In-situ immobilization of selenium within the saturated zones of backfilled pits at coal-mine operations Bianchin, Mario; Martin, Alan J.; Adams, Jack
g passive technologies at mine sites is challenging as a result of associated high flow rates. However, the chemical and hydraulic conditions of backfilled pits offer a potential means to bioremediate large volumes of water passively. Specifically, the oxidation demand associated carbonaceous waste materials in conjunction with long water residence times within backfilled pits can produce the suboxic conditions required to support the removal of Se from solution through a suite of microbially-mediated processes. Evidence for Se removal is provided from a study of a backfilled pit at a coal mine in Northern Canada. Redox conditions within the saturated backfill are mildly-suboxic, as inferred from low levels of oxygen, nitrate and the presence of dissolved Mn and Fe. Results show pronounced removal of dissolved Se within the saturated backfill, with concentrations decreasing from 40 μg/L (upgradient of saturated zone) to <1 μg/L within the saturated waste rock zone. The reduction in Se concentration was accompanied by a shift in Se speciation from dominantly selenate (SeVI) to selenite (SeIV) along the flow path. Nitrate-N values are also reduced to values below detection (<0.05 mg/L) in the saturated backfill. The data suggest that this relatively small backfilled pit achieves a Se removal rate ranging from 0.03 to 0.3 mg/day/m³wasterock with a hydraulic residence time of 0.3 to 3 years. Collectively, the data imply that the saturated backfilled zones are serving as an effective site for the bioremediation of Se under mildly suboxic condition.
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