The design and characterisation of a Z-scheme Bi2O2S/ZnO heterojunction photoanode for the photoelectrochemical removal of ciprofloxacin in synthetic and real wastewater

Towards the engineering of semiconductors for improved visible light photoelectrochemical degradation of organic pollutants in wastewater, we present the preparation of a Z-scheme heterojunction of Bi2O2S and ZnO nanoparticles for the removal of ciprofloxacin from wastewater. The Bi2O2S/ZnO nanocomposite heterojunction was synthesised via an in-situ hydrothermal method and deposited on FTO substrate to form the photoanode. The semiconductors and the photoanodes were characterised with electron microscopy, XRD, photo-electrochemistry, photoluminescence, XPS and UV/Vis diffusive reflectance techniques. The degradation of ciprofloxacin was enhanced by the heterojunction in comparison with the Bi2O2S and ZnO photoanodes. We attribute the enhanced performance to the presence of an internal electric field (IEF), the low rate of recombination of photogenerated electrons and holes, lower charge transfer resistance, and high photocurrent density of the heterojunction based on data from photoluminescence, electrochemical impedance spectroscopy, XPS, and photocurrent response. The extent of mineralisation of ciprofloxacin was calculated as 86 % from total organic carbon (TOC) measurement. LC-MS analysis provided the degradation pathway and premise for the reaction scheme, while quantitative structure–activity relationship (QSAR) model was used to predict the toxicity of the intermediate and by-product. The Bi2O2S/ZnO photoanode was stable after seven treatment cycles. The photoelectrochemical reactor was used in the treatment of ciprofloxacin in real wastewater with 87 % TOC removal. Thus, the paper reveals the performance of a unique Z scheme band gap engineering of semiconductors for application in the removal of pharmaceuticals in wastewater.