Photoelectrocatalytic processes for the elimination of organic dyes from model aqueous systems

Dye industries produce large quantities of wastewaters with visible colorization that cause non-esthetic pollution, serious health risks, and inhibit plant growth by impairing photosynthesis. Their bioaccumulation and recalcitrance can promote carcinogenicity, mutagenicity, and toxicity on aquatic beings and humans, requiring urgent decontamination. This article presents a comprehensive critical review of the development of photoelectrocatalysis (PEC) applied to remediate organic dyes in synthetic aqueous solutions. Photogenerated electron/hole pairs at the photoanode by photocatalysis are produced under light irradiation (UV light, Visible light, and sunlight). These pairs produce oxidants such as hydroxyl radical (•OH) from H2O oxidation with holes and superoxide (O2•-) from O2 reduction with electrons. The passage of electrons to the cathode prevents fast recombination of photogenerated charrier carriers. Physisorbed •OH was also formed at the photoanode by electrocatalysis from anodic H2O oxidation. The preparation methods of novel photoanodes, the PEC systems used, the effect of operating variables that affect discoloration performance of dyes, the mechanisms that generate the oxidants, and reusability are detailed. Application of TiO2, ZnO, and WO3 materials, Co and Fe oxides, Bi compounds, g-C3N4, PbO2, and other materials as photoanodes for organic dyes removal are described and analyzed. The performance of proposed hybrid PEC processes with O3, photoelectro-Fenton, sulfite, peroxymonosulfate, and persulfate was finally discussed.