Reinvestigating the role of reactive species in the oxidation of organic co-contaminants during Cr(VI) reactions with sulfite

Experimental work was undertaken in this study to re-investigate the mechanisms and active species responsible for oxidation of co-contaminants in the Cr(VI)/HSO₃⁻ reaction system. Batch experiments showed that the degradation rates of 4-chlorophenol (4-CP) correlated well with the rates of Cr(VI) reduction by sulfite in the same solutions, and that O_2(aq) was necessary for the oxidation of 4-CP. Multiple lines of evidences indicate that Cr(VI)/HSO₃⁻ reaction is a SO₄^[rad]−-based oxidation process. SO₃^[rad]− was generated in Cr(VI)/HSO₃⁻ system based on the electron spin resonance spectra, which could be transformed to secondary radicals (SO₄^[rad]−, SO₅^[rad]−, and HO[rad]). The contribution of SO₅^[rad]− was ruled out through almost complete inhibition of methanol (MeOH) on 4-CP degradation. Considering the negligible inhibition of tert-butanol (TBA) on 4-CP degradation, SO₄^[rad]− was identified to be reactive species in Cr(VI)/HSO₃⁻ process. This result was further verified by almost no degradation of nitrobenzene and the inhibiting effect of Cl⁻ in Cr(VI)/HSO₃⁻ process. This mechanism is beneficial to application of Cr(VI)/HSO₃⁻ system in wastewater treatment.