Fig. S3 illustrates the removal of HA from the solution, which is
determined by the UVevis spectrophotometer at 254 nm. After 4 h
of irradiation, we observe that the percent of HA removed is
approximately 56.0% for a pH of 3.0; the percent of HA removed for
pH levels of 6.5 and 9.0 are nearly the same (nearly 100%). A neutraland alkaline pH is more favorable for the generation of $OH via hole
oxidation of OH or hydroxy on the surface of the TiO2. Therefore, it
may cause a more efficient attack of the HA molecules, which are
adsorbed on the surface of the TiO2. However, the acidic functional
groups become less protonated, and the molecule begins to uncoil
as the pH increases. Hence, it becomes easier for the HA molecules
to be attacked by $OH (Liu et al., 2008). Palmer (Palmer et al., 2002)
determined that the HA molecules transformed into micelles at low
pH levels, which was considered to be undesirable in thephotodegradation process. Compared with the results on the size,
we determined that the equilibrium time of the HA removal is in
accordance with the change in particle time of. This result verifies
that the photocatalytic degradation of HA dominates the aggregation
behavior of TiO2 in our study.
Humic substances typically express three fluorophores, which
are in the excitation wavelength range of lexc ~ 200e550 nm and
the emission wavelength range of lemis ~ 200e550 nm. The fluorescence
center that was excited in the range of lexc ¼ 220e250 nmand emitted in the range of lexc ¼ 400e460 nm is referred to as a
fulvic-like fluorophore. The fluorescence center that was excited at
lexc ¼ 300e340 nm and emitted at lemis ¼ 400e460 nm is a humiclike
fluorophore. The fluorescence center that was excited in
lexc ¼ 270e280 nm and emitted at lemis ¼ 330e370 nm is referred
to as a protein-like fluorescence (tyrosine and tryptophan-like)
(Coble, 1996).
Fig. 4 demonstrates the 3DEEM fluorescence spectra in the first
240 min of the UV irradiation process at a pH of 3.0. At 0 min (thetime immediately after the 24 h adsorption process), the fluorescence
spectra display the presence of humic-like, fulvic-like and
protein-like fluorophores in the solution. However, the intensity of
the humic-like fluorophore is lower than that of the other two
fluorophores. After 60 min of irradiation, the humic-like fluorophore
disappears while the fluorescence intensities of the fulviclike
and protein-like fluorophores increase. As the irradiation time
increases (60e240 min), the intensities of the fulvic-like and
protein-like fluorophores remain constant.