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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 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-basedpaints, 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 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 metalchelate complexes. Chelating agents such as ethylenediaminetetraacetic

acid (EDTA) have been shown to form strong metalligand 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 non-specific chelating agent and

it reacts with other metals present in soil.Fig. 1shows the change in conditional stability constants with pH for various metalEDTA

complexes by assuming equal molar of metals present. As shown inFig. 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 metalEDTA complexes rather than PbEDTA 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.

Fig. 1.

Effect of pH on conditional stability constants of metalEDTA complexes (adapted fromKim and Ong, 1999). (The general conditional

stability equation is given by Ps=(CT,M)(CT,A) where Psis the conditional stability constant, CT,Mand CT,Aare the total concentrations of the metal

and the anion in all forms.)

Figure options

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 EDTAPb stoichiometric ratio were also investigated.

2. Materials and method

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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 organicdebris 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. Soilwater 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 H2O2to

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 the method

suggested byNelson 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

EDTAPb, 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 HNO3or 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 previouslyconducted 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 m 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 using Nanopure water at different pH values, adjusted with diluted HNO3or 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 EDTAPb stoichiometric ratio, applied EDTA concentration

varied from 0.0001 to 0.2 M giving an EDTAPb stoichiometric ratio of 0.1 to 100. All five soils were used to study the impact of

EDTAPb 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

EDTAPb 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.

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3. Results and discussions

Selected physicalchemical properties and the major cation concentrations in the five soils are presented inTable 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 700020 000 mg/kg. Soil D (rifle range)

is a sandy soil with a low specific surface area.Table 1.

Soil properties and major cations concentrations for the soil samples

Sample Pb Amorphous

Fe

Mn Al

(ppm)

Ca Mg Cu Zn Specific

surface

area (m2/g)

CEC

(meq/100

g)

%

Organic

carbon

pH

Artificiala 2413 9.7 11.6 0.75 8.16

Soil A 4180 544 384 25 940 19 580 6780 129 1860 37.3 17.8 2.26 8.55

Soil B 1247 31 720 91 707 103 900 115 557 1078 4.4 4.3 0.10 2.68

Soil C 13 260 316 2820 14 330 12 410 5270 30 86 15.4 17.5 2.52 8.13

Soil D 6238 328 21 2440 7450 436 279 70 0.7 5.8 0.18 8.47

a

Major metal cations were not measured.

Table options

Lead extraction efficiencies for different solution:soil ratios and EDTAPb stoichiometric ratios are present inFig. 2for 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 EDTAPb stoichiometric ratio while for Soil E (artificially contaminated), the change in

extraction efficiencies seemed to be quite steep for a small change in EDTAPb 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. 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).

Figure options

Fig. 3shows that the results of lead extraction from lead-contaminated soils and the artificially contaminated soil for EDTAPb

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

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the lead may be extracted for certain soils. Fig. 3shows 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.

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).

Figure options

As shown inFig. 3, EDTA concentrations above unit stoichiometric requirement was needed for most soils to maximize lead

extraction. For example, an EDTAPb stoichiometric ratio of at least 7 is needed to achieve the maximum lead extraction efficiency

for Soil E (artificially contaminated) while a EDTAPb 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 all even for very high EDTAPb 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 inthe 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 (EDTAPb ratio=8.3) was investigated as presented inFig. 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 solutionthe 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 (seeFig. 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 (EDTAPb ratio=330) at an adjusted solution pH of 8.7 where the effects of iron with EDTA is

minimized. Significant amount of 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 (se eFig. 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.

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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=8.3, 24-hour extraction period).

Figure options

Fig. 5.

Lead and iron extraction from Soil B (lead mine) with 0.2 M EDTA solution over time (stoichiometric ratio of EDTAPb=330, 15 days

extraction period).

Figure options

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.

Fig. 6,Fig. 7andFig. 8show 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 EDTAPb stoichiometric ratio of 0.78,

1.0 and 1.0 for Soil C (battery recycling), Soil D (rifle range) and Soil A (lead smelter). Extraction was conducted with water and the

EDTA solution for 1-day and 7-day extraction periods.

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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).

Figure options

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).

Figure options

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).

Figure options

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For Soil C (battery recycling), the percent of lead extracted around pH 6 after 7 days was 78% which was similar to the EDTAPb

stoichiometric ratio applied (seeFig. 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 7-day 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 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 inFig. 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 (seeTable 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 EDTAPb

stoichiometric ratio. Soil A (lead smelter) had a significant amount of zinc which may compete with lead (see Fig. 1). Fig. 9shows

that the percent of zinc extracted was dependent on the pH. It is probable that part of zinc may inhibit the lead extraction at low pH

values.

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).

Figure options

The experimental results obtained seemed to suggest that the addition of EDTA changed the solution properties resulting in anincrease 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

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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 EDTAPb 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 (

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equation(1)

Turn MathJaxon

equation(2)

Turn MathJaxon

The nascent Al3+ ions are very effective coagulants and form large networks of Al-O-Al-OH flocks, with a large surface area and

considerable absorption capacity (Shen et al., 2003). In our study Pb was almost entirely removed from the soil washing solution

while, to our surprise, some EDTA remained in the washing solution. This early result indicated separation of Pb from EDTA.

In the current work, the feasibility of electrochemical separation of EDTA and Pb in waste soil washing solution using an Al anode

and a single-chamber electrolytic cell was studied.

2. Materials and methods

2.1. Soil properties

Soil contaminated with 3980 60 mg kg1of Pb was collected from the 030 cm surface layer of a vegetable garden in the Meica

Valley, Slovenia. The Meica Valley has been exposed to more than three hundred years of active lead mining and smelting. For

standard pedological analysis, the pH in soils was measured in a 1/2.5 (w/v) ratio of soil and 0.01 M CaCl2water solution suspension.

Soil samples were analyzed for organic matter (as C content) by modified WalkleyBlack titrations (ISO 14235, 1998), cation

exchange capacity (CEC) by the ammonium acetate method (Rhoades, 1982)and soil texture by the pipette method (Fiedler et al.,

1964). The following values were obtained: pH 6.57, C content 8.2%, CEC 20.7 mval 100 g 1of soil, sand 51.0%, silt 42.5%, clay

6.5%. The soil texture was sandy loam.

2.2. Soil washing

The extraction of soil with EDTA solutions was performed in two scales. To obtain the washing solution for the electrochemical

treatment, we placed 0.5 kg of air-dried soil and 875 mL of aqueous solution (1:1.75 soil: washing solution ratio) of 75 mmol of EDTA(disodium salt) per kg of soil (43 mM EDTA), pH 4.3, in 1.5 L flasks. Soil was extracted on a rotating shaker (3040 GFL, Germany) for

72 h at 16 RPM. Approximately 400 mL of the washing solution was decanted from each flask after the soil was allowed to settle for

24 h. The decanted washing solution was filtered (filter paper was wide-pored, grade 388, density was 80 g m2).

The same procedure was used to extract the soil with the recycled EDTA solution, except that centrifugation at 2880 g for 5 min and

not decantation was used to separate the soil from the washing solution, to minimise solution loss. Fine particles were removed from

the solution by filtration as described above.

2.3. Electrochemical treatment of the soil washing solution

The electrolytic cell consisted of an Al anode placed between two stainless steel cathodes (distance = 10 mm), the overall anode

surface 63 cm2and the surface area ratio between the cathodes and anode 1:1. The electrodes were placed in 500 mL of

magnetically stirred soil washing solution in a 1.0 L flask. Current density was kept at 96 mA cm2, and the cell voltage measured with

a DC power supply (Elektronik Invent, Ljubljana, Slovenia). The electrode cell was cooled using a cooling mantle and tap water (flow

rate 250 mL min1) to keep the temperature of the treated washing solution below 35 C. The contact time of the electrochemical

treatment was calculated as the ratio of the electrode cell volume to the volume of the washing solution and multiplied by the

operation time (initially 30 min of operation time equalled 3.78 min of contact time). During the electrochemical treatment, the pH of

the washing solution was regulated to pH 6, 8 and 10 by drop-vice addition of 5 M NaOH and HCl. Samples (20 mL) of washing

solution were collected periodically and the pH and EC measured immediately. Samples were afterwards centrifuged at 2880 g for

10 min and the supernatant stored in the cold for further analysis of Pb and EDTA. The pellet (mainly Al hydroxide precipitate) was

suspended in 200 mL of deionised water acidified with 37% HCl to pH 1.5. The resulting solution with some finely suspended

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precipitate of EDTA, which is insoluble in acidic media, was stored in the cold for Pb determination. At the end of the electrochemical

treatment, the cathodes were etched with 30 mL of 65% HNO3to dissolve and later measure the concentration of electro-deposited

Pb. The Al anode was weighed before and after treatment of the washing solution to determine the amount of electro-corroded Al.

During the electrolysis, the surface of the Al anode was passivised by an oxide/hydroxide layer, which increased the potential

between the electrodes (Mouedhen et al., 2008). In order to break down this passive layer and reduce the power consumption, we

applied small amounts of Cl(as NaCl) when the voltage increased above 8 V (Chen, 2004).

To prepare the recycled EDTA solution for soil extractions, we electrochemically treated the washing solution at pH 10 for 24 min

(contact time) and separated the recycled EDTA solution from the Al hydroxide precipitate by centrifugation at 2880 g for 30 min.

2.4. Treatment of the soil washing solution with dosing Al-salt

A weight of 4110 mg of AlCl3was dosed into 100 mL of the soil washing solution with pH 10 and gently stirred for 22.68, 45.36 and

113.4 min, which corresponds to 1, 2 and 3-times the total contact time of the electrochemical treatment, respectively. The amount of

chemically dosed Al was the same as the molar amount of Al electro-corroded from the anode during electrochemical treatment.

During the coagulation treatment with Al dosing, the pH of the washing solution was kept at pH 10, using 5 M NaOH. The precipitate

was removed from the treated solution by centrifugation at 2880 g for 30 min, and the concentrations of Pb and EDTA in the

supernatant measured. Afterwards, the pH of the chemically treated washing solution was adjusted to 4.3 and the solution reused for

soil Pb extraction.

2.5. EDTA determination

The concentration of EDTA was determined spectrophotometrically according to the procedure ofHamano et al. (1993).The method

involves the reaction of EDTA in washing solution with Fe3+under acidic conditions to produce the Fe-EDTA chelate (trans-

complexation), followed by the removal of excess of Fe3+by chelate extraction in the aqueous phase using chloroform and N-benzoyl-

N-phenylhydroxylamine and the formation of a chromophore with 4,7-diphenyl-1,10-phenanthroline-disulfonic acid. Using a

spectrophotometer, absorbance was measured at 535 nm against a blank solution with the 4,7-diphenyl-1,10-phenanthroline-

disulfonic acid replaced with an equal volume of distilled water. The limit of EDTA quantification was 20 mg L1.

2.6. Pb determination

Air-dried soil samples (1 g) were ground in an agate mill, sieved through a 160 m mesh and digested in a glass beaker on a hotplate

with 28 mL of aqua regia solution (HCl and HNO3 in a 3:1 ratio (v/v)) for 2 h at 110 C. Condensation of evaporating fumes was

achieved via circulation of cool tap water through the glass tubes placed on top of the glass beakers. After cooling, digested samples

were filtered through Whatman no. 4 filter paper and diluted with deionised water up to 100 mL. The pseudo-total concentration of Pb

was determined by flame (acetylene/air) AAS with a deuterium background correction (Varian, AA240FS). The Pb in the solutions

was determined by AAS directly. A standard reference material used in inter-laboratory comparisons (Wepal 2004.3/4, Wageningen

University, Wageningen, Netherlands) was used in the digestion and analysis as part of the QA/QC protocol. The limit of

quantification for Pb was 0.01 mg L1. Reagent blank and analytical duplicates were also used where appropriate in order to ensure

accuracy and precision in the analysis.

2.7. Statistics

The Duncan multiple range test (Statgraphics 4.0 for Windows) was used to determine the statistical significance (P< 0.05) between

different treatments.

3. Results and discussion

3.1. Soil washing

Soil extraction with 75 mM EDTA per kg of dry soil removed 67.5% of Pb. The molar ratio between the Pb initially present in the soil

and the EDTA in the washing solution was 1:3.9. It is known that even strong chelants such as EDTA cannot remove heavy metals

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from the soil entirely, even at high molar ratios of EDTA vs. heavy metal concentration applied (Nowack et al., 2006). However, the

Pb residual in soil after stringent soil washing with EDTA is encapsulated in soil minerals or strongly bound to the non-labile soil

fractions and therefore essentially non-leachable and non-bioavailable (Udovic et al., 2009).

The concentrations of Pb and EDTA in the soil washing solution before treatment in the electrolytic cell were 1535 and

12 444 mg L1(33.4 mM), respectively. The pH of the washing solution before treatment was 7.91.

3.2. Electrochemical treatment of soil washing solution

Soil washing solutions were treated at various pH (6, 8 and 10). The pH of the solution tended to increase with treatment time, since

the electrochemical system generated enough OHat the electrode to counteract the H+released by the formation of Al hydroxides

as a net final product (Canizares et al., 2006). Solution treated at pH 10 consequently required very little pH adjustment. The voltage

between the electrodes also tended to increase with treatment time, regardless of the pH of the washing solution. The main reason

for the voltage increase was the passivisation of the Al anode surface by formation of an insulating film of Al oxide (Mouedhen et al.,

2008). In order to break down the passive film and thus to reduce the cell voltage surge and increase of power consumption, small

amounts of Cl(as NaCl) were applied (Chen, 2004), to keep the voltage close to initial 8 V. The amount of Al consumed from the Al

anode was 14.6 2.5, 12.0 0.4 and 9.4 0.4 g L1of solutions treated at pH 6, 8 and 10, respectively. The amount of electro-

corroded Al decreases with increasing pH. A higher aluminium current efficiency at higher alkaline conditions than at neutral is

generally found in electrochemical systems (Chen, 2004). The electro-conductivity of the washing solution increased from an initial

6.1 to up to 10.0 mS cm1(solution with pH 6). This increase followed the increasing concentration of charged Al hydroxide

(electrolyte) (data not shown) during the electrochemical process.

pH is an important operating factor influencing the performance of electrochemical processes (Chen, 2004). The effect of different pH

of electrochemical treatment on the dynamics of Pb removal and precipitation from the washing solution and on the mass balance of

Pb is shown inFig. 1andTable 1.During electrochemical treatment, metals complexed to EDTA could be removed from the soil

washing solution by absorption on Al hydroxide flocks (electrocoagulation). Metals (M) could also be released from the EDTA

complex after reduction reactions at the cathode (Juang and Wang, 2000), Eqs. (3)and(4).

equation(3)

Turn MathJaxon

equation(4)

Turn MathJaxon

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Fig. 1.

Removal of Pb from the washing solution (A) and accumulation of Pb in the precipitate (B). Washing solutions were electrochemically treated

at pH 6, 8 and 10. Error bars represent standard deviation from the mean value (n= 3).

Figure options

Table 1.

Balance of Pb after electrochemical treatment of the soil washing solution at different pH. Standard deviation from the mean value ( n= 3)

was calculated.

Treated washing solution Pb balance (%)

In solution Precipitated Electrodeposited

pH 6 30 6 20 7 33 5 83 10

pH 8 13 2 7 1 68 5 88 7

pH 10 11 3 15 2 62 5 88 7

Table options

Metals liberated from the EDTA complex could then be removed from the solution by direct electrodeposition on the cathode,

precipitation as insoluble hydroxides, or absorption and co-precipitation on Al hydroxide flocks according to the following reaction(Eq.5):

equation(5)

Turn MathJaxon

Table 1indicates that, in all electrochemical treatments, the majority of Pb was removed from solution by electrodeposition on the

cathode. However, at pH 8 and 10, a significantly higher amount of Pb was removed this way than at pH 6. This could again be

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3.3. Recycling and reuse of the treated EDTA soil washing solution

The efficiency of EDTA recycled from a washing solution electrochemically treated at pH 10 to extract Pb from the soil is shown

inFig. 3.After adjustment to pH 4.3 (pH of the fresh EDTA washing solution), the treated washing solution retained almost 90% of

the Pb extraction potential (from original soil) compared to freshly prepared EDTA solution of the same molarity and pH. In this

experiment (soil was extracted in two separate batches), fresh EDTA solution removed 63% of the Pb from the original soil and

further extraction using the treated solution on the once extracted soil removed an additional 24% of Pb (Fig. 3). In total, using EDTArecycling, almost 90% of Pb was removed from the soil. Interestingly, the potential of the treated washing solution to extract Pb from

previously (once) extracted soil was even higher than that of fresh EDTA solution, although the difference was not statistically

significant (P< 0.05),Fig. 3.

Fig. 3.

Removal of Pb from the original and previously extracted soil using fresh EDTA solution, EDTA solution after soil extraction but not treated,

and electrochemically treated EDTA solution. Soil washing solutions were treated at pH 10 and, for soil extraction, the solutions pH was

adjusted to 4.3. EDTA concentration was equal in all solutions (30 mM). Error bars represent standard deviation from the mean value (n= 3);

letters (a, b, c) denote statistically different Pb removal potential within the categories of original and extracted soil, according to the Duncan

test (p< 0.05).

Figure options

Since we used a high molar ratio of EDTA against soil Pb (as usual in soil washing;Nowack et al., 2006)only part of the EDTA in the

washing solution was complexed to Pb (approximately 22% calculated from data on Pb and EDTA concentration in the washing

solution, section 3.1.), some EDTA was presumably left in the original form or in various stages of protonation. Spent non-treated

washing solution was therefore expected to retain some Pb extraction potential, as indeed shown inFig. 3.

Table 2shows the potential of recycled EDTA for Pb extraction from the original soil through several steps of soil extraction and

washing solution treatment. First, fresh EDTA solution was used (in the 1st ext./treat. step) following by the use of electrochemically

treated washing solutions (2nd and 3rd ext./treat. step) for soil extraction. Washing solution treated once retained 86% and solution

treated twice 69% of the Pb extraction efficiency of fresh EDTA solution (calculated from data on percentages of Pb removed from

the soil presented inTable 2). The decrease of Pb extraction potential can be explained by the loss of EDTA from the solution (Table

2), mainly due to EDTA absorption into the soil solid phase (Nowack and Sigg, 1996). Some EDTA (

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Soil extraction/Solution

treatment

Pb

removed(%)

Initial EDTA

concentration (mM)

EDTA conc. after

extraction (mM)

EDTA conc. after

treatment (mM)

Lost EDTA

extraction (mM)(%)

Lost EDTA

treatment (%)

3. ext./treat. 49 22 15 / 32

Table options

3.4. Chemical dosing of Al

Electrochemical treatment of wastewaters with a sacrificial Al anode is an alternative to more commonly used chemical coagulation

of pollutants by dosing Al-salts. To compare these two methods, the same amount of Al (as AlCl3) was dosed in the washing solution

(pH 10) as the amount of Al electro-corroded during the corresponding electrochemical treatment. Chemical dosing (Table 3)

removed very little Pb from the washing solution, much less than electrochemical treatment (Fig. 1, Table 1), but did remove some

EDTA. The reason is that chemical coagulation with AlCl 3 removed Pb with EDTA complexes, while electrochemical treatment

liberated Pb and separated the EDTA. Chemical dosing did not lead to EDTA recycling; the Pb extraction potential of the chemically

treated solution (with pH adjusted to 4.3) was even lower than the extraction efficiency of non-treated washing solution with the same

pH (27 3% removed Pb,Fig. 3).

Table 3.

Pb and EDTA removal from the washing solution after chemical dosing of AlCl3at pH 10, and Pb extraction efficiency of chemically treated

soil washing solution (after pH was adjusted to 4.3). The treatment time of chemical dosing was selected as a 13 multiple of the

electrochemical treatment time (tel). Standard deviation from the mean value (n= 3) was calculated.

Treatment time (min) Pb removed (%) EDTA removed (%) Pb extraction potential (%)

23 (1 tel) 3 5 34 9 26 2

45 (2 tel) 4 5 27 4 23 6

113 (3 tel) 9 3 26 3 21 0

Table options

3.5. Cost and safety considerations

An accurate evaluation of the costs associated with soil remediation would require a pilot-scale experiment. However, the cost of Al

and electricity consumption, Al-hydroxide sludge disposal and treatment of the final spent washing solution (which represent the

major part of the material costs) can be extrapolated from the obtained data. If we assume two re-cycles of EDTA washing solution

treatment and reuse, then (including compensation for lost EDTA,Table 2)extraction of 1 ton of soil (with 75 mmol kg1of fresh and

recycled EDTA) would require 13.9 kg of EDTA. At a price of 1.3 per kg1EDTA (information obtained from a major European

EDTA producer) this translates into 18.1 . Treatment of the washing solution and EDTA recycling at a constant current density of

96 mA cm2and an average voltage of 8 V would require approximately 115 kW h and, at an approximate cost of 0.1per kW h, this

translates into 11.5. During the treatment/EDTA recycling, approximately 5 kg of Al would be expected to electro-corrode from the

anode. The current prices of Al in the open market are below 1.6 kg1, which equals 8 for spent Al. During the process,

approximately 20 kg of liquid Al hydroxide sludge was formed, calculated per ton of treated soil. The sludge can be deposited after

treatment, i.e., after solidification (and stabilisation of metals) with cement. The disposal cost of solid hazardous waste transportation

and disposal was assessed to be approximately 200per ton (Meunier et al., 2006), which adds an additional 4 to the total cost.

Since Al hydroxide is the major component of aluminium ore bauxite (together with AlO(OH) and Al2O3), it could perhaps be reused in

the Hall-Hroult process to recycle aluminium and avoid disposal costs. The spent washing solution contains EDTA and Pb, which

need to be completely removed before safe discharge. In a previous paper, we proposed an electrochemical advanced oxidation

process using a boron-doped diamond anode for EDTA degradation and removal of Pb from treated solution by (electro) precipitation

(Finzgar and Lestan, 2008). The electricity cost for this operation was estimated to be 10.3ton1of soil. The total estimated cost ofmajor material expenses for the treatment of 1 ton of contaminated soil thus amounts to approximately 52 . This cost does not

include capital investment in the equipment, which, in terms of electrochemical technologies, is characterised as relatively low (Chen,

2004). The cost seems favourable compared to the current cost of soil washing, which can go up to 350 per ton (Summergill and

Scott, 2005).

During the proposed remediation technique, some deposition of Al from the EDTA complex into the soil is expected. Aluminium is

known to reduce plant growth on acid soils, in which Al3+cations disturb root growth and function. However, Al constitutes

aluminosilicate minerals and sesquioxides and is naturally present in the soil. It is harmless to plants, immobile and non-toxic in pH-

neutral soils (Andersson, 1988).

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4. Conclusions

The following conclusions can be drawn from our study:

Electrochemical treatment of soil washing solution obtained after EDTA extraction of Pb contaminated soil, using an Al

anode in a conventional electrolytic cell at pH 10, efficiently separated EDTA and Pb.

Pb was relatively efficiently removed from the treated washing solution (>85%,Fig. 1), mostly by electrodeposition on the

cathode.

Electrochemical treatment separated EDTA in an active form. We demonstrated that, after treatment, the EDTA solution

retains almost all its Pb extraction potential.

Less than 10% of EDTA was lost during electrochemical treatment. More EDTA was lost from the solution due to

absorption onto the soil solid phases during soil extraction.

Chemical dosing of Al was not effective in separating Pb and EDTA in the washing solution. We conclude, therefore, thatelectro-reduction of EDTA on a cathode (Eq.(3))is essential for the exchange of Pb from the EDTA complex.

Electrochemical treatment of the washing solution with an Al anode at alkaline pH has potential for cost-effective recycling

and reuse of EDTA as a part of soil washing technologies.

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Kinetic extractions to assess mobilization of Zn Pb Cu and Cd in a metal-contaminated soil:EDTA v citrate

JrmeLabanowskia, Fabrice Monnab, Alain Bermondc, Philippe Cambiera, Christelle Fernandeza, Isabelle Lamya, Folkert van Oorta, , aINRA, UR 251 UnitPESSAC, RD 10, F-78026 Versailles Cedex, France bARTeHIS, UMR 5594 CNRS, Univ. de Bourgogne Centre des Sciences de la Terre, Bat. Gabriel, F-21000 Dijon, France cAgroParis Tech., Laboratoire de Chimie Analytique, 16 rue C. Bernard, 75231 Paris Cedex 05, France

Abstract

Kinetic EDTA and citrate extractions were used to mimic metal mobilization in a soil contaminated by metallurgical fallout. Modelingof metal removal rates vs. time distinguished two metal pools: readily labile (QM1) and less labile (QM2). In citrate extractions, total

extractability (QM1+ QM2) of Zn and Cd was proportionally higher than for Pb and Cu. Proportions of Pb and Cu extracted with EDTA

were three times higher than when using citrate. We observed similar QM1/QM2 ratios for Zn and Cu regardless of the extractant,

suggesting comparable binding energies to soil constituents. However, for Pb and Cd, more heterogeneous binding energies were

hypothesized to explain different kinetic extraction behaviors. Proportions of citrate-labile metals were found consistent with their

short-term, in-situmobility assessed in the studied soil, i.e., metal amount released in the soil solution or extracted by cultivated

plants. Kinetic EDTA extractions were hypothesized to be more predictive for long-term metal migration with depth.

Keywords

Kinetic extraction modeling;

Heavy metals; EDTA; Citrate; Metal mobility; Soils

1. Introduction

Fate of heavy metals in soils is of great environmental concern. They represent major risks regarding contamination of natural waters

after release by metal-bearing soil constituents and migration viathe soil solution downward to the water table (van Oort et al., 2006).

Studying soil solution collected in situfrom different horizons in metal polluted soils,Denaix et al. (2001)andCiteau et al.

(2003)reported the presence of metals either as predominantly mobile, water-soluble forms (Zn, Cd) or as colloidal forms (Pb, Cu).

Incorporation of metal pollutants in soils generally leads to changes in chemical speciation. Liberated metal ions are variously

trapped by a wide range of reactive soil constituents, i.e. organic matter, iron and manganese oxides and hydroxides, phyllosilicates,

phosphates, carbonates. A fraction of these metals can thereafter be (re)mobilized, either in dissolved or colloidal form before

migrating downward. Mobilization is defined here as the potential capacity of metals to be transferred from the solid phase into the

soil solution. It depends on the various links between metals and reactive sites of solid phase surfaces.

Numerous operational chemical extraction methods, including one-step extractions (Chaignon et al., 2003andFeng et al., 2005)and

sequential extraction procedures (McGrath, 1996,Gleyzes et al., 2002andKrishnamurti et al., 2002), have been developed to

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estimate the extractable metal pools in relation to the metal-bearing soil phases. Although informative, such extraction procedures

generally suffer from lacking selectivity with respect to the targeted chemical forms of metals. Alternatively, several authors have

hypothesized that kinetic metal extraction data might better reflect dynamics of metals in soils (Gutzman and Langford,

1993,Bermond et al., 1998,Lo and Yang, 1999andBermond et al., 2005)and in sediments (Fangueiro et al., 2002,Fangueiro et al.,

2005andGismera et al., 2004). For a given chemical agent, the kinetic extraction approach generates two kinds of data: (i) the

proportion of metals extracted with respect to the total metal content of the soil sample; and (ii) the kinetic behavior of metals. It has

been suggested that such kinetic extractions can be modeled by two first-order reactions, so-called TFOR model, which empirically

define two different pools. Although chemical extractants used for experiments do not perfectly mimic natural conditions, the first pool

of readily extracted metals, called labilebyFangueiro et al. (2005), and the second pool of more slowly extracted metals, called

less labile, might be reasonably attributed to potentially mobileand/or bioavailablemetal pools (Bermond et al., 2005). In this

view, Degryse et al. (2006)showed that the proportion of labile metals was well-correlated with metal uptake in plants. Particularly for

sandy textured soils, the distinction of two types of extractability with time is hypothesized to provide a good expression of the

diversity of metal-soil constituent interactions, i.e., to various chemical bond stabilities. In soils with strong aggregation phenomena

(i.e., clay soils), disaggregation phenomena due to chemical agents or physical stirring may, however, influence the extraction of

discrete kinetic pools.

Naidu and Harter (1998)stated that metals extracted by a mixture of organic acids are well-correlated with the mobile metal fraction

in the soil solution. Organic acids are likely to be more representative of a mobile metal fraction that is available for biota. Low

molecular weight organic acids, naturally exuded by plant roots or produced by microbial activity (Fox and Comerford, 1990), have

been hypothesized to influence nutrient mobilization (van Hees et al., 2002)or translocation of metals in soil profiles (van Hees and

Lundstrm,2000andLi et al., 2006). Ethylene diamine tetraacetic acid (EDTA) is a well-known strong chelating agent and has been

widely used in agronomy for estimating the total extractable metal pool (Alvarez et al., 2006andManouchehri et al., 2006). EDTA

was reported to remove metals organically bound, occluded in oxides, and associated with secondary clay minerals (Pay-Prez

et al., 1993). In contrast, citrate has been reported to be one of the dominant organic acids in the soil solution (van Hees et al., 2002).

Among the low-molecular-weight organic acids used to simulate metal mobilization, citrate presents a moderate metal complexation

strength compared to EDTA, according to thermodynamic data fromSillen and Martell (1964)andLindsay (1979)shown inTable 1.

Table 1.

Formation constants (log Kf) for metal-EDTA and metal-citrate complexes at 0.1 M ionic strength

Zn Pb Cd Cu

Me[EDTA]2

16.3 18.2 16.5 18.8

Me[citrate] 4.9 4.0 3.8 5.9

Me2[citrate]22 13.2

Me[citrate]24 6.5 6.1 5.4 8.1

The log Kfvalues presented in this table are derived from those ofSillen and Martell (1964)andLindsay (1979). They are recalculated to the

same ionic strength.

Table options

The present study aimed at estimating the labile fractions of different trace metals (Zn, Pb, Cu, and Cd) by applying kinetic EDTA and

citrate extractions on samples from a moderately metal-contaminated agricultural soil (Labanowski et al., 2007). Comparison of

EDTA and citrate extractions was used to quantify metal mobilization and to assess relationships between the energy of metal

binding and the strength of the used chemical extractant. By expressing the data in terms of metal removal rates vs. time, we first

checked the pertinence of fitting kinetic extractions with a TFOR model. Parameters provided by this modeling were then used to

compare the different kinetic metal pools with results on metal mobility obtained from field studies (Citeau et al., 2003) and pot

experiments (Mench et al., 2007)conducted on the same soil.

2. Materials and methods

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