L-cysteine boosted Fe(III)-activated peracetic acid system for sulfamethoxazole degradation: Role of L-cysteine and mechanism

Sulfamethoxazole (SMX), a widely used and frequently detected antibiotics, may pose threat to water safety and environmental sustainability. In this study, L-cysteine (L-Cys) was used to boost the Fe(III)/peracetic acid (PAA) system for SMX degradation. After the addition of L-Cys, the degradation efficiency of Fe(III)/PAA system for SMX increased from 26.2% to 96.7%. Appropriate increase in initial concentration of Fe(III), L-Cys or PAA promoted the SMX degradation. The role of L-Cys, including the promotion of Fe(III)/Fe(II) cycle and the improvement of pH suitability by complexation, in boosted SMX degradation was proposed. Results of methyl phenyl sulfoxide probe assays, electron paramagnetic resonance tests and quenching experiments showed that (FeO2+)-O-IV as well as HO center dot play major roles while carbon-centered radicals hardly contribute in SMX degradation. Degradation intermediates identified by LC/MS showed that SMX was degraded to less toxic small molecules through S-N bond breaking, S-C bond breakage, self-coupling and hydroxylation. Besides, L-Cys/Fe(III)/PAA system showed ideal tolerance to Cl- and humic acid, while HCO3- significantly inhibited the SMX degradation. Further, the satisfactory degradations of SMX were obtained in three real waters. This study provides a feasible strategy for enhancing the oxidation capacity of the Fe(III)/PAA system and an effective technique for the treatment of SMX in real waters.