Remediation of cadmium, lead and zinc in

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Journal of Hazardous Materials 366 (2019) 177–183

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Remediation of cadmium, lead and zinc in contaminated soil with CETSA and MA/AA

T



Zhenhua Xiaa, Shirong Zhanga, , Yaru Caoa, Qingmei Zhonga, Guiyin Wanga, Ting Lib, Xiaoxun Xua a b

College of Environmental Sciences, Sichuan Agricultural University, Wenjiang, 611130, PR China College of Resources, Sichuan Agricultural University, Wenjiang, 611130, PR China

A R T I C LE I N FO

A B S T R A C T

Keywords: Soil washing Heavy metal CETSA MA/AA Function groups

Soil washing, which is used to remove heavy metals from soil, is dependent on suitable washing agents. However, there is still a lack of economical, environmentally friendly washing agents with high removal efficiency. In this study, three washing agents, carboxyalkylthiosuccinic acid (CETSA), copolymer of maleic and acrylic acid (MA/AA) and ethylenediamine tetra acetic acid (EDTA), were used to remove heavy metals from contaminated soil. The influence of washing solution concentration, pH and washing time on heavy metals removal was also investigated. The cadmium (Cd), lead (Pb), and zinc (Zn) removal efficiencies increased as washing solution concentrations increased from 0 to 60 g L−1, while they declined as pH increased from 3 to 8. Despite fluctuations between 90 and 120 min, heavy metal removal efficiencies increased continuously from 10 to 90 min. The three agents also effectively reduced the potential risks of Cd, Pb, and Zn in contaminated soil, but only CETSA and MA/AA produced no significant changes in chemical properties. Fourier transform infrared spectroscopy revealed that the hydroxyl, carboxyl, carbonyl, methoxyl, and sulfur groups were related to the heavy metal ions from the soil colloids. Thus, CETSA and MA/AA were suitable washing agents for remediation of heavy metals contaminated soil.

1. Introduction Heavy metals contamination of soil, which is caused by multiple anthropogenic activities such as mining [1], chemical industrial processes, and smelting [2], is a global environment issue [3]. Heavy metals such as cadmium (Cd) and lead (Pb) are high persistence in soil because they are non-degradable [4,5]. As a result, these metals can cause damage to the human reproductive system and central nervous system because they can accumulate in organisms. Furthermore, excessive zinc (Zn) in soil could lead to the physiological dysfunction of plants and endanger human health via the food chain [6,7]. Hence, there is an imperative need to remediate heavy metals-contaminated soils. To date, technologies such as soil replacement or filling [8] electrodialysis [9], solidification [10], soil washing [11], and phytoremediation [12] have been widely applied for remediation of metalscontaminated soils. Among these, soil washing is regarded as one of the most effective treatments because of its simple operation and short running period [13]. Currently, soil washing is used as a sustainable remediation technique [14]. However, if a breakthrough could be made ⁎

in washing agents, this method could be transformed into a green remediation strategy [15]. Soil properties such as bulk density, texture and pH will affect the removal efficiencies of heavy metals [16]. Accordingly, these factors must be considered while striving for high removal efficiencies; however, we should also search for efficient and environmentally friendly agents [17]. Inorganic agents, surfactants, and chelators have been extensively used for soil washing [18]. Among these, inorganic agents were lowcost but might destroy the physical, chemical or biological properties of the soil [19]. Surfactants such as rhamnolipids are capable of moderate removal of heavy metals, whereas most surfactants are relatively expensive; therefore, it is difficult to use these in large-scale applications [20]. Chelating agents such as ethylenediaminetetraacetic acid (EDTA) display sufficient extraction ability [21], but often have poor biodegradability and could therefore lead to secondary pollution [22]. Therefore, exploring new low-cost and environmentally friendly washing agents is highly recommended. Low molecular weight organic acids such as citric or oxalic acid have been reported as alternatives to synthetic chelating agents because they are degradable and have little influence on soils [17]. In addition,

Corresponding author. E-mail address: [email protected] (S. Zhang).

https://doi.org/10.1016/j.jhazmat.2018.11.109 Received 30 June 2018; Received in revised form 9 November 2018; Accepted 29 November 2018 Available online 01 December 2018 0304-3894/ © 2018 Elsevier B.V. All rights reserved.

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experiments. All solutions were prepared with distilled water and the same experiments were conducted using distilled water as a control. Different washing conditions, including variations in the concentrations of agents, solution pH and washing time, were investigated as described below.

certain organic acid polymers are environmentally friendly. Therefore, these compounds may be promising materials for soil washing [23]. However, it is also important to search for organic acids with moderate cost for use as soil washing agents. Carboxyalkylthiosuccinic acid (CETSA) and copolymers of maleic and acrylic acid (MA/AA) are inexpensive biodegradable organic acids. One study indicated that CETSA displayed good water-soluble activity and bio-degradability [24]. In addition, MA/AA has been investigated for its ability to remove heavy metals from water [25]. Therefore, these two agents might be suitable for soil washing. Despite this, no studies have investigated their efficiencies for removal of heavy metals during soil washing [26,27]. Therefore, in this study, we investigated whether CETSA or MA/AA could remove heavy metals from soil while destroying as few soil nutrients as possible. The specific objectives of this study were to: (1) quantify removal efficiencies of heavy metals by CETSA or MA/AA; (2) observe the changes of geochemical fractions of heavy metals before and after soil washing; and (3) compare the changes of total nitrogen, phosphorus, potassium, available nitrogen, phosphorus and potassium in soil before and after washing with the agents.

(1) Washing agent concentrations: the three agents were investigated at concentrations of 10, 20, 40, 60, 80, and 100 g L−1. Concentration experiments were conducted at pH 5, with a washing time of 90 min and a soil: liquid ratio of 1: 10. (2) Washing solution pH values: solution pH values of 3.0, 4.0, 5.0, 6.0, 7.0, and 8.0 were investigated at an agent concentration of 60 g L−1, washing time of 90 min, and soil: liquid ratio of 1: 10. The pH values of the reaction systems were adjusted using dilute HNO3 or NaOH. (3) Washing times: washing times of 10, 20, 30, 60, 90, and 120 min were investigated at a solution concentration of 60 g L−1, pH of 5, and soil: liquid ratio of 1: 10. Upon completion of oscillation, the suspensions were immediately centrifuged at 3000 rpm for 5 min, after which the supernatants were analyzed for heavy metals by AAS. To ensure the authenticity and reliability of results, all experiments were conducted in triplicate. There was no significant weight loss during the solution configuration and transfer process.

2. Materials and methods 2.1. Soil sampling preparation Heavy metals contaminated soil samples were collected from the Tangjia Pb-Zn Mine in Hanyuan, Sichuan, China (29°24ʹN, 102°37ʹE). The samples were collected from the surface layer (0–20 cm), air-dried and then passed through a 2 mm screen prior to use in the experiments. The experimental soil was sandy clay with sand, silt and clay contents of 25.5%, 13.3% and 61.2%, respectively. The sample was digested with HNO3/HCl/HClO4 solution at a ratio of 1:2:2 (v/v/v), and the total Cd, Pb and Zn in the soil were determined to be 18.82, 2809.80, and 1175.63 mg kg−1, respectively, by atomic absorption spectrophotometry (AAS, Thermo Solaar M6, Thermo Fisher Scien-tific Ltd., USA). Standard reference material (GBW07405) was analyzed for QA/ QC during the digestion preparation procedure (accuracies within ± 5%).

2.4. Soil chemical analysis The soil washed by 60 g L−1 CETSA, MA/AA or EDTA was collected for drying, and their reacting conditions were 120 min and pH 5. The chemical properties were then evaluated as follows for comparison with the original soil: The pH of the soil was evaluated using a digital pH meter (pHS-3C, Shanghai INESA Scientific Instrument Co., Ltd., China) in a 1: 2.5 soil: deionized water mixture. Soil organic carbon (SOC) was measured by Walkley-Black titrations [30]. Total nitrogen (TN), total phosphorus (TP) and total potassium (TK) were determined by Kjeldahl method [31], molybdenum antimony colorimetry method [32] and flame photometry, respectively. Available nitrogen (AN) was determined by the alkali hydrolysis distilling method, available potassium (AK) was extracted using ammonium acetate [33,32] and available phosphorus (AP) was determined by the Olsen method [34].

2.2. Washing material preparation The washing agents CETSA (product no. 19343-85-2) and MA/AA (product no. 26677-99-6) were purchased from Qingdao Yousuo Chemical Technology Co., Ltd. (China). The mass fractions of CETSA and MA/AA were 70 and 48%, respectively, and their structures are presented in Fig. 1 [28,29].

2.5. Geochemical fractions of heavy metals in soil before and after washing 2.3. Effects of washing solution concentration, pH, and washing time on removal efficiency

Geochemical fractions of heavy metals were surveyed by the Tessier method [35,36]. The soil conducted at 120 min, 1:10 w/v and pH 5 were collected for drying, then used for extraction of the exchangeable, carbonate-bound, Fe-Mn oxides-bound, organo-bound, and residual fractions through five steps. After each step, the filtered liquid was collected and the content of each fraction was determined by atomic absorption spectrophotometry (AAS, Thermo Solaar M6, Thermo Fisher Scien-tific Ltd., USA). The Cd, Pb and Zn contents (mg kg−1) in the leachate were calculated as follows:

Three agents including CETSA, MA/AA, and EDTA (for comparison) were used in the experiment. Soils (2.00 g) were mixed with washing solution (20.00 mL), then shaken using a mechanical shaker at 200 rpm in acid-rinsed polycarbonate plastic bottles (100 mL) for a specific period of time. The suspensions were then centrifuged at 4200 rpm for 6 min, after which they were filtered through 0.45 μm filter membranes. The filtrates were then stored until used in follow-up

Fig. 1. Structures of CETSA (a) and MA/AA (b).

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M=

(C × V × D) m

(1)

R=

M × 100% MT

(2)

reducing the ability of the agents to bind to the target heavy metals. The heavy metal removal efficiencies of the three agents differed significantly at 100 g L−1 (p < 0.05) since various agents have different combining capacities for heavy metals in soil. Furthermore, differences in the removal efficiencies might be ascribed to different functional groups in the agents that could bind heavy metal ions to different degrees, such as carboxyl and amino groups [40,41]. The FTIR analysis results in the latter discussion might demonstrated the phenomena. The heavy metal removal efficiencies for Cd, Pb and Zn of CETSA were higher than those of the other two agents, being 52.39, 82.32 and 86.19% at a concentration of 100 g L−1. In the presence of MA/AA, the heavy metal removal efficiencies were slightly lower than those of CETSA, being 49.36, 64.90, and 78.14%, respectively. The heavy metal removal efficiencies of EDTA were lower than those of the other two agents, being 45.44, 38.29, and 34.11%, respectively. All three agents had the highest removal efficiency for Cd, followed by Zn and Pb, which is in accordance with the results of a study conducted by Torres et al. [42] Taken together, these findings indicate that the washing agents we used were better for removal of Cd from mining soil. Furthermore, CETSA or MA-AA was better than EDTA for heavy metals removal.

where R is the removal efficiency of Cd, Pb or Zn; M is the level of Cd, Pb or Zn in the leachate (mg kg−1); MT is the total amount of Cd, Pb or Zn in the soil (mg kg−1). 2.6. FTIR analysis The solutions of CETSA, MA/AA or EDTA before and after soil washing with solution at 60 g L−1 and pH 5 for 90 min at a solid: liquid of 1:10 were dried in an oven, then ground with spectroscopic grade KBr under hydraulic pressure of 400 kg cm−2. The samples were then identified using a PerkinElmer spectrometer (Spectrum Two, PerkinElmer Inc., USA) and the spectra were recorded in the range of 4000 to 400 cm−1. 2.7. Statistics analysis The experimental data were analyzed by SPSS Version 19.0 (SPSS Inc, Chicago, Ill, USA) using origin 9.0 to draw the data diagram. Oneway ANOVA was used to compare whether the metal removal efficiencies under different experimental conditions were significantly different and Fisher’s least significant difference test was employed to identify significant differences between means. A p < 0.05 was considered to indicate statistical significance.

3.2. Effects of the solution pH on the removal efficiencies Solution pH of the agents was another key factor influencing heavy metal removal efficiencies because it could determine the speciation of heavy metals in soil [43]. In this study, increasing the solution pH values resulted in removal efficiencies of Cd, Pb, and Zn generally decreasing, first sharply, then slowly, which was likely because the acidic conditions contributed to the dissociation of heavy metal ions from soil colloids [44]. Therefore, there might be differences in the removal rates of heavy metals under different pH values. There were significant differences in heavy metal removal efficiencies between pH 3 and pH 8 (p < 0.05). The removal efficiencies of Cd, Pb and Zn at a solution pH of 3 were as high as 51.52, 86.24 and 77.26% for CETSA, and 57.18, 62.95 and 77.20% for MA/AA, respectively. However, the removal efficiencies at pH 8 were only 8.75, 15.34 and 14.99% for CETSA and 22.87, 19.39, and 19.05% for MA/AA. As shown in Fig. 1, the two washing agents had significant differences in heavy metal removal efficiencies at different pH levels because of their large number of carboxylic groups (p < 0.05) (Figs. 3–6). Overall, pH values were negatively correlated with the removal efficiencies of heavy metals. There were two potential reasons for these results. The first possibility is that protons in the organic acid molecules activated the heavy metal ions in the soil, resulting in their desorption [45]. Moreover, because of the increase in pH values, the functional groups of organic acids that combined with heavy metal ions had a weaker ability to dissociate H+, causing a decline in the binding ability of functional groups in the agents [46]. Second, the negative surface

3. Results and discussion 3.1. Effect of the concentrations of the washing agents on heavy metal removal in soil Soils were washed with various concentrations of agents to investigate their effects on heavy metals removal (Fig. 2). The washing efficiencies generally increased, first rapidly and then steadily, with ascending concentrations (p < 0.05). The concentration was the key factor to the reaction [37]. Therefore, higher concentration of agents might lead to more heavy metal ions releasing from soil colloid during washing process [38]. The removal efficiencies of Cd, Pb and Zn were obviously reduced following treatment with a concentration of 80 g L−1, probably because most of the heavy metal ions in the soil had been removed [39]. These findings indicate that the concentration needs to be appropriate for the actual project because low concentrations might cause incomplete removal and high concentrations could lead to a surplus of washing agent. In addition, the removal efficiencies were still limited at high concentrations, which might be attributed to the combination of Fe3+ in the soil with washing agents, thereby

Fig. 2. Effects of concentrations of washing agents on soil Cd, Pb and Zn removal (CETSA, carboxyalkylthiosuccinic acids; MA/AA, copolymer of maleic and acylic acid; EDTA, ethylenediaminetetraacetic acid. Different lowercase letters in the same line represent significant differences between means according to Fisher’s LSD test at p < 0.05).

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Fig. 3. Effects of solution pH on soil Cd, Pb and Zn removal (CETSA, carboxyalkylthiosuccinic acids; MA/AA, copolymer of maleic and acylic acid; EDTA, ethylenediaminetetraacetic acid. Different lowercase letters in the same line represent significant differences between means).

3.4. Changes in geochemical fractions of heavy metals after washing

charge of the soil clay particles and organic matter could be reduced under low pH conditions, resulting in the dissolution of Fe-Mn oxidesbinding and leading to the formation of dissolved heavy metals, thereby enhancing mobilization of heavy metals [47].

The fractions of Cd, Pb and Zn in the soil differed significantly after washing with CETSA, MA/AA or EDTA (p < 0.05). Cadmium (Cd) showed significant differences from Pb or Zn after soil washing because it was mainly in the exchangeable fraction, with levels of 8.89 mg kg−1 being observed, while the contents of the other four fractions ranged from 2.54 to 3.75 mg kg−1. The changes in the fractions of the heavy metals showed similarities after washing with CETSA, MA/AA or EDTA. Among them, the exchangeable fraction decreased the most, ranging from 6.86 mg kg−1 to 7.07 mg kg−1, followed by the residual fraction, which had levels of only 0.29 to 0.86 mg kg−1. In the original soil, the residual fraction of Pb accounted for a large proportion, with a level of 483.19 mg kg−1. After extraction by CETSA, MA/AA or EDTA, the Pb exchangeable and carbonate-bound fractions decreased significantly (p < 0.05), with declines of 174.92, 171.35, and 158.13 mg kg−1 being observed for the exchangeable bound fraction and 95.29, 90.32, and 83.59 mg kg−1 for the carbonate-bound fraction, respectively. These results showed that during the initial extraction, the exchangeable and carbonate bound fractions of heavy metals were quickly acidsolved into solution, and soil washing could reduce the metal mobility by removing the labile fractions [51,52]. On the contrary, the organobound fraction of Pb in the soil decreased marginally after washing, showing that the three agents could only remove a small amount of organo-bound and residual fractions of Pb. The exchangeable, residual and Fe-Mn oxides-bound fraction of Zn accounted for the main components, with levels of 1027.06, 758.87 and 692.42 mg kg−1, respectively, being observed. Similarly, the changes in Zn fractions after soil washing were partially similar to those of Pb, with the exchangeable

3.3. Effects of the washing times on removal efficiencies The washing time experiment was in line with the earlier stages of the kinetic equilibrium process. Therefore, contact time played an important role in the remediation process [48]. In this study, as leaching time was prolonged, the overall removal efficiencies of Cd, Pb and Zn removal by the two agents first increased, then stabilized, peaking at 83.50, 94.13 and 80.37% for CETSA, and 83.55, 83.77 and 71.44% for MA/AA, respectively (Table 1). The removal efficiencies of the heavy metals increased rapidly at the beginning of the reaction under washing times ≤ 90 min. This might have been a result of the rapid release of metals in the initial stage of the washing, for the labile factions of the heavy metals played key roles [49]. After this point, the removal efficiencies of the heavy metals showed no remarkable change, indicating that combination of these agents with heavy metals had slowed because some recalcitrant fractions of the heavy metals predominantly depend on the long-term extraction. As a result, the soil washing process could be divided into two periods. In the first period, metals were desorbed rapidly, while in the second period they reacted slowly [50]. Therefore, the optimal washing time was 90 min for CETSA and 120 min for MA/AA based on the previous results.

Fig. 4. Effects of solution pH on soil Cd, Pb and Zn removal (CETSA, carboxyalkylthiosuccinic acids; MA/AA, copolymer of maleic and acylic acid; EDTA, ethylenediaminetetraacetic acid. Different lowercase letters in the same line represent significant differences between means). 180

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Fig. 5. Changes in the fractions of heavy metals before and after washing (CETSA, carboxyalkylthiosuccinic acids; MA/AA, copolymer of maleic and acylic acid; EDTA, ethylenediaminetetraacetic acid. Different lowercase letters in the same line represent significant differences between means).

Soil washing might cause changes in soil physical and chemical properties [53,54]. Therefore, it is necessary to investigate the chemical properties of soil after the washing process, including changes in the soil organic carbon, pH, total nitrogen, phosphorus, potassium, available nitrogen, phosphorus and potassium before and after soil washing [55]. After washing with CETSA, MA/AA and ETDA, the content of organic carbon in soil decreased all within 20%. When compared with the original soil, the pH of soil after washing with CETSA, MA/AA or EDTA showed no remarkable change. Moreover, total nitrogen, phosphorus, and potassium and available nitrogen, phosphorus and potassium of soil after extraction by CETSA or MA/AA decreased marginally by 1.1–7.84%. However, total phosphorus and potassium and available phosphorus and potassium in soil differed significantly after washing with EDTA when compared with the original soil (p < 0.05), with decreases of 14.1–31.9% being observed. Thus, these results indicated that EDTA washing might lead to a certain degree of soil nutrient loss, especially phosphorus and potassium, while CETSA or MA/AA had less effect on the soil.

spectroscopy could be used to qualitatively and quantitatively analyze the active groups of the agents [57]. In this study, the stretching vibrations of OeH were observed at 3389–3417 cm−1, while the asymmetric and symmetric stretching vibrations of the two agents were observed at 1573–1619 cm−1 and 1381–1454 cm−1, respectively. Stretching vibrations of CeO were observed at around 978–1281 cm−1. In addition, CETSA also had a unique CeS group at 621 cm−1. In general, the peaks of these groups changed to some degree or shifted to other positions after washing, indicating that heavy metals bound with reactive groups or had chemical interactions with the agents though the washing process [58]. For CETSA, new bands appeared at around 1715 cm−1 after washing, indicating that a chemical reaction might have occurred between the metal ions and carboxylic C]O [56]. Moreover, carboxylic OeH and CeS changed in amplitude after washing, and it can be inferred that these two groups played an important role in binding with heavy metals. For carboxylic CeO, the peak at 1150 cm-1 disappeared partially after washing and new bands emerged, suggesting that both heavy metal binding and chemical reactions occurred in the reaction process [57,58]. Copolymers of maleic and acrylic acid (MA/AA) contain large carboxyl groups, but the carboxylic OeH peak around 3417 cm-1 had a small amplitude of variation after the reaction, showing that OeH played a minor role in the washing process. Conversely, carboxylic C]O and CeO groups produced multiple new bands at 978–1281 cm−1 after washing, indicating that metal ions had reacted with MA-AA during the leaching process [57]. In general, the hydroxyl, carboxyl, carbonyl, methoxyl, and sulfur groups might be the main contributors to the heavy metal extraction, although each group reacted with heavy metals to different degrees.

3.6. FTIR analysis of agents before and after soil washing

4. Conclusions

The reactive functional groups in organic acids might bind with heavy metal ions during the washing process [56]. Therefore, FTIR

Carboxyalkylthiosuccinic acid (CETSA), copolymer of maleic and acrylic acid (MA/AA) and EDTA had high removal efficiencies for Cd,

state of Zn showing a downward pattern after washing (p < 0.05). However, the Fe-Mn oxides-bound and residual fraction of Zn in the soil only partially decreased after washing, with maximum declines of 533.49 and 549.28 mg kg−1, respectively, being observed. Finally, the risks posed by Cd, Pb and Zn were partially reduced after washing soil with the three agents because the exchangeable and carbonate fractions of Cd, Pb and Zn were all substantially reduced. 3.5. Changes in soil chemical properties before and after washing

Fig. 6. FTIR spectra of CETSA or MA-AA before and after soil washing (CETSA, carboxyalkylthiosuccinic acids; MA/AA, copolymer of maleic and acylic acid; A, before washing; B, after washing). 181

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Table 1 Changes in soil chemical properties before and after washing (CETSA, MA/AA or EDTA, concentration of 60 g L−1; pH, 5.0; solid/liquid, 1:20; washing time, 120 min). Characteristics Original pH SOC TN TP TK AN AP AK

(g kg−1) (g kg−1) (g kg−1) (g kg−1) (mg kg−1) (mg kg−1) (mg kg−1)

7.32 38.36 ± 0.56a 0.772 ± 0.02a 0.96 ± 0.04a 17.32 ± 0.47a 44.63 ± 1.25a 19.52 ± 0.58a 182.87 ± 5.92a

CETSA

MA/AA

EDTA

6.53 37.91 ± 0.71a 0.718 ± 0.03b 0.91 ± 0.02a 16.19 ± 0.61b 44.15 ± 2.01a 18.22 ± 0.46b 181.93 ± 4.49a

6.39 35.86 ± 0.49b 0.734 ± 0.03ab 0.89 ± 0.03a 15.77 ± 0.49b 43.19 ± 1.87b 17.99 ± 0.91b 174.81 ± 3.91b

6.45 31.21 ± 0.66c 0.601 ± 0.06c 0.63 ± 0.01b 13.61 ± 0.48c 39.21 ± 1.29c 13.29 ± 0.78c 157.16 ± 4.18c

CETSA, carboxyalkylthiosuccinic acids; MA/AA, copolymer of maleic and acylic acid; EDTA, ethylenediaminetetraacetic acid; SOC, soil organic carbon; TN, Kjeldahl nitrogen; TP, total phosphorus; TK, total potassium; AN, available nitrogen (ammonium-N + nitrate-N); AP, available phosphorus; AK, available potassium; Different lowercase letters in the same line represent significant differences between the means.

Pb and Zn in contaminated soil. Overall, heavy metals removal efficiencies of CETSA was higher than MA / AA and EDTA. The removal efficiencies of heavy metals by two kinds of organic acids were affected by the concentration, pH and time. The washing efficiencies showed upward trends with increasing concentrations, decreasing pH values and prolonged reaction times. After the soil was washed with organic acid chelating agents, the exchangeable and carbonate bonds with heavy metals were effectively removed. Therefore, the three agents could effectively remove heavy metals from soil. Moreover, CETSA and MA/AA had less effects on soil chemical properties when compared with EDTA. However, soil washing might cause changes in soil structure or microbial biomass. Therefore, their potential impacts on soil will be considered in subsequent studies.

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