Mechanism of photocatalytic disinfection using titania-graphenecomposites under UV and visible irradiation

Photocatalysis has been shown to be effective for the disinfection of water contaminated with pathogenic microorganisms. In order to increase the solar efficiency of photocatalysis on titanium dioxide (TiO₂₎ it is necessary to modify the TiO₂ so that visible photons may be utilised in addition to the UV. TiO2 – reduced graphene oxide composites (TiO₂-rGO) were prepared by the photocatalytic reduction of exfoliated graphene oxide (GO) using P25 (Evonik-Aeroxide) as the photocatalyst. The composites were tested for the inactivation of E. coli as the model microorganism under UV–Vis and visible only irradiation at relatively low light intensities to help elucidate the mechanism of disinfection. The results showed a 6 log inactivation of E. coli after 120 min of treatment with unmodified TiO₂-P25 and the same level of inactivation was achieved after 90 min with TiO₂-rGO under UV–Vis irradiation. Under visible irradiation only, the TiO₂-rGO gave a 5.3 log inactivation of E. coli following 180 min of treatment whereas the unmodified P25 gave only a 1.7 log-reduction in the same time, similar to that observed in the light control. Using probes, the main reactive oxygen species involved in the disinfection process were determined to be hydrogen peroxide, hydroxyl radicals, and singlet oxygen under UV–Vis irradiation; and only singlet oxygen under visible only irradiation. Scavenger studies were also performed to further elucidate the mechanism of disinfection.