Nitrogen-doped graphene as peroxymonosulfate activator and electron transfer mediator for the enhanced degradation of sulfamethoxazole

Abstract

Abstract Nitrogen-doped graphene (NRGO) can activate peroxymonosulfate (PMS). However, the activation mechanism of PMS by NRGO needs to be further investigated. In this study NRGO was prepared by a facile method, and its activation mechanism of PMS for the degradation of sulfamethoxazole (SMX) was investigated. The XPS spectrum showed that nitrogen loading reached 16.5% (atomic percentage). The SMX removal efficiency reached 91.7% at 240 min in the NRGO-activated PMS system. During the PMS activation process, NRGO acted as dual roles: PMS activator and electron transfer mediator. NRGO can activate PMS to form singlet oxygen. In addition, NRGO can act as the electron transfer mediator to enhance the SMX degradation. Density functional theory (DFT) calculations demonstrated that nitrogen doping increased the charge density of net positive carbon atoms and decreased the energy gap, further enhancing the PMS activation. The effects of pH, temperature and the concentrations of chloridion and humic acid were investigated. NRGO had good stability in activating PMS evidenced by the cycling experiments. Eight degradation intermediate products of SMX was identified. Among them, 3-amino-5-methyl-isoxazole and 4-nitro-SMX presented high abundance. The acute toxicity of treated SMX solution increased, suggesting that the degradation products of SMX formed in the non-radicals’ process should be concerned. This study provided an insight into the SMX degradation by NRGO-activated PMS including the mechanisms of PMS activation and SMX degradation.