248
of reducing rejection, suggesting defect formation in the active layer. Upon UV
irradiation, TiO 2 can degrade organic contaminants and inactivate microorganisms.
This helps reduce organic and biological fouling as well as remove contaminants
that are not retained by the membrane. However, the close adjacency between the
photocatalyst and the membrane may also lead to detrimental effects on polymeric
membrane materials, which needs to be addressed for long-term efficacy [48].
CNTs (unaligned) also found their application in TFN membranes due to their
antimicrobial activities. Tiraferri et al. covalently bonded SWNTs to a TFC membrane surface [304]. This approach is advantageous as it uses relatively small
amount of the nanomaterial and minimizes perturbation of the active layer. The
resulting TFN membrane exhibited moderate antibacterial properties (60% inactivation of bacteria attached on the membrane surface in 1-h contact time), potentially
reducing or delaying membrane biofouling.
Biologically Inspired Membranes
Many biological membranes are highly selective and permeable. Aquaporins are protein channels that regulate water flux across cell membranes. Their high selectivity and
water permeability make their use in polymeric membranes an attractive approach to
improve membrane performance. Aquaporin-Z from Escherichia coli has been incorporated into amphiphilic triblock-polymer vesicles, which exhibit water permeability
at least an order of magnitude over the original vesicles with full rejection to glucose,
glycerol, salt, and urea [163]. One potential design is to coat aquaporin-incorporated
lipid bilayers on commercial nanofiltration membranes. On this front, limited success
was achieved [150]. Aligned CNTs have been shown both experimentally and theoretically to provide water permeation much faster than what the Hagen-Poiseuille equation
predicts, owing to the atomic smoothness of the nano-sized channel, and the onedimensional single-file ordering of water molecules while passing through the nanotubes [125, 133]. It was predicted that a membrane containing only 0.03% surface area
of aligned CNTs will have flux exceeding current commercial seawater RO membranes [257]. However, high rejection for salt and small molecules is challenging for
aligned CNT membranes due to the lack of CNTs with uniform sub-nanometer diameter. Functional group gating at the nanotube opening has been proposed to enhance
the selectivity of aligned CNT membranes [214]. By grafting carboxyl functional
groups on sub-2 nm CNT openings, 98% rejection of Fe(CN 6 )
3−
was achieved at low
ionic strength by Donnan exclusion [94]. However, KCl rejection was only 50% at
0.3 mM, and decreased to almost zero at 10 mM. Grafting bulky functional groups at
the tube opening could physically exclude salts. However, steric exclusion will significantly reduce membrane permeability [242]. Thus at the current stage, aligned CNT
membranes are not capable of desalination. To achieve reliable salt rejection, the CNT
diameter must be uniformly smaller than 0.8 nm [124]. A key barrier for both aquaporin and aligned CNT membranes is the scale-up of the nanomaterial production and
membrane fabrication. Large-scale production and purification of aquaporins are very
challenging. To date, chemical vapor deposition (CVD) is the most common way to
13 Wastewater
of reducing rejection, suggesting defect formation in the active layer. Upon UV
irradiation, TiO 2 can degrade organic contaminants and inactivate microorganisms.
This helps reduce organic and biological fouling as well as remove contaminants
that are not retained by the membrane. However, the close adjacency between the
photocatalyst and the membrane may also lead to detrimental effects on polymeric
membrane materials, which needs to be addressed for long-term efficacy [48].
CNTs (unaligned) also found their application in TFN membranes due to their
antimicrobial activities. Tiraferri et al. covalently bonded SWNTs to a TFC membrane surface [304]. This approach is advantageous as it uses relatively small
amount of the nanomaterial and minimizes perturbation of the active layer. The
resulting TFN membrane exhibited moderate antibacterial properties (60% inactivation of bacteria attached on the membrane surface in 1-h contact time), potentially
reducing or delaying membrane biofouling.
Biologically Inspired Membranes
Many biological membranes are highly selective and permeable. Aquaporins are protein channels that regulate water flux across cell membranes. Their high selectivity and
water permeability make their use in polymeric membranes an attractive approach to
improve membrane performance. Aquaporin-Z from Escherichia coli has been incorporated into amphiphilic triblock-polymer vesicles, which exhibit water permeability
at least an order of magnitude over the original vesicles with full rejection to glucose,
glycerol, salt, and urea [163]. One potential design is to coat aquaporin-incorporated
lipid bilayers on commercial nanofiltration membranes. On this front, limited success
was achieved [150]. Aligned CNTs have been shown both experimentally and theoretically to provide water permeation much faster than what the Hagen-Poiseuille equation
predicts, owing to the atomic smoothness of the nano-sized channel, and the onedimensional single-file ordering of water molecules while passing through the nanotubes [125, 133]. It was predicted that a membrane containing only 0.03% surface area
of aligned CNTs will have flux exceeding current commercial seawater RO membranes [257]. However, high rejection for salt and small molecules is challenging for
aligned CNT membranes due to the lack of CNTs with uniform sub-nanometer diameter. Functional group gating at the nanotube opening has been proposed to enhance
the selectivity of aligned CNT membranes [214]. By grafting carboxyl functional
groups on sub-2 nm CNT openings, 98% rejection of Fe(CN 6 )
3−
was achieved at low
ionic strength by Donnan exclusion [94]. However, KCl rejection was only 50% at
0.3 mM, and decreased to almost zero at 10 mM. Grafting bulky functional groups at
the tube opening could physically exclude salts. However, steric exclusion will significantly reduce membrane permeability [242]. Thus at the current stage, aligned CNT
membranes are not capable of desalination. To achieve reliable salt rejection, the CNT
diameter must be uniformly smaller than 0.8 nm [124]. A key barrier for both aquaporin and aligned CNT membranes is the scale-up of the nanomaterial production and
membrane fabrication. Large-scale production and purification of aquaporins are very
challenging. To date, chemical vapor deposition (CVD) is the most common way to
13 Wastewater