Nitrite-mediated photodegradation of sulfonamides and formation of nitrated products

In this study, we systematically investigated the indirect photolysis of five SAs, i.e., sulfamethazine (SMZ), sulfamethoxazole (SMX), sulfathiazole (STZ), sulfapyridine (SPD), and sulfamethizole (SMT), under UV-A irradiation (365 nm) and mediated by nitrite (NO₂^‾) at environmentally relevant concentrations (0.005-0.1 mM). The SAs that are resistant to direct photolysis can be effectively removed in UV/NO₂^‾ system. SAs with a six-membered heterocyclic ring (i.e., SMZ and SPD) were degraded more quickly than those with a five-membered heterocyclic ring (i.e., SMX, STZ and SMT). The pseudo-first-order rate constants (k) at nitrite concentration of 0.1 mM followed the order of k_SPD (0.0265 min^-1) > k_SMZ (0.0245 min^-1) > k_SMX (0.0184 min^-1) > k_STZ (0.0176 min^-1) > k_SMT (0.0154 min^-1). A kinetic model was developed and the contributions of direct UV photolysis, OH, and RNS to SAs degradation in UV/NO₂^‾ system were calculated. At NO₂^‾ concentration of 0.1 mM, the contributions of OH and RNS for SAs removal were 29.17-46.53% and 52.33-63.28%, respectively. Main transformation pathways including hydroxylation and nitration were proposed, based on liquid chromatography mass spectrometry analysis of the degradation products and density functional theory calculation. However, Smile-type rearrangement which generated a SO₂-extrusion product was only observed in the degradation of SAs with a six-membered ring, which explains their higher degradation rate than those with a five-membered ring. The presence of natural organic matter (NOM) decreased the formation of nitrated products. Overall, these results will be helpful to understand the fate and the potential ecological risks of SAs in sunlit aquatic environments.