A novel in-situ chemical oxidation channel – Selective pH-dependence of refractory β-lactam antibiotics in the synergistic mechanism of persulfate and g-C3N4 under visible light

Abstract

The degradation characteristics of four typical structures of β-lactam antibiotics were studied during the synergistic oxidation mechanism of persulfate (PS) and mesoporous g-C3N4 (MCN) under visible light irradiation including amoxicillin (AMX), cefotaxime (CFX), meropenem (MER), and sulbactam (SUL). Notably, the degradation of SUL in the MCN/PS system exhibits a pH-dependence different from the other three antibiotics. In this work, a novel oxidation channel for such persistent organic pollutant was revealed. Initially, SUL was bound specifically with PS via the addition reaction caused by the electron transfer between the oxygen atoms of the sulfoxide group of SUL and the HSO3− group generated by the protonation of PS. The reaction was carried out under acidic conditions and generated the SUL-O-SO2H byproducts. Successively, the adduct was further oxidized by the free radicals and singlet oxygen until the intermediate products were completely mineralized. The environmental factors of the reaction, such as the presence of humic acid and the cation-anion interactions, were also considered. In this work, the degradation pathways of SUL, the hydrolysis of the β-lactam ring, the decarboxylation, and the deamination reaction based on the combination of DFT calculations and the HPLC-QTOF-MS/MS analysis are elucidated. The toxicity of the byproducts was also calculated via the USEPA TEST. The findings improve the understanding of the reaction mechanism of the persulfate and the photocatalytic systems and provide new ideas for the oxidative degradation of refractory organic contaminants.