246
Nanofiber Membranes
Electrospinning is a simple, efficient, and inexpensive way to make ultrafine fibers
using various materials (e.g., polymers, ceramics, or even metals) [64, 176]. The
resulting nanofibers have high specific surface area and porosity and form nanofiber
mats with complex pore structures. The diameter, morphology, composition, secondary structure, and spatial alignment of electrospun nanofibers can be easily
manipulated for specific applications [176]. Although nanofiber membranes have
been commercially employed for air filtration applications, their potential in water
treatment is still largely unexploited. Nanofiber membranes can remove micronsized particles from aqueous phase at a high rejection rate without significant fouling [270]. Thus, they have been proposed to be used as pretreatment prior to
ultrafiltration or reverse osmosis (RO). Functional nanomaterials can be easily
doped into the spinning solutions to fabricate nanoparticle-impregnated nanofibers
or those formed in situ [176]. The outstanding features and tunable properties make
electrospun nanofibers an ideal platform for constructing multifunctional media/
membrane filters by either directly using intrinsically multifunctional materials
such as TiO 2 or introducing functional materials on the nanofibers. For example, by
incorporating ceramic nanomaterials or specific capture agents on the nanofiber
scaffold, affinity nanofiber membranes can be designed to remove heavy metals and
organic pollutants during filtration.
Nanocomposite Membranes
A significant number of studies on membrane nanotechnology have focused on creating synergism or multifunction by adding nanomaterials into polymeric or inorganic membranes. Nanomaterials used for such applications include hydrophilic
metal oxide nanoparticles (e.g., Al 2 O 3 , TiO 2 , and zeolite), antimicrobial nanoparticles (e.g., nano-Ag and CNTs), and (photo)catalytic nanomaterials (e.g., bimetallic
nanoparticles, TiO 2 ).
The main goal of adding hydrophilic metal oxide nanoparticles is to reduce fouling by increasing the hydrophilicity of the membrane. The addition of metal oxide
nanoparticles including alumina, silica, zeolite, and TiO 2 to polymeric ultrafiltration
membranes has been shown to increase membrane surface hydrophilicity, water
permeability, or fouling resistance [18, 30, 217, 258]. These inorganic nanoparticles
also help enhance the mechanical and thermal stability of polymeric membranes,
reducing the negative impact of compaction and heat on membrane permeability
[84, 258].
Antimicrobial nanomaterials such as nano-Ag and CNTs can reduce membrane
biofouling. Nano-Ag has been doped or surface grafted on polymeric membranes to
inhibit bacterial attachment and biofilm formation on the membrane surface as well
as inactivate viruses [74, 216, 360]. However, its long-term efficacy against membrane biofouling has not been reported. Appropriate replenishment of nano-Ag
needs to be addressed for practical application of this technology. CNTs inactivate
13 Wastewater
Nanofiber Membranes
Electrospinning is a simple, efficient, and inexpensive way to make ultrafine fibers
using various materials (e.g., polymers, ceramics, or even metals) [64, 176]. The
resulting nanofibers have high specific surface area and porosity and form nanofiber
mats with complex pore structures. The diameter, morphology, composition, secondary structure, and spatial alignment of electrospun nanofibers can be easily
manipulated for specific applications [176]. Although nanofiber membranes have
been commercially employed for air filtration applications, their potential in water
treatment is still largely unexploited. Nanofiber membranes can remove micronsized particles from aqueous phase at a high rejection rate without significant fouling [270]. Thus, they have been proposed to be used as pretreatment prior to
ultrafiltration or reverse osmosis (RO). Functional nanomaterials can be easily
doped into the spinning solutions to fabricate nanoparticle-impregnated nanofibers
or those formed in situ [176]. The outstanding features and tunable properties make
electrospun nanofibers an ideal platform for constructing multifunctional media/
membrane filters by either directly using intrinsically multifunctional materials
such as TiO 2 or introducing functional materials on the nanofibers. For example, by
incorporating ceramic nanomaterials or specific capture agents on the nanofiber
scaffold, affinity nanofiber membranes can be designed to remove heavy metals and
organic pollutants during filtration.
Nanocomposite Membranes
A significant number of studies on membrane nanotechnology have focused on creating synergism or multifunction by adding nanomaterials into polymeric or inorganic membranes. Nanomaterials used for such applications include hydrophilic
metal oxide nanoparticles (e.g., Al 2 O 3 , TiO 2 , and zeolite), antimicrobial nanoparticles (e.g., nano-Ag and CNTs), and (photo)catalytic nanomaterials (e.g., bimetallic
nanoparticles, TiO 2 ).
The main goal of adding hydrophilic metal oxide nanoparticles is to reduce fouling by increasing the hydrophilicity of the membrane. The addition of metal oxide
nanoparticles including alumina, silica, zeolite, and TiO 2 to polymeric ultrafiltration
membranes has been shown to increase membrane surface hydrophilicity, water
permeability, or fouling resistance [18, 30, 217, 258]. These inorganic nanoparticles
also help enhance the mechanical and thermal stability of polymeric membranes,
reducing the negative impact of compaction and heat on membrane permeability
[84, 258].
Antimicrobial nanomaterials such as nano-Ag and CNTs can reduce membrane
biofouling. Nano-Ag has been doped or surface grafted on polymeric membranes to
inhibit bacterial attachment and biofilm formation on the membrane surface as well
as inactivate viruses [74, 216, 360]. However, its long-term efficacy against membrane biofouling has not been reported. Appropriate replenishment of nano-Ag
needs to be addressed for practical application of this technology. CNTs inactivate
13 Wastewater