Journal of Hazardous Materials 262 (2013) 464471

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Journal of Hazardous Materials

jou rn al hom epage: www.elsevier.com/locate/jhazmat

Chelant extraction of heavy metals from contaminated soils using new selective EDTA derivatives

Tao Zhang a , Jun-Min Liu b , Xiong-Fei Huang a , Bing Xia a , Cheng-Yong Su b , Guo-Fan Luo a , Yao-Wei Xu b , Ying-Xin Wu a , Zong-Wan Mao c , Rong-Liang Qiu a,d,

a School of Environmental Science and Engineering, Sun Yat-sen University, Guangzhou 510275, PR Chinab MOE Key Laboratory of Bioinorganic and Synthetic Chemistry, State Key Laboratory of Optoelectronic Materials and Technologies, School of Chemistry andChemical Engineering, Sun Yat-Sen University, Guangzhou 510275, PR Chinac MOE Key Laboratory of Bioinorganic and Synthetic Chemistry, School of Chemistry and Chemical Engineering, Sun Yat-Sen University, Guangzhou 510275, PR Chinad Guangdong Provincial Key Laboratory of Environmental Pollution Control and Remediation Technology, Guangzhou 510275, PR China

h i g h l i g h t s

Two EDTA derivatives were synthesized to enhance the selectivity of chelators. PDTA had the highest stability constants for Cu2+ and Ni2+ . PDTA had the highest overall selectivity for trace metals over major cations.

a r t i c l e i n f o

a b s t r a c t

Article history:Received 13 April 2013Received in revised form 30 July 2013Accepted 26 August 2013Available online 1 September 2013

Keywords:EDTA derivatives Potentiometric titration Cu(II)SelectivityExtraction

Soil washing is one of the few permanent treatment alternatives for removing metal contaminants. Ethylenediaminetetraacetic acid (EDTA) and its salts can substantially increase heavy metal removal from contaminated soils and have been extensively studied for soil washing. However, EDTA has a poor utilization ratio due to its low selectivity resulting from the competition between soil major cations and trace metal ions for chelation. The present study evaluated the potential for soil washing using EDTA and three of its derivatives: CDTA (trans-1,2-cyclohexanediaminetetraacetic acid), BDTA (benzyldiaminete- traacetic acid), and PDTA (phenyldiaminetetraacetic acid), which contain a cylcohexane ring, a benzyl group, and a phenyl group, respectively. Titration results showed that PDTA had the highest stability constants for Cu2+ and Ni2+ and the highest overall selectivity for trace metals over major cations. Equi- librium batch experiments were conducted to evaluate the efcacy of the EDTA derivatives at extracting Cu2+ , Zn2+ , Ni2+ , Pb2+ , Ca2+ , and Fe3+ from a contaminated soil. At pH 7.0, PDTA extracted 1.5 times more Cu2+ than did EDTA, but only 75% as much Ca2+ . Although CDTA was a strong chelator of heavy metal ions, its overall selectivity was lower and comparable to that of EDTA. BDTA was the least effective extractant because its stability constants with heavy metals were low. PDTA is potentially a practical washing agent for soils contaminated with trace metals 2013 Elsevier B.V. All rights reserved.

1. Introduction

Heavy metal contamination of soils, resulting from rapid indus- trialization, increased urbanization, modern agricultural practices, and inappropriate waste disposal methods, has become a serious problem worldwide [13]. The available remediation technologies for heavy metal-contaminated soils are mainly divided into two

Corresponding author at: School of Environmental Science and Engineering, SunYat-sen University, Guangzhou 510275, PR China. Tel.: +86 20 84113454;fax: +86 20 84113616.E-mail address: eesqrl@mail.sysu.edu.cn (R.-L. Qiu).

groups: namely immobilization, such as in situ chemical xation, and separation, such as soil washing. Chelant-enhanced soil wash- ing is a technology that is potentially useful for the economically feasible remediation of contaminated soils [48].The chelating agent ethylenediaminetetraacetic acid (EDTA) and its salts have been extensively studied for their potential use in soil washing [912]. They have been reported to appreciably increase the dissolution and mobilization of cationic heavy metals [13,14]. EDTA has low biodegradability in soil and a high efciency of metal extraction through the formation of thermodynamically sta- ble and soluble metalEDTA complexes [15,16]. In addition, recent advances in recovery and recycling techniques of used EDTA have enhanced its appeal [17].

0304-3894/$ see front matter 2013 Elsevier B.V. All rights reserved.http://dx.doi.org/10.1016/j.jhazmat.2013.08.069

Soil properties.

SoilpHOM (%)CEC (cmol kg1 )Sand (%)Silt (%)Clay (%)Cu (mg kg1 )Ni (mg kg1 )Pb (mg kg1 )Zn (mg kg1 )

GD7.35.19.5613110138826507236

466T. Zhang et al. / Journal of Hazardous Materials 262 (2013) 464471

A majority of the literature focused on demonstrating the reme- diation capabilities of EDTA has found that extraction of heavy metals was faster and more complete with increased quantities of added EDTA. Competition between the major cations of the soil (e.g., Ca2+ , Mg2+ , and Fe3+ ) and minor cations for chelation by EDTA may be one of the factors affecting the efciency of trace metal extraction [1823]. As illustrated in previous studies [24,25], when Ca solubility in calcareous soils is raised, the effectiveness of EDTA extraction is diminished signicantly, increasing the cost of reme- diation. For non-calcareous soils, Fe and Al dissolution may be more crucial, in view of their high tendency for complexation (i.e., large stability constants). Excessive addition of chelating agent can cause extensive dissolution of soil minerals and organic matter, leading to alteration of soil physical and chemical properties and even dis- integration of soil structure, which could render the soil unt for future use for vegetation or construction. Therefore, there has been a growing need to develop highly selective chelating agents for the extraction of heavy metal ions from polluted soils.In previous studies, EDTA has been modied to improve its selectivity in chelating target metal ions [2628]. Highly selective EDTA derivatives have a wide range of application in the elds of analytical chemistry, biology and medicine, as well as in many industrial processes. In this study, two new EDTA derivatives were designed and synthesized with the goal of enhancing its selectivity as a chelating agent. These EDTA derivatives contain a phenyl or benzyl group directly bonded to the nitrogens of the ethylenedi- amino group, and thus potentially are more sterically constrained than the parent compound. The objective of this study was to char- acterize these EDTA derivatives in aqueous solution and assess their potential as selective washing agents. Batch experiments were con- ducted to determine their efcacy in the simultaneous extraction of trace metal ions and major cations from contaminated soils and to investigate the extraction mechanisms.

2. Materials and methods

2.1. Soil Characteristics

Soil samples were collected from 0.7 to 1.7 m below the ground surface at a demolished industrial site in South China, air-dried atroom temperature (2030 C), and passed through a 2 mm sieve.The soil properties in Table 1 were the average of three replicates.Various soil physical and geochemical characterization tests were carried out. The physical and chemical characteristics of the soil are shown in Table 1. The metal concentrations in soil were determined by acid digestion with HFHClO4 HNO3 and induc- tively coupled plasma optical emission spectrometry (ICP-OES) measurement (5300DV, Perkin Elmer). Soil pH was determined using a 1:5 soil-to-water ratio and pH meter. The CEC of the sample

was determined by the ammonium acetate/sodium acetate method[29]. Organic matter content was determined by heating the dried samples at 350 C for 5 h [30]. The particle size distribution wasestablished by mechanical sieving, followed by the hydrometer method [31].

2.2. Synthesis of EDTA derivatives

Phenyldiaminetetraacetic acid (PDTA) was synthesized as illus- trated in Fig. 1 according to a previously published procedure [32]. A solution of 1,2-diaminobenzene (1.1 g, 10 mmol), ethyl bro- moacetate (6.7 mL, 60 mmol), sodium iodide (1.3 g, 8.5 mmol), and diisopropylethylamine (8.3 mL 50 mmol) in 10 mL acetonitrile was reuxed under nitrogen for 7 h, then cooled and poured into 50 mL water. The resulting mixture was extracted with dichloromethane(3 40 mL). The extract was dried and was concentrated to givebrown oil, which was puried by ash chromatography usingpetroleum ether/ethylacetate (10:1, v/v) as eluant. The product obtained was 2.1 g (45%) of a white solid (A) (Fig. 1); a mixtureof ethanol, complex A, and NaOH (14.0 M) were reuxed at 78 Cfor 24 h. The solution was cooled to room temperature, ethanol was evaporated, and the residue was washed with 6.0 M HCl solution and dried under high vacuum to give a white solid (B) (Fig. 1).Benzyldiaminetetraacetic acid (BDTA) was synthesized as illus- trated in Fig. 2. To a mixture of 1,2-bis(bromomethyl)benzene (1.0 mmol) in 5.0 mL dry CH3 CN was added diethyliminodiacetate (2.0 mmol) and anhydrous K2 CO3 (10 mmol). The resulting reac- tion mixture was stirred at room temperature for 5 h, then the solids were ltered off and solvent was removed under vacuum. The residue was again dissolved in ethyl acetate and washed with water and saturated NaOH solution, which was puried by ash chromatography using petroleum ether/ethylacetate (10:1, v/v) as eluant. Light yellowish oil (A) (Fig. 2) was obtained by evaporation of the organic phase. Then the residue was dissolved in the solu- tion of KOH. The mixture was stirred at room temperature for 24 h, and then resulting mixture was poured into ice water, after whichit was acidied with concentrated HCl (pH 1) to obtain a whitesolid (B) (Fig. 2).

2.3. Potentiometric titration

An automatic titrator (Metrohm 702GPD Titrino) coupled to a Metrohm electrode was used and calibrated according to the Gran method [33,34]. The electrode system was calibrated with buffers and checked by titration of HClO4 with NaOH solution(0.01 M). The thermostated cell contained 25 mL of 1.0 mM species (free lig- and/metal ligand complex) in aqueous solution with ionic strength maintained at 0.10 M by potassium nitrate. All titrations were car-ried out on the aqueous solutions under nitrogen at 298 0.1 K, and

Fig. 1. Synthesis of PDTA.

Fig. 2. Synthesis of BDTA.

initiated by adding xed volumes of 0.10 M standard NaOH in small increments to the titrated solution. Triplicate measurements were performed, for which the experimental error was below 1%. The titration data were tted from the raw data with the Hyperquad2000 program to calculate the log and the pKa values of species[3538].

2.4. Batch experiments

In the batch experiments, contaminated soil was mixed with a measured volume of chelating agent solution in 50-mL polyethyl- ene tubes. To probe the inuence of EDTA derivative concentration on metal mobility in the washed soil, every 1.000 g of soil was mixed with 20 mL chelating agent solution of different concentrations (0.005 M, 0.01 M, 0.02 M, 0.05 M and 0.1 M) in tubes. The tubes were agitated using a thermostatic shaker at 180 rpm at room tempera-ture (25 2 C) for a given time. The suspensions were centrifugedat 5000 rpm for 10 min and the supernatants were then lteredthrough a 0.45 m membrane. All solutions were stored in amber vials at 4 C prior to analysis.Another two sets of batch equilibrium experiments were con- ducted to study the inuence of solution pH and liquid-to-soil ratio on metal extraction by the EDTA derivatives. An initial chelator concentration of 0.02 M was used in both sets. In the rst set, theliquid-to-soil ratio was 20:1 (i.e., 20 mL g1 soil) while the solutionpH were adjusted using NaOH or HNO3 to pH values of 3.0, 4.0, 5.0,6.0, 7.0, 8.0, 9.0, 10.0, 11.0, and 12.0. In the second set, the solu- tion pH was kept at pH 6.0 while ve liquid-to-soil ratios of 2.5:1(2.5 mL g1 ), 5:1 (5 mL g1 ), 10:1 (10 mL g1 ), 20:1 (20 mL g1 ), and40:1 (40 mL g1 ) were studied. These ratios were achieved using aconstant solution soil mass of 1.0 g.

2.5. Analytical methods

The 1 H NMR spectra of EDTA derivatives was obtained by Mercury-Plus 300 1 H Nuclear Magnetic Resonance (VARIAN, USA). We determined the molecular weight of PDTA and BDTA as 339 and364, respectively, with electro spray ionization mass spectroscopy (negative ion mode). The heavy metals of concern (Cu, Ni, Zn and Pb) and soil component elements (Ca and Fe) were measured by induc- tively coupled plasma optical emission spectrometry (ICP-OES).

3. Results and discussion

3.1. Acidbase properties of EDTA derivatives

The protonation constants of PDTA and BDTA in water were determined with a view to assessing their acidbase properties, since these properties control what species exist in solution at various pH values. Both PDTA and BDTA have six potential sites that can bind with a proton, including the two nitrogens of the ethylenediamino groups and the four carboxyl groups. However, only three deprotonation events were observed by potentiometry (Table 2). For comparison, Table 2 also includes the literature val- ues and determined values for EDTA. The agreement of the obtained

pKa values with the protonation constants of EDTA (2.67, 6.16, and10.26) is acceptable.The pKa1 value reects the acidity of the carboxyl groups, and the pKa2 and pKa3 values characterize the basicity of the amino nitrogens. The pKa2 and pKa3 values of CDTA and BDTA were similar to those of EDTA. The pKa3 value of CDTA was higher as a result of the electron-donating effect of the cyclohexane ring attached to the amino nitrogens.The obtained pKa1 value of PDTA (2.79 0.01), correspondingto the protonation of carboxyl groups, was lower than that ofEDTA (3.05 0.02). The lower pKa1 value of PDTA indicates that theacidity of the carboxyl groups was increased due to the electron-withdrawing effect of the aromatic group.Similarly, PDTA had lower pKa2 and pKa3 values in comparison with EDTA, indicating that the basicity of the nitrogen atoms is signicantly lower for PDTA relative to EDTA [39]. This may be attributed to the lower electronic density of the nitrogens in the former due to the presence of a phenyl group, since the electron- withdrawing effect is increased when an aromatic group is directly bonded to nitrogens of an ethylenediamino group [32].In total, three deprotonation events were observed for the free ligands PDTA and BDTA (Fig. 3 and Table 2). At pH < 4.0, the

Fig. 3. Distribution plots of species of the free ligands PDTA (1.0 mM) and BDTA (1.0 mM).

Table 2Deprotonation constants (pKa ) of EDTA, CDTA, PDTA, and BDTA.

EquilibriumEDTAaCDTAaEDTAbPDTAbBDTAb

H3 Y /H2 Y2 (pKa1 )2.673.533.05 0.022.79 0.013.56 0.03

H2 Y2 /HY3 (pKa2 )6.166.156.42 0.023.70 0.028.31 0.05

HY3 /Y4 (pKa3 ) 10.26 12.40 10.50 0.03 5.95 0.03 10.00 0.06 a Literature values.b Determined values.

468T. Zhang et al. / Journal of Hazardous Materials 262 (2013) 464471

percentage of species H3 Y/H2 Y2 was higher for BDTA than forPDTA, which is consistent with the pKa2 values of BDTA and PDTA. Similarly the percentage of BDTA existing as the species HY3 (i.e.,with one nitrogen deprotonated) at pH < 5.0 was approximately zero, in accordance with the high pKa3 values of BDTA.

3.2. Selectivity of EDTA derivatives

The metalligand (ML) complexation equilibrium constant, KML , expresses the ligands afnity toward the target metal (reaction (I)).

M2+ 2 ML + 2K+ H2 Y 2H + MY (I)

The equilibrium constants of PDTA and BDTA complexes with a range of metals were determined by pH potentiometric titration, with the calculated results for the [MPDTA] complex summa- rized in Table 3. The [MBDTA] complex, however, did not exist in distribution plots of species, except for CdBDTA(OH), which demonstrated that MBDTA was not thermodynamically stable (Table 4). We deduced that the structure of BDTA allowed it to form

Table 3Equilibrium constants (log KML ) of EDTA and derivatives in aqueous solution.

MetalsEDTAaCDTAaPDTAbPb(II)17.8820.2420.30 0.05Cu(II)18.7021.9224.84 0.07Cd(II)16.3619.8418.18 0.03Zn(II)16.4419.3517.99 0.09Ni(II)18.5220.2023.28 0.05Hg(II)21.5024.7919.73 0.09Ca(II)10.6113.1511.25 0.05 Mg(II) 8.83 11.07 10.41 0.08 a Literature values.b Determined values.

a seven-membered ring complex with most metal ions, but not a stable chelate compound with a ve-membered or six-membered ring.Table 3 also shows that PDTA has higher selective capability for Cu2+ and Ni2+ , relative to Cd2+ , Zn2+ , Ca2+ , and Mg2+ , than do the conventional chelators EDTA and CDTA. The differences in the log KML values for Cu2+ and Ni2+ complexes with PDTA, CDTA, and

Fig. 4. Extraction efciency of different concentrations of chelating agents (liquid/solid = 20; contact time, 24 h; temperature, 25 C).

Table 4Equilibrium constants of MBDTA in aqueous solution at 298 0.1 K.

Species Equilibrium constants

log log KMLCdL 7.41 0.08

Table 5pKtarget and SR values of EDTA and EDTA derivatives.Chelating agents pKtarget SR EDTA 18.23 1.88CDTA 21.06 1.74PDTA 20.72 2.11CdL(OH) 4.67 0.07 3.57 CaL(OH) 0.35 0.02 MgL(OH) 0.30 0.05 ZnL(OH) 1.28 0.04 NiL(OH) 2.27 0.04 PbL(OH) 1.89 0.02 HgL(OH) 0.90 0.05

The selectivity of chelating agents toward heavy metals can be quantitatively computed on the basis of the selectivity ratio (SR), which is dened as: pKtarget CuL(OH)

SR =

pKambient

(3)

EDTA are related to the steric constraints created by the benzene ring in PDTA and the cyclohexane ring in CDTA [40] that are not

According to formula (3), a strong chelator will have large pKtarget (>12) and pKambient values, whereas a strong and selec- tive chelator toward heavy metals will have a large pKtarget and a relatively small pKambient , and thus a large SR value (>2) [42].present in EDTA. Because the radii of Ni2+ and Cu2+ are similar,

The pKtarget

and SR were calculated and summarized in Table 5,PDTA does not exhibit good selectivity between the two [41].In evaluating EDTA derivatives, we determined the equilibrium constants (log KML ) of PDTA for complexation with six divalent tar- get metals, including Pb, Cu, Cd, Zn, Ni, and Hg [42], and with two ambient divalent cations, Ca and Mg.pKPb + pKCu + pKCd + pKZn + pKNi + pKHg

which may serve to guide in the selection of chelators that are both effective and selective toward the six target metals. The results demonstrated that CDTA and PDTA were strong chelators, each having as large a pKtarget (>12) as EDTA. More interestingly, PDTA, compared with EDTA and CDTA, had a larger SR value (>2), showing that it has a higher selectivity toward heavy metals.pKtarget =

pKambient =

(1)6

pKCa + pKMg (2)2

3.3. Comparison of heavy metal removal with different chelators

3.3.1. Inuence of concentrationEDTA and its derivatives in a range of concentrations were used to extract the heavy metals Cu, Ni, Pb, and Zn from a contaminated

Fig. 5. Extraction efciency using chelator solutions with different liquid/soil ratios (contact time, 24 h; temperature, 25 C).

Fig. 6. Inuence of pH on extraction of metals using chelators (initial concentration, 0.02 M; liquid/solid = 20; contact time, 24 h; temperature, 25 C).

soil. The extent of removal of the four metals increased with increasing concentration of the chelators from 0.005 to 0.10 M, but the rate of increase for most metalchelator combinations was less steep above a chelator concentration of 0.02 M (Fig. 4). At a chela- tor concentration of 0.02 M, the extraction of Cu was 62% by PDTA,59% by CDTA, 48% by EDTA, and 8.6% by BDTA, while at the high- est chelator concentration (0.10 M), these values increased to only73%, 65%, 60%, and 10%, respectively. Over the entire concentra- tion range, addition of PDTA caused the extraction increase slightly compared with EDTA due to the low concentration of Ni in the soil, from 4.4% to 15% by PDTA and from 2.2% to 11% by EDTA, respec- tively. Pb extraction by PDTA increased from 2.0% to 10%, while the corresponding Pb values for EDTA were much higher, increasing from 8.1% to 21%. Zn showed a trend similar to that of Pb, with extraction values reaching 13% for PDTA and 28% for EDTA at the highest chelator concentration. In light of these results, a chelator concentration of 0.02 M was selected for further studies.As the chelator concentration was increased, only a small frac- tion of the added chelator was incorporated into metalchelant complexes, while the excess remained in the free form or was com- plexed with other cations (Ca, Mg, Fe, Al, etc.) [43]. Similarly, when EDTA is used for soil washing, not all EDTA added to soil binds the target metals, because other ions in the soil such as Ca and Fe also interact with EDTA due to their high concentrations and the relatively high stability of their complexes [44]. Above 0.02 M, PDTA was the most efcient of the tested chelators at extracting Cu from soil. This result can be attributed not only to stronger chela- tion of Cu by PDTA compared to the other chelators, but also to the weaker chelation between PDTA and other soil cations (Ca, Fe, etc.) and heavy metals, as evidenced by the pKML constants of theircomplexes (Table 3). In fact, PDTA had the second lowest Pb and

Zn extraction efciencies, only slightly higher than those of BDTA, which is unable to form a stable chelate compound with a ve- or six-membered ring.

3.3.2. Inuence of liquid-to-soil ratioAt a constant chelator concentration of 0.4 mmol g1 soil, theextraction efciency of the target metals was measured as the liquid-to-soil ratio increased from 2.5:1 to 40:1. For EDTA and CDTA, increasing the liquid-to-soil ratio over this range generally increased the efciency of extraction of Cu (from 21% to 54% by EDTA and from 24% to 61% by CDTA), Ni (from 0 to 12% by EDTA and from 0% to 10% by CDTA), Pb (from 8% to 18% by EDTA and from

Fig. 7. Distribution plots of species of 1.0 mM Cu:PDTA as a function of pH at 0.10 M KNO3 and (298 0.1) K.

Fig. 8. Solubilization of Ca and Fe by EDTA and EDTA derivatives (a: initial concentration, 0.02 M; liquid/solid = 20; contact time, 24 h; temperature, 25 C. b: initial pH of 6.0;liquid/solid = 20; contact time, 24 h; temperature, 25 C).

6% to 17% by CDTA), and Zn (from 8% to 17% by EDTA and from 6% to24% by CDTA), respectively (Fig. 5). However, for PDTA and BDTA, extraction of the four metals was found to be independent of the liquid-to-soil ratio.The extraction efciency, as determined by the mole ratio of extracted potentially toxic trace metals to chelant inputs [45], increased with increasing liquid-to-soil ratio because a larger amount of organic matter could be dissolved from a greater mass of soil [8,43]. In addition, Cu has a high afnity toward organic mat- ter, and the hydrophobic aromatic groups present in PDTA may have facilitated soil organic matter dissolution, making Cu quickly extractable and independent of the liquid:soil ratio.

3.3.3. Inuence of solution pHAs shown in Fig. 6, extraction efciency was highest at pH 3.0, and generally decreased as solution pH was increased from pH 3.0 to pH 12.0. At pH 12.0, 24% of Cu was removed by PDTA but almost none was removed by EDTA and BDTA.The important mobilization processes of heavy metals in soil include: (1) acidication, (2) competitive adsorption of other metal ions or anions, (3) reductive and non-reductive dissolution of the solid phase, and (4) complexation of metal ions by ligands [46]. Metals bound to soil hydrous oxides can often be retrieved simply by lowering pH because protons can promote oxide dissolution. Hydrogen ions are also weak competitive cations which can replace the adsorbed heavy metals via a cation exchange mechanism, andas the H+ ion concentration increases, the particle surface generallybecomes increasingly protonated and acquires a positive charge, thus promoting desorption of metals [13]. Therefore this is why heavy metal extraction is higher at lower values of solution pH. In addition, the pH-potentiometric titration results showed that the equilibrium species of Cu at a 1:1 metal:PDTA ratio are pri- marily ML, ML(OH), and ML(OH)2 (Fig. 7). The concentration of free Cu2+ was very low in aqueous solution, and Cu is almost completely complexed above pH 3.5. As pH is increased, one water molecule is deprotonated but remains in the inner coordination sphere of the complexed Cu2+ , forming a stable chelate compound, which explains why even at a solution pH of 12.0, 24% of Cu was removed by PDTA. In practice, however, the pH is normally controlled within the range of 5.09.0 in order to eliminate adverse changes in the soils chemical and physical structure brought about by too acidic or alkaline a solution [14]. As pH increased from 5.0 to 9.0, the extrac- tion efciency of Cu by PDTA decreased from 72% to 56%, but the decrease was from 52% to 35% for EDTA. Thus, at constant pH, PDTA is more efcient at extracting Cu from soil than is EDTA.

3.3.4. Solubilization of major soil mineral elementsThe solubilization of soil mineral elements (i.e., Ca, and Fe) by EDTA and its derivatives was also measured (Fig. 8). EDTA solubi- lized a large amount of Ca from the soil, with values that decreasedfrom 13 400 mg kg1 to 4510 mg kg1 as pH rose from pH 4.5 to pH9.0, while the amount of Ca solubilized by PDTA was considerably lower (10 500 mg kg1 to 2900 mg kg1 over the same pH range).Similarly, at pH 6.0 and a chelator concentration of 0.05 M, EDTAsolubilized more Fe than did PDTA, with values of 227 mg kg1 and102 mg kg1 , respectively.Some researchers have found that co-dissolution of soil Ca is an important factor that can result in a low degree of heavy metal complexation by EDTA [20,21]. Another study showed that Ca2+ is the main competitive cation, because CaCO3 is strongly dissolved in the EDTA leaching solution under acidic conditions [20,25]; thus, concentrations of Ca2+ in the leaching solution are very high com- pared to those of the target heavy metals. However, at pH 7.0, PDTA solubilized 25% less Ca and extracted 54% more Cu from the con- taminated soil than did EDTA (Figs. 6 and 8). These results can be partly attributed to the low metal selectivity of EDTA, caus- ing potential chelation of all of the exchangeable cations present in the soil [18], and partly to the selective complexing ability of PDTA for Cu. Thus, the minerals present in soil may not interfere with PDTA extraction of Cu, conrming the potential applicabil- ity of PDTA for remediation of heavy metal-contaminated soil containing high concentrations of mineral elements such as Ca and Fe.

4. Conclusions

Two new EDTA derivatives, BTDA and PDTA, were synthesized and their metalligand complexation equilibrium constants and selective capabilities in aqueous media were investigated, along with those of EDTA and CDTA. Titration results showed that PDTA had the highest stability constants for Cu and Ni and the highest overall selectivity for trace metals over major cations. Equilibrium batch experiments were conducted to evaluate the efcacy of the EDTA derivatives at extracting Cu, Zn, Ni, Pb, Ca, and Fe from a con- taminated soil. At pH 7.0, PDTA extracted 1.5 times more Cu than did EDTA, but only 75% as much Ca. Although CDTA was a strong chelator of heavy metals, its overall selectivity was lower and com- parable to that of EDTA. BDTA was the least effective extractant because its stability constants with heavy metals were low. PDTA is potentially a practical washing agent for soils contaminated with trace metals, especially Cu.

Acknowledgements

The project was supported by National Natural Science Foun- dation (No. 41171374), National Funds for Distinguished Young Scientists of China (No. 41225004), Guangdong Province Higher Vocational Colleges & Schools Pearl River Scholar Funded Scheme, the Ministry of Environmental Protection of China (No. 201109020) and the Research Fund Program of Guangdong Provincial Key Laboratory of Environmental Pollution Control and Remediation Technology (No. 2011K0007).

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