Enhanced Removal of Hexavalent Chromium in the Presence of H_2O_2 in Frozen Aqueous Solutions

摘要

The reductive transformation of Cr(Ⅵ) to Cr(Ⅲ) by H_2O_2 in ice was compared with that in water. The reduction of Cr(Ⅵ) was significant at -20 ℃ (ice), whereas the reduction efficiency was very low at 25 ℃ (water). This enhanced reduction of Cr(Ⅵ) in ice was observed over a wide range of H_2O_2 concentration (20-1000 μM), pH (3-11), and freezing temperature (-10 to -30 ℃). The observed molar ratio of consumed [H_2O_2] to reduced [Cr(Ⅵ)] in ice was in close agreement with the theoretical (stoichiometric) molar ratio (l.S) for H_2O_2-mediated Cr(Ⅵ) reduction through proton-coupled electron transfer (PCET). The synergistic increase in Cr(Ⅵ) reduction in water by increasing the H_2O_2 and proton concentrations confirms that the freeze concentration of both H_2O_2 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 H_2O_2 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.