Nonradical transformation of sulfamethoxazole by carbon nanotube activated peroxydisulfate: Kinetics, mechanism and product toxicity

In this work, the transformation of sulfamethoxazole (SMX) by peroxydisulfate (PDS) activated with one multi-walled carbon nanotube (CNT) via a nonradical pathway was examined, and the underlying reaction mechanism was explicated. The nonradical PDS/CNT process exhibited considerable reactivity toward SMX, and the oxidation of SMX was enhanced by the decrease of solution pH. The addition of background inorganic ions (i.e., bicarbonate and chloride ion) showed ignorable impacts on SMX transformation, while natural organic matter resulted in the decrease of SMX oxidation rate. The stoichiometric efficiency of SMX degraded vs PDS consumed in the presence of CNT was obviously higher compared to that obtained in the case of zero-valent iron (a common heterogeneous metal activator for PDS). The oxidation of sub-structural model compounds in the PDS/CNT process unveiled that the main reactive functional group of SMX molecule for attack by nonradical reactive species was aniline moiety, while the isoxazole moiety could also be slowly oxidized. A total of six oxidation products were identified during SMX treatment by the PDS/CNT process, and their generation pathways mainly included hydroxylation, isoxazole ring opening, and sulfonamide N–S bond cleavage. Ecotoxicity analysis by ECOSAR showed that the toxicity of these products was generally lower than parent SMX. Consequently, this work demonstrate that the emerging PDS/CNT oxidation technology is likely a prospective candidate for destruction of SMX in water and wastewaters.