Degradation of progesterone by coexisting free radical and nonradical pathways in the CuO/HNTs-PS system

Progesterone (PGT), a widely used progestogen, has become a refractory pollutant present in urban runoff. The removal of PGT in wastewater is urgent owing to its endocrine disruption. In this study, a nano-CuO loaded halloysite nanotube (CuO/HNTs) was prepared by a simple hydrothermal method, which could be used as a catalyst to activate persulfate (PS) for the degradation of PGT. The characterizations showed that the nano-CuO was uniformly loaded on the HNTs to form corn-like structures with a large specific surface area. The optimal PGT degradation efficiency (100%) was achieved with the addition of 0.5 g/L catalyst and 7 mM PS. The effects of pH, HCO3– and Cl- on the catalytic activity of the CuO/HNTs-PS system were investigated. The results showed that 1) pH had little effect due to the buffer effect of Al2O3 in HNTs, and nearly neutral solutions were obtained after reaction even in strong acidic or alkaline conditions, 2) low concentrations of HCO3– and Cl- had almost no impact on PGT degradation. Furthermore, the degradation mechanism was explored by quenching reactions, XPS, EPR studies and electrochemical experiments. The results demonstrated that the reaction was divided into the free radical pathway (SO4•- and •OH generation by the mutual transformation of PS, Cu(I), Cu(II), and Cu(III)) and the nonradical pathway (electron transfer process from the organic pollutant to the CuO/HNTs). In addition, the byproducts of PGT in the process were determined by LC-MS, and the possible transformation pathways were proposed. The CuO/HNTs-PS system might provide some new insights for the remediation of wastewaters.