After the 24-h adsorption process, the intensities of all the fluorophores
at a pH of 6.5 are stronger than those at a pH of 3.0
because the adsorption amount of the HA by TiO2 decreases with
the increasing pH levels (Erhayem and Sohn, 2013). Furthermore,
Au (Au et al., 1999) determined that the HA molecules would be
stretched linearly due to the repulsive interactions between the HA
molecules under neutral and alkaline conditions; more fluorophores
would be exposed in the solution, and the fluorescence
intensity would be enhanced. When the irradiation begins, the
intensities of the fulvic-like and humic-like fluorophores decrease
due to the photocatalytic degradation of the HA. However, the intensity
of the protein-like fluorophore increases slightly during first
the 30 min of irradiation, which indicates that certain intermediates
of the protein-like fluorescence substances are generated.
The humic-like fluorophore disappears after 60 min of
irradiation, and the fulvic-like and protein-like fluorophores
decrease with an increase in the irradiation time. In the UV irradiation
process, the particles aggregate to a larger size of approximately
1300.0 nm after 4 h of UV irradiation, and all of the
fluorescence intensities are weaker compared to those at a pH of
3.0. In this case, the aggregation size is considerably larger than that
at a pH of 3.0, thus, indicating that these by-products are important
for the stability of the TiO2 nanoparticles.