MICROBIAL SELENIUM REDUCTION RATE AND COMMUNITY RESPONSE IN CARBON-AMENDED COAL MINE WASTE ROCK, BC AND ALBERTA, CANADA

Microbial reduction rates of SeO₄^2- as a function of initial O₂ concentrations and carbon amendments were investigated using anaerobic bioreactors containing drill core material and corresponding site water. Ten grams of moist solid drill core material was added to 100 mL of site water in serum bottles, prepared in an anaerobic manner, and supplemented with SeO₄^2- (0.2 mM), and yeast extract (1.6 mM C). The gas headspace within the bioreactors was supplemented with 0, 2, 8, 40, 80 or 800 μmoles of O₂, respectively. Incubations were conducted in the dark at 10 or 20 ± 1 °C and samples for analyses were collected at 1 to 3 d intervals. Carbon amendments included A) no carbon, killed, B) no carbon, C) no carbon, plus macro and micro inorganic nutrients, D) organic acids and yeast extract, E) cyanobacterial biomass, F) methanol, G) molasses and H) wood chips. Killed control samples were generated by autoclaving twice.

The most rapid rates of reduction were observed in the cyanobacteria and the organic acid/YE treatments, where complete SeO₄^2- reduction (in both treatments) occurred by 18 d. The addition of methanol and molasses stimulated reduction, but at slower rates compared to the cyanobacteria and organic acid/YE treatments. Complete reduction with molasses was observed by about 45 d, and reduction with methanol was completed in 47 d. Wood chips did not stimulate reduction; perhaps a result of poor bioavailability of lignocellulose within the timeframe of the experiments. Higher initial oxygen availability increased the lag time before SeO₄^2- reduction began, and in some cases slightly decreased rates SeO₄^2- reduction. These results suggest that low oxygen levels, and some labile carbon amendment, may be needed to support increased rates of SeO₄^2- reduction.