Given the widespread use of TiO2, its release into aquatic systems and complexation with dissolved organic matter (DOM) are highly possible, making it important to understand how such interactions affect photocatalytic activity under visible light. Here, we show that humic acid/TiO2 complexes (HA/TiO2) exhibit photoactivity (without significant electron-hole activation) under visible light through ligand-to-metal charge transfer (LMCT). The observed visible-light activities for pollutant removal and bacterial inactivation are primarily linked to the generation of H2O2via the conduction band. By systematically considering molecular-scale interactions between TiO2 and organic functional groups in HA, we find a key role of phenolic groups in visible-light absorption and H2O2 photogeneration. The photochemical formation of H2O2 in river waters spiked with TiO2 is notably elevated above naturally occurring H2O2 generated from background organic constituents due to LMCT contribution. Our findings suggest that H2O2 generation by HA/TiO2 is related to the quantity and functional group chemistry of DOM, which provides chemical insights into photocatalytic activity and potential ecotoxicity of TiO2 in environmental and engineered systems.
Keywords: humic acid; hydrogen peroxide; ligand-to-metal charge transfer (LMCT); titanium dioxide nanoparticles; visible light.