Peroxymonosulfate activation by black TiO2 nanotube arrays under solar light: Switching the activation mechanism and enhancing catalytic activity and stability

Abstract

Black TiO2 nanotube arrays (black TNAs) suffer from the low activity and deactivation for peroxymonosulfate (PMS) activation, which limit their application in the oxidative destruction of organic pollutants in water. Here, we report an efficient, environmentally benign, and cost-effective method to enhance the catalytic activity and prevent the deactivation of black TNAs in PMS activation by utilizing solar energy. Optical absorption and electrochemical analysis and density functional theory calculations demonstrated that abundant oxygen vacancies (estimated to be 26%) on the black TNAs surface markedly improved solar light absorption and electrical conductivity and played a critical role as a catalytic active site for PMS activation. As a result, the solar light-irradiated black TNAs/PMS system exhibited the higher phenol degradation rate (k = 0.0488 min−1) and total organic carbon (TOC) removal efficiency (~70%) compared to other TNAs systems. These results were ascribed to the switching of the reaction mechanism from non-radical mechanism to radical-involved. Black TNAs oxidized organic pollutants by mediating electron transfer from organics to PMS in the dark (i.e., a non-radical pathway). On the other hand, PMS activation under solar light irradiation involved the production of highly reactive sulfate and hydroxyl radicals (i.e., radical pathway), markedly improving the degradation and mineralization of organics. Additionally, the solar light-irradiated black TNAs showed relative pH-independence for PMS activation and durable catalytic performance without the loss of activity during the repetitive reaction cycles.