Selenate Removal Research Articles

Differences in uptake and translocation of selenate and selenite by the weeping willow and hybrid willow

Due to its essentiality, deficiency, and toxicity to living organisms and the extensive use in industrial activities, selenium (Se) has become an element of global environmental and health concern. Se removal from contaminated sites using physical, chemical, and engineering techniques is quite complicated and expensive. The goal of this study was to investigate uptake and translocation of Se in willows and to provide quantitative information for field application whether Se phytoremediation is feasible and ecologically safe. Intact pre-rooted plants of hybrid willows (Salix matsudana Koidz x alba L.) and weeping willows (Salix babylonica L.) were grown hydroponically and treated with selenite or selenate at 24.0 +/- 1 degrees C for 144 h. Removal of leaves was also performed as a treatment to quantify the effect of transpiration on translocation and volatilization of Se. At the end of the study, total Se in the hydroponic solution and in different parts of plant tissues was analyzed quantitatively by hydride generation-atomic fluorescence spectrometry. The capacity of willows to assimilate both chemical forms of Se was also evaluated using detached leaves and roots in sealed glass vessels in vivo. Translocation efficiency of Se in both plants was estimated. Significant amounts of the applied selenite and selenate were eliminated from plant growth media by willows during the period of incubation. Both willows showed a significantly higher removal rate for selenate than for selenite (p < 0.05). Substantial differences existed in the distribution of both chemical forms of Se in plant materials: lower stems and roots were the major sites for accumulation of selenite and selenate, respectively. Translocation efficiency for selenite was significantly higher than that for selenate in both willow species (p < 0.01). Compared to the intact trees, remarkable decrease in the removal rate of both chemical forms of Se was found for willows without any leaves (p < 0.01). Volatilization of Se by plant leaves was estimated to be approximately 10% of the total applied selenite or selenate. Significant reduction (>20%) of selenate was observed in the sealed vessel with excised roots of willows, whereas trace amounts of selenite were eliminated from the hydroponic solution in the presence of roots. Detached leaves from neither of them reduced the concentration of selenite or selenate in the solution. Due to the significant difference in the removal rate and the distribution of the two chemical forms of Se in plant materials, the conversion of selenate to selenite in hydroponic solution prior to uptake and within plant tissues is unlikely. An independent uptake and translocation mechanisms are likely to exist for each Se chemical species. Uptake of selenate is mediated possibly through an active transport mechanism, whereas that of selenite may possibly depend on plant transpiration. Uptake velocities of selenite are linear (zero-order kinetics), while selenate removal processes obey first-order kinetics. In experiments with detached leaves in closed bottles, the cuticle of leaves was the major obstacle to extract both chemical forms of Se from the hydroponic solution. Phytovolatilization is a biological process playing an important role in Se removal. Although faster removal rates of selenate than selenite from plant growth media were observed by both willow species, selenite in plant materials was more mobile than selenate. Significant decrease in removal rates of both chemical forms of Se was detected for willows without any leaves. Significant differences in extraction, assimilation and transport pathways for selenite and selenate exist in willow trees. Phytoremediation of Se is an attractive approach of cleaning up Se contaminated environmental sites. More detailed investigation on the assimilation of Se in plant roots and transport in tissues will provide further biochemical evidence to explain the differences in uptake and translocation mechanisms between selenite and selenate in willows. A relevant phytoremediation scheme can then be designed to clean up Se contaminated sites. Willows show a great potential for uptake, assimilation and translocation of both selenite and selenate. Phytotreatment of Se is potentially an efficient and practical technology for cleaning up contaminated environmental sites.

Selenate removal in methanogenic and sulfate-reducing upflow anaerobic sludge bed reactors

This paper evaluates the use of upflow anaerobic sludge bed (UASB) bioreactors (30 °C, pH=7.0) to remove selenium oxyanions from contaminated waters (790 μg Se L −1) under methanogenic and sulfate-reducing conditions using lactate as electron donor. One UASB reactor received sulfate at different sulfate to selenate ratios, while another UASB was operated under methanogenic conditions for 132 days without sulfate in the influent. The selenate effluent concentrations in the sulfate-reducing and methanogenic reactor were 24 and 8 μg Se L −1, corresponding to removal efficiencies of 97% and 99%, respectively. X-ray diffraction (XRD) analysis and sequential extractions showed that selenium was mainly retained as elemental selenium in the biomass. However, the total dissolved selenium effluent concentrations amounted to 73 and 80 μg Se L −1, respectively, suggesting that selenate was partly converted to another selenium compound, most likely colloidally dispersed Se 0 nanoparticles. Possible intermediates of selenium reduction (selenite, dimethylselenide, dimethyldiselenide, H 2Se) could not be detected. Sulfate reducers removed selenate at molar excess of sulfate to selenate (up to a factor of 2600) and elevated dissolved sulfide concentrations (up to 168 mg L −1), but selenium removal efficiencies were limited by the applied sulfate-loading rate. In the methanogenic bioreactor, selenate and dissolved selenium removal were independent of the sulfate load, but inhibited by sulfide (101 mg L −1). The selenium removal efficiency of the methanogenic UASB abruptly improved after 58 days of operation, suggesting that a specialized selenium-converting population developed in the reactor. This paper demonstrates that both sulfate-reducing and methanogenic UASB reactors can be applied to remove selenate from contaminated natural waters and anthropogenic waste streams, e.g. agricultural drainage waters, acid mine drainage and flue gas desulfurization bleeds.

Bioconversion of Selenate in Methanogenic Anaerobic Granular Sludge

The capacity of anaerobic granular sludge to remove selenate from contaminated wastewater was investigated. The potential of different types of granular sludge to remove selenate from the liquid phase was compared to that of suspended sludge and contaminated soil and sediment samples. The selenate removal rates ranged from 400 to 1500 microg g VSS(-1) h(-1), depending on the source of biomass, electron donor, and the initial selenate concentration. The granular structure protects the microorganisms when exposed to high selenate concentrations (0.1 to 1 mM). Anaerobic granular sludge "Eerbeek," originating from a UASB reactor treating paper mill wastewater, removed about 90, 50, and 36% of 0.1, 0.5, and 1 mM of Se, respectively, from the liquid phase when incubated with 20 mM lactate at 30 degrees C and pH 7.5. Selenite, elemental Se (Se(o)), and metal selenide precipitates were the conversion products. Enrichments from the anaerobic granular sludge "Eerbeek" were able to convert 90% of the 10-mM selenate to Se(o) at a rate of 1505 microg Se(VI) g cells(-1) h(-1), a specific growth rate of 0.0125 g cells h(-1), and a yield of 0.083 g cells mg Se(-1). Both microbial metabolic processes (e.g dissimilatory reduction) as well as microbially mediated physicochemical mechanisms (adsorption and precipitation) contribute to the removal of selenate from the Se-containing medium.

Selenium speciation in anaerobic granular sludge

Chromatographic (IC-CD, GC-FID) and spectroscopic (XRD) techniques that allow the specific determination of several selenium species present or formed during bioremediation processes of selenate contaminated drinking, ground, or wastewaters have been established. The developed techniques are shown to be applicable in determining selenium species in the range of target concentrations for emissions and are thus appropriate to characterize bioremediation processes. The applied techniques offer advantages regarding short analytical times without loss of satisfactory accurateness towards more sophisticated methods. By means of IC-CD, selenate and selenite can be detected specifically to concentrations far below allowance levels for metal finishing industries (20 µg L−1 selenate, 40 µg L−1 selenite) within 16.0 min. Thus, the removal of selenate from the liquid phase by anaerobic granular sludge was quantified by IC-CD, and the concomitantly formed red precipitates were analysed by XRD. Hexagonal elemental selenium was found to be present in the XRD analysis. Even if the precipitate contained iron and zinc, as shown by ICP-OES, no iron- or zinc-containing selenium minerals were detected by XRD. The GC-FID method described determines dimethylselenide and dimethyldiselenide at a detection limit of 1 ng and 2 ng, respectively, without further chemical derivatization within 7.4 min. Thus, the GC-FID offers adequate detection of two major volatile organic selenium species potentially formed during bioremediation of selenium contaminated waters.

Removal of selenate from sulfate‐containing media by sulfate‐reducing bacterial biofilms

A biofilm-selected strain of a Desulfomicrobium sp. removed selenate from solution to sub-micromolar concentrations during growth on lactate (or hydrogen) and sulfate. Under sulfate-limited growth conditions, selenium was enzymatically reduced to selenide. Under excess sulfate conditions, selenate removal was primarily by enzymatic reduction to elemental selenium. Sequestration by biofilms was greater under the latter condition. Experiments with washed cell suspensions showed that high sulfate concentrations inhibited cell-specific selenate reduction, but when growing cells were exposed to selenate, the biomass increase achieved during incubations with abundant sulfate resulted in more rapid selenate removal. The addition of small amounts of sulfite, or thiosulfate, ameliorated this inhibition. Nitrate also inhibited selenate reduction in washed cell suspensions, apparently due to a general oxidizing effect. These results suggest that where biofilm-based sulfate-reducing bacteria (SRB) bioreactors are considered for the treatment of mixed metalliferous wastes that contain selenium oxyanions, adequate selenate removal should be achievable under a range of environmental conditions. The form and fate of the precipitated product will, however, be influenced by the dominant reduction pathway, which is controlled by environmental variables.

グリーンラスト／フェライト循環処理法によるセレン汚染水の高度処理

Removal of selenium compound, especially hexavalent selenate ion, is one of the most difficult water treatment techniques and an effective method has not yet been established. In this research we propose a novel selenium removal process called Recycled Green rust/Ferrite Process, composed of an addition of Fe source, an alkaline conditioning of a concentrated slurry, an anaerobic chemical reaction stage, and a return of the settled slurry after solid-liquid separation. A mixed Fe2+-Fe3+ hydroxide compound, green rust (GR), reduces selenate to elemental selenium, while GR is consequently oxidized to magnetite Fe3O4. Therefore the black-colored magnetic slurry containing GR and magnetite contributes to the removal of selenate. Repeated batch-mode selenium removal tests showed that the residual selenium concentration gradually decreased with the number of batch test increased. Addition of ferrous sulfate or ferrous chloride (resulting Fe2+ concentration 600 mg/L) was eligible for Fe source for GR formation. Stable selenium removal was achieved when the number of batch test exceed 15 times. Optimum selenium removal ratio reached 99.95% where initial Se concentration 2.1mg/L decreased to 0.001mg/L. In contrast, little Se removal occurred when the combination of ferrous sulfate and elemental iron powder (each Fe2+ and Fe0 concentration was 300 mg/L) was used. During the reaction, oxidation-reduction potential leveled off -700mV vs Ag/AgCl and no dissolved oxygen was detected, indicating a strong reducing condition suitable for the reduction of selenate. Ninety percent of selenium disappeared as soon as the GR formed, and then selenate removal rate followed a first-order kinetics depending on the slurry concentration. These experimental results indicated that the proposed process served an advanced treatment technique for selenate-contaminated water.

Removal of Selenate in River and Drainage Waters by Citrobacter braakii Enhanced with Zero-Valent Iron

A cost-effective remediation method is needed to remove selenium (Se) from Se-contaminated water. In this study, a selenate [Se(VI)]-reducing bacterium, Citrobacter braakii, that is capable of using molasses as a carbon source to reduce Se(VI) from natural river and drainage waters was isolated. During an 8-day experiment, 87-97% of the added Se(VI) in New River water and White River water, California, was reduced to elemental Se [Se(0)] or transformed to organic Se. In highly saline drainage water, removal of Se(VI) by C. braakii was limited, with 20% Se(VI) removal in a 7-day experiment. Addition of zero-valent iron (ZVI) into these waters along with C. braakii inoculation significantly enhanced the removal of Se(VI) and reduced the formation of organic Se. This study suggests that the combination of a bacterial treatment using inexpensive molasses and ZVI can effectively remove Se from natural river water and agricultural drainage waters.

Selenate Removal from Sulfate Containing Aqueous Solutions

The performance of selenium remediation agents, mainly γ alumina sorbents and zerovalent metals is greatly inhibited in the presence of large concentrations of sulfate found in selenium-rich agricultural drainage water. It is therefore imperative to remove sulfates from these contaminated waters, prior to their treatment for selenium removal. This paper presents data on enhanced selenium removal resulting from a two stage process involving the use of barium chloride in the first stage followed by a selenium remediation agent in the second. Barium chloride is known to stoichiometrically precipitate out sulfates. In addition to the sulfate removal from the contaminated solutions, a significant amount of selenate immobilization in the precipitated solids was achieved. When a loading of 15 g l−1 of BaCl2 was employed, 100 % selenate removal from an aqueous solution (containing 1 mg l−1 selenate, 4 g l−1 of sulfate and 2.5 g l−1 of chloride) was achieved by the use of BaCl2 alone. Bimetallic NiFe particles, γ alumina and activated carbon were employed to further remove selenium in the second stage. A solution containing 1 mg l−1 and 4 g l−1 of selenate and sulfate, respectively, when treated sequentially with 10 g l−1 of BaCl2 and NiFe powder showed 100 % selenium removal.

Effect of arsenate and molybdate on removal of selenate from an aqueous solution by zero-valent iron

Zero-valent iron (ZVI) is an inexpensive agent that can remove many common environmental contaminants. We investigated the effects of arsenate [As(V)] and molybdate [Mo(VI)] on the removal of selenate [Se(VI)] (1000 μg/L) in a 5 mM Cl − and SO 4 2− solution by ZVI. Analysis of selenium (Se) species in a control experiment revealed that there was no detectable selenite [Se(IV)] in the solution and Se(IV) was the only dominant form of Se in the PO 4 3− extract, revealing that the removal of Se(VI) by ZVI appeared to be partly attributed to the reduction of Se(VI) to Se(IV) by Fe(II) oxidized from ZVI, followed by rapid adsorption of Se(IV) to Fe oxyhydroxes (Fe OH). Addition of As(V) in a range of 5 to 25 mg/L in the solution did not affect Se(VI) removal. As(V) was removed at much faster rates than Se(VI). It took 16 h for ZVI to completely remove As(V) from the solutions, whereas it needed 31 h to remove 99% of the added Se(VI). Addition of Mo(VI) to the solution significantly affects the removal of Se. After 16 h, 99%, 98%, and 73% of the added Mo(VI) was removed from the solutions with 10, 50, and 200 mg/L Mo(VI) added, respectively. Whereas about 18%, 38%, and 78% of the added Se(VI) still remained in the solutions, respectively. This study suggests that a sequence for rapid element removal by ZVI is As(V) > Mo(VI) > Se(VI) and that ZVI can be used to effectively remove a high concentration level of As(V), Mo(VI), and Se(VI) from water.

Removal of Selenate from Water by Zerovalent Iron

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 microg 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.

Removal of Selenate by Fe and NiFe Nanosized Particles

Irrigation drainage and industrial wastewaters often contain elevated levels of toxic anions such as selenate, arsenate, and chromate. Immobilizing the contaminants via reaction with zero-valent iron has been proven to be an effective remediation method. Ultrafine metallic and bimetallic particles were prepared by borohydride reduction of aqueous salt solutions and characterized. Nanosized zero-valent NiFe and iron particles rapidly reduced and immobilized selenate from aqueous solutions. Nearly 100% selenate removal was obtained in 5 h. The data show that, at identical solids concentrations, the use of NiFe particles accomplished 42 and 56% greater removals, respectively, than the use of Fe and Ni particles individually. At low selenate concentrations, the selenium removal rate followed the first-order rate kinetics that shifted to zeroth-order at higher concentrations. The maximum selenate removal (87% in 30 min at a solids concentration of 0.5 g/L) was obtained by bimetallic particles containing 70 at....

Fe 3+ coordinated to amino-functionalized MCM-41: an adsorbent for the toxic oxyanions with high capacity, resistibility to inhibiting anions, and reusability after a simple treatment

Fe 3+ coordinated to amino ligands fixed on MCM-41 mesoporous silica works as a strong adsorbent for toxic oxyanions. The maximum adsorption amounts were 1.56, 0.99, 0.81, and 1.29 mmol g −1 for arsenate, chromate, selenate, and molybdate, respectively. When the initial concentrations of oxyanions were less than 1 mmol l −1, they were removed completely by adsorption and the distribution coefficients K d were found to be more than 200,000. Inhibition of oxyanion adsorption by abundant competing anions found in nature, NO 3 −, SO 4 2−, PO 4 3−, and Cl −, was evaluated at adsorption saturation. Among these anions, the adsorption of the oxyanions was inhibited most in the presence of PO 4 3−, with which the selectivities for the target oxyanions were still more than 80%. The other coexisting anions, NO 3 −, SO 4 2−, and Cl −, had little influence on adsorption of the oxyanions except in the case of selenate removal from sulfate solution. The high ability and selectivity to the target oxyanions are attributed to specific interactions between Fe 3+ and the oxyanions. The acid treatment and re-coordination of Fe 3+ lead to a reactivation of the used adsorbent, in which 87–90% of the original adsorption capacity was obtained and the oxyanion/Fe stoichiometries were not changed.

Removal of selenate in simulated agricultural drainage water by a rice straw bioreactor channel system.

Removal of selenium (Se) from agricultural drainage water is important in protecting wetland wildlife. Three flow-through bioreactor channel systems (BCSs), each with three channels filled with rice (Oryza sativa L.) straw, were set in the laboratory to determine removal of selenate [Se(VI)] (1020 microg L(-1)) from drainage water with a salinity of 10.4 dS m(-1), a pH of 8.1, and a nitrate (NO3-) range of 0 to 100 mg L(-1). Results showed that the rice straw effectively reduced Se(VI) during 122 to 165 d of the experiments. Calculation of Se mass in the three BCSs showed that 89.5 to 91.9% of the input Se(VI) was reduced to red elemental Se [Se(0)], where 96.6 to 98.2% was trapped in the BCSs. Losses of each gram of rice straw were almost equal to the removal of 1.66 mg of Se from the drainage water as a form of red Se(0), indicating that rice straw is a very effective organic source for removing Se(VI) from drainage water.

Characterization of selenate removal from drainage water using rice straw.

Removal of selenium (Se) from agricultural drainage water is very important for protecting wildlife in wetland systems. We conducted a series of experiments on selenite [Se(IV)] adsorption and selenate [Se(VI)] reduction to determine Se removal from drainage water amended with 1000 microg/L of Se(VI) or Se(IV) and 5 g of rice (Oryza sativa L.) straw. Under sterile conditions, the added Se(IV) was not adsorbed to the rice straw within 2 d of the experiment and the added Se(VI) was not reduced within 14 d. In contrast, added Se(VI) in a nonsterile rice-straw solution was reduced rapidly, from 930 microg/L at Day 3 to 20 microg/L at Day 5, with an increase in unprecipitated elemental Se [Se(0)] and total Se(0). In the last several days of the experiments, unprecipitated Se(0) was the major Se form in the rice-straw solution, with a small amount of organic Se(-II). This study showed that Se removal from drainage water in the presence of rice straw involves a two-step process. The first is the microbial reduction of Se(VI) to Se(IV) and then to colloidal Se(0). The second is flocculation and precipitation of colloidal Se(0) to the bottom of the experimental flasks and the surface of rice straw.

Characterization of Selenate Removal from Drainage Water Using Rice Straw

Characterization of Selenate Removal from Drainage Water Using Rice Straw

Factors affecting removal of selenate in agricultural drainage water utilizing rice straw

Microbial reduction of selenate [Se(VI)] to elemental selenium [Se(0)] is a useful technique for removing Se from agricultural drainage water. A series of batch experiments were conducted in the laboratory to determine the effects of pH (5–10), NO 3 − (100–500 mg/l), and SO 4 2− (0–5 000 mg/l) on the removal of Se(VI) from drainage water with 1000 μg/l of Se(VI) and different amounts (1–4 g) of rice straw. Results showed that rice straw was very effective in creating a reducing environment (Eh=−205 to −355 mV) in the first 3 days of the pH-effect experiments. The optimum conditions for rapid Se(VI) removal from drainage water were a pH range of 6–9, high amounts of SO 4 2− (1 000–5 000 mg/l), low amounts of NO 3 − (100 mg/l) and high amounts of rice straw (3–4 g). Under these conditions, it took 5–7 days to reduce 93–95% of the added Se(VI) to Se(0). This study indicates that rice straw may be an inexpensive reducing agent to remediate Se(VI)-dominant San Joaquin Valley drainage water in the field.

Interaction of selenate with a wetland sediment

Selenate removal from solution through various biogeochemical processes occurred in experiments with wetland sediment. The removal of selenate from solution to sediment was rapid and substantial. About 44–80% of added selenate was accumulated in sediment in a 72-h incubation. Different biogeochemical processes competed in removing selenate from water. Reduction of selenate to elemental Se dominated removal with a reduction rate of 0.147 μg/g/h. Formation of organically-bound Se (organic material Se and adsorbed organic Se) also removed Se from water; the rate of Se binding to organic material was 0.065 μg/g/h. In contrast, selenate adsorption was relatively less important due to a low adsorption rate, 0.013 μg/g/h. Volatilization removed little Se from water and sediment. These results show that Se reduction to the elemental form and binding of Se to organic material are the most important processes removing selenate from water and fixing it in wetland sediment.

Reduction Potential of Selenate in Wetland Sediment

Laboratory and field experiments were conducted to examine the reduction potential of dissolved selenate in wetland sediment at Benton Lake, Montana. Results showed that selenate reduction in wetland sediment was a microbially mediated process. This process proceeded rapidly and removed more than 50% of added selenate from solution to the sediment during the first day of the experiment. The reduction potential of selenate in sediment was positively correlated to Se concentrations in sediment, but not to sediment organic C content. This process occurred at the sediment surface because the depth of diffusion of selenate was limited, resulting in most Se accumulating in the top 2 cm of the sediment. The results from this study indicate that selenate reduction in sediment dominates selenate removal from water.

Electrokinetic Remediation. I. Modeling of Simple Systems

Abstract A One-Dimensional Model Is Developed For Simulating The Electrokinetic Treatment Of Saturated Porous Media Contaminated With An Ionic Salt. Simulations Of Simple, Unenhanced Electrokinetic Treatment For The Removal Of A Nonamphoteric Salt Such As Cadmium Sulfate Exhibit A Severe Drop-Off In Electric Current And In Remediation Rate After About 50–60% Of The Cation Has Been Removed. Simulation Of Electrokinetic Treatment In Which The Oh− Generated In The Cathode Compartment Is Partially Neutralized By The Addition Of Acid Show Rapid And Complete Removal Of The Cation. Partial Neutralization Of H+ In The Anode Compartment By Addition Of Base Results In Immobilization Of The Toxic Metal As The Solid Hydroxide, Although This Should Be A Useful Technique For The Removal Of Arsenate And Selenate.

The Removal of Selenium from Water by Slow Sand Filtration

The feasibility of selenium removal from drainage water by slow sand filtration (SSF) was investigated. Two anaerobic laboratory-scale slow sand filters, operated in parallel, received synthetic feed solution. Selenate reduction and subsequent selenium removal were monitored during five experimental filtrations. The results suggest that selenium removal occurred by the dissimilatory reduction of selenate to elemental selenium. This reduction was independent of sulfate. Selenium removal efficiencies were governed by the hydraulic loading rate (HLR).