Inhibitory Effect of Sulfite on the SO₄·^--Induced Transformation of Selected Organic Contaminants in Sulfite-Based Advanced Oxidation Processes

Sulfite (S(IV))-based advanced oxidation processes (AOPs), with SO₄^·-being the primary oxidant, have attracted increasing attention for degrading organic contaminants in water. However, the side effect of S(IV) as a reductant on the SO₄^·--induced transformation of organic contaminants has never been revealed. For the first time, we demonstrated such an effect of S(IV) by taking the MnO₂/S(IV) system as the surrogate S(IV)-based AOP. Multiple evidence confirmed that SO₄^·-is the sole active oxidant in the MnO₂/S(IV) system. The relative-rate experiments in the UV/S₂O₈^2-system, a SO₄^·--dominated process with the presence of excess tert-butanol, and the MnO₂/S(IV) system showed that S(IV) can inhibit the SO₄^·--induced transformation of some organic contaminants, with the inhibiting effect being strongly dependent on the solution pH. Flash photolysis experiments and reduction potential calculations clarified the necessary reaction conditions for the inhibiting effect of S(IV) to occur, i.e., the reduction of oxidation intermediate of organic contaminants (TC^·+) by S(IV) to parent compound (TC) is exergonic (ΔE > 0), and its rate is faster or comparable to that of the transformation of TC^·+to stable products. This work updates the fundamental understanding of the degradation of organic contaminants in S(IV)-based AOPs.