Advanced Oxidation Process Based on the Cr(lll)/Cr(VI) Redox Cycle

摘要

Oxidative degradation of aqueous organic pollutants, using 4-chlorophenol (4-CP) as a main model substrate, was achieved with the concurrent H2O2-mediated transformation of Cr(III) to Cr(VI). The Fenton-tike oxidation of 4-CP is initiated by the reaction between the aquo-complex of Cr(III) and H2O2, which generates HO* along with the stepwise oxidation of Cr(III) to Cr(VI). The Cr(III)/H2O2 system is inactive in acidic condition, but exhibits maximum oxidative capacity at neutral and near-alkaline pH. Since we previously reported that Cr(Vl) ' can also activate H2O2 to efficiently generate HO*, the dual role of H2O2 as an oxidant of Cr(IH) and a reductant of Cr(Vl) can be utilized to establish a redox cycle of Cr(III)-Cr(VI)-Cr(IlI). As a result, HO* can be generated using both Cr(IU)/H2O2 and Cr(VI)/H2O2 reactions, either concurrently or sequentially. The formation of HO* was confirmed by monitoring the production of p-hydroxybenzoic acid from [benzoic acid + HO*] as a probe reaction and by quenching the degradation of 4-CP in the presence of methanol as a HO* scavenger. The oxidation rate of 4-CP in the Cr(IH)/H2O2 solution was highly influenced by pH, which is ascribed to the hydrolysis of Cr"'(H2O), into Cr"'(H2O)n.m(OH)m and the subsequent condensation to oligomers. The present study proposes that the Cr(III)/H2O2 combined with Cr(VI)/H2O2 process is a viable advanced oxidation process that operates over a wide pH range using the reusable redox cycle of Cr(III) and Cr(VI).