Enhanced Redox Conversion of Chromate and Arsenite in Ice

The redox transformation of trace elements is critically affected by properties of the environmental media. While the environmentally important redox chemical reactions in aquatic environments have been extensively studied, those in the ice phase have been studied in only a few cases. In this work, chromium and arsenic species were selected as the model inorganic oxyanion contaminants for the study of redox chemical transformation in ice. We investigated (1) the reduction of hexavalent Cr(VI) (as chromate) by model organic acids (e.g., citric and oxalic acid) and (2) the simultaneous removal of Cr(VI) and As(III) (as arsenite) in ice phase in comparison with their counterparts in aqueous solution. The reduction of Cr(VI) by various organic acids (electron donors) was negligible in ambient aqueous solution but was significantly accelerated in ice. The simultaneous reduction of Cr(VI) and oxidation of As(III) in ice phase proceeded stoichiometrically, whereas their mutual conversion was insignificant in aqueous solution. The enhanced redox conversion of Cr(VI)/As(III) in ice is ascribed to the freeze concentration of both electron donors (e.g., organic acids, arsenites) and protons in the ice crystal grain boundaries. When the concentrations of both electron donors and protons were highly raised to an extreme, the removal rates of Cr(VI) in aqueous solution approached to those in ice. This specific combination of Cr(VI)/As(III) redox couple may provide an example that represents innumerable redox conversion reactions that could be greatly accelerated in ice/snow-covered or frozen environments.