Immobilization of Se(VI) in mine drainage by permeable reactive barriers: column performance

The potential for immobilization of Se in mine drainage water using a permeable reactive barrier was investigated by a column study, in which the reactive components were zero valent Fe, municipal leaf compost, sawdust, and wood chips. These components were mixed with silica sand and gravel. Trace amounts of creek sediment were added to serve as a source of anaerobic bacteria. The influent concentration (40 mg L−1) of image decreased to less than 2 mg L−1 within one week and to less than 0.014 mg L−1 within 1 month during passage through the column. In the column, the concentrations of image also were reduced from 620 to 220 mg L−1. After 2 months, cell populations of image-reducing bacteria, estimated using the MPN method, were in the range of 106–107 cells g−1. Isotopic analysis of S showed δ34S = −9.19‰ for the input solution, and δ34S = −4.69‰ for the output solution. This change in isotopic ratio is attributed to the preferential utilization of image32SO42- over image by SO4-reducing bacteria in the column. Geochemical calculations indicate that image is stable in the influent water, and that conditions within the column favour reduction of image to metallic Se or image, and the reduction of image to S2−.