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.