Cr(VI) Formation via Oxyhalide-Induced Oxidative Dissolution of Chromium Oxide/Hydroxide in Aqueous and Frozen...

The oxidative dissolution of Cr(III) species (Cr₂O₃ and Cr(OH)₃) by oxyhalide species, which produces hexavalent chromium (Cr(VI)), was studied in aqueous and frozen solution. The oxyhalide-induced oxidation of Cr(III) in frozen solution showed a different trend from that in aqueous solution. Cr(VI) production was higher in frozen than aqueous solution with hypochlorous acid (HOCl) and bromate (BrO₃^-) but suppressed in frozen solution with hypobromous acid (HOBr) and periodate (IO₄^-). In particular, bromate markedly enhanced Cr(VI) production in frozen solution, whereas it had a negligible activity in aqueous solution. On the contrary, periodate produced Cr(VI) significantly in aqueous solution but greatly suppressed it in frozen solution. Bromate was found to be much more concentrated in the ice grain boundary than periodate according to both chemical and Raman spectral analyses. The oxidative transformation of Cr(III) to Cr(VI) was accompanied by the concurrent and stoichiometric reduction of oxyhalide species. Dissolved O₂ had little effect on the oxidative dissolution, but dissolved organic matter retarded the oxidation of Cr₂O₃ in both aqueous and frozen conditions. This study proposes that the oxyhalide-induced oxidation of Cr(III) (particularly by bromate) in frozen conditions might have a significant effect on the generation of Cr(VI) in the frozen environment.