UV/p-benzoquinone/peroxymonosulfate process for efficient degradation of sulfamethoxazole: Significance of the peroxymonosulfate anion radical generated by single electron transfer

This study demonstrated the synergy between peroxymonosulfate (PMS) and p- benzoquinone ( p -BQ) in the sulfamethoxazole (SMX) degradation under the ultraviolet (UV) irradiation. In the UV/ p -BQ/PMS process, the SMX degradation showed positive dependence on the initial dosages of PMS, p -BQ and SMX. The variation of the observed pseudo-first-order rate constant ( k obs ) exhibited aligned with the speciation of SO 5 2− across pH 7–10, and the anomalous decline of k obs emerged at pH 10–12. It suggested that SO 5 2− might be significant for the SMX degradation. The quenching experiments and the electron paramagnetic resonance spectroscopy confirmed the dominant reactive oxygen species (ROS) (i.e., sulfate radicals (SO 4 •– ), hydroxyl radicals (OH • ), singlet oxygen ( 1 O 2 ), peroxymonosulfate anion radical (SO 5 •– )) in the UV/ p -BQ/PMS process. The generation of SO 5 •– might be attributed to the single electron transfer (SET) mechanism from PMS to the triplet excited-state of p -BQ, which subsequently triggered the formation of SO 4 •– and 1 O 2 via disproportionation. The possible degradation pathways of SMX were identified by LC-MS, including the hydroxylation, the cleavage of S N bond, the nitration and the oxidation of amino group. Compared to p -BQ, methyl- p -benzoquinone, with an electron donating group, was found to suppress the activation of PMS, which validated the rationality of the SET mechanism. While tetrachloro-1,4-benzoquinone, with an electron withdrawing group, inhibited the SMX degradation by scavenging the generated radicals. Furthermore, the coexisting ions made no diffidence to the SMX degradation. The results provided the critical insights into the role of quinones in the PMS-based advanced oxidation processes for water treatment.