A simple Cr(VI)-S(IV)-O2 system for rapid and simultaneous reduction of Cr(VI) and oxidative degradation of organic pollutants.

Hexavalent chromium (Cr(VI)), a heavy-metal contaminant, can be easily reduced to less toxic trivalent chromium (Cr(III)) by sulfite ions (S(IV)). However, S(IV) has not drawn as much attention as the ferrous ion has. We report herein a novel Cr(VI)-S(IV)-O2 system containing sulfite ions that rapidly and simultaneously reduces Cr(VI) and oxidize organic pollutants in the presence of oxygen in aqueous solutions. This Cr(VI)-S(IV)-O2 system contains the initiator Cr(VI), the reductant S(IV), and the oxidant O2, which produce oxysulfur radicals (mainly SO4(-) and SO5(-)) and hydroxyl radicals (OH). The Cr(VI)/S(IV) molar ratio, pH, and oxygen content play important roles in the entire reaction system. Acidic conditions (pH 3.0) facilitated degradation of organic compounds and reduction of Cr(VI) as well. In addition, experiments of rapid degradation of several kinds of organic pollutants such as azo dye (acid orange 7, AO7), aniline, phenol, bisphenol A etc were also conducted. Preliminary results show that the removal rates of the analogs of phenols or aromatic amines in this Cr(VI)-S(IV)-O2 system have a relationship with the electronic parameters (Hammett constant, σ) of the substituted groups. Thus, the Cr(VI)-S(IV)-O2 system, provides an excellent strategy of "waste control by waste" for removing multiple industrial contaminants.