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TECHNICAL REPORTS

Heavy Metals in the Environment

Removal of Selenate from Water by Zerovalent Iron

Department of Environmental Sciences, University of California, Riverside, CA 92521-0424

^* Corresponding author (william.frankenberger{at}ucr.edu)

Received for publication May 25, 2004.

	ABSTRACT

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Zerovalent iron (ZVI) has been widely used in the removal of environmental contaminants from water. In this study, ZVI was used to remove selenate [Se(VI)] at a level of 1000 µg L^–1 in the presence of varying concentrations of Cl^–, SO^2–₄, NO^–₃, HCO^–₃, and PO^3–₄. Results showed that Se(VI) was rapidly removed during the corrosion of ZVI to iron oxyhydroxides (Fe_OH). During the 16 h of the experiments, 100 and 56% of the added Se(VI) was removed in 10 mM Cl^– and SO^2–₄ solutions under a closed contained system, respectively. Under an open condition, 100 and 93% of the added Se(VI) were removed in the Cl^– and SO^2–₄ solutions, respectively. Analysis of Se species in ZVI–Fe_OH revealed that selenite [Se(IV)] and nonextractable Se increased during the first 2 to 4 h of reaction, with a decrease of Se(VI) in the Cl^– experiment and no detection of Se(VI) in the SO^2–₄ experiment. Two mechanisms can be attributed to the rapid removal of Se(VI) from the solutions. One is the reduction of Se(VI) to Se(IV), followed by rapid adsorption of Se(IV) to Fe_OH. The other is the adsorption of Se(VI) directly to Fe_OH, followed by its reduction to Se(IV). The results also show that there was little effect on Se(VI) removal in the presence of Cl^– (5, 50, and 100 mM), NO^–₃ (1, 5, and 10 mM), SO^2–₄ (5 mM), HCO^–₃ (1 and 5 mM), or PO^3–₄ (1 mM) and only a slight effect in the presence of SO^2–₄ (50 and 100 mM), HCO^–₃ (10 mM), and PO^3–₄ (5 mM) during a 2-d experiment, whereas 10 mM PO^3–₄ significantly inhibited Se(VI) removal. This work suggests that ZVI may be an effective agent to remove Se from Se-contaminated agricultural drainage water.

Abbreviations: Fe_OH, iron oxyhydroxides • Se(0), elemental selenium • Se(IV), selenite • Se(VI), selenate • ZVI, zerovalent iron

	INTRODUCTION

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ELEVATED SELENIUM (Se) concentrations in agricultural drainage water have been found in many sites of California. In the San Joaquin Valley, Se in the drainage water is frequently found at a concentration level of 140 to 1400 µg L^–1 (Amweg et al., 2003; Cantafio et al., 1996; Sylvester, 1990, p. 119–124). In the Salton Sea region, elevated Se is in the range of 3 to 300 µg L^–1 in the subsurface drainwater (Setmire and Schroeder, 1998). In an effort to minimize environmental impacts of Se, the State of California Water Resources Control Board (1989) has recommended an interim maximum mean monthly Se concentration of 2 to 5 µg L^–1 in rivers and wetlands receiving agricultural drainage water. So far, no treatment technology has proven economically feasible for meeting this criterion.

Several biotreatment systems have been used to treat Se-contaminated agricultural drainage water in the San Joaquin Valley. In a pilot-scale Se bioremediation system using a Se(VI) reducer, Thauera selenatis, and acetate in liquid phase as an electron donor, Cantafio et al. (1996) reported that bacterial reduction of Se(VI) to elemental selenium [Se(0)] proceeded rapidly in a series of four columns filled with Jaeger (Houston, TX) Tri-Packs and/or silica sand. About 98% of Se(VI) and Se(IV) in agricultural drainage water was reduced. However, high costs make it less feasible to use acetate as an electron donor and carbon source for bacteria to reduce Se(VI) to Se(0) during full-scale operation in the field.

By using economical organic carbon sources, an algal–bacterial selenium removal system (ABSRS) has been used as an economic way to remove large amounts of Se from drainage water (Lundquist et al., 1995). However, increase in concentration of the most bioavailable organic Se in the treated water creates greater bioavailability of Se to biota than that in the influent (drainage water) (Amweg et al., 2003), which could propose greater toxicological risk to biota if the treated water flows into nearby wetlands. Therefore, there is a need for alternative techniques to remove Se from drainage water.

Zerovalent iron (ZVI) is an inexpensive and moderately strong reducing agent (Genin et al., 1998). It is used as a catalyst for chemical synthesis in industrial applications (Campbell, 1988) and is capable of removing many common environmental contaminants, such as Cr(VI), U(VI), and NO^–₃ (Alowitz and Scherer, 2002; Farrell et al., 1999; Huang et al., 1998; Powell et al., 1995; Qiu et al., 2000). The corrosion of ZVI is an electrochemical process during which iron is oxidized to soluble Fe²⁺. Reaction of Fe²⁺ with OH^– forms Fe(OH)₂, which can be further oxidized to green rust I [Fe₃(II)Fe(III)(OH)₈Cl] ([Fe₄(III)Fe₂(II)(OH)₁₂][CO₃·2H₂O]), green rust II ([Fe₄(III)Fe₃(II)(OH)₁₂][SO₄·2H₂O]), magnetite (-Fe₃O₄), lepidocrocite (-FeOOH), ferrihydrite Fe(OH)₃, and goethite (-FeOOH) (Furukawa et al., 2002; Genin et al., 1998; Phillips et al., 2003). The green rust can also serve as a reducing agent to abiotically reduce Se(VI) to Se(IV) and Se(0) (Myneni et al., 1997; Refait et al., 2000). Ferrihydrite and goethite are also strong adsorbents that can be used to effectively remove Se(IV) from water (Balistrieri and Chao, 1987; 1990). Therefore, ZVI may be an inexpensive potential agent to remove Se(VI) from Se-contaminated water.

This study was conducted to determine the removal of Se(VI) from water by ZVI in the presence of varying concentrations of Cl^–, SO^2–₄, NO^–₃, HCO^–₃, and PO^3–₄. The removal process was characterized in a series of batch experiments.

	MATERIALS AND METHODS

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Selenium standards used in this study included Se(IV) (selenite reference standard solution) purchased from Fisher Scientific (Hampton, NH) and Se(VI) (Na₂SeO₄) from Sigma (St. Louis, MO). Elemental Se was obtained by the chemical reaction of Se(IV) with ascorbic acid (Sigma) (Combs et al., 1996). Fifteen milliliters of 1000 mg L^–1 of Se(IV) was added to a 50-mL glass beaker, followed by the addition of 0.05 g of ascorbic acid. The beaker was slightly shaken by hand for less than 0.5 min and then red Se(0) was formed. The red Se(0) solution was passed through a 0.1-µm filter, followed by washing six times with deionized water to remove ascorbic acid and unreacted Se(IV). Elemental Se accumulated on the filter membrane was transferred to a 20-mL glass vial with deionized water and was then sonicated for 15 min before use. Other chemicals, such as sodium sulfate (Na₂SO₄), sodium chloride (NaCl), sodium hydroxide (NaOH), and sodium phosphate (NaH₂PO₄ and Na₂HPO₄), were purchased from Fisher Scientific. Zerovalent iron (40–60 mesh) was obtained from Peerless Metal Powers and Abrasive (Detroit, MI) and was used as received. The surface area of ZVI particles was 1.63 m² g^–1.

Stability Test of Selenium Species in a Phosphate Anion Extractant
Stability of Se species in extractants needed to be tested to correctly extract Se adsorbed in ZVI–Fe_OH described below. In this study, the term "Fe oxyhydroxides" is defined to represent all corrosion products of ZVI in NaCl and Na₂SO₄ solutions. Phosphate and NaOH, the most commonly used extractants in Se speciation studies (Jackson and Miller, 2000), were selected for extracting Se species in ZVI–Fe_OH. The extractant solution containing 0.35 M of PO^3–₄ with a pH range of 7 to 12.6 was used. In the test, 1.25 g of ZVI was added to each 40-mL glass vial, followed by 1 mL of 1 mM Cl^– and SO^2–₄ solution. The vials were opened to the atmosphere and stood overnight allowing the formation of Fe_OH (yellowish-brown color) on the surface of some ZVI particles before the addition of the extractants. Afterward, 24 mL of the extractant was added to each vial, followed by spiking with standard Se(VI), Se(IV), and Se(0) in different vials to a final concentration of 1000, 1000, and 2600 µg L^–1, respectively. The vials were tightly capped and horizontally shaken in a gyrotory shaker for 2 and 5 h at 180 rpm. Then, solution samples were collected for analysis of Se species after centrifugation (Microfuge 11; Beckman, Fullerton, CA) for 14 min at 12000 rpm to precipitate ZVI–Fe_OH.

Characterization of Selenate Removal
A series of time course experiments were conducted in the laboratory to determine the removal of Se(VI) by ZVI in a 10 mM Cl^– and a 10 mM SO^2–₄ solution. These experiments were designed to examine whether Se(VI) can be removed from solution. Therefore, pH and Eh were not controlled during the experiments and dissolved O₂ in solution was not removed before the experiments. In a closed contained experiment, 25 mL of 10 mM Cl^– or SO^2–₄ solution containing 1000 µg L^–1 of Se(VI) was placed to 40-mL EPA glass vials, followed by the addition of 1.25 g of ZVI. The vials were tightly capped and placed horizontally in a gyrotory shaker for shaking at a speed of 180 rpm. The vials were removed from the shaker at 0.125, 0.25, 0.5, 1, 2, 4, 7, and 16 h. Each sample was passed through a 0.2-µm membrane filter (Fisher Scientific) into another 40-mL glass vial for Se species, pH, and Eh analysis. The ZVI–Fe_OH residue in the vials was washed two times with 5 mL of deionized water and refiltrated. The ZVI–Fe_OH on the filter membrane was replaced into the original vials for extraction of accumulated Se to ZVI–Fe_OH particles. During the extraction, 25 mL of 0.35 M PO^3–₄ solution with a pH of 12.6 was added to each vial. The vials were tightly capped, placed horizontally in a gyrotory shaker, and shaken for 2 h at a speed of 180 rpm. Then, solution samples were collected for Se species analysis by centrifugation for 14 min at 12000 rpm to precipitate ZVI–Fe_OH. All of the experiments were run in triplicate at room temperature (21 ± 1°C).

In an open system experiment, 25 mL of 10 mM Cl^– or SO^2–₄ solution containing 1000 µg L^–1 of Se(VI) was placed in 50-mL Pyrex flasks, followed by the addition of 1.25 g of ZVI. The flasks were not capped and shaken in a gyrotory shaker at a speed of 180 rpm. The sample collection and extraction of accumulated Se to ZVI–Fe_OH were the same as the closed system experiment described above.

Effects of Different Anions on Selenate Removal
Effects of different anions (Cl^–, SO^2–₄, HCO^–₃, PO^3–₄, and NO^–₃) on Se(VI) removal by ZVI were examined in a series of batch experiments under aerobic conditions. In these experiments, 150 mL of 1000 µg L^–1 of Se(VI) containing one of the anions was placed in 250-mL Pyrex flasks, followed by the addition of 5 g of ZVI. These anions included a Cl^– and SO^2–₄ level of 5, 50, and 100 mM, and a HCO^–₃, PO^3–₄, and NO^–₃ range of 1, 5, and 10 mM. The flasks were not capped and shaken in a gyrotory shaker at a speed of 180 rpm. We collected 1.5 mL of the water samples in each flask at 0.5, 1, 2, 4, 7, 24, 31, and 48 h for Se species analysis. Before analysis, the samples were centrifuged for 14 min at 12000 rpm to precipitate ZVI–Fe_OH. The pH and Eh in the samples were not determined. All of the experiments were run in triplicate at room temperature (21 ± 1°C). The rate constant of Se(VI) removal by ZVI in these experiments was calculated using a simple first-order kinetics equation: dSe(VI)/dt = –kSe(VI), where k is the rate constant of Se(VI) removal. Selenium data that were close to zero were not used because these data were not in a linear range in the rate constant calculation.

Analysis
Redox potential and pH in the filtered samples were immediately measured after collection with a 720A pH/ISE meter (Thermo Orion, Beverly, MA). pH was measured using an Accumet pH combination electrode (Fisher Scientific). The redox potential was measured with an Accumet combination platinum electrode (Ag/AgCl). The measured potential (Eh_measured) was converted to potential in the solution (Eh_actual) relative to a standard H electrode as Eh_actual = Eh_measured + 224.4 mV (Jayaweera and Biggar, 1996).

Only total soluble Se and Se(IV) were monitored in the filtered samples and PO^3–₄ extract because there were no organic materials and organic Se in the solution–ZVI–Fe_OH system. Selenite in the samples was determined in a 6 M HCl solution. Total soluble Se in the samples was determined after a reduction of Se(VI) to Se(IV) in 6 M HCl (8.9 mL) added with 0.1 mL of 5% K₂S₂O₈ at 90°C for 15 min. Selenium concentrations in all prepared solutions were analyzed by hydride generation atomic absorption spectrometry (HGAAS) (Zhang et al., 1999b). The Se(VI) concentration was calculated as the difference between total soluble Se concentration and Se(IV). Estimation of nonextractable Se was performed as the differences between the added Se and sum of the total soluble Se and total extractable Se in each sample.

	RESULTS

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Stability of Selenium Species in the Phosphate Anion Solutions with Different pH
Extraction effectiveness of adsorbed Se from soil constituents is commonly related to the composition and concentrations of extractants used (Jackson and Miller, 2000). Selenite adsorption results through its replacement of the surface hydroxyl groups and its adsorption decreases with increasing OH^– concentrations (as the pH increases). Phosphate is considered to have a similar affinity with Se(IV) binding on soil constituents (Balistrieri and Chao, 1987; Goldberg, 1985) and it can replace Se(IV) on adsorption sites. In this study, stability of Se species [Se(0), Se(VI), and Se(IV)] in a 0.35 M PO^3–₄ solution with a pH range of 7 to 12.6 was tested in a ZVI–Fe_OH system (Fig. 1) . Selenate was stable in the PO^3–₄ solutions during a 2- and 5-h reaction, indicating that Se(VI) was not removed by ZVI–Fe_OH. Selenite was also stable in the PO^3–₄ solutions with a pH range of 9 to 12.6 during a 2-h reaction. About 49 and 94% of the added Se(IV) were recovered in the pH 7 and 8 solutions, respectively. A similar reaction of Se(IV) was observed in the PO^3–₄ solutions with a pH range of 7 to 11 during a 5-h reaction. In the pH 12 and 12.6 solutions, 6 and 14% of the added Se(IV) were oxidized to Se(VI). Elemental Se was relatively stable in the PO^3–₄ solutions during a 2-h reaction with less than 3% of the added Se(0) being oxidized to Se(IV). Oxidation of the added Se(0) increased to 1.5 to 8% during a 5-h reaction. Considering stability of the Se species in the PO^3–₄ solutions and high efficiency of the extraction of adsorbed Se species in a high pH solution (Jackson and Miller, 2000; Zhang et al., 1999a), the PO^3–₄ solution with the highest pH (12.6) tested and a 2-h extraction period were selected for extracting Se accumulated to ZVI–Fe_OH on the removal of Se(VI) in the Cl^– and SO^2–₄ solutions.

View larger version (32K): [in this window] [in a new window]   Fig. 1. Stability of Se species in a 0.35 M PO3–4 solution containing zerovalent iron (ZVI) and iron oxyhydroxides (FeOH) for 2- (left figures) and 5-h reactions (right figures). The pH range of PO3–4 solution was 7 to 12.6. Error bars show one standard deviation (n = 3).

View larger version (34K): [in this window] [in a new window]   Fig. 2. Removal of Se(VI) in 10 mM Cl– (left figures) and SO2–4 (right figures) solutions by zerovalent iron (ZVI) under a closed condition. (A) Changes in pH and Eh during the removal of Se(VI). (B) Removal of Se(VI) in the solutions. (C) Selenium species in the ZVI and iron oxyhydroxides (FeOH) extracted by a 0.35 M PO3–4 solution (pH 12.6). Error bars show one standard deviation (n = 3).

Under the open condition (Fig. 3) , total soluble Se in the 10 mM Cl^– solution decreased rapidly from 1000 to 46.9 µg L^–1, with an increase in pH from 5.7 to 7.6 and a drop of Eh from 0.62 to a range of 0.34 to 0.43 V during the first 2 h of the reaction. Total soluble Se dropped to 4 µg L^–1 during rest of the experiment. pH and Eh stabilized at the levels of 6.9 to 7.6 and 0.34 to 0.43 mV, respectively. Selenate was the only Se species in the solution during the 16 h of the experiment. Total PO^3–₄–extractable Se increased rapidly to 12.8 mg kg^–1 during the first 4 h of the experiment, and then decreased to 2.95 mg kg^–1 at 16 h. Selenite was the major form of Se in the extract, accounting for 59 to 99%. Selenate was relatively low, having a peak of 2.38 mg kg^–1 at 0.5 h.

View larger version (35K): [in this window] [in a new window]   Fig. 3. Removal of Se(VI) in 10 mM Cl– (left figures) and SO2–4 (right figures) solutions by zerovalent iron (ZVI) under an open condition. (A) Changes in pH and Eh during the removal of Se(VI). (B) Removal of Se(VI) in the solutions. (C) Selenium species in the ZVI and iron oxyhydroxides (FeOH) extracted by a 0.35 M PO3–4 solution (pH 12.6). Error bars show one standard deviation (n = 3).

Effects of Anions on Selenate Removal
Effect of Cl^–, SO^2–₄, HCO^–₃, PO^3–₄, and NO^–₃ on Se(VI) removal by ZVI under aerobic conditions is illustrated in Fig. 4 and 5 . Only Se(VI) (total Se) is presented in Fig. 4 and 5 because no measurable Se(IV) was detected in the samples. Selenate in the all of the solutions decreased during the experiments. More than 99% of the added Se(VI) was removed in the Cl^– solutions (5, 50, and 100 mM) (Fig. 4 and Table 1). In contrast, 99, 87.4, and 79% of the added Se(VI) was removed in the 5, 50, and 100 mM SO^2–₄ solutions, respectively.

View larger version (28K): [in this window] [in a new window]   Fig. 4. Effect of Cl– and SO2–4 concentrations on the removal of Se(VI) by zerovalent iron (ZVI). Error bars show one standard deviation (n = 3). Inserted small figures show the first-order kinetics of Se(VI) removal.

View larger version (24K): [in this window] [in a new window]   Fig. 5. Effect of HCO–3, PO3–4, and NO–3 concentrations on the removal of Se(VI) by zerovalent iron (ZVI). Error bars show one standard deviation (n = 3). Inserted small figures show the first-order kinetics of Se(VI) removal.

	DISCUSSION

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The major mechanism for the rapid removal of Se(VI) by ZVI–Fe_OH can be attributed to the reduction of Se(VI) to Se(IV) by Fe(II) oxidized from ZVI (Murphy, 1988), followed by rapid adsorption of Se(IV) to Fe_OH due to lower redox potentials of Fe(III) and Fe(II) than that of Se(VI) and Se(IV) (Genin et al., 1998) and stronger adsorption of Se(IV) to Fe_OH than Se(VI) (Balistrieri and Chao, 1987, 1990). Zingaro et al. (1997) reported that 99% of added Se(VI) was rapidly reduced after a 7-h reaction with Fe(II) at an initial Fe(II) to Se(VI) molar ratio of 9. Manning and Burau (1995) extracted both Se(VI) and Se(IV) from Fe precipitates after a reaction of Fe(II) with Se(VI) in 0.1 M NaClO₄ solution. In this study, little Se(IV) was found in both 10 mM Cl^– and SO^2–₄ solutions during the removal of Se(VI), revealing that Se(IV) formed from the reduction Se(VI) to Se(IV) by ZVI–Fe(II) was rapidly adsorbed to Fe_OH. Analysis of Se species in the PO^3–₄ extract shows that Se(IV) increased rapidly in the Fe_OH, with a rapid decrease of Se(VI) in the Cl^– and SO^2–₄ solutions.

Another mechanism that cannot be ruled out for the removal of Se(VI) might be the direct adsorption of Se(VI) to Fe_OH, followed by the reduction of adsorbed Se(VI) to Se(IV). Several studies have reported that Se(VI) can be adsorbed to Fe_OH via surface complexes of outer-sphere layer or inner-sphere layer, and/or both layers (Hayes et al., 1987; Manceau and Charlet, 1994; Su and Suarez, 2000). Refait et al. (2000) reported that SO^2–₄ in green rust II , one of the Fe corrosion products, can be replaced by SeO^2–₄ to form GR and Se(VI) can be reduced to Se(IV) after the replacement. In a study on abiotic reduction of Se(VI) by green rust using X-ray absorption near edge structure (XANES) and extended X-ray absorption fine structure (EXAFS) spectroscopy, Myneni et al. (1997) reported that Se(VI) can be incorporated into the interlayers of green rust to form bidentate binuclear and edge-sharing complex with structural Fe(II) and reduced immediately to Se(IV). In this study, we observed greenish Fe_OH on the filter membranes after filtration at the initial period of the experiments, showing the existence of green rust I in the Cl^– solution and green rust II in the SO^2–₄ solution. Analysis of Se species in the ZVI–Fe_OH revealed that extracted Se(VI) from ZVI–Fe_OH accounted for as high as 30 to 40% of total extractable Se during first 0.5 h of the Cl^– experiment and then decreased rapidly to zero, providing evidence that Se(VI) was adsorbed to Fe_OH. These studies indicate that Se(VI) can be reduced rapidly to Se(IV) when it is adsorbed to Fe_OH (e.g., green rust I and II).

Further reduction of Se(IV) to Se(0) might occur in the Fe_OH. Calculation of Se mass balance showed that nonextractable Se increased with time. At the end of the experiment, nonextractable Se was 17.3 and 9.75 mg kg^–1 in the Cl^– and SO^2–₄ experiments under an open condition, respectively. Under a closed contained system, it was 6.86 and 3.78 mg kg^–1 in the Cl^– and SO^2–₄ experiments, respectively. By using XANES and EXAFS spectroscopy, Myneni et al. (1997) detected that Se(0) existed in the green rust after a reduction of Se(VI). Using XANES, Roberson (1999) also reported that Se(IV) and Se(0) was observed in Fe_OH during a study on the removal of Se(VI) by ZVI. After a reaction of Se(VI) with Fe(II), Zingaro et al. (1997) identified Se(0) in the Fe precipitates using X-ray photoelectron spectroscopy (XPS) and transmission electron microscopy (TEM)–energy dispersive spectroscopy (EDS) analysis. These results may suggest that nonextractable Se in the present study might partly be a form of Se(0), which cannot be extracted by a basic PO^3–₄ extractant in a short period of the time (2 h). The increase of nonextractable Se may reveal a further reduction of Se(IV) to Se(0). Part of nonextractable Se might be attributed to the formation of Fe(II)–Se(IV) precipitates [Fe₂(SeO₃)₃ and Fe₂(OH)₄SeO₃] in the both Cl^– and SO^2–₄ solutions (Elrashidi et al., 1987).

Experimental conditions affect the removal of Se(VI) by ZVI. Under a closed contained system, ZVI in the vials can be reciprocally moved during shaking due to horizontal placement of the vials in a shaker. Rapid reaction of ZVI–Fe_OH with Se(VI) resulted in a removal of 97 and 40% of the added Se(VI) during the first hour of the experiment in 10 mM Cl^– and SO^2–₄ solutions, respectively. Removal of Se(VI) was slow in the SO^2–₄ solution during the rest of the experiment. One possible reason is that ZVI was aerobically oxidized rapidly to Fe(II) (2Fe⁰ + 2H₂O + O₂ = 2Fe²⁺ + 4OH^–) at an initial period of time when dissolved O₂ and air existed in the closed contained vials. After consuming O₂, anaerobic reactions of ZVI took place (Fe⁰ + 2H₂O = Fe²⁺ + 2OH^– + H₂), which produced H₂. Accumulation of H₂ limited continuous corrosion of ZVI under a closed contained system, thus slowing the production of Fe_OH used for the removal of Se(VI). Under an open condition, ZVI was not mobilized at the bottom of the flasks. Relatively slow corrosion of ZVI resulted in relatively slower removal of Se(VI) at an initial period of the experiment than that under the closed contained condition. However, continuous corrosion of ZVI by dissolved O₂ removed all added Se(VI) in the Cl^– solution at 7 h, and removed greater amounts of Se(VI) in the SO^2–₄ solution at end of the experiment than that in the closed contained system. Improvement of Se removal from aqueous solution in the presence of dissolved O₂ by ZVI can also be found in Murphy's (1988) study, in which he reported that Se(VI) removal was much greater under open conditions than that under a condition of purging with argon. Recently, Meng et al. (2002) reported that >97% of SeCN^– was removed from water by ZVI in the presence of dissolved O₂, whereas only 8% of SeCN^– was removed in water purged with N₂ after 7h.

Relatively slower removal of Se(VI) in the SO^2–₄ solutions than in the Cl^– solutions can be partially attributed to the similarities in the chemical properties of SO^2–₄ and Se(VI). In this study, the concentrations of SO^2–₄ were much greater than Se(VI). High SO^2–₄ concentrations can compete for adsorption sites on Fe_OH, thus delaying and limiting the removal of Se(VI). Relatively slow removal of Se(VI) in the SO^2–₄ solutions might also be partially attributed to rapid corrosion of ZVI in the SO^2–₄ solutions (Hunt, 2000), resulting in a higher pH. This higher pH in the SO^2–₄ solution can decrease Se adsorption, causing a relatively slow removal of Se(VI).

Higher PO^3–₄ levels significantly affected the removal of Se(VI) by ZVI–Fe_OH. In comparison with Se(VI) removal in the 5 mM Cl^– solution, 1 mM PO^3–₄ only had a slight effect on the removal of Se(VI). Increases in the concentrations of PO^3–₄ to 5 and 10 mM significantly reduced Se(VI) removal, with low k values of 0.037 and 0.011 h^–1, respectively. This decrease of Se(VI) removal might be attributed to the reaction of Fe(II) with PO^3–₄. The formation of Fe-PO^3–₄ minerals [e.g., Fe₃(PO₄)₂(H₂O)₈, vivianite] consumes Fe(II) used for reducing Se(VI), resulting in a lower removal of Se(VI). The presence of PO^3–₄ can also compete for adsorption sites of Fe_OH, thus reducing adsorption of Se (Balistrieri and Chao, 1987; Goldberg, 1985).

Higher levels of HCO^–₃ can also reduce the removal of Se(VI) in HCO^–₃–CO^2–₃–ZVI–Fe_OH. Reaction of Fe(II) with CO^2–₃ can form FeCO₃ (siderite) that is often found in ZVI barriers used for removing environmental contaminants from ground water (Phillips et al., 2003), and reduce the amounts of Fe(II) available for reducing Se(VI). In this study, increasing the concentrations of HCO^–₃ from 1 to 10 mM led to a 18% decrease of Se(VI) removal in the ZVI–Fe_OH system.

Nitrate is commonly considered as a competitive electron acceptor in bacterial reduction of Se(VI) to Se(0) because the redox potential of NO^–₃ to N₂ in an aquatic system is very similar to that of Se(VI) to Se(IV) and much higher than Se(IV) to Se(0) (Masscheleyn and Patrick, 1993). However, this study reveals that NO^–₃ in a range of 1 to 10 mM had little effect on the removal of Se(VI) by ZVI–Fe_OH in the Se–NO^–₃–ZVI system. Like the 5 mM Cl^– solution, about 100% of the added Se(VI) was removed from solutions containing 1, 5, and 10 mM of NO^–₃ during the first 24 h of reaction. The most likely reason is that the major reduction of Se(VI) to Se(IV) was caused by Fe(II) so that the removal of Se(VI) is controlled by oxidation of ZVI to Fe(II) by dissolved O₂ under an open condition. The presence of NO^–₃ did not significantly affect oxidation of ZVI to Fe(II) by dissolved O₂, thus resulting in little effect of NO^–₃ on Se(VI) removal by ZVI.

	CONCLUSIONS

TOP ABSTRACT INTRODUCTION MATERIALS AND METHODS RESULTS DISCUSSION CONCLUSIONS REFERENCES

 
Results from this study reveal that Se(VI) can be removed from aqueous solutions with varying concentrations of Cl^–, SO^2–₄, NO^–₃, HCO^–₃, and PO^3–₄. During the 16 h of the experiment, 100 and 56% of the added Se(VI) were removed in 10 mM Cl^– and SO^2–₄ solutions under a closed contained system, respectively. Under an open condition, 100 and 93% of the added Se(VI) were removed in the Cl^– and SO^2–₄ solutions, respectively. The experiments also showed that there was little effect of Cl^– (5, 50, and 100 mM), SO^2–₄ (5 and 50 mM), NO^–₃ (1, 5, and 10 mM), HCO^–₃ (1 and 5 mM), and PO^3–₄ (1 mM) and a slight effect of SO^2–₄ (100 mM), HCO^–₃ (10 mM), and PO^3–₄ (5 mM) on Se(VI) removal. Only higher PO^3–₄ (10 mM) levels significantly inhibited Se(VI) removal by ZVI. In California, Se-contaminated drainage water contains an average concentration of 987 (Cl^–), 2282 , 97 , 214 , and 0.12 mg L^–1 (Oswald et al., 1989), which are much lower levels than those tested in our study. Our work suggests that ZVI may be an inexpensive agent that can be used to treat Se-contaminated drainage water.

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