277
MF [49, 132, 144, 222, 278, 280], UF [227, 293, 294, 298, 310], and nanofiltration
(NF) [13, 226–229], depending on the targeted colloidal size and final water quality
requirement. The MF membrane is useful when the colloidal size is in the range of
0.1–5 mm, while both UF and NF target a smaller particle size range. The photooxidation efficiency of the contaminants was reported to be higher when an immobilized PM was used, rather than in the case of PMRs with suspended catalyst particles
[229]. It was, however, reported that immobilizing the photocatalyst particles might
cause severe destruction to the membrane structure owing to their close contact with
both UV light and hydroxyl radicals [48]. In view of this, the hybridization configuration of the membrane process using photocatalysts in suspension appears to be the
more promising arrangement. This PMR configuration has been well described in
the literature for water-phase degradation of humic and fulvic acids, bisphenol A,
phenol, 4-nitrophenol, 4-chlorophenol, gray water, para-chlorobenzoate, river
water, and dyes [48, 95, 132, 145, 222, 226, 227, 229, 278, 280, 293, 294, 298, 310].
With these PMRs, one of the main operational issues is the transmembrane pressure, which determines both the filtration rate and operating costs. It was known that
the PMR treatment costs increase if the photocatalysts with small particle and colloidal size are used. With both the MF and UF membrane filtration, the fine photocatalyst particles can cause membrane fouling and subsequently reduce membrane
permeate flux. Fu et al. [95] utilized a spherical ball-shaped TiO 2 particle that promotes separation, recovery, and reuse while prolonging the membrane life span as
the particles do not cause pore blockage. Besides, the surface charge properties of
the photocatalyst particles can also be manipulated to prevent membrane pore
blockage. Xi and Geissen [332] integrated a thermoplastic membrane module of
cross-flow MF and found that the low permeate flux occurred when the operating
pH varied from the isoelectric point of the TiO 2 particles used. This is owing to the
pH-induced coagulation–flocculation state of TiO 2 that declines the rate of permeate
flux. This was resolved by maintaining the operating pH close to the isoelectric
point of TiO 2 by adding certain electrolytes to the TiO 2 slurry. Even with such control strategies, the quality of permeate is low owing to the rapid penetration of small
molecules through the membrane used.
Recently, different hybridizations of PMRs with dialysis, pervaporation, and
direct-contact membrane distillation (MD) have been used (Fig. 13.9) [16, 41, 232].
Pervaporation is a physical process where usually a selective organophilic membrane was used to act as a selective barrier for the molecules to be degraded.
Augugliaro et al. [14] observed that a synergistic effect occurs when pervaporation
was used, where the intermediates from the degradation of 4-chlorophenol (i.e.,
hydroquinone, benzoquinone) could be selectively permeable without competing
with 4-chlorophenol for photocatalytic reaction. Other types of organophilic membrane have also been investigated, such as polymeric UF membranes and polyethersulfonate NF membranes [18, 171]. A stronger rejection impact on the membrane is
usually associated with the use of the UF or NF membranes. The choice of membrane for an efficient hybridization depends on the organic molecular size, pH, or
electrostatic interaction and Donnan exclusion phenomenon [221].
Recent Developments in Photocatalytic Water Treatment Technology
MF [49, 132, 144, 222, 278, 280], UF [227, 293, 294, 298, 310], and nanofiltration
(NF) [13, 226–229], depending on the targeted colloidal size and final water quality
requirement. The MF membrane is useful when the colloidal size is in the range of
0.1–5 mm, while both UF and NF target a smaller particle size range. The photooxidation efficiency of the contaminants was reported to be higher when an immobilized PM was used, rather than in the case of PMRs with suspended catalyst particles
[229]. It was, however, reported that immobilizing the photocatalyst particles might
cause severe destruction to the membrane structure owing to their close contact with
both UV light and hydroxyl radicals [48]. In view of this, the hybridization configuration of the membrane process using photocatalysts in suspension appears to be the
more promising arrangement. This PMR configuration has been well described in
the literature for water-phase degradation of humic and fulvic acids, bisphenol A,
phenol, 4-nitrophenol, 4-chlorophenol, gray water, para-chlorobenzoate, river
water, and dyes [48, 95, 132, 145, 222, 226, 227, 229, 278, 280, 293, 294, 298, 310].
With these PMRs, one of the main operational issues is the transmembrane pressure, which determines both the filtration rate and operating costs. It was known that
the PMR treatment costs increase if the photocatalysts with small particle and colloidal size are used. With both the MF and UF membrane filtration, the fine photocatalyst particles can cause membrane fouling and subsequently reduce membrane
permeate flux. Fu et al. [95] utilized a spherical ball-shaped TiO 2 particle that promotes separation, recovery, and reuse while prolonging the membrane life span as
the particles do not cause pore blockage. Besides, the surface charge properties of
the photocatalyst particles can also be manipulated to prevent membrane pore
blockage. Xi and Geissen [332] integrated a thermoplastic membrane module of
cross-flow MF and found that the low permeate flux occurred when the operating
pH varied from the isoelectric point of the TiO 2 particles used. This is owing to the
pH-induced coagulation–flocculation state of TiO 2 that declines the rate of permeate
flux. This was resolved by maintaining the operating pH close to the isoelectric
point of TiO 2 by adding certain electrolytes to the TiO 2 slurry. Even with such control strategies, the quality of permeate is low owing to the rapid penetration of small
molecules through the membrane used.
Recently, different hybridizations of PMRs with dialysis, pervaporation, and
direct-contact membrane distillation (MD) have been used (Fig. 13.9) [16, 41, 232].
Pervaporation is a physical process where usually a selective organophilic membrane was used to act as a selective barrier for the molecules to be degraded.
Augugliaro et al. [14] observed that a synergistic effect occurs when pervaporation
was used, where the intermediates from the degradation of 4-chlorophenol (i.e.,
hydroquinone, benzoquinone) could be selectively permeable without competing
with 4-chlorophenol for photocatalytic reaction. Other types of organophilic membrane have also been investigated, such as polymeric UF membranes and polyethersulfonate NF membranes [18, 171]. A stronger rejection impact on the membrane is
usually associated with the use of the UF or NF membranes. The choice of membrane for an efficient hybridization depends on the organic molecular size, pH, or
electrostatic interaction and Donnan exclusion phenomenon [221].
Recent Developments in Photocatalytic Water Treatment Technology