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 (3)p-BQ*, the oxidation of HSO3- and SO32- by the first triplet state of p-BQ ((3)p-BQ*, the major excited species during the excitation) could result in the generation of HSO3 center dot, SO3 center dot 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 eaq in the water treatment applications.