282
TiO 2 photocatalysis, which include Lactobacillus acidophilus, E. coli, Saccharomyces
cerevisiae, Chlorella vulgaris, Streptococcus faecalis and aureus, Enterobacter cloacae, total coliforms, Candida albicans, Fusarium solani, Aspergillus niger, and
others [119, 135, 196, 212, 336].
Although the electrostatic link between the catalyst particles and microorganisms was reported to exist, a subsequent microbial cell adsorption and eventual
penetration of the catalysts through the cell wall are highly dependent on the mean
particle size. Since the relative sizes between the bacteria and catalysts are significantly different, the charged TiOH 2
+ clusters might not come into full contact with
the bacteria. Sichel et al. [291] found that the TiO 2 catalyst is actually adsorbed onto
fungal spores rather than the reverse setting. This is owing to the fact that the fungal
spores are larger (a few order of magnitudes) than the catalyst particles. A small
decrease in microbial loadings was observed during the initial dark homogenization
period with catalyst particles. This is owing to the catalyst agglomeration and
subsequent sedimentation of the agglomerates. The rate of bacterial adsorption in
this instance was found to directly relate to the bactericidal activity of the catalyst
used. It was also reported that the adsorption phenomena during the dark and irradiated phase will act indifferently and thus a strong conclusion cannot be made.
Besides, the interaction between the catalyst particles itself also exists and is
dependent on the operating pH. The particle size of the same catalysts can vary from
300 nm to 4 mm depending on the distance from the PZC of TiO 2 [205]. At
pH = PZC, the neutral surface charge of the catalyst particles is unable to produce
the interactive rejection for solid–liquid separation. Thus, this induces catalyst
aggregation where the catalyst becomes larger, leading to catalyst sedimentation
[28]. This physical property is usually manipulated in the hybridized PMR system,
where the pH of the treated wastewater is neutralized to pH 7 for the subsequent
recovery of catalyst particles. The larger TiO 2 clusters can settle faster than the
smaller one. It was reported that with such neutralization strategy, almost 97% of
the catalysts can be recovered in the settling tank. The remaining TiO 2 catalysts can
be recovered via the downstream MF system. Similarly, the water pH will also
affect the effective separation in the PMR system where inappropriate control of
water pH promotes the electrostatic repulsion, the Donnan exclusion phenomena,
and thus the rejection tendency for the membrane used [225, 283]. It must be
stressed that appropriate pH control strategies must be implemented at every different location of a photocatalytic water treatment process for efficient photocatalytic
reaction to proceed.
Temperature
Numerous studies have been conducted on the dependence of photocatalytic reaction on the reaction temperature [45, 87, 96, 236, 274]. Although heat energy is
inadequate to activate the TiO 2 surface, the understanding on such dependency
could be extrapolated when operating the process under natural sunlight illumination. Most of the previous investigations stated that an increase in photocatalytic
13 Wastewater
TiO 2 photocatalysis, which include Lactobacillus acidophilus, E. coli, Saccharomyces
cerevisiae, Chlorella vulgaris, Streptococcus faecalis and aureus, Enterobacter cloacae, total coliforms, Candida albicans, Fusarium solani, Aspergillus niger, and
others [119, 135, 196, 212, 336].
Although the electrostatic link between the catalyst particles and microorganisms was reported to exist, a subsequent microbial cell adsorption and eventual
penetration of the catalysts through the cell wall are highly dependent on the mean
particle size. Since the relative sizes between the bacteria and catalysts are significantly different, the charged TiOH 2
+ clusters might not come into full contact with
the bacteria. Sichel et al. [291] found that the TiO 2 catalyst is actually adsorbed onto
fungal spores rather than the reverse setting. This is owing to the fact that the fungal
spores are larger (a few order of magnitudes) than the catalyst particles. A small
decrease in microbial loadings was observed during the initial dark homogenization
period with catalyst particles. This is owing to the catalyst agglomeration and
subsequent sedimentation of the agglomerates. The rate of bacterial adsorption in
this instance was found to directly relate to the bactericidal activity of the catalyst
used. It was also reported that the adsorption phenomena during the dark and irradiated phase will act indifferently and thus a strong conclusion cannot be made.
Besides, the interaction between the catalyst particles itself also exists and is
dependent on the operating pH. The particle size of the same catalysts can vary from
300 nm to 4 mm depending on the distance from the PZC of TiO 2 [205]. At
pH = PZC, the neutral surface charge of the catalyst particles is unable to produce
the interactive rejection for solid–liquid separation. Thus, this induces catalyst
aggregation where the catalyst becomes larger, leading to catalyst sedimentation
[28]. This physical property is usually manipulated in the hybridized PMR system,
where the pH of the treated wastewater is neutralized to pH 7 for the subsequent
recovery of catalyst particles. The larger TiO 2 clusters can settle faster than the
smaller one. It was reported that with such neutralization strategy, almost 97% of
the catalysts can be recovered in the settling tank. The remaining TiO 2 catalysts can
be recovered via the downstream MF system. Similarly, the water pH will also
affect the effective separation in the PMR system where inappropriate control of
water pH promotes the electrostatic repulsion, the Donnan exclusion phenomena,
and thus the rejection tendency for the membrane used [225, 283]. It must be
stressed that appropriate pH control strategies must be implemented at every different location of a photocatalytic water treatment process for efficient photocatalytic
reaction to proceed.
Temperature
Numerous studies have been conducted on the dependence of photocatalytic reaction on the reaction temperature [45, 87, 96, 236, 274]. Although heat energy is
inadequate to activate the TiO 2 surface, the understanding on such dependency
could be extrapolated when operating the process under natural sunlight illumination. Most of the previous investigations stated that an increase in photocatalytic
13 Wastewater