No abstract

A facultative bacterium capable of removing the selenium (Se) oxyanions selenate (SeO 4 2؊) and selenite (SeO 3 2؊) from solution culture in flasks open to the atmosphere was isolated and studied with the goal of assessing its potential for use in bioremediation of seleniferous agricultural drainage water. Elemental Se (Se 0) was confirmed as a product of the reaction. The organism, identified as Enterobacter cloacae and designated strain SLD1a-1 (ATCC 700258), removed from 61.5 to 94.5% of added SeO 4 2؊ (the primary species present in agricultural drainage water) at concentrations from 13 to 1,266 M. Equimolar amounts of nitrate (NO 3 ؊), which interferes with SeO 4 2؊ reduction in some organisms, did not influence the reaction in growth experiments but had a slight inhibitory effect in a washed-cell suspension. Washed-cell suspension experiments also showed that (i) SeO 3 2؊ is a transitory intermediate in reduction of SeO 4 2؊ , being produced and rapidly reduced concomitantly; (ii) NO 3 ؊ is also reduced concomitantly and at a much higher rate than SeO 4 2؊ ; and (iii) although enzymatic, reduction of either oxyanion does not appear to be an inducible process. Transmission electron microscopy revealed that precipitate particles are <0.1 m in diameter, and these particles were observed free in the medium. Evidence indicates that SLD1a-1 uses SeO 4 2؊ as an alternate electron acceptor and that the reaction occurs via a membrane-associated reductase(s) followed by rapid expulsion of the Se particles.

Abstract-ReductionSeO 4 useful information for optimization of a bioreactor using E. cloacae SLD1a-1 to treat Se-contaminated water.

Hexavalent chromium [Cr(VI)] is a common environmental pollutant that is mobile in soils and is a known mutagen The trivalent form [Cr(III)] has no known mutagenic properties and is highly insoluble and immobile above pH 5 5 Reduction to the trivalent state thus represents an effective mechanism for detoxification and immobilization of Cr(VI) in soil/water systems In this study, we conducted experiments to examine various parameters involving aerobic reduction of Cr(VI) in a field soil We found that organic matter content, bioactivity, and oxygen status were important factors Under optimum conditions, the soil reduced 96% of added Cr(VI) under aerobic, field moist conditions The pH of the system was shown to have little effect Both biological and nonbiological processes were influential, and, of 20 chromate resistant bacterial cultures isolated from soils, 9 were found to actively reduce Cr(VI) to Cr(III) in liquid media Our study suggests that organic amended soils can readily reduce Cr(VI) and could promote excellent removal efficiency either as a primary treatment technique, or in conjunction with bioreactor type wastewater treatment systems

A cost‐effective method is needed for removing chromate from cooling water blowdown, wastewater effluent, and contaminated groundwater. Experiments were conducted to determine the effectiveness of treating Cr‐contaminated water by using the water for irrigation, and that reduction of Cr(VI) to Cr(III) would occur in a soil amended with organic matter and irrigated to promote low oxidation/reduction status. The Cr(III) would then precipitate as oxides and hydroxides, and be immobilized and rendered plant unavailable. Samples of a field soil (mixed, thermic Typic Torripsamments) overlying a contaminated groundwater site were placed in pots and irrigated for 20 wk with water containing 1000 µg L−1 Cr(VI). Treatments included plants (alfalfa; Medicago sativa L.) vs. no plants, each at three organic matter Ioadings ‐ 0, 12, and 50 Mg ha−1 dried cattle manure (0, 5.5, and 21.8 g manure kg−1 soil). The drainage waters were collected weekly and analyzed for total Cr, Cr(VI), pH, dissolved O2, dissolved organic C, and electrical conductivity. The removal percentages of Cr(VI) from the enriched water ranged from 51 to 98% and increased with increasing organic matter loading. Chromium concentrations in the drainage water were consistently <50 µg L−1 in the organic‐amended soil. Daily irrigation yielded lower drainage water Cr concentrations than weekly irrigation (at a constant weekly volume) due to increased residence time of the water in the bioactive zone. Chromate adsorption accounted for <1% of the total immobilized Cr and the amount taken up by the alfalfa shoots was <0.5% of the total added. This method shows promise as a cost‐effective treatment for Cr‐contaminated groundwater.

Oxidation of elemental selenium (Se°) leads to increased solubilizalion and enhanced bioavailability. In this work, laboratory microcosm experiments were conducted to study oxidation of Se° in soil and liquid cultures. Major objeclives were to examine the oxidation rates of four San Joaquin Valley, California soils, and to assess the contribution of biological vs. chemical processes. For these experiments, red, crystalline Se° was prepared by both chemical and biological synthesis, and its presence was confirmed by synchrotron-based x-ray absorption spectroscopy. The amount of Se° oxidized over 1125 d was from 1 to 10% of Se° in soils spiked to 250 mg Se° kg -1 and approximately half that in soils spiked to 100 mg Se° kg -1. First order rate conslanls for oxidation of Se° were from 0.05 to 0.32 yr ~ and 0.04 to 0.39 yr -~ at 250 and 100 mg Se° kg -~ soil, respectively. The amount of Se° oxidized was generally correlated with prior exposure of the soil lo Se. Products included either selenite (SeO]-), or both SeO]-and selenate (SeO~-). Biotic processes were shown to be of major importance. Both heterotrophic and autotrophic oxidation were observed, and an inorganic C source (NaHCOa) was favored relative lo glucose. This study demonstrates that Se° oxidation in soils is largely biotic in nature, occurs at relatively slow rates and yields both SeO~-and SeO~-.Abbreviations: XANES, x-ray absorption near-edge structure; AASHG, atomic absorption spectroscopy with hydride generation; LOD, limit of detection. 836

No abstract

No abstract