removal of trace and major metals by soil washing with na2edta and oxalate

SOILS, SEC 3: REMEDIATION AND MANAGEMENT OF CONTAMINATED OR DEGRADED LANDS •RESEARCH ARTICLE

Removal of trace and major metals by soil washingwith Na2EDTA and oxalate

Rongliang Qiu & Zeli Zou & Zhihao Zhao &

Weihua Zhang & Tao Zhang & Hanying Dong &

Xiange Wei

Received: 31 December 2008 /Accepted: 23 March 2009 /Published online: 8 April 2009# Springer-Verlag 2009

AbstractBackground, aim, and scope Various metals such ascationic metals (Cu, Pb, Zn) and anionic metals (As, Cr)often coexist in real soils, and normal soil washingtechniques for the removal of cationic metals with asingle-washing reagent make it rather difficult to simulta-neously remove all of them. Oxalate could effectivelyremove anionic As and EDTA could effectively remove thecationic metals, so it was possible to remove all coexistingcationic and anionic metals by washing with the combina-tion of Na2EDTA and oxalate. The objective of this studywas to (1) discuss the possibility of removing five metals,As, Cd, Cu, Pb, and Zn, effectively from the soil bywashing with Na2EDTA-combined oxalate; (2) optimizedthrough the consecutive washing.Materials and methods The soil sample was collected, air-dried, and sieved. The typical extraction tests wereconducted in 50-mL-capacity polyethylene tubes. Tubescontaining 1 g samples and a measured volume ofextractant (L/S=20 mL g−1) were stirred for a given time.The soil was first treated with 0.01 M Na2EDTA and 0.1 Moxalate individually to compare the removal efficiencies.Then, consecutive extractions were performed so that 1 g ofsoil was treated by four cycles of washing with 2 h for eachwashing. To optimize the combination of Na2EDTA andoxalate, three methods of consecutive washing weredesigned: washing with Na2EDTA followed by oxalate,

for two rounds (A); washing with oxalate followed byNa2EDTA, for two rounds (B); and washing with theadmixture of oxalate and Na2EDTA for four cycles (C).Results After 24 h washing, Na2EDTA removed only 2.3%of As from the soil, while oxalate removed 59.9%. Incontrast, the removal of Pb for oxalate was 1.5% while it was27.4% for Na2EDTA. A large amount of Ca (379.72 mg l−1)was released when washing with Na2EDTA; while thereleased Ca was far lower (15.86 mg l−1) with oxalate. Incontrast, washing with oxalate resulted in a large amount ofAl (123.34 mg l−1) and Fe (305.9 mg l−1) in the solution, farhigher than washing with Na2EDTA (14.03 mg l−1 for Aland 38.40 mg l−1 for Fe). The consecutive experimentsindicated that after four cycles of washing, threemethods could effectively remove all of both cationicand anionic metals, and the accumulative removals offive metals were 54.7–65.6% for As, 28.6–33.8% forCd, 80.3–86.6% for Cu, 15.8–42.9% for Pb, and 43.2–45.2% for Zn, respectively. In the first cycle of methodA, the total molar concentration of major elements was11.53 mmol l−1, higher than the molar concentration ofNa2EDTA. Arsenic released was linearly correlated withthe concentrations of Fe and Al in the eluate to asignificant degree and the correlation coefficients (r)were 0.942 and 0.920, respectively.Discussion The reason for the low As removal forNa2EDTA was that the anionic form of As does not allowfor the formation of a stable complex with Na2EDTA. Thereason for the low removal of Pb by oxalate was due to theformation of low-solubility lead oxalate precipitates. Ex-tract analyses indicated that major elements were extractedby Na2EDTA and oxalate as well. Oxalate is able topromote greater Al/Fe/Mn dissolution than an equivalentlevel of EDTA through ligand-promoted oxide dissolutionand reductive dissolution mechanisms.

J Soils Sediments (2010) 10:45–53DOI 10.1007/s11368-009-0083-z

Responsible editor: Jianming Xu

R. Qiu (*) : Z. Zou : Z. Zhao :W. Zhang : T. Zhang :H. Dong :X. WeiSchool of Environmental Science and Engineering,\Sun Yat-sen University,135 Xin Gang Xi Road,Guangzhou 510275, Chinae-mail: [email protected]

Method A removed more Cd and Pb, but a little less Cu,Zn, and As than methods B and C. Since it could removeall of the five metals effectively, but the other two methodscould remove only 15.8–20.7% of Pb, it was considered asthe best method to decontaminate this studied soil. In thefirst cycle of method A, the total molar concentration ofmajor elements was higher than the molar concentrations ofNa2EDTA. It was reasonable to assume that some of themajor substances in the washing solutions were thereforenot complexed to chelant. In the first cycle, a large amountof Ca (388.7 mg l−1) was released when washing withNa2EDTA (method A), while the concentration of Ca wasfar lower for method B (11.38 mg l−1) and C (6.51 mg l−1).It seemed that oxalate could inhibit the release of Cathrough the formation of insoluble CaOx, which is notEDTA-exactable. No matter with which method, the releaseof Fe and Al was large when washing with oxalate, while itwas small when washing with Na2EDTA. Arsenic releasedwas significantly linearly correlated with the concentrationsof Fe and Al in the eluate and the correlation coefficients (r)were 0.942 and 0.920, respectively. It further confirmed theconclusion that the releasing of As can be mainly attributedto the release of Fe and Al oxides.Conclusions After 24 h washing, Na2EDTA led to highremoval for cations such as Cu, Pb, Zn, and Cd, but less Asremoval. On the contrary, oxalate removed more As, Cd,Cu, and Zn than Na2EDTA, while the removal of Pb wasvery low (1.5%). Four cycles of washing with threemethods combining Na2EDTA and oxalate washing couldeffectively remove both cationic and anionic metals. About96.54–99.20% of chelant was found binding with majorelements in the eluate. Washing with Na2EDTA followedby oxalate (method A) was considered to be the bestmethod to decontaminate the studied soil. Arsenic releasedby washing was significantly correlated with the concen-trations of Fe and Al in the eluate, indicating that therelease of As was mainly ascribed to the release of Fe andAl oxide.Recommendations and perspectives The removal of metalsin this study is efficient, but the high cost of chelatingreagents such as EDTA has precluded their field applica-tion. Since the adsorption mechanism between the metaland the specific soil fraction differs, the lack of under-standing the chemistry of soil metal speciation, interparticleextraction dynamics, and spent extractant recycling techni-ques have limited this promising technology to small-scaleapplications. The behavior of the individual fractionsduring the extraction process, the selectivity between targetheavy metals and chelators, and the reuse and recoverabilityof chelators will be the major tasks for further studies.

Keywords EDTA . Extraction . Heavymetals . Oxalate .

Soil washing

1 Background, aim, and scope

Soil contaminated with heavy metals, such as lead,chromium, copper, zinc, arsenic, mercury and cadmium,are commonly found as the result of numerous industrialactivities, including mining, smelting, automobile batteryproduction, vehicle emission, and landfilling of industrialwaste and fly ash from incineration processes (Hong et al.2002). Since heavy metals do not degrade and are toxic tobiological systems (Kim et al. 2003), they must bedemobilized in the soil to decrease their bioavailability ortaken out from the ecosystem. Soil washing with strongacids or chelators or organic ligands is considered as one ofthe most suitable on-site, ex situ techniques to removeheavy metals from the ecosystem (Peters 1999). However,strong acid (such as HCl) washing has been found todecrease soil productivity and to lead to the adversechanges in the chemical and physical structure of soilsdue to mineral dissolution (Reed et al. 1996).

Thus far, chelator EDTA is recognized as the mosteffective synthetic chelating agent to remove heavy metalsfrom soils for the following four reasons: (1) EDTA has agreat chelating ability for cationic metals (especially Pb,Cd, Cu, and Zn); (2) EDTA washing process can treatdifferent types of soils; (3) EDTA is low biodegradable, soit can be recovered and reused (Dermont et al. 2008); and(4) EDTA does not induce a strong acidification whenexacting the metals in soils. The ability of EDTA to extractmetals without inducing a strong acidification of the soil isa very desirable characteristic. But EDTA resists biodegra-dation and is adsorbed on the soil particles, thus making thetreated soils unfit for agricultural use (Wasay et al. 2001).Otherwise, EDTA is not effective in removing arsenic,because arsenic often presents As (III) and As (V) inanionic form and EDTA does not allow for the formation ofstable chelates with As (Tokunaga and Hakuta 2002).

Organic ligands such as citrate and oxalate also have theability to complex with various heavy metals withoutleaving any toxic effect. Oxalate (Ox) was a promising soilextractant because it is biodegradable, naturally occurring,relatively inexpensive, and forms moderately stable metalcomplexes. Moreover, Ox has the capability to attack anddissolve hydrous oxides (Stumm 1992), so it could beadvantageous for washing soil where the contaminatingmetals are significantly associated with oxide soil compo-nents (Elliott and Shastri 1999).

Previous studies on soil washing mainly focused on thesoils contaminated with single or several cationic metals.However, the investigation of some demolished industrialsites indicated that various metals such as cationic metals(Cu, Pb, Zn) and anionic metals (As, Cr) often coexist inreal soils. These coexisting metals have contrasting charac-teristics, and normal clean-up techniques, which were

46 J Soils Sediments (2010) 10:45–53

developed for the removal of cationic metals fromcontaminated soils and it is rather difficult to simultaneous-ly remove all of them with a single-washing reagent(Assink and Rulkens 1987; Tampouris et al. 2000).However, limited effort has been made to focus on suchproblems so far. Since previous studies showed that oxalatecould effectively remove anionic As (Peters 1999; Tao et al.2006) and EDTA could effectively remove the cationicmetals, it was possible to remove all coexisting cationic andanionic metals by washing with the combination ofNa2EDTA and oxalate. Therefore, the objective of thisstudy was to (1) discuss the possibility to remove the fivemetals, As, Cd, Cu, Pb, and Zn, effectively from the soil bywashing with Na2EDTA-combined oxalate; and (2) tooptimize the consecutive washing.

2 Materials and methods

2.1 Soil samples

The soil samples were collected from 0.7 to1.7 m below theground surface at a demolished industrial site in South China,where the warehouse of a fertilizer plant was previouslylocated. The collected soil samples were air-dried at roomtemperature (20–30°C). The metal contents of all the sampleswere measured, and a sample with a high level of contami-nants was selected for this study (Table 1). The selectedsample was sieved using a 10 mesh sieve (U.S. No. 10) toremove stones and large materials, then thoroughly mixed toensure uniformity and stored in a plastic bag at roomtemperature for subsequent experiments.

Various soil physical and geochemical characterization testswere carried out. The physical and chemical characteristics ofthe soil are shown in Table 1. The total metal content wasdetermined after acid digestion (HF/HClO4/HNO3). Soil pHwas determined by using a 1:5 soil to water ratio and pHmeter. The CEC of the sample was determined by theammonium acetate (NH4OAc)/sodium acetate (NaOAc)method (Van Reeuwijk 1992). Organic matter content wasdetermined by heating the dried samples at 350°C for 5 h(Ball 1964). The point of zero charge (pHzpc) of the soil wasobtained from an acidimetric–alkalimetric titration followingthe procedure described by Kummert and Stumm (1980). Theparticle size distribution was established by mechanicalsieving, followed by the hydrometer method (Blake andHartage 1986). All heavy metal analyses were performedusing a Horizon 5310 inductively coupled plasma opticalemission spectrometer (ICP-OES).

2.2 Washing procedure

The typical extraction tests were conducted in 50-mL-capacitypolyethylene tubes. Tubes containing 1 g samples and ameasured volume of extractant were stirred for a given time,using an end-over-end shaker at a speed of 180 rpm at a roomtemperature (28–33°C). At the end of the extraction, thesuspensions were centrifuged at 5,000 rpm for 10 min andthen the supernatant was filtered through a 0.45 mm mem-brane for heavy metal analysis. The concentrations of metalsunder investigation were measured by ICP-OES and graphitefurnace atomic absorption spectrometry (GFAAS). All testswere performed in triplicate and the results were presented asaverages of the triplicate extracts.

The soil was first treated with 0.01 M Na2EDTA and0.1 M oxalate, individually to compare the removalefficiencies of heavy metals. Studies by many researchersindicated that 24 h were more than sufficient to reachsteady state conditions (Elliott and Brown 1989; Peters andShem 1992), so we set the washing time at 24 h and theliquid/soil ratio as 20 mL/g. Consecutive extractions werethen performed so that 1 g soil was treated by four cycles ofwashing with 2 h for each washing. To optimize thecombination of Na2EDTA or oxalate, three methods of2-h consecutive washing were designed: washing withNa2EDTA first, then followed by oxalate, for two rounds(A); washing with oxalate then followed by Na2EDTA, fortwo rounds (B); and washing with the admixture of oxalateand Na2EDTA (10 mL 0.01 M Na2EDTA add 10 mL 0.1 Moxalate for 1 g soil) for four cycles (C). In these threemethods, the same dose of reagents (40 mL 0.01 MNa2EDTA and 40 mL 0.1 M oxalate) was used for 1 g ofsoil in the whole consecutive washing. We compared theheavy metal removal of the three washing methods (A, B,and C) to determine the most proper one.

Table 1 Physicochemical characteristics of the soil sample

Soil properties Value

Sand (%) 55.8

Silt (%) 28.9

Clay (%) 15.3

Organic content (%) 5.1

pH 7.31

pHzpc 2.01

Specific gravity 2.54

CEC (cmol kg−1) 13.26

Metal content

As (mg kg−1) 70.41

Cd (mg kg−1) 1.73

Cu (mg kg−1 ) 325.01

Pb (mg kg−1) 254.35

Zn (mg kg−1) 258.56

J Soils Sediments (2010) 10:45–53 47

3 Results

3.1 Batch washing with EDTA and oxalate

3.1.1 Comparison of heavy metal removal

The removal efficiencies of metals by 0.01 M Na2EDTA or0.1 M oxalate solution after a 24-h washing are shown inFig. 1. The stability constants of EDTA-metals, oxalate-metals, and the solubility products (Ksp) by the metaloxalate are also shown in Table 2. As shown in Fig. 1, aftera 24-h washing, Na2EDTAwas found to remove only 2.3%of As from the soil, while oxalate removed 59.9% of As. Incontrast, the removal of Pb by oxalate was only 1.5% while27.4% was removed via Na2EDTA. From Fig. 1, 47.9% ofCd, 65.4% of Cu, and 22.9% of Zn were removed byoxalate while 24.2% of Cd, 53.7% of Cu and 16.8% of Znwere removed by Na2EDTA.

3.1.2 Comparison of major elements releasing

The concentrations of major elements in the eluates werealso monitored in order to estimate the degree of simulta-neous dissolution of soil compounds, such as CaCO3, Al–Fe–Mn oxides, etc., and the results are listed in Table 3. Itindicated the major elements were extracted by EDTA andoxalate. A large amount of Ca (379.72 mg l−1) was releasedwhen washing with Na2EDTA; while the released Ca wasfar lower (15.86 mg l−1) when treated with oxalate. Incontrast, washing with oxalate resulted in a large amount ofAl (123.34 mg l−1) and Fe (305.9 mg l−1) in the solution,far higher than washing with Na2EDTA (14.03 mg l−1 forAl and 38.40 mg l−1 for Fe).

3.2 Consecutive washings with Na2EDTA combinedwith oxalate

3.2.1 Heavy metal removal in consecutive washings

In general, Na2EDTA is effective in removing metal cationsbound to exchangeable, carbonate, and organic fractions,while oxalate is more effective than Na2EDTA in removingthe metals associated with Fe–Mn oxides of soil for it canattack the hydrous oxides (Elliott and Shastri 1999). Thus,if washing with Na2EDTA combined with oxalate, themetals bound to these four fractions can be removedeffectively. To determine the best washing combination,we compared the heavy metal removal of three treatmentmethods (A, B, and C) as described in Section 2.2. Theresults are shown in Fig. 2. From the figure, four cycles ofwashing with these three methods all could remove bothcationic and anionic metals effectively, and the accumula-tive removal of five metals were 54.7–65.6% for As, 28.6–33.8% for Cd, 80.3–86.6% for Cu, 15.8–42.9% for Pb and43.2–45.2% for Zn, respectively.

3.2.2 Major elements released in consecutive washings

The concentrations of major elements in the eluates forthree methods of consecutive washings are listed in Table 4.From the table, in the first cycle of method A, the totalmolar concentrations of major elements ([Me]t) was11.53 mmol l−1, higher than the molar concentrations ofNa2EDTA (10 mmol l−1). In the next step, however, [Me]tis less than the reagent (Na2EDTA or oxalate), and the samesituation existed in every cycle washing of method B andC. From Table 4, the molar levels of major elements are tentimes or more higher than the trace metals molarities ineach washing, and most of the chelant (96.54–99.20%) istherefore involved in binding with major elements.

4 Discussion

The removal of As for Na2EDTAwas reasonably low, sincethe anionic form of both As(III) and As(V) does not allowfor the formation of a stable complex with Na2EDTA. Thereason for the low removal of Pb by oxalate waspresumably due to the formation of a low-solubility leadoxalate precipitate (PbC2O4, Ksp ¼ 2:74� 10�11). In con-trast, the solubility of other metal oxalate compounds, suchas CdOx, ZnOx, was sufficiently high so that no similarprecipitation occurred to quantitatively hinder the Cd, Cu,and Zn removal from the particular soil. As shown inTable 2, the stabilities of complex for Na2EDTA and metalsare dramatically higher than the corresponding complex foroxalate and metals, for example ZnEDTA2− (log K=16.5) is

Fig. 1 The removal efficiency of 0.01 M of Na2EDTA and 0.1 Moxalate (liquid/soil ratio 20, contact time 24 h)


far higher compared to ZnOx− (log K=3.4), whichgenerally indicates that Na2EDTA forms stronger chelatecomplexes with free metals than oxalate. However, oxalatecould remove more Cd, Cu, and Zn than Na2EDTA. Itseems that the removal efficiencies were not very consistentwith the complex constants, and the superiority of oxalateover Na2EDTA in releasing Cu, Cd, and Zn from the soilmay involve a comparison of the ability for these twoligands to release the metals associated with the variousfractions of soil constituents.

Traditionally, it was typically assumed for EDTA that itcould not release metals firmly bound in crystal lattices,such as iron oxides and the silicate matrix (Pickering 1986;Schramel et al. 2000). However, some researchers reportedthe metals bound to Fe/Mn oxides or residual fraction werealso released by chelating agents EDTA, NTA, and DTPAtoo (Lim et al. 2004; Kirpichtchikova et al. 2006).Borggaard (1979) also found that EDTA can selectivelysolubilize Fe oxides from soils, but required weeks tomonths for a complete reaction. Oxalate and relatedcarboxylic acids such as EDTA can attack Al and Feoxides through ligand-promoted oxide dissolution mecha-nisms. They can form inner-sphere, ring-type surfacecomplexes with Fe, and the resulting shift of electrondensity toward the central Al/Fe ions weakens the linkbetween Al/Fe and the solid lattice, thus promotingdetachment of Al/Fe into solution (Elliott and Shastri1999). The dissolution rate of ligand-promoted Al/Fe/Mnoxide depends on the number of carboxylate groups on theligand (Blesa et al. 1994), and the maximum rate isobserved for ligands with two or three carboxylate groups.As a dicarboxylate, Ox is able to promote greater Al/Fe/Mnoxide dissolution than an equivalent level of EDTA.Otherwise, as a strong reducing agent, oxalate is morelikely to release the metals trapped in iron–manganeseoxides through reductive dissolution mechanism by chang-ing the oxidation degree of Fe (III) and Mn (IV) on Mn andFe oxyhydroxides of soils (Song and Greenway 2004).

Na2EDTA caused a great loss of Ca in the soil, butoxalate caused a far larger amount of Al and Fe loss andhigher release of Mg and Mn than Na2EDTA. Otherresearchers have also found an important co-dissolution ofsoil Ca, thus resulting in a low degree of heavy metalcomplexation by EDTA (Theodoratos et al. 2000; Papas-siopi et al. 1999). A recent study (Palma and Ferrantelli2005) showed that Ca2+ is the main competitive cation,because CaCO3 is strongly dissolved in the EDTA leachingsolution at pH 4.5; thus, concentrations of Ca2+ in theleaching solution are very high compared to that of thetarget heavy metals. However, despite the high concentra-tion of Ca in washing solutions, extensive interference ofCa with Pb, Zn, and Fe-EDTA complexation was not likelyto occur (Finžgar and Leštan 2007). For Ca forms, muchless stable complexes with EDTA (log K=10.65; Martelland Smith 1976) than Pb, Zn, or Fe ions presumably didnot interfere with Pb and Zn complexation (Papassiopi et al.1999). The far lower amount of Ca released for oxalateindicated that oxalate could inhibit the release of Ca throughthe formation of insoluble CaOx. The concentration data ofAl, Fe, andMn in the eluate showed EDTA can really dissolvepoorly crystalline Fe oxyhydroxides, and washing withoxalate leads to more impact on soil physical–chemicalproperties. Oxalate can dissolve oxides more effectively thanNa2EDTA, and thus releases more oxide-bound heavymetals, such as Cu, Cd, and Zn, even if the stabilities oftheir oxalate complexes were lower than their EDTAcomplex. Arsenic in soil and sediment was reported to bemainly bound to Al and Fe oxides and clay minerals due tosorption mechanisms (Tokunaga and Hakuta 2002; Goldberg2002), and this conclusion was also further confirmed herein,since the much higher As removal was found accompaniedwith a much higher concentration of Al, Fe in the eluatewhen washing with oxalate.

For the studied soil, which was contaminated with bothcationic and anionic metals, if using only Na2EDTA, theanionic As could not be effectively removed; if only using

Log K for complex (I=0, 18°C) Oxalate solubility product Ksp (I=1M, 25°C)Metal EDTA Oxalate

Cd 16.5 2.73 9.00×10−8

Cu 18.8 4.51 2.30×10−8

Pb 17.7 4.16 2.74×10−11

Zn 16.5 3.40 1.35×10−9

Table 2 Equilibria constants forEDTA and oxalate (Martell andSmith 1976; CRC 1976)

Reagent Al (mg l−1) Ca (mg l−1) Fe (mg l−1) Mg (mg l−1) Mn (mg l−1)

Oxalate 123.34 15.86 305.90 32.46 2.56

Na2EDTA 14.03 379.72 38.40 14.22 1.71

Table 3 The concentration ofmajor elements in the eluate


oxalate, a low Pb removal could occur. Therefore, threemethods of washing with a combination of EDTA andoxalate seems to be a promising way to decontaminate thefive metals of As, Cd, Pb, Cu, and Zn from the soil. Theresults showed that the accumulative removals of fivemetals for three methods were different. For As, 54.7% ofAs was removed with method A, which is less than withmethods B (65.1%) and C (65.6%). It suggests that the firstaddition of oxalate either individually or with EDTA, led tomore As removal, since oxalate always leads to theincreased oxide release from soils. For Cd, method Acaused a little higher removal than did B and C. Afterthe first round (the second cycle), the release of Cd from thesoil almost reached its maximum available level, and therelease of Cd in the next round was little, no matter iftreatment A, B, or C was used. After the weakly boundmetals are extracted first, the release of the remaining,strongly bound metals is presumably related to thedissolution of mineral constituents of sediment (includingoxides and silicates) initially retaining the contaminants(Polettini et al. 2007; Tsang et al. 2007). For Cu, washingwith admixture accumulatively could remove 86.6% from

the soil, a little higher than A (80.3%) and B (81.7%),indicating that a synergistic action for Cu removal mayexist when washing with the admixture of Na2EDTA andoxalate. For example, after the first cycle, 72% of Cu wasremoved by method C, while the removals were 54.0% and66.8% for A and B, respectively. For Pb, method A attaineda far higher removal efficiency (42.9%) than methods B(15.8%) and C (20.7%). In the first cycle of method B,oxalate first formed the insoluble PbOx, thus hindering therelease of Pb in the following washing cycles so that the Pbremoval was only 10.0% by the sequent Na2EDTAwashing, much lower than that for the first cycle of methodA (26.11%). It indicates the formed insoluble PbOx afteradding oxalate may not be EDTA-extractable. Whenwashing with the admixture (method C), part of the Pbcomplexes with Na2EDTA and cannot react with oxalate toform PbOx precipitates, thus resulting in a little higherremoval of Pb (20.7%) than the first oxalate washing withmethod B (15.8%). For Zn, the three methods obtainedalmost the same accumulative removal efficiencies (43.2–45.2%). In short, method A removed more Cd and Pb, but alittle less Cu, Zn, and As than with methods B and C. Since

Fig. 2 The accumulativeremoval efficiencies of washingmethods A, B, and C


it could remove all of the five metals effectively and theother two methods could remove only 15.8–20.7% of Pb,washing with Na2EDTA followed by oxalate (method A)was the best method to successfully decontaminate thestudied soil.

It is reasonable to assume that some of the major metalsin the washing solutions were therefore not complexed tochelant when the metals molarities exceed the chelantmolarities, and complexed to the chelant when lower thanthe chelant molarities. In our experiments, the molarities ofmajor elements were ten times or higher than the tracemetal molarities in each washing, and most of the chelant(96.54–99.20%) is therefore binding with major elements.Finžgar and Leštan (2007) reported that most of Ca in thesolutions was therefore not complexed to EDTA, for molaramounts of Ca removed from the soil system with eight-step leaching were higher than the amount of appliedEDTA. Similarly, Papassiopi et al. (1999) reported that Cais the predominantly dissolved element in the solution,binding almost 89% of the available EDTA, while Zn andPb utilize only 5.7% of the chelate ions. When the metalmolarities exceed the chelant molarities, the Ca–EDTAinitially formed will be displaced by the Al and Fe cationsin the soil solution after a given time (Manouchehri et al.2006). Therefore, Catlett et al. (2002) indicated that thetotal Ca concentration in solution cannot be considered asCaEDTA2−, because there is likely to be a significantamount of free Ca ion and inorganic Ca complexes insolution. Thus, with a lack of EDTA, CaEDTA2− iscalculated by taking the total EDTA concentration and

subtracting the sum of the other metal-EDTA (Al, Fe, Mg,Mn, Cd, Cu, Pb, and Zn) concentrations. For oxalate,because it does effectively attack hydrous oxides, sufficientquantities of the extractant (Ox) are necessary to accom-plish both ligand-assisted dissolution of the hydrous oxidesand keep the contaminating metals in solution via com-plexation (Elliott and Shastri 1999).

It seemed that oxalate could inhibit the release of Cathrough the formation of insoluble CaOx, which is notEDTA-exactable. In the first cycle, a large amount of Ca(388.7 mg l−1) was released when washing with Na2EDTA(method A), while it was far lower for methods B(11.38 mg l−1) and C (6.51 mg l−1). In the second cycle,the Ca concentrations in the eluate were 11.12 and3.56 mg l−1 with methods B and C, respectively, still farlower than 388.7 mg l−1. The removal of Mg and Mndecreased with the washing steps in three methods, andthe total releasing amount were almost the same. Nomatter in what washing method, when washing oxalate,the release of Fe and Al was large; while it was smallwhen washing with Na2EDTA. The effect of oxalate uponthe increasing solubility of As from the soils could mainlybe ascribed to the dissolution of amorphous Fe/Al oxides,and oxalate extractable iron generally reflects the amor-phous iron present in the soil (McKeague and Day 1966;Davranche and Bollinger 2001). Based on the data listed inTable 3, it was found that arsenic released was significantly(p<0.01, n=12) linearly correlated with the concentrations ofFe and Al in the eluate with all three methods and thecorrelation coefficients (r) were 0.942 and 0.920, respectively.

Table 4 The concentration of major elements in the eluate of the three methods of washing

Cycles Reagents Al Ca Fe Mg Mn [Me]ma Rm

b

(mg l−1) (mg l−1) (mg l−1) (mg l−1) (mg l−1) (mmol l−1) (%)

Method A

1 0.01 M Na2EDTA (1) 17.45 388.68 26.93 15.84 2.00 11.53 98.39

2 0.1 M oxalate (2) 56.88 7.76 173.16 14.42 1.07 6.02 98.40

3 0.01 M Na2EDTA (3) 35.03 4.33 100.50 5.10 0.81 3.43 98.92

4 0.1 M oxalate (4) 14.88 8.82 60.91 3.52 0.30 2.01 99.08

Method B

1 0.1 M oxalate (1) 96.54 11.38 207.31 23.00 2.49 8.57 97.13

2 0.01 M Na2EDTA (2) 26.05 11.12 68.64 6.08 1.06 2.74 98.29

3 0.1 M oxalate (3) 15.56 5.83 40.81 8.21 0.25 1.80 98.84

4 0.01 M Na2EDTA (4) 13.26 8.70 53.57 3.71 0.24 1.83 99.09

Method C

1 Admixture (1) 85.14 6.51 181.47 21.06 1.96 7.47 96.54

2 Admixture (2) 21.62 3.56 72.94 9.16 0.83 2.59 98.47

3 Admixture (3) 20.34 3.58 85.61 6.49 0.66 2.66 99.13

4 Admixture (4) 19.30 4.11 87.74 5.85 0.58 2.64 99.20

a [Me]m: the sum of the major metal-EDTA molar concentrations. It represents [Al]+[Ca]+[Fe]+[Mg]+[Mn]bRm: the molar ratio of the major metals and the total metals in the solution. It represents [Me]m×100/([Me]m+[As]+[Cd]+[Cu]+[Pb]+[Zn])


It further confirmed the conclusion that the releasing of Ascan be mainly attributed to the release of Fe and Al oxides.

5 Conclusions

After 24-h washing, washing with Na2EDTA led to highremoval efficiencies for cations such as Cu, Pb, Zn, and Cd,but less for As removal. In contrast, washing with oxalateremoved more As, Cd, Cu, and Zn than Na2EDTA, whilethe removal of Pb was very low (1.5%). Therefore, washingwith single Na2EDTA or single oxalate seem impossible tosuccessfully remove the coexisting As, Cd, Cu, Pb, and Znin soil. Four cycles of washing with three methodscombining EDTA and oxalate washing could remove bothcationic and anionic metals effectively, the accumulativeremovals of five metals were: 54.7–65.6% for As, 28.6–33.8% for Cd, 80.3–86.6% for Cu, 15.8–42.9% for Pb and43.2–45.2% for Zn. About 96.54–99.20% of chelant wasfound binding with major elements. Method A wasconsidered as the best method to successfully decontami-nate the studied soil for it could remove all of the fivemetals effectively and the other two methods could removeonly 15.8–20.7% of Pb.

About 96.54–99.20% of chelant was found binding withmajor elements in the four washing cycles of the threemethods. If oxalate was added first, it can inhibit the releaseof Ca but result in a large amount of Al and Fe releasedfrom the soil. Arsenic released by washing was significant-ly correlated with the concentrations of Fe and Al in theeluate, indicating the releasing of As was mainly ascribedto the release of Fe and Al oxide. Arsenic released wassignificantly linearly correlated with the concentrations ofFe and Al in the eluate with all three methods and thecorrelation coefficients (r) were 0.942 and 0.920, respec-tively. It further confirmed the conclusion that the releasingof As can be mainly attributed to the release of Fe and Aloxides.

6 Recommendations and perspectives

The removal of metals by this study is quite efficient, butthe high cost of chelating reagents such as EDTA hasprecluded its field application. Since the adsorptionmechanism between the metals and the specific soilfractions (exchangeable, carbonate, Fe/Mn oxides, organicmatter, and residual) differs, the metal removal capabilityfrom individual soil phases using chelating agents alsodiffers. And when the soil properties change, the washingresults may be different. The lack of understanding thechemistry of soil metal speciation, interparticle extractiondynamics and spent extractant recycling techniques have

limited this promising technology to small scale applica-tions. To keep treatment costs low, more detailed studies onthe behavior of the individual fractions during the extrac-tion process are necessary, and the selectivity betweentarget heavy metals and chelators, and the reuse andrecoverability of chelators will be the major task for furtherstudies.

Acknowledgments The authors wish to thank the Ministry ofEnvironmental Protection of the People's Republic of China forgranting a special project (National Survey of Soil Status andPollution Control. Project ID: 1440800011) to support this research.

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