Photoelectrochemical Degradation of Selected Organic Pollutants on Tungsten Trioxide Photoanodes

This study describes the photoelectrochemical degradation of model aromatic pollutants, viz. a herbicide (“monuron”, 3-(4-chlorophenyl)-1,1-dimethylurea) and an antimicrobial preservative (benzoic acid) on WO3 films prepared by aerosol pyrolysis. Crystalline WO3 films had a monoclinic structure, 4 μm thickness and a photocurrent density of 1 mA cm−2 at 1.2 V vs. RHE and illumination under 1 sun (simulated). The chemical and photoelectrochemical stability of WO3 films at various pH was evaluated. The dissolution rate at pH 1 and 4 was low (0.5 nm·h−1), whereas at pH 6 an increase in the dissolution rate to 15 nm·h−1 was observed. The course of the photoelectrochemical oxidation of selected pollutants was examined using HPLC. The achieved Faradaic efficiency of photoelectrooxidation of 1 mM benzoic acid divided by the number of charge equivalents per 1 mol of oxidised compound R, f/z, was 13.1 % and 12.2 % at pH 1 and 4, respectively, and for 1 mM monuron 7.2 % at pH 1 and 7.5 % at pH 4. Although the concentration of pollutants decreased significantly, UV–VIS and TOC (total organic carbon) analyses showed the prevalent presence of intermediates during the degradation. During the degradation of benzoic acid, hydroxybenzoic acid (salicylic acid) was found by HPLC as an early intermediary product.