Chemosphere 109 (2014) 1–6 Contents lists available at ScienceDirect Chemosphere journal homepage: www.elsevier.com/locate/chemosphere Technical Note Influence of the selective EDTA derivative phenyldiaminetetraacetic acid on the speciation and extraction of heavy metals from a contaminated soil Tao Zhang a,b, Hang Wei b, Xiu-Hong Yang b,c,⇑, Bing Xia b, Jun-Min Liu a, Cheng-Yong Su a,⇑, Rong-Liang Qiu b,c a MOE Key Laboratory of Bioinorganic and Synthetic Chemistry, State Key Laboratory of Optoelectronic Materials and Technologies, School of Chemistry and Chemical Engineering, Sun Yat-sen University, Guangzhou 510275, PR China b School of Environmental Science and Engineering, Sun Yat-sen University, Guangzhou 510275, PR China c 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 PDTA significantly enhanced extraction of Cu from contaminated soil. Complexation-promoted and dissolution of SOM were the mechanisms for Cu extraction. 2+ High selectivity of PDTA for Cu a r t i c l e avoided unwanted dissolution of soil minerals. i n f o Article history: Received 21 October 2013 Received in revised form 14 February 2014 Accepted 15 February 2014 Keywords: Chelating agent Soil washing Dissolution Soil organic matter Heavy metal speciation a b s t r a c t The development of more selective chelators for the washing of heavy metal contaminated soil is desirable in order to avoid excessive dissolution of soil minerals. Speciation and mobility of Cu, Zn, Pb, and Ni in a contaminated soil washed with phenyldiaminetetraacetic acid (PDTA), a derivative of EDTA, were investigated by batch leaching test using a range of soil washing conditions followed by sequential extraction. With appropriate washing conditions, PDTA significantly enhanced extraction of Cu from the contaminated soil. The primary mechanisms of Cu extraction by PDTA were complexation-promoted dissolution of soil Cu and increased dissolution of soil organic matter (SOM). PDTA showed high selectivity for Cu(II) over soil component cations (Ca(II), Mg(II), Fe(III), Mn(II), Al(III)), especially at lower liquid-tosoil ratios under PDTA deficiency, thus avoiding unwanted dissolution of soil minerals during the soil washing process which can degrade soil structure and interfere with future land use. PDTA-enhanced soil washing increased the exchangeable fractions of Cu, Zn, and Pb and decreased their residual fractions, compared to their levels in unwashed soil. Ó 2014 Elsevier Ltd. All rights reserved. 1. Introduction The available technologies for remediating heavy metal -contaminated soils are largely divided into two groups: immobilization (e.g., in situ chemical fixation) and separation, which includes soil washing (Khodadoust et al., 2004). Soil washing separates contaminants from the bulk soil in either or both of the following ways: solubilizing contaminants using chelating ⇑ Corresponding authors. Address: School of Environmental Science and Engineering, Sun Yat-sen University, Guangzhou 510275, PR China. Tel.: +86 20 8411 3454; fax: +86 20 8411 3616 (X.-H. Yang). E-mail address: [email protected] (X.-H. Yang). http://dx.doi.org/10.1016/j.chemosphere.2014.02.039 0045-6535/Ó 2014 Elsevier Ltd. All rights reserved. agents or acid, or concentrating contaminants into a small volume of soil through particle size separation (FRTR, 1994; Maturi and Reddy, 2008). Among the chelating agents that can be used to solubilize contaminants, EDTA presents the following advantages: a low degree of biodegradability in groundwater (Mulligan et al., 2001) and soil (Sims et al., 1984; Peters, 1999; Abumaizar and Smith, 1999), and a high capacity for complexing with heavy metals (Sims et al., 1984; Tejowulan and Hendershot, 1998; Martinez and Motto, 2000). Thus, EDTA is a promising washing agent for metal-contaminated sites. However, a significant part of heavy metals usually remains at aged sites in washed soils, especially if the soil is rich in organic matter or clay minerals, inasmuch as these soil solid phases often 2 T. Zhang et al. / Chemosphere 109 (2014) 1–6 have a strong affinity for the target heavy metals (Reddy and Chinthamreddy, 2000; Pichtel et al., 2001; Finzgar and Lestan, 2007; Zhang et al., 2008; Zou et al., 2009). As reported in previous studies, in aged contaminated sites where heavy metals are primarily bound to oxides or organic matter, EDTA-promoted dissolution can play a substantial role in the metal removal (Yip et al., 2009). EDTA-promoted dissolution from oxides occurs in two steps: fast adsorption of free or complexed EDTA onto specific surface sites via surface complexation, which can destabilize metal–oxygen bonds in the mineral structure, followed by ratelimiting metal detachment from the oxide. If heavy metals are present in chemically stable mineral forms or bound to non-labile soil fractions, they are less mobile and less bioavailable, and hence less toxic. The mobility and bioavailability of these heavy metals in washed soils may change with different combinations of washing conditions, which have been neglected in most research on soil washing with EDTA, although EDTA has been proven to effectively increase the availability of heavy metals in the soil phase when being used to enhance the phytoextraction efficiency of potential metal-accumulators. In fact, enhanced mobility of heavy metals and nutrient deficiency have been suggested as major concern about EDTA application by technical meetings of European Union Member State Representatives (ECB, 2004). At high concentrations, EDTA has been found to dissolve indigenous oxides, carbonates, and organic matter, and to appreciably alter both the physical structure and chemical properties of soils (Tsang et al., 2007), which could render the soil unfit for future use for vegetation or construction. However, if too dilute, EDTA is unable to release the majority of the labile fractions of metals. Therefore, there is a need to develop highly selective chelating agents for the extraction of heavy metal ions from polluted soils. In earlier work comparing the soil washing abilities of EDTA with three of its derivatives, we showed that phenyldiaminetetraacetic acid (PDTA), which contains a phenyl group, had the highest stability constants for Cu and Ni and the highest overall selectivity for trace metals over major soil cations (Zhang et al., 2013). To avoid chelator-induced mental movement into groundwater, there is a need for further research on how metal mobility and speciation are influenced by chelating agent concentration and liquid-to-soil ratio. Therefore, the objective of this study is to investigate the influences of soil-to-solution ratio and PDTA concentration on the mobility and speciation of heavy metals in a contaminated soil subjected to PDTA-enhanced soil washing. 2. Materials and methods 2.1. Materials and soil characteristics PDTA used in this research was synthesized by our group according to a previously described method (Wang and Qian, 2006). Pure water obtained using a Milli-Q system was used for the preparation of all solutions. The studied soil was collected from 0.7 to 1.7 m below the ground surface at a demolished industrial site in the north of Guangzhou city, China, air-dried at room temperature (20–30 °C), and passed through a 2-mm sieve. The soil was comparable to sandy loam (61% of sand, 30% of silt, 9% of clay by mass) according to particle size distribution obtained by sieving and hydrometer methods, and organic matter content was determined by heating the dried samples at 350 °C for 5 h (Ball, 1964). The soil pH was 7.3, measured at a 1:5 soil-to-water ratio. The cation exchange capacity (CEC) of the soil was 9.5 cmol kg 1, as determined by NH4–Na exchange (Van Reeuwijk, 1992). The metal concentrations in soil were determined by acid digestion with HF–HClO4–HNO3 and inductively coupled plasma optical emission spectrometry (ICP-OES) (5300DV, PerkinElmer). The physical and chemical characteristics of the soil are shown in Table 1. 2.2. Batch experiments In the batch experiments, 1 g of contaminated soil was mixed with a measured volume of chelating agent solution at pH 6.0 in 50-mL polyethylene tubes and shaken at 180 rpm in a thermostatic shaker at room temperature (25 ± 2 °C) for 2 h. To probe the influence of PDTA concentration and liquid-to-soil ratio on metal mobility, a range of chelating agent concentrations (0.5–20 mM) and liquid-to-soil ratios (5:1 to 20:1) was used. Detailed operating parameters are listed in Table 2. The washing solution and soil were separated by centrifuging at 5000 rpm for 10 min, and heavy metals of concern (Cu, Ni, Pb, and Zn) and soil component elements (Ca, Fe, Mg, Al, and Mn) in the supernatant were measured by ICPOES. In order to investigate the effect of the PDTA soil washing treatments on heavy metal speciation, metal species in the washed soil were fractionated by a traditional sequential extraction scheme (detailed extraction reagents and operating conditions are listed in Table SM-1 in Supplementary Material (SM)). All experiments were performed in at least triplicate. 3. Results and discussion 3.1. Heavy metal removal by PDTA washing Fig. 1 shows the removal efficiencies of Cu, Ni, Pb, and Zn for the different PDTA washing treatments. As a whole, the extraction efficiencies of these metals were low. One potential reason was that for most of the treatments (Exp. 1–6), the mass of added chelating agent was less than the stoichiometric requirement for 1:1 metalchelating agent complexation which was about 2 mM PDTA at a 20:1 solution:soil ratio. Moreover, the strong bonding between these metals and soil particles, which are rich in clay minerals and organic matter in the aged site, can result in low metal removal. As shown in Fig. 1, the removal efficiencies of the target metals except Pb generally increased with decreasing liquid-to-soil ratio. With the same dosage of chelator, decreasing the liquid-to-soil ratio meant the chelating agent concentration was increased. The use of a high concentration of chelating agent can increase the extraction efficiency of trace metals, especially for Cu. In general, the metal-chelating agent complex is the dominant mechanism for most cationic metal removal, and this process often depends on chelating agent concentrations, especially when the amount of chelating agent is insufficient for complete complexation (Peters, 1999). Under PDTA-deficiency conditions (Exp. 1–6), decreasing the liquidto-soil ratio from 20:1 to 5:1 increased the extraction efficiency of Cu from 3% to 5% and from 4% to 9% when the mass of chelating agent was 0.01 and 0.02 mmol g 1 soil, respectively. Similar results Table 1 Characteristics of the studied soil. Soil properties Value pH Organic matter content (%) CEC (cmol kg 1) Sand (%) Silt (%) Clay (%) Metal content (mg kg 1) Cu Ni Pb Zn 7.3 5.1 9.5 61 30 9 1388 26 507 236 T. Zhang et al. / Chemosphere 109 (2014) 1–6 3 Table 2 Washing combinations of PDTA-enhanced soil washing. Index Liquid-to-soil ratio PDTA concentration (mM) Exp. Exp. Exp. Exp. Exp. Exp. Exp. Exp. Exp. 20:1 10:1 5:1 20:1 10:1 5:1 20:1 10:1 5:1 0.5 1.0 2.0 1.0 2.0 4.0 5.0 10.0 20.0 1 2 3 4 5 6 7 8 9 Fig. 2. Dissolution of soil organic matter during PDTA-washing. (e.g. Ni, Pb, Zn), Cu shows a stronger affinity for organic matter (Tandy et al., 2006), which suggested the dissolution of SOM from soils is another probable mechanism for Cu extraction by PDTA. 3.2. Dissolution of soil component elements during PDTA washing Fig. 1. Metal removal among different washing combinations (the operating conditions of Exp. 1–9 are listed in Table 2). have been obtained by varying the solution volume or the soil mass (Yan et al., 2010). As Cu-PDTA is a relatively strong complex (Zhang et al., 2013) and there was insufficient PDTA to complex with all the heavy metals adsorbed by soil particles, the readily extracted fraction of Zn and Pb were displaced from PDTA via metal exchange with adsorbed Cu (Tsang et al., 2009). With a higher PDTA-to-metal molar ratio (under the operating conditions of Exp. 7–9), the number of moles of PDTA available for extracting Cu from the soil increased, resulting in much higher extraction efficiency of Cu than other metals. Under excessive chelating dosage conditions, metal resorption was indiscernible, and enhanced soil mineral elements (such as Ca and Mg) dissolution might have influence on chelating agent speciation (Yip et al., 2009) due to the higher selective capability of PDTA for Cu(II) (logKCu-PDTA = 24.8), relative to Ca(II), and Mg(II) (logKCa-PDTA = 11.3, logKMg-PDTA = 10.4). Thus, the effects of liquid-to-soil ratio on metal extraction were statistically minor under sufficient PDTA. Previous studies have indicated that polyvalent metals ions may serve as cross-linking agents within the organic phase by binding to multiple functional groups from different strands of humic macromolecules (Yang et al., 2001; Tipping, 2002). As a consequence, SOM (soil organic matter)-bound heavy metals can also be removed from the soil along with the released SOM (McBride et al., 1997). The quantity of SOM released from the soil by PDTA washing increased with PDTA concentration when the solution:soil ratio was held constant (Fig. 2). Expressed as UV adsorption intensity, the dissolved SOM increased from 0.41 in Exp. 1 (0.5 mM PDTA) to 3.32 in Exp. 7 (5 mM PDTA). In contrast, the released SOM by distilled water was 0.039. Since the concentration of SOM is commonly found to be linearly correlated to UV absorption (APHA, 1998), it is evident that PDTA greatly enhances the release of SOM from the soil into solution. Compared to other heavy metals The concentrations of soil component elements in solution after PDTA washing are listed in Table 3. The most significant dissolution of Ca occurred during PDTA-enhanced soil washing, while comparatively few amounts of Fe, Mn, Al, and Mg were released. Similar phenomena have been observed for soil washing with EDTA (Theodoratos et al., 2000; Kim et al., 2003; Palma and Mecozzi, 2007; Polettini et al., 2007; Qiu et al., 2010). As chelating agent concentration is increased, the excess amount might form complexes with soil component elements, leading to the enhanced release of Al, Mn, and Fe from soil (Tsang et al., 2007). The amount of Ca decreased from 1031 to 96 mmol kg 1 when the PDTA concentration increased from 0.5 to 5 mM, while the amounts of Fe, Mn, Al, and Mg solubilized by PDTA increased with increasing concentration of PDTA. It has been reported that the Ca complex itself can mediate Fe and Al dissolution and that this reaction has slower kinetics than that of Ca dissolution (Nowack and Sigg, 1997). Consequently, Ca can be re-adsorbed on soil particles, while Fe and Al concentrations in the washing solution slightly increase (Palma, 2009). Under PDTA deficiency and at a constant dosage of PDTA, the amount of soil component elements solubilized by PDTA generally decreased with decreasing liquid-to-soil ratio, especially for Ca, and competition between Ca and heavy metals for chelation by PDTA may be present. Ca was dissolved initially due to proton-promoted dissolution in an acidic environment, since the initial pH of the PDTA solution was 6.0, lower than that of the soil (pH = 7.3). The dissolved Ca can quickly complex with PDTA (Palma and Ferrantelli, 2005), but an exchange reaction subsequently occurs between Ca-PDTA and Cu-PDTA, because the highest overall selectivity for trace metals over major cations (Zhang et al., 2013). Thus, the extraction efficiency of heavy metals increased, but the solubilization of soil component elements decreased with decreasing liquid-to-soil ratio under PDTA deficiency, which could avoid the destruction or alteration of the soil structure. 3.3. Effect of PDTA washing on metal speciation The metals in the soils washed in Exp. 1, 4, 5, 6, and 7 as well as in the unwashed soil were fractionated by sequential extraction to investigate how metal speciation is affected by PDTA-enhanced 4 T. Zhang et al. / Chemosphere 109 (2014) 1–6 Table 3 Release of soil component elements during PDTA-enhanced soil washing. Index Exp. Exp. Exp. Exp. Exp. Exp. Exp. 1 2 3 4 5 6 7 Concentration of soil componential elements in washing solution (mmol kg 1 ) Fe Mn Al Mg Ca 0.021 ± 0.004 0.007 ± 0.006 0.005 ± 0.001 0.055 ± 0.028 0.047 ± 0.018 0.011 ± 0.003 0.483 ± 0.019 0.045 ± 0.008 0.050 ± 0.008 0.030 ± 0.006 0.128 ± 0.015 0.080 ± 0.060 0.109 ± 0.098 0.241 ± 0.029 0.065 ± 0.045 0.006 ± 0.003 0.037 ± 0.004 0.368 ± 0.016 0.060 ± 0.013 0.072 ± 0.003 0.883 ± 0.029 3.453 ± 0.096 3.241 ± 0.041 3.364 ± 0.400 3.803 ± 0.132 3.724 ± 0.087 3.567 ± 0.934 4.269 ± 0.152 1030.7 ± 0.3 57.7 ± 0.2 57.4 ± 1.3 953.5 ± 1.4 1019.7 ± 3.7 83.3 ± 0.2 95.6 ± 0.1 soil washing. The metal fractions are defined in Table SM-1. The results presented in Fig. 3 are expressed as percentages of the individual metal contents in the unwashed soil. This soil was collected from a former industrial site that had been inactive for 10 year, wherefore most weakly bound exchangeable metals had been leached or transformed by weathering into more strongly bound forms, resulting in the very low proportions of all heavy metals of interest in the exchangeable fractions (S1), especially those of Ni and Pb. Ni, Pb and Zn in the unwashed soil were primarily bound to Fe/Mn oxides of low crystallinity (S3) or in the residual fraction (S5). The Cu in the unwashed soil was predominantly in the most stable fraction (S5), followed by the S4 fraction, the S3 fraction and the S2 fraction, while S1 was negligible. The low level of metals in S1 was an important reason for the generally low observed metal extraction efficiencies. In the other four fractions, metal speciation was found to vary between the different metals, suggesting that there were different mechanisms for metal retention in soils and metal removal by PDTA. The higher extraction efficiency of Cu compared to Ni, Pb, and Zn was likely attributable to the high Cu loading and more Cu in the weakly bound S2 and S3 fractions. Increased PDTA concentration generally decreased the S2 of Cu, Pb, and Zn in washed soil. Decreased liquid-to-soil ratio under PDTA deficiency increased the S2 of Cu but had little effect on the other metals due to the enhanced exchange reaction between Ca-PDTA and Cu-PDTA. When the metal in S3 fraction is extracted by washing, portions of Fe, Mn, and Al oxides binding the heavy metals are released into solution (Table 3). Larger portions of Cu and Ni in the S3 fraction were substantially reduced than that of Zn by PDTA washing at higher PDTA concentrations, due to the formation of more stable Cu-PDTA and Ni-PDTA complexes despite the highest proportion of Zn in S3 fraction. As illustrated in Fig. 3, the proportion of Cu in the S4 fraction of washed soil decreased markedly as PDTA concentration increased or liquid-to-soil ratio decreased due to the enhanced release of SOM from the soil into the solution (see Fig. 2). The metal fractions Fig. 3. Metal fractions affected by PDTA-enhanced soil washing (the metal percentages are calculated based on their individual contents in the unwashed soil). T. Zhang et al. / Chemosphere 109 (2014) 1–6 in S2, S3, or S4 were found to increase for some washing combinations for all heavy metals of interest, indicating that portions of these metals were redistributed during the washing procedure. Some fractions were destabilized and readsorbed on the carbonates, Fe/Mn/Al oxides, or SOM and sulfides. The residual metal fraction (S5) was expected to be very stable and cannot generally be removed or changed. However, in Exp. 7 of this study, where a high PDTA dosage was employed, the S5 fraction of all four metals was lower than it was in unwashed soil. Similar results were obtained in a study of soil washing with EDTA (Lei et al., 2008). A caveat that must be kept in mind when interpreting the results of the speciation experiments is that, as has been reported in previous study (Yong and Mulligan, 2004), the determination of heavy metal fractions by sequential extraction is only operationally defined, and is generally considered to be more qualitative than quantitative in providing insight into metal distribution and mobility. The apparent metal redistribution revealed by sequential extraction reflects only the increase in the chemical availability of these metals after PDTA washing. 4. Conclusions Heavy metal speciation and mobility in a contaminated soil washed with PDTA, a derivative of EDTA, were investigated by batch leaching tests using a range of soil washing conditions followed by sequential extraction. With appropriate washing conditions, PDTA significantly enhanced extraction of Cu from contaminated soil. The primary mechanisms of Cu extraction by PDTA were complexation-promoted dissolution of soil Cu and increased dissolution of SOM. PDTA showed high selectivity for Cu(II) over soil component cations (Ca(II), Mg(II), Fe(III), Mn(II), Al(III)), especially at lower liquid-to-soil ratios under PDTA deficiency, thus avoiding unwanted dissolution of soil minerals during the soil washing process. PDTA-enhanced soil washing increased the exchangeable fractions of Cu, Zn, and Pb and decreased their residual fractions, compared to their levels in unwashed soil. A cost-effective strategy for the use of this agent would be a continuous washing system where the contaminants were precipitated from the washing solution and the PDTA was reused. Soil adsorption and biodegradation of PDTA should also be investigated before this method is used in soil remediation. Acknowledgements The project was supported by National Natural Science Foundation (No. 41171374, 21272292), 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). Appendix A. 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