A Novel Combination of Anaerobic Bioleaching and Electrokinetics for

Nov 5, 2009 - This study provides evidence that a hybrid method integrating anaerobic bioleaching and electrokinetics is superior to individual method...
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Environ. Sci. Technol. 2009 43, 9354–9360

A Novel Combination of Anaerobic Bioleaching and Electrokinetics for Arsenic Removal from Mine Tailing Soil KEUN-YOUNG LEE,† IN-HO YOON,† B Y U N G - T A E L E E , ‡ S O O N - O H K I M , * ,§ A N D K Y O U N G - W O O N G K I M * ,† Department of Environmental Science and Engineering, Gwangju Institute of Science and Technology (GIST), Gwangju 500-712, Republic of Korea, Department of Chemistry and Geochemistry, Colorado School of Mines, 1500 Illinois Street, Golden, Colorado 80401, and Department of Earth and Environmental Sciences and Research Institute of Natural Science, Gyeongsang National University, Jinju 660-701, Republic of Korea

Received June 7, 2009. Revised manuscript received October 23, 2009. Accepted October 24, 2009.

This study provides evidence that a hybrid method integrating anaerobic bioleaching and electrokinetics is superior to individual methods for arsenic (As) removal from mine tailing soil. Bioleaching was performed using static reactors in batch tests and flow conditions in column test, and each test was sequentially combined with electrokinetics. In the bioleaching, indigenous bacteria were stimulated by the injection of carbon sources into soil, leading to the mobilization of As with the concurrent release of Fe and Mn. Compared with the batchtype bioleaching process, the combined process showed enhanced removal efficiency in the equivalent time. Although the transport fluid bioleaching conditions were inadequate for As removal, despite long treatment duration, when followed by electrokinetics the combined process achieved 66.5% removal of As from the soil. The improvement of As removal after the combined process was not remarkable, compared with single electrokinetics, whereas a cost reduction of 26.4% was achieved by the reduced duration of electrokinetics. The As removal performance of electrokinetics was significantly dependent on the chemical species of As converted via microbial metal reduction in the anaerobic bioleaching. The synergistic effect of the combined process holds the promise of significant time and cost savings in As remediation.

Introduction The introduction of arsenic (As) into the environment has significantly adverse effects and the major sources are mining and smelting operations, As-containing pesticides and herbicides, and various industrial activities that use As (1). Concern has been raised, particularly in some South Asian countries, over the danger represented by As-contaminated * Address correspondence to either author. Phone: +82-55-7516275 (S.O.K.); +82-62-970-2442 (K.W.K.). Fax: +82-55-757-2015 (S.O.K.); +82-62-970-2434 (K.W.K.). E-mail: [email protected] (S.O.K.); [email protected] (K.W.K.). † Gwangju Institute of Science and Technology (GIST). ‡ Colorado School of Mines. § Gyeongsang National University. 9354

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groundwater due to its potential for immediate human exposure (2, 3). Biotic and abiotic processes can easily transform immobile or insoluble As into a soluble form under environmental conditions (4, 5), thereby allowing As in the soil to enter the groundwater and eventually the food chain. Therefore, active remediation of As-contaminated soils is of great importance and such strategies fall into two main categories: chemical solidification/stabilization (6) and separation processes by washing/flushing, electrokinetics, and phytoremediation (7, 8). Anaerobic As bioleaching, which has been represented by microbial metal reduction, is ubiquitous in subsurface environments (1, 9-12), including mining-impacted areas (5, 13-16). In our recent works, the dissolution of As, along with Fe and Mn, under anaerobic conditions was significantly enhanced by activating the indigenous bacteria in several tailing soils collected from different sites, although the extent of mobilization varied depending on the forms of As, the original biomass, and the utilized carbon sources (15, 16). Though such a phenomenon could be interpreted as an extension of As contamination, from a different viewpoint, it could be applied as an active remediation technique for the microbial extraction of As from soils, such as the previously reported examples of microbial Fe and Mn reductions for other heavy metals (17, 18). Because As associated with such Fe and Mn (hydro)oxides forms a high proportion in tailing soils, as well as other As-contaminated soils, anaerobic bioleaching is favorable to mobilize As from soils. Although electrokinetic remediation technology has been shown in laboratory and field studies to be a promising method for simultaneously recovering multiple metal contaminants, the mobility and removal efficiency of As can be low relative to other heavy metals (19, 20). In common environments, the major As species are oxyanions; As(V) as H2AsO4- and HAsO42-, As(III) as H2AsO3-, and a neutral hydroxy complex, As(III) as H3AsO30. Due to the specific characteristics of these species, their mobility is affected by pH and the valence states, more so than with other heavy metals, which introduces some difficulties in As removal using electrokinetics and other techniques. Thus, the development of remediation technology specifically focused on As removal in soils is a priority. As(III) is more mobile than As(V) in nature, and the redox chemistry of As needs to be considered in any attempt to elevate the applicability of electrokinetics. Generally, anionic and neutral forms of As migrate toward the anode and cathode, respectively, in electrokinetics. To achieve the desired process efficiency and system performance, several studies have reported enhancement schemes via electrolyte conditioning (7, 21, 22). Based on the results of these studies, alkaline conditions appear the most favorable for As electromigration, with the drawback that As migration is very slow and time-consuming (19). Currently, most combinations of electrokinetics and bioremediation, termed bioelectrokinetics, have been used to introduce microorganisms, water, and nutrients into lowpermeability soil by applying an electric field to enhance the biodegradation of organic pollutants (23). While this approach has been actively studied, little research has focused on combination approaches for heavy metal contaminants, probably due to the nondegradable nature of heavy metals. Maini et al. (24) first reported microbially enhanced electrokinetic remediation for the removal of a heavy metal, in which soils artificially contaminated by Cu were treated by the sequential application of aerobic bioleaching and electrokinetics. Although their study demonstrated the ap10.1021/es901544x CCC: $40.75

 2009 American Chemical Society

Published on Web 11/05/2009

FIGURE 1. Experimental scheme of the individual and combined processes.

TABLE 1. Experimental Conditions, Removal Efficiency, and Mass Balance of the As Removal Processes bioleaching experiment

comment

type

electrokinetics

duration current (days) (mA)

voltage (V)

duration power consumption total As (days) (kWh/ton) removal (%) recovery (%)c

until 2 days after 2 days control 1 2 3a control 4 5 6b

no carbon batch BL only batch BL only batch BL and EK no carbon column BL only EK only column BL and EK

batch batch batch batch column column column

14 8 20 8 20 28 28

10

43.2 ( 21.5

3.9 ( 2.3

12

74.3

10 10

45.4 ( 16.9 62.9 ( 32.7

3.9 ( 2.4 4.4 ( 2.9

44 16

150.9 111.1

3.88 31.1 ( 4.7 35.0 ( 2.7 68.2 ( 6.3 0.38 17.3 ( 15.6 63.6 ( 2.3 66.5 ( 3.7

98.3 105.7 97.6 97.0

a

The soil pretreated by batch-type bioleaching was transferred into the electrokinetic soil cell. b The soil cell used in the column-type bioleaching experiment was directly connected with electrode compartments in the electrokinetic apparatus. c Recovery ) (amount of As in both electrolytes/amount of As removed from soil) × 100.

plicability of the electrokinetic process for the removal of heavy metals through the amendment of the soil using inexpensive sulfur addition, the decreased soil pH induced by bacterial sulfur oxidation, while suitable for normal heavy metals, is not suitable for As, as mentioned above. In the present study, therefore, the combination of anaerobic bioleaching and electrokinetic remediation was applied to highly As-contaminated mine tailing soils. Bioleaching was performed using static reactors in batch tests and flow conditions in column tests, and each test was sequentially combined with electrokinetic treatment. The main study objectives were to evaluate the process efficiency of both the combined process and its individual components in terms of As removal and cost expenditure, and to investigate the effects of the accompanying geochemical and microbial transformations of As on the process efficiency.

Experimental Section Soil. As-contaminated mine tailing soil was taken from the abandoned Songcheon gold mine located in the mideastern region of the Korean Peninsula. Collected at a depth of 0-30 cm from the surface of the tailing-dumping area, a composite, representative sample was prepared by mixing five samples taken within a 10 m2 area and the sample was stored without

drying to ensure survival of the indigenous bacteria. The tailing soil was predominantly sandy loam (soil texture according to the U.S. Department of Agriculture) with a 0.89% mass loss on ignition (400 °C for 24 h) and pH 5.7-6.2 (5 g soil in 50 mL water). The Fe/As minerals in this soil were arsenopyrite (FeAsS) as the primary mineral and scorodite (FeAsO4 · 2H2O) as the secondary mineral (see Supporting Information (SI) Figures S1 and S2), with a total As concentration of 4,023 ( 71 mg kg-1. Bioleaching Batch Experiments. A diagram of the individual and combined processes is shown in Figure 1 and all As removal and control experiments are summarized in Table 1. Three hundred grams of the soil sample were mixed with 300 mL of leaching solution, containing 100 mM of glucose and lactate, at a ratio of 2:1 (v/v) as carbon sources, and the pH of the solution was adjusted to ∼7 using 0.1 M NaOH. This mixture was placed in two rectangular, polypropylene, 1.2 L, bioleaching reactors, agitated in a rotating shaker (180 rpm) at 30 °C under anaerobic conditions for 8 days (experiments 1 and 3) and 20 days (experiment 2), with distilled water but without carbon sources as a control. All experiments repeated in triplicate. Following these experiments, soil samples were taken directly from the soil bed for further analysis. VOL. 43, NO. 24, 2009 / ENVIRONMENTAL SCIENCE & TECHNOLOGY

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Bioleaching Column Experiments. Arsenic was removed under transport conditions in duplicate using a cylindrical column (20 cm length, 4 cm diameter) packed with soil for a total mass of 370 ( 5 g (bulk density ) 1.47 ( 0.05 g cm-3), with the same 10 mM carbon source used in the batch experiments, and deoxygenated by constant N2 purging. The influent at the bottom of the column had a continuous flow of 18.2 mL h-1 (4.1 pore volume/day, pore water velocity ) 1.45 cm h-1) for 28 days (experiments 4 and 6), and the control had distilled water as the influent. The process performance was monitored periodically by collecting the effluent and measuring the pH, Eh, and concentrations of free As, Fe, and Mn. After treatment, the soil cell was divided into four sections for sample collection (Figure 1B). Electrokinetic Experiments. Two experiments combining the bioleaching and electrokinetic processes (experiments 3 and 6) and one using only electrokinetics (experiment 5) were performed for the specified durations. A soil sample of batch-type bioleaching that had been pretreated for 8 days was transferred into the electrokinetic soil cell in experiment 3 (Figure 1A and C), by discarding the leaching solution. The column bioleaching apparatus was initially designed to be connected to the electrode compartments (Figure 1B and C). Experiment 5 was conducted using electrokinetics without any bioleaching pretreatment. Apart from these variations, the experimental conditions of the three electrokinetic applications were the same. For electrical applications, the cathodes were stainless steel and anodes titanium-coated stainless steel. Mesh-type electrodes lined with filter paper to allow the passage of ions and water were located between the soil cell and electrode compartments. Each electrode compartment was connected to an electrolyte reservoir, and the solutions were circulated by peristaltic pumps (Figure 1C). In all experiments, 0.1 M Na2CO3 and 0.1 M NaOH were used as the anolyte and the catholyte solutions, respectively. Such a combination of electrolytes has been shown to promote the As removal efficiency in contaminated field soils from our previous research (7) and by others (21). The electrolyte solutions were refreshed regularly to maintain consistent pH conditions. A current density on the electrode surface area of 0.79 mA cm-2 was applied with monitoring of the voltage changes of the soil cell. Both electrolyte solutions were periodically collected to analyze the amount of accumulated As in both electrode compartments. After electrokinetic processing, soil samples were obtained from the four soil bed sections to determine the residual concentrations of As and the final soil pH. The pore water samples were also collected from each section and passed through a silica-based, anionexchange resin (LC-SAX SPE Tube, Supelco) to separate the anionic and neutral forms of As (25). The As passing through the resin was regarded as neutral As, and anionic As concentrations were calculated by subtracting the neutral As from the total As. Analytical Methods. The residual concentrations of As 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 on a shaker for 1 h (26). The chemical form of each As fraction in the tailing soils was examined using a sequential extraction technique, as suggested by Wenzel et al. (27). The sequential extraction of Fe and Mn was originally suggested by Tessier et al. (28). 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 As, Fe, and Mn. As concentrations were determined using graphite furnace-atomic absorption spectroscopy while flame-AAS was used for Fe and Mn (see SI for the quality control of the analysis). 9356

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FIGURE 2. Results of experiment 3. (A) Variation of soil pH within 6 days at each section during electrokinetics, (B) final soil pH after bioleaching process (BL) and combined bioleaching and electrokinetics (BL and EK) in the four sections of the cell, (C) transported water volume in the cathode compartment by electroosmotic flow, and (D) accumulated As in each electrolyte. XANES Analysis. The speciation of As in the tailing soil was investigated using X-ray absorption near edge structure (XANES) spectroscopy (see SI for the detailed analytic method).

Results Batch Bioleaching and Electrokinetics. The final removal efficiencies in terms of residual As concentrations in the tailing soils from each experiment are presented in Table 1. Compared to the abiotic control, a significant amount of As leaching was observed with the batch-type bioleaching simulating biotic processing stimulated by a carbon source injection. Arsenic was moved to the leaching solution at 31.1 and 35.0% efficiency within 8 and 20 days, respectively. Previous investigations have shown that the tailing soil used here contained a relatively high fraction of bioavailable As (16). The main As species in the leaching solution was As(III) (up to 80%), which was attributed to the microbial reduction of As(V) originally present in the soil. Twenty-day bioleaching (experiment 2) did not show a significant improvement in removal efficiency compared to 8 days (experiment 1), suggesting that batch-type bioleaching reached a stationary phase within 8 days (see SI Figure S3). Compared with the single batch-type bioleaching, the combined processes of bioleaching and electrokinetics (experiment 3) showed significantly enhanced removal efficiency (68.2%) in the same time (Table 1). The soil’s slightly decreased pH of 5.2 after bioleaching pretreatment gradually increased during the electrokinetic process, to exceed pH 12 by the end of the experiment. These highly alkaline conditions

FIGURE 3. Results of experiment 4 on column-type bioleaching (BL) and control column (Cont). Variation in (A) pH and Eh of effluent solution, (B) released amount of Fe and Mn, and (C) removed amount of As from soil column. resulted from the cathode production of hydroxide ions reinforced by the added NaOH solution while the anodeproduced hydrogen ions were counteracted by the Na2CO3 solution. Migration of hydroxide ions gradually increased the soil pH over time, progressing from sections 1 to 4 (Figure 2A and B). However, a buffering effect in the early stages (within 4 days) degraded the As removal and the pH increase. The anionic As electromigration was initiated after 4 days (Figure 2D), at which time most of the As was rapidly removed toward the anode while a small amount of neutral As was transported toward the cathode by electroosmosis. Column Bioleaching and Electrokinetics. Similar to the batch bioleaching results, there was a significant difference in As removal between the column-type bioleaching (experiment 4) and the control (Table 1). Furthermore, the pH and Eh variations of the effluent solution were insignificant in the control, which lacked a carbon source, while those of the bioleaching experiments significantly decreased with time (Figure 3). Although continuous N2 gas purging promoted anaerobic conditions inside the soil column, no dissolution of Fe, Mn, or As occurred for nearly 4 days, indicating a lag phase for the initiation of anaerobic bioleaching in columntype processing. After 4 days, As removal progressed with the release of both Fe and Mn. Up to 90% of the dissolved Fe, Mn, and As was in a reduced form, Fe(II), Mn(II), and As(III) (See SI Table S1), which corresponded with the decreased Eh value. Despite the significant dissolution of As in the columntype bioleaching process, the removal efficiency of As after 28 days remained inadequate (at 17.3%), whereas the combined process (experiment 6) showed enhanced As removal efficiency of 66.5% after continuous electrokinetic treatment for 16 days (Figure 1B and C). In single electrokinetics for 44 days (experiment 5), the increase of soil pH was efficiently driven, and the pH was increased from neutral to 10-12 within 2 days (Figure 4A), which enabled the As

FIGURE 4. Results of experiment 5. (A) Variation of soil pH within 6 days at each section during electrokinetics, (B) final soil pH after single electrokinetics (EK) in the four sections of the cell, (C) transported water volume in the cathode compartment by electroosmotic flow, and (D) accumulated As in each electrolyte. removal to begin almost immediately and reach a final As removal efficiency of 63.6% (Figure 4D). In both combined and single treatments, the normal electroosmosis occurred throughout the process, leading to the accumulation of small amounts of neutral As in the catholyte solution (Figures 2C and 4C). Chemical Forms of Fe, Mn, and As. The results of the sequential extraction of Fe and Mn showed that their oxide fractions were significantly reduced after both anaerobic bioleaching processes (Figure 5), which strongly indicates that the indigenous bacteria, stimulated by the added carbon sources, mainly utilized the oxide fractions of Fe and Mn as electron acceptors. The results (see SI Figure S5) confirmed that the As relative to such oxide fractions was removed from the soil. Although the As fraction of the poorly crystalline hydrous oxides remained in the soil after bioleaching, this fraction was transformed into weakly bound forms which were quickly removed in the electrokinetic steps. Such results were reflected to the reduction of the treatment duration of combined process, despite the total As removal efficiencies of combined process and single electrokinetics were not significantly different from each other. As Speciation in Soil and Pore Water. The As species was predominantly As(V) in initial tailing soil (Figure 6A). The significant amount of As(III) transformed from As(V) in the anaerobic bioleaching was reflected in the XANES peak as a shoulder in front of the As(V) peak, and the As(III) in the soil was sequentially removed in the electrokinetic step, as indicated by the shrunken As(III) peaks. In examining the distribution of residual As in the pore water collected from each section, the results indicated that anionic and neutral VOL. 43, NO. 24, 2009 / ENVIRONMENTAL SCIENCE & TECHNOLOGY

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FIGURE 5. Accumulative concentrations (mg/kg) of Fe and Mn by sequential extraction before and after bioleaching experiments. Each step of the sequential extraction was in terms of the exchangeable fraction (step 1), carbonate and specifically adsorbed fraction (step 2), Fe-Mn (hydro)oxides (step 3), organic matter and sulfides (step 4), and the residual fraction (step 5). Results of batch-type bioleaching (Batch BL) for 8 days (8 d) and 20 days (20 d), and column-type bioleaching (Column BL) at all sections (1, 2, 3, and 4).

FIGURE 7. Biogeochemical reaction model in (A) anaerobic bioleaching and (B) electrokinetics.

FIGURE 6. As speciation in soil and pore water after experiment 3. (A) Derivative As K-edge XANES spectra of the As standard materials (As powder for As(0), sodium (meta) arsenite (NaAsO2) for As(III) and sodium arsenate (Na2HAsO4 · 7H2O) for As(V)) and of the tailing soil before (Initial) and after (BL) batch-type bioleaching, and all sections (1, 2, 3, and 4) after batch-type bioleaching with electrokinetics, and (B) distribution of anionic and neutral As concentrations in pore water collected at all sections (1, 2, 3, and 4). As were transported in electric fields toward the anode by electromigration and toward the cathode by electroosmosis, respectively (Figure 6B). In an alkaline environment (pH > 9), As(III) as well as As(V) predominantly exist as anionic forms, and it was confirmed here that the dominant species of As was anionic, migrating toward the anode.

Discussion Reaction in Anaerobic Bioleaching. Direct evidence of microbial reduction in anaerobic bioleaching was clearly 9358

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presented. The biogeochemical model in this step is presented here by extracting a reasonable reaction path from previous suggestions (29, 30) (Figure 7A). The model consisted of the tailing soil initially containing As, mainly As(V), associated with Mn and Fe (hydro)oxides such as pyrolusite, ferrihydrite, goethite, hematite, and magnetite. This initial state was visually detected by spectroscopic analysis in a previous investigation (see SI Figures S1 and S2). The injected organic carbon was oxidized at the expense of such (hydro)oxides as electron acceptors, thus leaching Mn(II), Fe(II), and As(III) into solution, as confirmed by the speciation analysis conducted in this study and a previous investigation (16). Some of the dissolved As(III) was separated by discarding the solution from the soil, and a considerable portion of residual As(III) was readsorbed onto the soil particle, as detected in the XANES peak (Figure 6). Reaction in Electrokinetics. The critical process in electrokinetics for As removal from soil is the ligand exchange reaction of hydroxide ions with anionic As. Moreover, the mobilization of anionic As is enhanced by the increased negative charge on the soil surface when the pore water pH is above the point of zero charge (PZC) (31, 32). Anionic As released from the soil migrates toward the anode in the electric field. The dominant reaction in electrokinetics is modeled in Figure 7B. This series of processes explains the effect of the anaerobic bioleaching pretreatment in accelerating the As removal rate in the electrokinetic step. As aforementioned, both As(III) (H2AsO3-) and As(V) (HAsO42-) exist as oxyanions at pH > 9 and previous investigations have reported the diffusion coefficient of H2AsO3- to be up to three times higher than that of HAsO42- (diffusion coefficients based on molar mobility, 1.03 × 10-9 m2s-1 and 0.32 × 10-9 m2s-1, respectively) (33). In other words, the transformation of As(V) to As(III) by anaerobic bioleaching is preferable to alkaline electrokinetic process. Even though the buffering effect initiated by the bioleaching pretreatment was a side effect that obstructed the increase of soil pH, the As conversion through microbial metal reduction was clearly sufficient to reduce the duration of electrokinetics owing to the enhanced electromigration. The present study results have demonstrated that the As removal performance of

electrokinetics is significantly dependent on the species and chemical forms of As in the soil. Beneficial Effects of the Combined Process. Table 1 shows the average voltages until 2 days and after 2 days, as well as the power consumption in each electrokinetic experiment. The electrical resistance and voltage across the soil bed were high due to the initially dominant dissociation and desorption mechanisms and, with the increasing dominance of the migration of hydroxyl ions and dissolved and/ or desorbed As species, the voltage gradually decreased. The electrical energy per unit volume (E, kWh/m3) for electrokinetics was calculated by E)

I(∆V)TR vs

(1)

where I is the applied current, ∆V the voltage, TR the duration, and vs the soil volume. The power consumption (kWh/ton) in Table 1 is based on a specific gravity of tailing soil of 1.6 ton/cm3. At a power cost of $0.06/kWh, the expenditures for electricity of experiments 5 and 6 are $9.1 and $6.7 per ton, respectively. The 26.4% cost reduction is achieved by the combination of electrokinetics with column-type bioleaching. The extra cost for each process does not markedly affect the cost effectiveness, because both processes require the operation of two pumps (for injection and extraction of leaching solution in columntype bioleaching, and for circulation of the two electrolytes in electrokinetics) and only a small amount of chemicals and water (for glucose/lactate in bioleaching, and NaOH/ Na2CO3 in electrokinetics). On the other hand, a cost reduction of 50.5% was achieved by using the combined process of electrokinetics and batchtype bioleaching. It is improper to compare this ex situ type combination with single electrokinetics because of the difference of extra cost. However, the significant reduction of total process duration (20 days) achieved by using this combination, as well as the slightly improved removal efficiency (68.2%), proves its technical advantage. In the bioleaching processes performed here, the use of inexpensive carbon sources, with their less hazardous effects compared with chemical washing, was a desirable feature. The promotion of anaerobic bioleaching and the significant enhancement of As mobility were clearly demonstrated. However, the process drawbacks included the ex situ nature of the batch-type bioleaching, despite its rapidly achieved high efficiency, which decreased the cost effectiveness in terms of the excavation and transportation of contaminated field soils. Moreover, single processing by column-type bioleaching, used to simulate in situ fluid conditions, exhibited low efficiency despite the long treatment duration and mostly only increased the mobility of solid phase As. We consequently concluded that this technique was not appropriate for single flushing treatments for As. On the other hand, the sequential application of electrokinetic remediation incorporated with in situ bioleaching demonstrated a capability to solve this problem, with enhanced removal efficiency. Furthermore, the synergistic effects of this combination promise reduced treatment duration and cost. This novel combination is therefore considered a more efficient technology than the individually applied processes.

Acknowledgments This work was supported by Korea Ministry of Environment as “The GAIA Project”. We gratefully acknowledge three anonymous reviewers who provided constructive and invaluable comments for improving the quality of the manuscript.

Note Added after ASAP Publication There was an error in the fourth paragraph of the Results section in the version of this paper published ASAP November 5, 2009; the corrected version published ASAP November 10, 2009.

Supporting Information Available Additional information on the detailed analytical methods and the experimental results cited in the text. This material is available free of charge via the Internet at http:// pubs.acs.org.

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