Enhanced Removal of Hexavalent Chromium in the Presence of H2O2 in Frozen Aqueous Solutions

Abstract

The reductive transformation of Cr(VI) to Cr(III) by H₂O<sub>2</sub in ice was compared with that in water. The reduction of Cr(VI) was significant at -20 °C (ice), whereas the reduction efficiency was very low at 25 °C (water). This enhanced reduction of Cr(VI) in ice was observed over a wide range of H₂O₂ concentration (20-1000 μM), pH (3-11), and freezing temperature (-10 to -30 °C). The observed molar ratio of consumed [H2O2] to reduced [Cr(VI)] in ice was in close agreement with the theoretical (stoichiometric) molar ratio (1.5) for H₂O₂ -mediated Cr(VI) reduction through proton-coupled electron transfer (PCET). The synergistic increase in Cr(VI) reduction in water by increasing the H₂O₂ and proton concentrations confirms that the freeze concentration of both H₂O₂ and protons in the liquid brine is primarily responsible for the enhanced Cr(VI) reduction in ice. In comparison, the one-electron reduction of Cr(VI) to Cr(V) and subsequent reoxidation of Cr(V) to Cr(VI) is the major reaction mechanism in aqueous solution. The reduction efficiency of Cr(VI) by H₂O₂ in the frozen aqueous electroplating wastewater was similar to that in the frozen aqueous deionized water, which verifies the enhanced reduction of Cr(VI) by freezing in real Cr(VI)-contaminated aquatic systems.