Self-Purifying Chloride-Mediated Sequential Nitrate Reduction-Oxidation Enabled by a Co-Oxygen Vacancy Tandem Photoelectrocatalyst

Abstract

The treatment of industrial high-salinity nitrate wastewater remains a considerable challenge, particularly for low-concentration nitrate (<100 ppm), wherein conventional biological methods exhibit limited efficiency. Herein, we report a tandem Co-OV/TiO2@TP photoelectrocatalyst fabricated in situ on a titanium substrate that integrates oxygen vacancies (OVs) and cobalt (Co) sites to drive a sequential reduction–oxidation process for deep nitrate removal. Density functional theory (DFT) calculations and in situ characterization reveal that OVs serve as preferential sites for nitrate adsorption, while adjacent Co sites promote water dissociation to generate active hydrogen species (*H), synergistically enabling reduction of nitrate (NO3-) to ammonium (NH4+). Critically, the chloride ions naturally present in the wastewater are oxidized in situ to generate ClO-, which subsequently converts NH4+ to N2, achieving complete nitrate removal. This catalyst achieves an impressive NO3--N removal efficiency of 98% and nearly 100% N2 selectivity under neutral conditions with visible light irradiation at -1.5 V (vs. SCE), showing a 67% enhancement over the OV-catalyst. Moreover, this system demonstrates excellent chloride tolerance, wide pH adaptability, and sustained performance over 26 consecutive cycles (104 h), offering an efficient strategy for the self-purifying treatment of industrial high-salinity nitrate wastewater.