Selectively photoelectrocatalytic reduction of oxygen to hydroxyl radical and singlet oxygen: Mechanism and validation in coal wastewater

The fine-tuning of generating hydroxyl radical ((OH)-O-center dot) and singlet oxygen (O-1(2)) was firstly proposed via the selectively photoelectrochemical (PE) reduction of oxygen with Cu-coordinated carbon-nitride-electrode (CuCN). Complete removal efficiency and trace Cu leaching (<0.01 ppm) were achieved during treatment of dimethyl phthalate and atrazine, which were mainly oxidized by (OH)-O-center dot and O-1(2), respectively. Besides, no toxic intermediates was detected in the whole degradation process. The overall oxygen reduction pathway in PE system was demonstrated by electron paramagnetic resonance measurements and quenching tests. The valence band position of CuCN was designed to be more negative (-1.5 V-SHE) for favoring the photocatalytic oxygen reduction. Specifically, Cu-N-2-C as active sites optimized the Gibbs free energy of intermediate *OOH (AG(*OOH) = 3.99 eV) to improve the reactivity of two-electron oxygen reduction. The CuCN/photoelectrochemical system is applied for treatment of coal water slurry gasification (CWS) wastewater with high concentration of salt. The TOC of CWS was successfully decreased from 127 to 1.9 mg.L-1 in 3 h with a low specific energy consumption of 0.024-0.05 kW.h.g(-1).TOC-1. The treated CWS wastewater can be used for recovery and utilization of salt as chemical raw materials. This work highlights the important role of synergistic photo-electro-catalytic effect in regulating the generation of active oxygen species via oxygen reduction pathway, which can be a promising way especially for salinity coal wastewater remediation.