The physical and chemical nature of elemental selenium in the environment

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Selenium (Se) is an inorganic contaminant of high current importance in North America. It is discharged accidentally into the aquatic environment from some major industrial processes, including coal-fired power plants, petroleum refineries and various types of mining operations (e.g. coal, gold and uranium). In some receiving environments, there is concern about ecotoxicological effects arising from Se discharges, which may lead to Se bioaccumulation in aquatic organisms, and to reproductive effects in aquatic birds and predatory fish that can ultimately endanger the survival of entire local populations.******The US Environmental Protection Agency and Environment Canada are in the process of re-evaluating and (most likely) lowering their current ambient Se water quality criteria. Consequently, most industrial operations are currently evaluating or installing Se removal systems. Bacterial reduction reactors, which mimic the processes occurring in ambient water bodies when Se is removed to the sediments, are currently the most popular type of treatment system considered/installed. These systems are supposed to convert soluble Se compounds into insoluble elemental Se, which is removed from the water and disposed of with solid residues. While these treatment systems are often very effective at lowering the total Se concentration in the process waters, a small fraction of Se passes through these systems, and may still raise ecotoxicological concerns. Of particular interest is the chemical nature of the released Se, and preliminary studies suggest that some of this Se might be present in the form of suspended colloidal elemental Se with very small particle size. However, there are currently no analytical methods available to measure colloidal elemental Se, or characterize its properties with respect to molecular composition or particle size.******In the proposed research program, we will develop such methods, and then quantify and characterize elemental Se generated by biological reduction, as well as chemical processes, in laboratory studies, and later on in natural water-sediment systems, as well as in industrial Se removal plants. The results of our studies will determine for the first time if, and in what exact form, colloidal elemental Se plays a role in these processes. This will lay the foundation for assessing the fate of elemental Se discharged from industrial treatment operations into ambient waters, since it is possible that elemental Se is unavailable to organisms and will sediment rapidly, but may also be able to convert back into soluble Se species in the water column. For the industrial stakeholders, our research will provide part of the assessment whether biological reduction is the right treatment technique in the long term, while for the Canadian public, it will help determine if continued low-level Se emissions from such treatment processes pose danger to aquatic organisms.