Efficient elimination of sulfamethoxazole by UV/peroxymonosulfate/sulfite process: Activation of peroxymonosulfate by sulfite via nucleophilic addition

Sulfamethoxazole (SMX), a common broad-spectrum antibiotic, has received widespread attention due to its adverse effects and ecological risks. Herein, the UV/PMS/sulfite process was proposed to efficiently remove SMX in the aqueous solution. Under the conditions of pH 9.0, PMS concentration of 300 µM, and sulfite concentration of 50 µM, the removal rate of SMX reached 52.7 % within 13 min. There was a similar trend between the change in the degradation rate constants and the speciation of HSO₃^–, HSO₅^– as pH varied. HSO₃^– had a greater impact on the removal efficiency than SO₃^2–, which implied that PMS might not be activated through the single electron transfer. The scavenging experiments and the electron paramagnetic resonance revealed that sulfate radicals (SO₄^·–), peroxysulfate radicals (SO₅^·–), hydroxyl radicals (OH^·), and singlet oxygen (¹O₂) might lead to the degradation of SMX. The quantum chemical calculations showed that the efficient transformation of PMS was attributed to the nucleophilic addition of HSO₅^– to HSO₃^–, which precisely explained the reason for the highest k_obs at pH 7.0. The cleavage of peroxide bond of the adduct led to the generation of sulfite radicals (SO₃^·–) and SO₄^·–. Then, SO₃^·– could directly react with the dissolved oxygen in water to produce SO₅^·–, and SO₅^·– had the potential to convert into SO₄^·– and ¹O₂ through the self-reaction. These studies may offer some understandings into the activation mechanism of PMS, and it has potential practical application for the degradation of other pollutants.