270
TiO 2 in the treated water [338]. The catalyst recovery can be achieved through
process hybridization with conventional sedimentation, cross-flow filtration, or various membrane filtrations [80, 90, 353, 357]. Coupled with the pH control strategy
close to the isoelectric point for induced coagulation, it was reported that the microfiltration (MF) hybridization can recover the remaining 3% of the catalyst particles
for reuse [205]. Several important operating issues with slurry TiO 2 still remain
even with a membrane integration process. These include the types of membrane,
pore size and blockage, regeneration or back-washing, and fouling [173, 227, 332].
A number of studies have utilized micron-size immobilizers for catalyst fixation
that enhance surface contact with contaminants and prevent membrane fouling or
pore blocking with rapid back-washing [332, 355]. These immobilizers include
catalyst fixation onto activated carbon, mesoporous clays, fibers, or even the membrane itself [63, 165, 170, 358]. The following subsections outline a few catalyst
immobilization strategies that are suitable for the use of slurry reactor or membrane
reactor or both.
Mesoporous Clays
Natural clays have been used intensively as the support for TiO 2 owing to their high
adsorption capacity and cost-effectiveness. Figure 13.5 shows the TiO 2 crystal being
deposited on a clay material [63]. Different types of clays have been investigated,
which include bentonite, sepiolite, montmorillonite, zeolite, and kaolinite [63, 100,
164, 299, 333]. Although these clays are catalytically inactive, their superior adsorption capacity has been attractive for increasing the surface contact during photocatalysis reaction. It was proposed that the natural clays should not be used directly to
immobilize TiO 2 . This is owing to the presence of different surface or latticebounded impurities that might diffuse and further affect the TiO 2 efficiency of the
immobilized layer [63]. In addition, if these impurities are not removed, the polar
molecules in the aqueous environment might initiate an internal reaction within the
clay structure that results in clay swelling [63]. The swelling will be profound in
certain type of clays, where van der Waals forces hold the entire clay in a turbostatic
array. This is undesirable, particularly if the photocatalytic reactions take place in a
reactor where the hydrodynamics may be strongly affected and consequently leading to the loss of photoactivity. Other factors that might need to be taken into consideration if pillared clays are used as the immobilizer substrate include the density
of the clays, particle size distribution range, and complementary photoreactor system used. The use of mesoporous clays as the support for nano-size TiO 2 has been
successfully demonstrated in a number of studies, including the slurry or membrane
processes [62, 298].
13 Wastewater
TiO 2 in the treated water [338]. The catalyst recovery can be achieved through
process hybridization with conventional sedimentation, cross-flow filtration, or various membrane filtrations [80, 90, 353, 357]. Coupled with the pH control strategy
close to the isoelectric point for induced coagulation, it was reported that the microfiltration (MF) hybridization can recover the remaining 3% of the catalyst particles
for reuse [205]. Several important operating issues with slurry TiO 2 still remain
even with a membrane integration process. These include the types of membrane,
pore size and blockage, regeneration or back-washing, and fouling [173, 227, 332].
A number of studies have utilized micron-size immobilizers for catalyst fixation
that enhance surface contact with contaminants and prevent membrane fouling or
pore blocking with rapid back-washing [332, 355]. These immobilizers include
catalyst fixation onto activated carbon, mesoporous clays, fibers, or even the membrane itself [63, 165, 170, 358]. The following subsections outline a few catalyst
immobilization strategies that are suitable for the use of slurry reactor or membrane
reactor or both.
Mesoporous Clays
Natural clays have been used intensively as the support for TiO 2 owing to their high
adsorption capacity and cost-effectiveness. Figure 13.5 shows the TiO 2 crystal being
deposited on a clay material [63]. Different types of clays have been investigated,
which include bentonite, sepiolite, montmorillonite, zeolite, and kaolinite [63, 100,
164, 299, 333]. Although these clays are catalytically inactive, their superior adsorption capacity has been attractive for increasing the surface contact during photocatalysis reaction. It was proposed that the natural clays should not be used directly to
immobilize TiO 2 . This is owing to the presence of different surface or latticebounded impurities that might diffuse and further affect the TiO 2 efficiency of the
immobilized layer [63]. In addition, if these impurities are not removed, the polar
molecules in the aqueous environment might initiate an internal reaction within the
clay structure that results in clay swelling [63]. The swelling will be profound in
certain type of clays, where van der Waals forces hold the entire clay in a turbostatic
array. This is undesirable, particularly if the photocatalytic reactions take place in a
reactor where the hydrodynamics may be strongly affected and consequently leading to the loss of photoactivity. Other factors that might need to be taken into consideration if pillared clays are used as the immobilizer substrate include the density
of the clays, particle size distribution range, and complementary photoreactor system used. The use of mesoporous clays as the support for nano-size TiO 2 has been
successfully demonstrated in a number of studies, including the slurry or membrane
processes [62, 298].
13 Wastewater