Catalysis of first triplet p-benzoquinone for electron ejection from sulfite: Significance of single electron transfer

In this study, the combination of p-benzoquinone (p-BQ) and sulfite under the ultraviolet (UV) irradiation demonstrated markedly superior efficiency in the ejection of hydrated electrons (e_aq⁻) compared to the individual UV/p-BQ and UV/sulfite processes. Theoretical calculations reveal that the 1,2- and 1,4-addition of bisulfite (HSO₃⁻) to p-BQ was unlikely to occur kinetically, and the 1,4-addition of sulfite (SO₃²⁻) might be the major pathway for the reaction with p-BQ. Via single electron transfer from the highest occupied molecular orbitals (HOMOs) of HSO₃⁻ and SO₃²⁻ to the HOMO–1 of ³p-BQ*, the oxidation of HSO₃⁻ and SO₃²⁻ by the first triplet state of p-BQ (³p-BQ*, the major excited species during the excitation) could result in the generation of HSO₃^•, SO₃^•− and semiquinone radical anion. This mechanism could elucidate the synergism of p-BQ and sulfite for ejecting e_aq⁻ in the water under the UV irradiation. Given the abundance of quinones in natural waters, these findings may be helpful for the utilization of sulfite to efficiently release e_aq⁻ in the water treatment applications.