Heavy Metal Removal from Shooting Range Soil by Hybrid


Nov 19, 2010 - This study presents a method for heavy metal removal from a shooting range soil by a newly suggested hybrid technology...
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Environ. Sci. Technol. 2010, 44, 9482–9487

Heavy Metal Removal from Shooting Range Soil by Hybrid Electrokinetics with Bacteria and Enhancing Agents KEUN-YOUNG LEE AND KYOUNG-WOONG KIM* School of Environmental Science and Engineering, Gwangju Institute of Science and Technology (GIST), Gwangju 500-712, Republic of Korea

Received August 1, 2010. Revised manuscript received November 5, 2010. Accepted November 5, 2010.

This study presents a method for heavy metal removal from a shooting range soil by a newly suggested hybrid technology. Active bioaugmentation was performed using Acidithiobacillus thiooxidans in the bioleaching step, and each test was sequentially combined with acid-enhanced and EDTA-enhanced electrokinetics. The results of the bioleaching processes indicated that S-oxidizing bacteria enhanced the mobility of heavy metals in the soil, based on their chemical forms. This process improved the final removal efficiencies of Cu and Zn in the hybrid electrokinetics. In the case of Pb, however, anglesite (PbSO4) has been easily formed in the bioleaching step from sulfate, a byproduct of S oxidation. Despite the potential negative effect on combining acid-enhanced electrokinetics, this problem was overcome by the application of an electrokinetic EDTA injection. Moreover, this method showed enhanced removal efficiency for Pb (92.7%) that was superior to that of an abiotic process. This hybrid method of EDTA-enhanced bioelectrokinetics demonstrated an adequate removal efficiency of heavy metals, especially Pb, with lower power consumption and eco-friendly soil conditions.

Introduction Heavy metal contamination of soils is a great worldwide problem. Especially, the use of shooting ranges produces a great quantity of soil contaminated with heavy metals. The use of Pb shots and bullets is a significant source of Pb contamination because Pb is still the most suitable primary material used in ammunition (1-3). In addition, copper (Cu), zinc (Zn), and antimony (Sb) can be considered as secondary metal contaminants of potential concern (1, 4, 5). Through various geochemical weathering reactions, such as oxidation, carbonation, dissolution, and reprecipitation, the particulate heavy metals deposited in the soils have been transformed to more mobile and bioavailable forms (3-5). Consequently, shooting ranges increase the ecotoxicological risks of surrounding environment with contaminating soils, surface water, groundwater, terrestrial, and aquatic biota. Previous investigations have shown that one of the most common remediation techniques is soil excavation and disposal, in which all shooting range soil is considered waste (2). Various physical separation techniques have also been applied as alternative methods suitable for the removal of particulate heavy metals (bullets, pellets, and fragments of them) from * Corresponding author phone: +82-62-715-2821; fax: +82-62715-2434; e-mail: [email protected] 9482

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soil (2, 5, 6). While these methods avoid the disadvantages of whole disposal because the treated soil can be returned to the site, they are not complete for fine soil (5, 6). Heavy metals transformed by weathering processes can remain as a residue at high concentration in the treated soil (1, 3, 4). According to previous literatures, shooting range soils have also been treated by solidification/stabilization (S/S) as a nonremoval technology and phytoremediation, soil washing, and electrokinetics as removal technologies (2, 5-7). All available soil remediation methods to date have their advantages and disadvantages; therefore, active remediation of shooting range soils is of great importance, and innovative strategies are still required for removing heavy metals from contaminated soil. In an attempt to remove heavy metals from soil and sediment, a microbially mediated leaching process, termed bioleaching, has been proposed due to its several advantages such as low cost, low energy requirement, nonhazardous byproducts, and effective metal mobilization (8-12). One of the most intensively used microorganisms in this process is an acidophilic Acidithiobacillus thiooxidans which is a bacterium that gains energy to support its growth and maintenance from the oxidation of reduced sulfur compounds and that uses CO2 as a cellular carbon source (10, 11). Two mechanisms of bioleaching with A. thiooxidans have been proposed (10, 11). In the first, bacteria directly interact with sulfide minerals and mobilize heavy metals coexisted with them, as follows A.thiooxidans

+ MeS2 + H2O + 3.5O2 98 Me2+ + 2SO24 + 2H

(1) where MeS2 is the insoluble metal sulfide, and Me2+ is the free metal ion. Overall the reaction produces hydrogen ions, which decreases the soil pH. Such soil acidification allows leaching of heavy metals from soil to pore water, which is the second indirect mechanism of this process. For practical promotion of S oxidation and soil acidification, elemental S (S0) has been injected into the soil, either with bacteria (bioaugmentation) (8) or without bacteria (biostimulation) (9), when the amount of oxidizable and bioavailable S compounds is not sufficient in the contaminated soil. Although a number of previous studies have proven the effectiveness of bioleaching for heavy metal mobilization, it suffers the drawback of requiring kinetic energy to completely separate mobilized metal contaminants from submerged soil (10). Additionally, several previous studies have shown that the overall Pb removal efficiency has not been sufficient relative to other heavy metals (8, 9, 11). The electrokinetic process has been widely applied for decontaminating soil in laboratory and field studies. To achieve the desired process efficiency and system performance, studies have reported enhancement schemes via electrolyte conditioning with acid catholyte for heavy metals (13-16) or alkaline anolyte for anionic contaminants such as As (13, 17, 18). As we have previously reported, although acid-enhanced electrokinetics has been shown to be a promising technology for simultaneously recovering multiple metal contaminants from soil, Pb migration is very slow and time-consuming because of the low mobility of Pb and great adsorption capacity and affinity of Pb for the soil surface (13). Several studies have examined the electrokinetics enhanced by chelating agents (15, 19, 20), such as ethylenediaminetetraacetic acid (EDTA) which has most often been tested (21). As the major transport mechanism for the 10.1021/es102615a

 2010 American Chemical Society

Published on Web 11/19/2010

TABLE 1. Experimental Conditions of Hybrid Electrokinetics electrokinetics soil pretreatment

electrolyte conditioning

experiment

bioleaching

moisture content (%)

current density (mA/cm2)

Acid EK Acid BioEK EDTA EK EDTA BioEK

nonea A. thiooxidansb none A. thiooxidans

15.8 ( 0.5 17.1 ( 0.2 16.9 ( 0.4 15.0 ( 0.1

2 2 2 2

a Initial shooting range soil was used without bioleaching. thiooxidans was transferred into the electrokinetic soil cell.

charged compounds in the electric field is electromigration, the anionic EDTA is mainly added in the cathode compartment. Based on the results of previous investigations, EDTA has shown good selective chelation with Pb, Cu, and Zn and has occasionally achieved competent removal efficiency (19, 20). This method is therefore a candidate as a remediation technology for metal-contaminated soil in shooting ranges. Little research has yet been performed on bioelectrokinetics for the removal of heavy metals from soil, while its application for organic pollutants has been actively studied (22, 23). In our recent work, a method integrating anaerobic bioleaching and electrokinetics was superior to individual methods for As removal from tailing soil (18). While this synergistic effect was observed to offer significant time and cost savings for the process, this combination has been limited to As removal. Maini et al. (16) first reported electrokinetic remediation enhanced by S oxidizing mechanism for the removal of Cu, in which soils were treated by the sequential application of S amendment (biostimulation) and electrokinetics. Even though their study also demonstrated the cost effectiveness achieved with this combination, its efficiency for Pb removal has not been confirmed. The main study objective was to investigate the feasibility of hybrid electrokinetics for heavy metal removal from shooting range soil. Active bioaugmentation was performed using A. thiooxidans in the bioleaching step, and each test was sequentially combined with acid-enhanced and EDTAenhanced electrokinetics. The overall bioelectrokinetics results were compared with those of (abiotic) electrokinetics, which enabled the process efficiency to be evaluated in terms of heavy metal removal and cost expenditure.

Experimental Section Soil. Heavy metal-contaminated soil was taken from an abandoned military shooting range located in the Civilian Control Line of South Korea. Collected at a depth of 0-30 cm from the surface of the target area, a composite, representative sample was prepared by mixing ten samples taken within a 30 m2 area, and the sample was stored without drying to ensure survival of the indigenous bacteria. The soil sample was sieved to pass through a no. 10 sieve with a 2 mm diameter size to remove gravel fraction. The treated soil was predominantly sandy loam (soil texture according to USDA) with a 3.1% of water content (dried at 105 °C for 24 h) and less than 0.005% of organic matter content (calculated from measured loss on ignition at 400 °C for 24 h). The initial soil pH was 4.9-5.9 (5 g of soil in 50 mL of water), and the cation exchange capacity was 17.9 meq/100 g (from USEPA method 9081). The initial Pb, Cu, and Zn concentrations were 3529 ( 89 mg/kg, 209 ( 34 mg/kg, and 78 ( 9 mg/kg, respectively. Bioleaching Experiments. A strain of A. thiooxidans (ATCC 8085), S-oxidizing bacteria, was used in the experiments. After precultivation for 10 days, which was the logphase of bacterial growth (data not shown), the bacterial solution was used in the bioleaching step. Six hundred grams

b

anode

cathode

duration (days)

0.01 M HNO3 0.01 M HNO3 0.1 M Na2CO3 + 0.03 M EDTA 0.1 M Na2CO3 + 0.03 M EDTA

0.1 M HNO3 0.1 M HNO3 0.1 M EDTA 0.1 M EDTA

20 20 20 20

The soil pretreated by bioleaching with S0 and A.

of the soil sample was mixed with 200 mL of bacterial solution and 2% (w/w) of S0. This mixture was placed in a small aerobic reactor and incubated at 25 °C in static condition (without agitation and aeration). Such an experimental setting simulated the application of low solid-liquid ratio in the field site where the slurry-phase bioreactor cannot be guaranteed. Also, it was assumed that the oxygen in soil and pore water is sufficient to maintain an aerobic environment in the reactor. Such a bioaugmentation experiment with A. thiooxidans was performed for 20 days in parallel with the biostimulation experiment which used only sterilized ATCC 125 medium and S0 as a leaching solution. All experiments were repeated in triplicate. The concentrations of sulfate and heavy metals, pH, and Eh in the leaching solutions were monitored. Following these experiments, soil samples were collected directly from the soil bed for total digestion and sequential extraction. The removal of heavy metals and the changed binding strength of heavy metals in soil were evaluated. (Bio)Electrokinetic Experiments. A diagram of the electrokinetic device is shown in Figure S1, and four sets of experiments are summarized in Table 1. Two sets of electrokinetics were performed by electrolyte conditioning with HNO3 and were regarded as acid-enhanced EK and BioEK. Using the conventional electrolyte conditioning method to decrease soil pH, the acid electrolyte in the cathode compartment neutralized the hydroxide ions generated by cathodic water electrolysis and prevented alkaline conditions from developing within the soil bed. To maintain the pH in the catholyte at less than 2, the appropriate amount of concentrated HNO3 was added into the catholyte reservoir. In addition, two other electrokinetic experiments were conducted for electrokinetic injection of EDTA and its soilflushing: EDTA-enhanced EK and BioEK. EDTA has been commonly added to the catholyte (15, 19, 20). To prevent soil acidification near the anode compartment, Na2CO3 was added to the anolyte with a little EDTA, which acted to buffer the rapid change of pH in the electrodes (19). For (abiotic) electrokinetics, initial shooting range soil was used without any pretreatment. For bioelectrokinetics, on the other hand, soil pretreated by bioleaching with S0 and A. thiooxidans was transferred into the electrokinetic soil cell. Apart from these, all the other conditions of the experiments on electrokinetics were identical. A rectangular-shaped electrokinetic cell (14 cm in length, 5 cm2 of cross-sectional area) was used with around 50 g of soil and a water content of 15.0-17.1%. A constant current (2 mA/cm2) was applied to all experiments for the same duration of 20 days, with monitoring of the voltage changes of soil cell. Electroosmotic flow was also observed by measuring the volume of water transported across the soil cell. The variations in the soil pH were monitored regularly during electrokinetic experiments. Both electrolyte solutions were periodically collected to analyze the heavy metal concentrations. After the electrokinetic process, soil samples VOL. 44, NO. 24, 2010 / ENVIRONMENTAL SCIENCE & TECHNOLOGY

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FIGURE 1. Results of bioleaching experiments on biostimulation (dotted line) and bioaugmentation (solid line). Variations in (A) pH and Eh of the leaching solution and (B) concentrations of Pb, Cu, Zn, and SO4. were obtained directly from three sections of soil bed and two sections of kaolin bed, and collected soil samples were analyzed by the aforementioned equivalent methods. Analytical Methods. The residual concentrations of Pb, Cu, and Zn in the soil after each experiment were investigated by digesting the collected soil samples with aqua regia, composed of concentrated HNO3 and HCl (1:3, v/v), and heated to 70 °C for 1 h (24). The chemical form of each heavy metal in the soils was examined using sequential extraction technique, as suggested by Tessier et al. (25). Although each corresponding fraction cannot be exactly defined by each step in the sequential extraction, its results can be used to speculate on the binding strength of the heavy metals. The heavy metal concentrations in all samples were determined by inductively coupled plasma-optical emission spectrometry (ICP-OES, Perkin-Elmer, USA).

Results and Discussion Bioleaching. To determine the optimal conditions for application of bioleaching to the target shooting range soil, a number of preliminary tests were conducted in batch-type reactors (Figures S2 and S3). In the biostimulation results, the oxidation of S0 was insufficient for the experimental duration. In addition, the bioaugmentation at the low temperature of 5 °C was not appropriate for survival of A. thiooxidans. There was no significant difference among the methods of oxygen supply, which strongly supports the feasibility of the static-type bioleaching process and suggests that the oxygen in soil and pore water is sufficient to maintain an aerobic environment in the static reactor. In the main bioleaching processes, with experimental conditions obtained from preliminary tests, S-oxidation was accompanied by the decrease of pH and the increase of Eh (Figure 1A). Compared to the biostimulation, Cu and Zn leaching with SO4 generation was observed in the bioaugmentation with S0 and A. thiooxidans injection. However, although Pb was the major metal contaminant with the highest concentration in the soil, its bioleaching was negligible in both processes (Figure 1B). Several previous results have shown a similar trend with this result (8, 9, 11), which strongly supports that the bioleaching process using S-oxidation mechanism is insufficient for Pb removal. However, based on the chemical forms of heavy metals in soil, the exchangeable fractions of all target heavy metals, including Pb, were significantly increased in the bioaug9484

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FIGURE 2. Results of acid-enhanced electrokinetics (Acid EK; left) and acid-enhanced bioelectrokinetics (Acid BioEK; right). The horizontal axis represents all sections of the electrokinetic cell, including the anolyte (+), kaolin section near the anode (1), three soil sections (2-4), kaolin section near the cathode (5), and catholyte (-). (A) Initial soil pH (dotted line) and its variation within 20 days during electrokinetics (solid line). The arrow represents the temporal variation. (B) Distribution of initial (dotted line) and residual (solid line) concentrations of Pb. (C) Distribution of initial (dotted line) and residual (solid line) concentrations of Cu (filled symbol) and Zn (empty symbol). mentation (Figure S4). S-oxidizing bacteria enhanced the mobility of the heavy metals in the solid phase, even though the heavy metals were not completely separated into the liquid phase. Therefore, although the bioleaching process itself may not be effective for treating shooting range soil, it can be an effective pretreatment step in the bioelectrokinetics for heavy metal removal. Acid-Enhanced (Bio)Electrokinetics. For (abiotic) electrokinetics, initial shooting range soil was used without any pretreatment, whereas the soil after bioaugmentation was transferred into the electrokinetic soil cell for bioelectrokinetics. In general, the acid-enhanced scheme of electrokinetics is derived from fast soil acidification. The migration of hydrogen ions entering the soil to the cathode gradually and progressively decreased the soil pH over time from sections 1 to 5 (Figure 2A). Because the soil pH was lower than the soil PZC (point of zero charge), a reverse electroosmotic water flow occurred in both processes (data not shown). In the acid-enhanced electrokinetics, the heavy metals were effectively removed within 20 days (Figure 2B and 2C). This result demonstrated the viability of the electrokinetic process as a strong technology for the treatment of metal-contaminated field soil. However, while the removal efficiencies of Cu and Zn in the bioelectrokinetics were higher than those in the electrokinetics, Pb showed a different result (Figure 2B). Much of the Pb, as the dominantly exchangeable fraction, remained in the soil. This result indicated that the exchangeable fraction of Pb formed in the bioleaching step still remained as the same fraction in the extremely acidic condition of the electrokinetic step.

FIGURE 3. (A) EXAFS spectrum of Pb in the soil samples (solid line) and the best fitting by combination of several Pb standard materials (dotted line). Initial soil (Initial), soil after bioleaching (Bio), and soil after acid-enhanced bioelectrokinetics (Acid BioEK) were analyzed. (B) Contribution percentages of Pb minerals for the best fit in the EXAFS spectrum of each soil sample. Massicot (M), cerussite (C), anglesite (A). To explain this result, mineralogical and chemical forms of Pb before and after the processes were identified by the EXAFS spectrum of Pb in the soil samples (Figure 3). The initial shooting range soil mainly consisted of massicot (PbO) and cerussite (PbCO3), minerals that have mostly been regarded as the weathering products from Pb bullets and pellets in shooting range soil (3, 4). On the other hand, after the bioleaching process the main mineral constituent was changed to anglesite (PbSO4), and this transformation provided around 90% of the contribution for the best fit (Figure 3B). This direct measurement provides a clear explanation for the chronic drawback on the Pb removal in the bioleaching process and strongly supports the complete crystallization of the reaction product. Moreover the solubility product of this complex was very low, 1.58 × 10-8, compared to other metal sulfate complexes, e.g., CdSO4 (5.01 × 10-3), CuSO4 (3.98 × 10-3), and ZnSO4 (7.94 × 10-3) (26). EDTA-Enhanced (Bio)Electrokinetics. Two other electrokinetic processes, EDTA-enhanced EK and BioEK, were conducted for the electrokinetic injection of EDTA and its soil-flushing. The two or more ligands contained within chelating agents induce the migration of the anionic EDTA in the cathode compartment toward the anode in the electric field. Soil pH was increased for 20 days due to the electrolyte conditioning, and the variation in the soil pH in EDTAenhanced bioelectrokinetics was favorably progressed from 2 to 8.6 in a relatively short time (Figure 4A). In the neutral and alkaline soil conditions, the electroosmosis was reinforced from the anode to the cathode in both processes (data not shown), whereas the effect of the direction of water flow was negligible for the metal removal, because the electromigration is the predominant mechanism for the transport of charged complexes (18-20). The residual concentrations of Pb after EDTA-enhanced electrokinetics demonstrated that this method utilizing electrokinetic EDTA injection after bioleaching was successful in overcoming the problem that occurred in the previous bioelectrokinetic process (Figure 4B). The formation of anglesite did not negatively affect this combination. Moreover, the heavy metal removal efficiencies of the method were similar to those of acid-enhanced

FIGURE 4. Results of EDTA-enhanced electrokinetics (EDTA EK; left) and EDTA-enhanced bioelectrokinetics (EDTA BioEK; right). The horizontal axis represents all sections of the electrokinetic cell, including the anolyte (+), kaolin section near the anode (1), three soil sections (2-4), kaolin section near the cathode (5), and catholyte (-). (A) Initial soil pH (dotted line) and its variation within 20 days during electrokinetics (solid line). The arrow represents the temporal variation. (B) Distribution of initial (dotted line) and residual (solid line) concentrations of Pb. (C) Distribution of initial (dotted line) and residual (solid line) concentrations of Cu (filled symbol) and Zn (empty symbol). electrokinetics and better than those of EDTA-enhanced (abiotic) electrokinetics (Figure 4B and 4C). Reactions in Hybrid Electrokinetics. Two methods using different chemicals (HNO3 or EDTA) showed reverse directions of Pb migration in the electric field, because the difference in the desorption and/or dissolution reactions. While the ion exchange reaction of heavy metals with hydrogen ions is dominant in acidic condition, the ligandpromoted dissolution of heavy metals with EDTA is dominated in EDTA-enhanced condition, as follows Me2+ + H2Y2- f MeY2- + 2H+

(2)

where Me includes Pb, Cu, and Zn, and Y represents the unprotonated EDTA4- (27). Therefore, Pb2+ was removed toward the cathode in acid-enhanced (bio)electrokinetics, and PbEDTA2- was removed toward the anode in EDTAenhanced (bio)electrokinetics (Figure 5A and 5B). These results also demonstrate the relatively good performance on Pb removal of the acid-enhanced electrokinetics and EDTAenhanced bioelectrokinetics, as confirmed previously. Figure 5C shows the result of chemical leaching tests, which could explain the dissolution characteristics of Pb in several conditions with different Pb complexes and extractants. Our analytical results strongly supported the bioleaching process-induced transformation of the dominant Pb complex in the soil to anglesite. The difference between the two soils was compared. First, the use of 0.5 M MgCl2 and 0.1 M HNO3 as the extractants gave opposite dissolution characteristics. While Pb in the initial soil was mainly contained in acid-soluble fractions, Pb in anglesite was VOL. 44, NO. 24, 2010 / ENVIRONMENTAL SCIENCE & TECHNOLOGY

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FIGURE 5. (A) Accumulated Pb in each electrolyte during acid-enhanced (bio)electrokinetics. (B) Accumulated Pb in each electrolyte during EDTA-enhanced (bio)electrokinetics. (C) Result of chemical leaching test. Dissolution rate (%) of Pb by several extractants on the soil before (Initial) and after (After Bio) bioleaching. favorably solubilized not by 0.1 M HNO3 but rather by 0.5 M MgCl2, which is a condition of higher ionic strength. This result agreed exactly with the poor Pb-removal performance shown by the acid-enhanced bioelectrokinetics in step 1 of the sequential extraction. However, when the acid concentration of the extractant was increased to 0.5 M, Pb in anglesite was completely dissolved into the solution, which indicated that more hydrogen ions are required to mobilize Pb in anglesite and that a longer process duration must be allowed for the sufficient removal of Pb in the acid-enhanced bioelectrokinetics. In the EDTA-leaching tests, Pb in anglesite was completely dissolved, whereas the dissolution of Pb in the initial soil was not enough at the same condition. This result is also in good agreement with the result of EDTAenhanced (bio)electrokinetics. Many studies have reported that PbSO4 was successfully dissolved by EDTA and sometimes more readily dissolved than PbO and/or PbCO3 at neutral condition because of the differences of aging and surface area (19, 28). The initial forms of Pb in the shooting range soil were more aged than the fresh PbSO4 formed after the bioleaching process, and, furthermore, the PbSO4 particles

might have provided more attack sites for the ligand on their surface, compared to the initial Pb embedded in the soil. Evaluations. The final removal efficiencies in terms of residual heavy metals concentrations in the soil from each experiment are presented in Table 2. While the Pb removal efficiencies varied due to the aforementioned reasons, the bioleaching process improved the removal efficiencies of Cu and Zn in both acid- and EDTA- enhanced combinations. Both acid-enhanced electrokinetics and EDTA-enhanced bioelectrokinetics showed that the total Pb concentration in the shooting range soil satisfied the Korean standard limit for soil. Although the Pb removal efficiencies were similar between two methods, we finally demonstrate that the latter process was the preferable method due to the following reasons. First, the final soil condition after EDTA-enhanced bioelectrokinetics must be neutral because the chelating reaction of EDTA and heavy metals is maximized in the neutral condition (pH 6-8), and this condition differs markedly from that of the competitive method, acid-enhanced electrokinetics. Because acidic soil itself can be harmful to the ecosystem, it has to be treated again, which is the most critical drawback of normal electrokinetics using acid electrolyte. In order to restore the soil pH of the finished electrokinetics, neutralizing electrokinetics was additionally performed in this study (see the SI for the detailed experimental method), and the soil pH was favorably increased to a level similar to that of EDTA-enhanced bioelectrokinetics, pH 8.8, within 4 days (Figure S7). However, the extremely high voltage gradient required during this step increased the power consumption in the acid with neutral electrokinetic process (Table 2). Therefore, EDTA-enhanced bioelectrokinetics (738.9 kWh/ ton) was more cost-effective than acid-enhanced electrokinetics (994.9 kWh/ton) for the optimal treatment of the soil, as the second reason. Third, this method is less concerned about the codissolution of soil components comparing with acid-enhanced method. The use of acids in high concentrations can affect the soil structure, which is an adverse effect in the environmental point of view. However, EDTA enables the selective extraction for Pb, Cu, and Zn mainly in the neutral condition (pH 7-10) (29). In the field of removal technology for shooting range soil, existing methods have their advantages and disadvantages. While phytoremediation is the most environmentally friendly technology, it requires long-term treatment for the highly contaminated soil (2, 7). Although soil washing, the most practical technique for metal-contaminated soil, might be considered as an alternative, it suffers the drawback of being unfeasible for a soil-containing fines (silt/clay) content in excess of 30-50% (6). The most representative advantage of the electrokinetic process is its superior performance on the fines fraction of soil (14, 20, 23). Here, we have confirmed that the hybrid electrokinetics presented in this study is a method to effectively remove heavy metals from fines as

TABLE 2. Removal Efficiency and Power Consumption of the Processes removal (%)a experiment

Pb

Cu

Zn

Korean standard limit for soilb

power consumption (kWh/ton)c

Acid EK Acid BioEK EDTA EK EDTA BioEK Acid EK + Neutral EK

92.2 7.6 63.1 92.7

66.3 71.3 24.4 64.3

36.6 52.6 23.5 45.9

satisfy exceed exceed satisfy

489.5 134.2 889.5 738.9 994.9

a Removal efficiency was calculated from the residual concentrations of heavy metals in the soil after each experiment. Korean standard limits for nonresidential area are 400 mg/kg of Pb, 500 mg/kg of Cu, and 600 mg/kg of Zn. c The power consumption was calculated as described in ref 18. The voltage gradient of each experiment was measured (Figures S5 and S6). A specific gravity of 1.5 ton/m3 was used for soil. b

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well as sand fractions (54.69% of sand, 45.31% of fines in the sieved soil). The cost-effectiveness was also discussed above in comparison with the acid-enhanced electrokinetics. However, because that is only the results of lab-scale experiments, the evaluations from pilot and field-scale studies are needed. Also, more comprehensive evaluation considering specific components, such as application of bacteria, use of chemicals, and post-treatment of waste electrolyte, should be further performed.

Acknowledgments This work was supported by Korea Ministry of Environment as “The GAIA Project”. The authors acknowledge Dr. In-Ho Yoon who provided EXAFS analysis and comments on the data. We also gratefully acknowledge three anonymous reviewers who provided constructive and invaluable comments for improving the quality of the manuscript.

Supporting Information Available Detailed experimental methods on cultivation of bacteria, electrokinetic cell, quality control for analysis, EXAFS analysis, chemical leaching test, and neutralizing electrokinetics. Figure S1 illustrates the electrokinetics apparatus; Figures S2 and S3 show the results of preliminary bioleaching experiments; Figure S4 presents the results of sequential extraction on the soil before and after bioleaching experiments; Figures S5 and S6 show the changes in the voltage gradient during acid-enhanced, EDTA-enhanced, and neutralizing electrokinetics; and Figure S7 presents the variation in the soil pH of neutralizing electrokinetics. This material is available free of charge via the Internet at http:// pubs.acs.org.

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