In situ microbial reduction of selenate in backfilled phosphate mine waste, s.e. Idaho
Kirk, Lisa Marie Bithell
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The reduction of selenium (Se) by microbes is controlled by oxygen (O 2)-availability within mixed deposits of shale, chert, and mudstone mined from the Phosphoria Formation in S.E. Idaho. Waste rock and groundwater from backfilled mine pits, which have been studied using geochemical, microbial cultivation, and molecular methods, host native populations of selenate-(SeO 4 2-) and selenite-(SeO 3 2-) reducing bacteria that are highly similar to the genera Dechloromonas, Stenotrophomonas, Anaeromyxobacter, and Ralstonia. These bacteria rapidly reduced more than 95% of soluble SeO 4 2- concentrations. Reduction occurred within a consortium of slow-growing, cold-tolerant, hydrocarbon-degrading, and nitrate-(NO 3-), iron-(Fe 3+), and manganese-(Mn 4+) reducing bacteria, including the genera Polaromonas and Rhodoferax, which appeared to use the naturally-occurring hydrocarbon present in the rock. Most-probable number estimates of SeO 4 2--reducers were highest in saturated sediments and in unsaturated shale, and were very low in unsaturated chert and mudstone. Selenium reduction was studied in microaerophilic, saturated native chert, shale, and mixed run-of-mine sediments inoculated with live groundwater cultures, with sampling and analysis of total Se, Fe, Mn; Se speciation; NO 3- and sulfate (SO 4 2-); dissolved organic carbon and total nitrogen(N); and mineralogy. Following an O 2- and N-dependent lag, SeO 4 2- was reduced within 100 hours under saturated, suboxic conditions at rates that varied depending on lithotype and temperature. The microbial community shifted during reduction as well, from phylotypes associated with the Fe-reducing Rhodoferax and HC-degrading Sphingomonas and SeO 4 2--reducing Dechloromonas genera to include members of the SeO 3 2-reducing genus Ralstonia. A unique biogeochemical Se reduction pathway was suggested in chert experiments, where Se reduction proceeded more rapidly and produced SeO 3 2- and elemental Se products, relative to the shale, wherein reduction was slower and produced more reduced selenide minerals. Results of these experiments offer insight into the results of in situ monitoring in backfill at multiple locations in S.E. Idaho, and potentially explain differences in Se solubility at these locations. Strategic management of rock and water in constructed mine wastefacilities to limit O 2 recharge can thus promote SeO 4 2- reduction by communities of indigenous organisms using available carbon and other electron donors. This offers a sustainable, design-based approach to natural attenuation of Se in mined rock.