Removal of selenate from water by chemical reduction

Abstract

A previously unknown chemical reaction in which selenate is reduced to elemental selenium may prove to be the basis of a chemical process to remove selenate from water systems and to control selenate in agricultural wastewater. The reaction occurs between selenate and ferrous hydroxide under alkaline conditions, producing magnetic iron oxides (magnetite and maghemite) and elemental selenium. The reaction rate reaches a maximum at about pH 9, dropping sharply below pH 8 or above pH 10. In the presence of excess ferrous hydroxide, magnetite is the predominant product. As the ratio of selenate to ferrous hydroxide increases, more maghemite seems to form. The reaction may proceed in a stepwise fashion, with selenate first reduced to selenite and then to selenium. The Gibbs free energy of the selenate reduction reaction is calculated to be -83.1 kcal/mol, assuming magnetite is the final iron oxide produced. The elemental selenium remains trapped in the iron oxide. This chemistry may find application in the recovery and refining of selenium and provide insight into the selenium cycle in the environment.

Selenate removal by zero-valent iron in oxic condition: the role of Fe(II) and selenate removal mechanism

“…They suggested two mechanisms regarding the rapid removal of Se(VI) from a solution. The first of these involves the reduction of Se(VI) to Se(IV), followed by a rapid adsorption of Se(IV) to oxyhydroxides (Murphy 1988;Zingaro et al 1997). The other mechanism involves the adsorption of Se(VI) directly to oxyhydroxides, followed by its reduction to Se(IV) (Myneni et al 1997).…”

The reduction of selenium(IV) by hydrogen sulfide in aqueous solutions

“…Possible reductants for Se(IV) also include zerovalent iron (Olegario et al, 2010), sodium dithionite (Geoffroy and Demopoulos, 2009), ascorbic acid (Shaker, 1996) and iodide (Ericzon et al, 1990). Many technological applications involving some of the above reductants have been proposed for the removal of selenium from process and mine wastewater and agricultural drainage water (Murphy, 1988;Zingaro et al, 1997;Zhang et al, 2008;Gonzalez et al, 2010) as well as for bioremediation of polluted sites (Hunter and Kuykendall, 2005;Antonioli et al, 2007;Ikram and Faisal, 2010). Recently, Geoffroy and Demopoulos (2011) investigated the removal of Se(IV) from wastewaters by precipitation with sodium sulfide.…”

Immobilization of selenate by iron in aqueous solution under anoxic conditions and the influence of uranyl

“…The iron will thereby come in contact with anoxic groundwater and corrode as the SNF matrix dissolves and releases fission products such as selenium. Iron and certain iron corrosion products have been shown to reduce and thus immobilize dissolved selenite and selenate [4][5][6][7][8][9][10]. To the best of our knowledge data on the immobilization of selenate by iron and the effect of uranyl present in solution is however scarce.…”