Ag2O/TiO2 nanostructures for the photocatalytic mineralization of the highly recalcitrant pollutant iopromide in pure and tap water

TiO2 was modified by the deposition of Ag2O nanoparticles to increase the photocatalytic degradation of iopromide in water under UV-C (lambda = 254 nm) and UV-A/visible light irradiation (380-800 nm) using pure and tap water. Several loadings of Ag2O were deposited on TiO2, namely 0.03, 0.15, 0.25, 0.65, 1.0, 1.15, 1.35 and 1.8 wt. %. XRD, TEM, BET, ICP-OES, XPS, DRS and cathodoluminescence spectroscopy were carried out to characterize the materials, while semiconducting properties of the composite were elucidated through electro-chemical and photoelectrochemical characterization. Under UV-C light irradiation, the Ag2O/TiO2 heterostructures showed higher mineralization of iopromide (up to 86%, using the 1.15 wt. % Ag2O/TiO2 material) than unmodified materials (37% for TiO2 and 14% for Ag2O), indicating a synergistic effect by the combination of both compounds in the composite. Under UV-A/visible light irradiation, mineralization achieved with the 1.15 wt. % Ag2O/TiO2 material decreased up to 65%, which was again higher than that obtained for its single components. Stability of the photocatalyst was observed through three consecutive reaction cycles under UV-A/visible light irradiation. In tap water, environmentally relevant concentrations of iopromide were tested (Co= 50 mu g L-1), resulting in a high degradation rate, while mineralization dramatically decreased because of the matrix effect. Some by-products were identified by mass spectroscopy and a possible degradation path was proposed. The outstanding photocatalytic activity of the Ag2O/TiO2 materials was explained by the electron trap effect exerted by Ag2O, along with the appearance of different silver species (Ag2O, Ag2O2 and Ag degrees) throughout the photocatalytic reaction, enhancing the mobility of the charge carriers and thus the generation of reactive species on the photocatalyst surface.