New Insights into the Mechanism of the UV/Sulfite Process: Formation of SO₂•– Radicals and Their Derivatives under Acidic Conditions

The UV/sulfite process shows great potential for reductively degrading or eliminating pollutants. While its mechanism in neutral and alkaline environments has been well-elucidated, the reaction pathway under acidic conditions remains unclear. Herein, we report the novel findings of the formation of reductive SO2•– radicals and their derivatives in the UV/sulfite process at pH levels below 4. Mechanistic investigation revealed that H• radicals and SO3•– radicals formed by the photolysis of sulfite under acidic conditions, with the H• radicals being scavenged by sulfite to produce SO2•– radicals. Subsequently, these SO2•– radicals are transformed into dithionite, thiosulfate, hydrogen sulfide, and elemental sulfur through a series of intricate reactions. This study is expected to expand the potential application of the UV/sulfite process under acid conditions.