possible that the low pH of the soil may result in the precipitation of some of the applied EDTA or that the EDTA may have reacted with other metal ions present in the soil. To further investigate the possible causes, the molar amount of various extracted metals from Soil B (lead mine) using Nanopure water and 0.005 M EDTA (EDTA–Pb ratio ¼ 8.3) was investigated as presented in Fig. 4. The pHs of the solutions were 3.15 for the 0.005 M EDTA solution and 2.70 for the water. This ﬁgure shows that signiﬁcant amount of iron was extracted with 0.005 M EDTA solution––the amount of iron extracted was approximately equal to 90% of the applied molar amount of EDTA. For calcium the extracted molar amount with water and EDTA was about three molar times higher than the applied EDTA. For other metals such as zinc, copper, and magnesium, the dissolved amounts for both solutions were similar which means that the dissolved metal ions may be due to the low pH of the solution. Lead was not extracted at all with both water and 0.005 M EDTA solution. It is probable based on the results obtained that the presence of iron at low pH (see Fig. 1) may have played a role in suppressing lead extraction at low values. In a separate experiment (24-hour extraction), lead was not extracted at all from Soil B (lead mine) using 0.2 M EDTA solution (EDTA–Pb ratio ¼ 330) at an adjusted solution pH of 8.7 where the eﬀects of iron with EDTA is minimized. Signiﬁcant amount of
Fig. 4. Molar amounts of metals extracted with water and 0.005 M EDTA solution at pH 3 (for Soil B (lead mine)) (stoichiometric ratio of EDTA–Pb ¼ 8.3, 24-hour extraction period).
Fig. 5. Lead and iron extraction from Soil B (lead mine) with 0.2 M EDTA solution over time (stoichiometric ratio of EDTA–Pb ¼ 330, 15 days extraction period).
calcium was found to be extracted. To investigate this further, 0.2 M EDTA solution was applied to Soil B (lead mine) for up to 15 days at a pH of 4.3. Approximately 10% of lead was extracted in 15 days (see Fig. 5). However, iron extraction from Soil B (lead mine) seemed to mirror that of lead extraction, both reaching a constant percent extracted after 13 days. As more iron was extracted, more lead was extracted. A possible reason for this observation is that lead ions may be strongly adsorbed or occluded in the iron preventing lead ions from complexing with the applied EDTA. Therefore, as the iron dissolved, more lead becomes available for complexation with EDTA. To assess the eﬀect of lead species on the lead extraction with EDTA, an eﬀort was conducted to analyze the species of lead in Soil B (lead mine) using X-ray diﬀraction method. However, information on the lead species present could not be obtained due to detection limitation of analytical equipment. It is probable that the lead species present in Soil B (lead mine) had low solubility such as lead sulﬁde (Clevenger et al., 1991). The extraction results of Soil B (lead mine) suggest that lead extraction might be inhibited by a combination of factors such as type of lead species present, the location of lead within the matrix and possible competition with other metal ions presented in the soil. Figs. 6–8 show the lead, iron, and calcium extraction eﬃciencies for Soil C (battery recycling), Soil D (riﬂe range) and Soil A (lead smelter). The EDTA concentrations used were 0.005, 0.002 and 0.003 M giving a EDTA–Pb stoichiometric ratio of 0.78, 1.0 and 1.0 for Soil C (battery recycling), Soil D (riﬂe range) and Soil A
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Fig. 6. Lead, iron and calcium extraction eﬃciencies for Soil C (battery recycling) with water and 0.005 M EDTA solution for diﬀerent reaction times (solid lines are best ﬁt lines).
Fig. 7. Lead, iron and calcium extraction eﬃciencies for Soil D (riﬂe range) with water and 0.002 M EDTA solution for diﬀerent reaction times (solid lines are best ﬁt lines).
(lead smelter). Extraction was conducted with water and the EDTA solution for 1-day and 7-day extraction periods. For Soil C (battery recycling), the percent of lead extracted around pH 6 after 7 days was 78% which was similar to the EDTA–Pb stoichiometric ratio applied (see Fig. 6). This may imply that for Soil C (battery recycling) most of the applied EDTA appeared to be complexed with lead around pH 6. However, much less
lead was extracted for 1-day extraction period for pH greater than 6. As pH increased, kinetics seemed to be a factor controlling the extraction of lead from the soil. For pH value less than 6, the extraction eﬃciencies of lead were slightly lower but were similar for 1-day or 7day extraction period. The lower pH may solubilize other ions such as iron and calcium which then may compete with lead for EDTA ligand sites. Note that the extraction of iron and calcium was similar for 1-day or
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Fig. 8. Lead, iron and calcium extraction eﬃciencies for Soil A (lead smelter) with water and 0.003 M EDTA solution for diﬀerent reaction times (solid lines are best ﬁt lines).
7-day extraction period. It is interesting to note that the total molar concentration of calcium dissolved at pH less than 8 was much larger than the applied EDTA molar concentration. Even at pH higher than 10, the dissolved molar concentrations of calcium ion was approximately 30% of the molar concentration of applied EDTA. However, as shown in Fig. 1, calcium and iron may not be a factor at these high pH values. The metal ion that may compete with lead for EDTA ligand sites at high pH is copper (Sommers and Lindsay, 1979). But copper concentrations in the soil tested were very low (see Table 1) in comparison to the lead concentrations. For Soil D (riﬂe range) at the 1-day extraction period and pH value less than 6, the percent of lead extracted was between 60% and 80%. After 7 days more lead was extracted showing that steady state conditions were not achieved after one day. The extraction eﬃciency of lead decreased gradually up to pH value of 8 where about 43% of lead was extracted after 7 days. An interesting observation is that the percent of lead extracted in water at low pH was similar to that of EDTA extraction. This implies that the majority of the lead extracted at low pH may be due to the pH of the solution. On the other hand the percent of iron extracted with water at low pH was much lower than that for lead. This may imply that the EDTA present may have assisted the extraction of iron more than that of lead. Unlike Soil C (battery recycling) where extraction of iron was completed in one day, more iron was extracted after 7 days just as for the lead. This result may imply that some of the lead may be occluded in the iron.
As in the case of Soil A (lead smelter), only 50% of lead in soil were extracted in 7 days for pH values less than 6 for unit EDTA–Pb stoichiometric ratio. Soil A (lead smelter) had a signiﬁcant amount of zinc which may compete with lead (see Fig. 1). Fig. 9 shows that the percent of zinc extracted was dependent on the pH. It is
Fig. 9. Zinc extraction eﬃciencies for Soil A (lead smelter) with water and 0.003 M EDTA solution for diﬀerent reaction times (solid lines are best ﬁt lines).
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probable that part of zinc may inhibit the lead extraction at low pH values. The experimental results obtained seemed to suggest that the addition of EDTA changed the solution properties resulting in an increase in the dissolution of major metals. The dissolution of metals would result in a corresponding increase in anion concentrations. It is probable that the released anions may form soluble ion pair complexes with the dissolved metals. For example, calcium, magnesium and manganese can easily form ion pair complexes with phosphate, carbonate and sulfate ions in solution (Bohn et al., 1985). Another possible reason for high calcium and magnesium extraction by EDTA is that the iron oxides and hydroxides compounds which were dissolved by EDTA have high adsorption capacities for these metal ions. Therefore, the concentrations of the metals in solution increased correspondingly when the iron oxides were solubilized. 4. Conclusion Based on the experimental results, it was shown that a solution:soil ratio as low as 3:1 had similar extraction results as that of a solution:soil ratio of 10:1 and that the extraction eﬃciencies were dependent on the molar concentration of EDTA present. Using diﬀerent lead-contaminated soils, results of the experiments showed that lead extraction eﬃciencies for soils were diﬀerent for a given stoichiometric ratio but if suﬃciently large amount of EDTA was applied, most of the lead may be extracted for certain types of soils. To maximize extraction, the EDTA–Pb stoichiometric ratio needed varied from as low as 7 for artiﬁcially contaminated soil to as much as 20 for contaminated soils from a lead smelting facility. For one of the soils tested (Soil B (lead mine)), very low amounts of lead (<10%) were extracted even with an EDTA–Pb stoichiometric ratio of 300. The molar concentrations of all the extracted metals due to the addition of EDTA were more than the molar concentration of applied EDTA. For pH less than 6, dissolved ferric ions appeared to compete with EDTA, therefore, reducing the lead extraction eﬃciency. In addition, for pH value less than 6, other metal ions such as zinc and copper may compete with lead. Although the nature of the metal species present in the contaminated soils was not known, the experimental results appeared to indicate that besides competition other mechanisms such as occlusion of lead in the iron oxides and the type of lead species may play a role. References Austin, G.A., Brandvold, L.A., Hawley, J.W., Renault, J., 1993. Lead contamination at an old smelter site at Socorro, New Mexico: Part I particle size and depth of contamination. Mining Engineering 45 (4), 389–395.
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