Photoelectrochemical degradation of organic compounds via formed reactive chlorine and sulfate species by WO₃-based photoanodes

Contamination of aquatic environment and drinking water with various organic compounds and microorganisms has negative effect on human and animal health. Conventional cleaning systems consume a lot of energy and are not fully effective, so new cleaning systems are being sought. Photoelectrochemical (PEC) oxidation systems are based on the light-assisted degradation of organic materials on semiconductor electrodes. In this work tungsten oxide (WO₃) photoanodes were formed using hydrothermal synthesis. Synthesis was optimized via adjustment of temperature and addition of polyethylene glycol as a structure-directing agent. X-ray diffraction and scanning electron microscopy analysis were performed to evaluate the phase composition, crystallite size and morphology of the layers. PEC activity of the coatings was evaluated using cyclic voltammetry and chronoamperometry. Faradaic efficiencies of light-induced production of reactive sulfate and reactive chlorine species in 0.1 M Na₂SO₄ and 0.1 M NaCl electrolytes were in the range of 62–93% and 91–96%, respectively. All coatings demonstrated excellent stability during 4 h lasting photoelectrolysis performed in both electrolytes, however photocurrents in chloride medium were found to be almost twice higher compared to sulfate electrolyte. The efficiency of rhodamine B photoelectrochemical degradation in 0.1 M Na₂SO₄ was significantly lower than in 0.1 M NaCl, however diclofenac was more efficiently decomposed in sodium sulfate electrolyte. Based on a comparative study of PEC and chemical oxidizing treatments, it has been proposed that highly reactive chlorine, sulfate and hydroxyl radicals are the key promoters of photoelectrochemical degradation of organic molecules, however efficiency of degradation is strongly dependent on the nature of organic molecule.