The behavior of photoactive TiO2 nanoparticles in an aquatic environment under UV irradiation was
investigated. When there was no UV light irradiation, the attachment of humic acid (HA) onto the TiO2
nanoparticles improved their stability due to an increase in the electrostatic and steric repulsions between
the particles. However, our study demonstrated that UV light clearly influenced the aggregation of
TiO2 nanoparticles. Half an hour of UV irradiation caused the particles to aggregate from 331.0 nm to
1505.0 nm at a pH of 3.0. Similarly, the particles aggregated from 533.2 nm to 1037.0 nm at a pH of 6.5
and from 319.0 nm to 930.0 nm at a pH of 9.0. The aggregation continued with increased irradiation time,
except for the condition at pH 3.0, which demonstrated disaggregation. Furthermore, we determined
that the photocatalytic degradation of the HA dominated the behavior of TiO2 in our study. From the
results of HA removal and 3DEEM fluorescence spectra data for the solution, a change in the HA was in
accordance with the size change of the TiO2. The results illustrated that the UV irradiation affected the
behavior of light-active nanomaterial (such as TiO2) in an aquatic system, thus influencing their
bioavailability and reactivity.