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 SeO42- 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 SO42- also were reduced from 620 to 220 mg L−1. After 2 months, cell populations of SO42--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 32SO42- over 32SO42- by SO4-reducing bacteria in the column. Geochemical calculations indicate that SeO42- is stable in the influent water, and that conditions within the column favour reduction of SeO42- to metallic Se or SeO32-, and the reduction of SO42- to S2−.