Chromate-Induced Activation of Hydrogen Peroxide for Oxidative Degradation of Aqueous Organic Pollutants

The oxidation of organic compounds in water was investigated with using chromate as an activator of H2O2. 4-chlorophenol (4-CP) was used as a main model substrate, and its degradation was successfully achieved at circumneutral pH. Unlike the traditional Fenton-based activation of H2O2 that is mainly limited to acidic condition, the oxidative capacity of the proposed Cr(VI)/H2O2 system is active over a wide range of pH 3−11. H2O2 substitutes the oxo ligands of chromate by the peroxo ligands and, subsequently, converts chromate(VI) into a tetraperoxochromate(V) complex. The instantaneous disproportionation between chromium-coordinated peroxo ligands initiates the generation of HO• that are responsible for the degradation of organic compounds in the Cr(VI)/H2O2 system. The oxidation rate of 4-CP and the in situ generated concentration of peroxochromate(V) decreased with increasing pH. The generation of HO• in the Cr(VI)/H2O2 solution was confirmed by monitoring the production of p-hydroxybenzoic acid from the oxidation of benzoic acid as a probe reaction and by quenching the degradation of 4-CP in the presence of methanol as a radical scavenger. The oxidation of 4-CP investigated at different H2O2 concentrations and pH indicated the pH-dependent competition between peroxo ligand exchange and dissociation reactions. The proposed Cr(VI)/H2O2 process can be ideally suited for the treatment of chromate-contaminated wastewaters with recalcitrant organic compounds. The degradation of 4-CP in actual Cr(VI)-contaminated wastewater was successfully demonstrated in the presence of added H2O2. The Cr(VI)/H2O2 system is proposed as a viable advanced oxidation process.