247
bacteria upon direct contact [33]. High bacterial inactivation (>90%) has been
achieved using polyvinyl-N-carbazole-SWNT nanocomposite at 3 wt% of SWNT
[2]. As CNTs are insoluble in water and not consumed, there is no need for replenishment. However, as direct contact is required for inactivation, long-term filtration
experiments are needed to determine the impact of fouling on the antimicrobial
activity of CNTs. Addition of oxidized MWNT at low weight percentage (up to
1.5 wt%) also increases the hydrophilicity and permeability of polysulfone membranes [57].
Photocatalytic nanoparticle-incorporated membranes (a.k.a. reactive membranes) combine their physical separation function and the reactivity of a catalyst
toward contaminant degradation. Much effort has been devoted to develop photocatalytic inorganic membranes consisting of nanophotocatalysts (normally nanoTiO 2 or modified nano-TiO 2 ) [55]. Metallic/bimetallic catalyst nanoparticles such as
nano zerovalent iron (nZVI) and noble metals supported on nZVI have been incorporated into polymeric membranes for reductive degradation of contaminants, particularly chlorinated compounds [330, 331]. nZVI serves as the electron donor and
the noble metals catalyze the reaction.
Thin-Film Nanocomposite (TFN) Membranes
Development of TFN membranes mainly focuses on incorporating nanomaterials
into the active layer of thin-film composite (TFC) membranes via doping in the
casting solutions or surface modification. Nanomaterials that have been researched
for such applications include nano-zeolites, nano-Ag, nano-TiO 2 , and CNTs. The
impact of nanoparticles on membrane permeability and selectivity depends on the
type, size, and amount of nanoparticles added.
Nano-zeolites are the most frequently used dopants in TFN and have shown
potential in enhancing membrane permeability. The addition of nano-zeolites leads
to more permeable, negatively charged, and thicker polyamide active layer [184].
One study reported that water permeability increased up to 80% over the TFC membrane, with the salt rejection largely maintained (93.9 ± 0.3%) [141]. TFN membranes doped with 250 nm nano-zeolites at 0.2 wt% achieved moderately higher
permeability and better salt rejection (>99.4%) than commercial RO membranes
[186]. It was hypothesized that the small, hydrophilic pores of nano-zeolites create
preferential paths for water. However, water permeability increased even with porefilled zeolites, although less than the pore-open ones, which could be attributed to
defects at the zeolite-polymer interface. Nano-zeolites were also used as carriers for
antimicrobial agents such as Ag, which imparts antifouling property to the membrane [185]. The zeolite TFN technology has reached the early stage of commercialization. QuantumFlux, a seawater TFN RO membrane, is now commercially
available (www.nanoH2O.com).
Incorporation of nano-TiO 2 (up to 5 wt%) into the TFC active layer slightly
increased the membrane rejection while maintaining the permeability [91]. When
the concentration of nano-TiO 2 exceeded 5 wt%, the water flux increased at the cost
Current and Potential Applications for Water and Wastewater Treatment
bacteria upon direct contact [33]. High bacterial inactivation (>90%) has been
achieved using polyvinyl-N-carbazole-SWNT nanocomposite at 3 wt% of SWNT
[2]. As CNTs are insoluble in water and not consumed, there is no need for replenishment. However, as direct contact is required for inactivation, long-term filtration
experiments are needed to determine the impact of fouling on the antimicrobial
activity of CNTs. Addition of oxidized MWNT at low weight percentage (up to
1.5 wt%) also increases the hydrophilicity and permeability of polysulfone membranes [57].
Photocatalytic nanoparticle-incorporated membranes (a.k.a. reactive membranes) combine their physical separation function and the reactivity of a catalyst
toward contaminant degradation. Much effort has been devoted to develop photocatalytic inorganic membranes consisting of nanophotocatalysts (normally nanoTiO 2 or modified nano-TiO 2 ) [55]. Metallic/bimetallic catalyst nanoparticles such as
nano zerovalent iron (nZVI) and noble metals supported on nZVI have been incorporated into polymeric membranes for reductive degradation of contaminants, particularly chlorinated compounds [330, 331]. nZVI serves as the electron donor and
the noble metals catalyze the reaction.
Thin-Film Nanocomposite (TFN) Membranes
Development of TFN membranes mainly focuses on incorporating nanomaterials
into the active layer of thin-film composite (TFC) membranes via doping in the
casting solutions or surface modification. Nanomaterials that have been researched
for such applications include nano-zeolites, nano-Ag, nano-TiO 2 , and CNTs. The
impact of nanoparticles on membrane permeability and selectivity depends on the
type, size, and amount of nanoparticles added.
Nano-zeolites are the most frequently used dopants in TFN and have shown
potential in enhancing membrane permeability. The addition of nano-zeolites leads
to more permeable, negatively charged, and thicker polyamide active layer [184].
One study reported that water permeability increased up to 80% over the TFC membrane, with the salt rejection largely maintained (93.9 ± 0.3%) [141]. TFN membranes doped with 250 nm nano-zeolites at 0.2 wt% achieved moderately higher
permeability and better salt rejection (>99.4%) than commercial RO membranes
[186]. It was hypothesized that the small, hydrophilic pores of nano-zeolites create
preferential paths for water. However, water permeability increased even with porefilled zeolites, although less than the pore-open ones, which could be attributed to
defects at the zeolite-polymer interface. Nano-zeolites were also used as carriers for
antimicrobial agents such as Ag, which imparts antifouling property to the membrane [185]. The zeolite TFN technology has reached the early stage of commercialization. QuantumFlux, a seawater TFN RO membrane, is now commercially
available (www.nanoH2O.com).
Incorporation of nano-TiO 2 (up to 5 wt%) into the TFC active layer slightly
increased the membrane rejection while maintaining the permeability [91]. When
the concentration of nano-TiO 2 exceeded 5 wt%, the water flux increased at the cost
Current and Potential Applications for Water and Wastewater Treatment