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 H2O2
via 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.