Photocatalytic production of 1O2 and ·OH mediated by silver oxidation during the photoinactivation of Escherichia coli with TiO2

Highlights: ► Metallic silver enhances the Ag–TiO2 photoinactivation of Escherichia coli under Vis irradiation. ► 1O2 and ·OH were identified in Vis irradiated Ag–TiO2 suspensions. ► UV oxidized the Ag metallic species in the material decreasing photoactivity. ► Dark contact of the UV oxidized material with E. coli regenerates the photocatalyst. - Abstract: Ag loaded TiO2 was applied in the photocatalytic inactivation of Escherichia coli under ultraviolet (UV) and visible (Vis) light irradiations. Ag enhanced the TiO2 photodisinfecting effect under Vis irradiation promoting the formation of singlet oxygen and hydroxyl radicals as identified by EPR analyses. Ag nanoparticles, determined on TEM analyses, undergo an oxidation process on the TiO2's surface under UV or Vis irradiation as observed by XPS. In particular, UV pre-irradiation of the material totally diminished its photodisinfection activity under a subsequent Vis irradiation test. Under UV, photodegradation of dichloroacetic acid (DCA), attributed to photoproduced holes in TiO2, was inhibited by the presence of Ag suggesting that oxidation of Ag0 to Ag+ and Ag2+ is faster than the oxidative path of the TiO2's holes on DCA molecules. Furthermore, photoassisted increased of Ag+ concentration on TiO2's surface enhances the bacteriostatic activity of the material in dark periods. Indeed, this latter dark contact of Ag+–TiO2 and E. coli seems to induce recovering of the Vis light photoactivity promoted by the surface Ag photoactive species.