Degradation and mineralization of oxytetracycline in pure and tap water under visible light irradiation using bismuth oxyiodides and the effect of depositing Au nanoparticles

Abstract Different bismuth oxyiodide materials (BiOI, Bi4O5I2, Bi7O9I3 and Bi5O7I) were synthesized via the solvothermal method. For BiOI, microspheres (M.S.) and nanocrystals were obtained. Decrease in the iodine loading was achieved by varying the synthesis conditions, such as the solvent, temperature and heating time. Gold metallic nanoparticles were deposited on the surface of the BixOyIz materials via the deposition–precipitation route. The physical, chemical, optic and electrochemical properties of the obtained materials were assessed and compared, finding notable structural and optoelectric changes as the loading of iodine decreased in the semiconductor. In fact, lessening the iodine loading resulted, not only in the increase of the band gap value, but in the shift of the valence band toward more oxidative potential. In contrast, no changes in the semiconductor properties of the BixOyIz materials were observed upon the deposition of metallic Au nanoparticles. Regarding the photocatalytic activity, the use of BiOI M.S. led to the highest degradation and mineralization rate of the antibiotic oxytetracycline (OTC) upon 5 h of visible light irradiation. Deposition of Au nanoparticles reduced the photo activity of the materials, even when low metal loadings were tested. BiOI M.S. showed certain stability throughout three consecutive reactions cycles. High photocatalytic performance was observed when environmentally relevant conditions –i.e., tap water and Co = 50 µg/L– were tested. Through mass spectrometry some degradation intermediaries were identified and a photocatalytic degradation route of OTC was proposed.