Environmental Engineering Reference
In-Depth Information
As a strong oxidizing agent, ozone possesses high reactivity with organic compounds responsible for the fouling of mem-
branes in water treatment [129]. Furthermore, the integration of ozonation with other oxidation processes, such as photocataly-
sis, in the degradation of organic substances was successful [130-132]. polymeric membranes are not feasible in the combination
of ozonation and membrane processes because of their poor stability when exposed to ozone [133]. Meanwhile, ceramic mem-
branes with superior thermal properties and high resistance against corrosion and chemicals are suitable in combined ozonation
and membrane processes for water or wastewater treatment [133-135].
17.3.3.1 Polymeric Membranes with Photocatalytic Functionalities
Under UV irradiation, polymeric membranes incor-
porated with TiO
2
nanoparticles exhibit excellent photocatalytic and separation properties in various applications [136-138]. In
the removal of rb5 dye under UV via photolysis and photocatalysis mechanisms, composite pVDF/TiO
2
demonstrated a higher
rate of rb5 removal than a neat pVDF membrane [138].
Meanwhile, You et al. [137] investigated the photocatalytic and separation performances of a pAA plasma-grafted pVDF
membrane with self-assembled TiO
2
nanoparticles in the removal of rb5 dye. The results showed that the flux behavior
improved significantly after UV irradiation, which activated the photodegradation of strongly attached foulants. The resulting
membrane also demonstrated the capability for photocatalytic removal of rb5 dye.
Song et al. [136] reported that polyethylene glycol (peG)-pVDF membranes doped with TiO
2
nanoparticles exhibited
improved removal of nOM and reduced membrane fouling. In addition, the peG-TiO
2
-doped pVDF membranes also exhib-
ited excellent self-cleaning ability because of their photocatalytic properties.
17.3.3.2 Ceramic Membranes with Photocatalytic Functionalities
In addition to polymeric membranes, ceramic mem-
branes have been employed in the fabrication of catalytic membranes for the integrated oxidation and separation processes
under UV irradiation. Ma et al. [139] fabricated an asymmetrical inorganic membrane by coating silicon (Si)-doped TiO
2
layers
on a commercial Al
2
O
3
membrane by a sol-gel technique and found that the removal efficiency of the dye reactive red eD-2b
was enhanced significantly under UV irradiation than with photocatalysis or membrane separation alone because of simulta-
neous separation and degradation of the pollutant.
Similar results were reported by Ma et al. [140] using a composite membrane incorporated with Ag-doped TiO
2
nanoparti-
cles (Ag-TiO
2
/HAp/Al
2
O
3
) for humic acid (HA) removal by microfiltration-coupled photocatalysis. Owing to the synergistic
effect of the photocatalytic degradation of foulants and filtration, HA removal and the antifouling property of the fabricated
membrane was enhanced under UV irradiation.
In addition to employing doped TiO
2
nanoparticles in the integrated oxidation and separation processes, liu et al. [141]
deposited Ag nanoparticles on electrospun TiO
2
nanofibers by in situ polyol synthesis in the fabrication of Ag/TiO
2
nanofiber
membranes. remarkably high permeation flux was exhibited by the Ag/TiO
2
nanofiber membrane. The Ag/TiO
2
nanofiber
membranes exhibited bacterial inactivation up to 99.9% and dye degradation up to 80% under solar irradiation of 30 min.
Hong et al. [142] demonstrated a different approach in the degradation of organic solvents using needle-shape TiO
2
nanofi-
bers inside a micro-channeled Al
2
O
3
-ZrO
2
composite porous membrane system through a sol-gel technique for the integrated
photocatalytic filtration process. In this study, TiO
2
nanofibers were grown on the surface frame of the micro-channeled Al
2
O
3
-
ZrO
2
composite membrane, and the interior of the micro-channeled pores were fully covered. notably, the specific surface area
of the TiO
2
-coated membrane system was significantly increased by over 100-fold by following this procedure compared with
that of the noncoated membrane.
In addition to TiO
2
nanoparticles, Ke et al. [143] studied the performance of Ag nanoparticles doped with Al
2
O
3
nanofiber
filtration membranes for the removal of organic dyes. excellent results were obtained in removing organic dyes under the
combination of photocatalysis and filtration processes because of the ability of Ag nanoparticles in stimulating photocatalytic
degradation under visible light irradiation.
Membrane processes combined with ozonation are only feasible with ceramic membranes because of their excellent physical
and chemical properties. Zhu et al. [144] combined filtration with ozonation to treat wastewater from a major wastewater
treatment plant to eliminate organic compounds using a tubular ceramic membrane (α-Al
2
O
3
) deposited with TiO
2
nanoparticles
via a dip-coating technique. The permeate flux increased significantly because of the reduced membrane fouling contributed by
the ozonation process. The resulting membrane not only functions as a physical barrier to separate the particulates, but also
simultaneously breaks down the organic matter catalytically.
Aside from TiO
2
nanoparticles, byun et al. [127] investigated the performance of catalytic membranes coated with differ-
ent metal oxides (MnO or Fe
2
O
3
nanoparticles) in a hybrid ozonation-ceramic membrane filtration system. The MnO-coated
membrane exhibited the best performance in the filtration of water obtained from a borderline eutrophic lake. The membrane
exhibited the fastest recovery in permeate flux when ozone was applied and the highest reduction in the total organic carbon
