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Reduction and immobilization of selenium in wetland sediment Hodaly, Al Henry

Abstract

Elevated levels of selenium have been found in stream waters downstream of surface coalmines in the Elk River Basin (McDonald and Strosher, 1998). The fate of Se in slow-moving side-channels and wetlands in the Elk River Valley is important because these types of ecosystems are some of the most important feeding and breeding habitats for fish and wildlife. The focus of this study was to observe the potential for selenium uptake in the sediments of these wetlands. If successful, wetlands could represent an effective passive remediation technology for the area. Sediment from Goddard Wetland was used in this study. Monitoring by the Elkview Mine found soluble selenium concentrations as high as 106 μg/L in the Goddard Tailing Pond, which feeds directly into Goddard Wetland. Sediment samples were inoculated into Thauera selenatis (T.sel) and sulphate-reducing bacteria (SRB) growth media. After approximately 3 days a bright-red precipitate (Se°) was seen in the T.sel medium, and after 13 days the SRB medium turned jet-black (FeS), confirming the presence of these microorganisms in the sediment. In both media, selenate concentrations decreased significantly (619 ppb -> 15 ppb and 364 ppb ->22 ppb, respectively) confirming the ability of these organisms to reduce selenate. Adsorption experiments performed with sterilized sediment showed that selenite adsorbed very strongly (78-100% removal from solution) to sediment, while selenate did not adsorb at all. A semi-continuous microcosm experiment was run to determine whether these selenium immobilizing/removal mechanisms would occur in an open system representative of the Goddard Wetland. Results clearly demonstrated that Se concentrations in solution decreased markedly after spiking; within one week, average concentrations dropped from 401 μg/L to 37 μg/L There was no significant difference between organically amended and unamended microcosms, possibly because the sediment used in these trials already had a high organic matter content (TOC = 26%; Walkley-Black C = 16%).

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