Nano-Porous Graphene as Free-Standing Membranes
45
2 State-of-the-Art Membranes Versus Ideal Membranes
2.1 State-of-the-Art Membranes, Properties, and Limitations
The function of RO membranes is to extract fresh water from pressurized saltwater.
Membrane technologies for water purification and desal-ination are largely based on
size exclusion [15]. Most of Sect. 2.1 is reproduced with permission from Ref. [15],
Copyright 2016, Macmillan Publishers Limited.
A wide range of membrane materials, both polymeric and inorganic, have been
used in RO [55–60]. Polymeric membranes include polysulfone (PSF), polyethylene
(PE), polytetrafluoroethylene (PTFE), polypropylene (PP), polyvinylidene fluoride
(PVDF) [61], while inorganic membranes including silica, carbon, metal-organic
frameworks (MOFs) and various metal oxides [62]. Polymeric membranes are the
dominant material of desalination technology today, essentially owing to their high
processability and low cost. These membranes are categorized into phase inversion
membranes, track etching membranes and thin-film composite (TFC) polyamide
membranes [15]. Due to the stochastic nature of the phase inversion process, a wide
range of membranes formed by this method, have polydisperse pore size distributions, that adversely influence the selectivity of the resulting active layer [15]. Tracketched membranes, porous systems, consisting of a thin polymer foil with channels
from surface to the surface, have a major limitation. Their porosity must be maintained below 5% to prevent pore overlap and consequently, the low porosity results
in low water permeability. TFC polyamide membranes consist of a non-porous,
highly crosslinked polyamide selective layer and an underlying porous support layer,
usually made of PSF. For porous membranes, flow transpires through the active layer
and selectivity is inherently linked with active layer pore size [15]. For non-porous
and dense membranes, water and solute transports are dominated by the solution–
diffusion model [20]. Water and solute molecules separate into the active layer of
the membrane, diffuse through the polymer matrix down their chemical potential
gradients and desorb into the permeate solution [15].
TFC membranes possess much higher water permeability and salt rejection
compared to the first-generation RO membranes (asymmetric cellulose acetate-based
membranes) which were developed more than 5 decades ago [63]. Furthermore, TFC
membranes proved to be stable over a wide pH range (pH 2–11) in continuous operation [15, 26]. The combined performances of the TFCs has led to their widespread
utilization in desalination applications to date, however, commercial membranes
still suffer from fairly low permeability and low stability to cleaning chemicals that
are inevitable for fouling mitigation [4, 64]. Membrane fouling is the build-up of
substances on the membrane surface or within the membrane pores. While inorganic
membranes solve most of the challenges involved with polymeric membranes, they
endure low scalability and low mechanical strength, which obstructs their large-scale
applications [65].
45
2 State-of-the-Art Membranes Versus Ideal Membranes
2.1 State-of-the-Art Membranes, Properties, and Limitations
The function of RO membranes is to extract fresh water from pressurized saltwater.
Membrane technologies for water purification and desal-ination are largely based on
size exclusion [15]. Most of Sect. 2.1 is reproduced with permission from Ref. [15],
Copyright 2016, Macmillan Publishers Limited.
A wide range of membrane materials, both polymeric and inorganic, have been
used in RO [55–60]. Polymeric membranes include polysulfone (PSF), polyethylene
(PE), polytetrafluoroethylene (PTFE), polypropylene (PP), polyvinylidene fluoride
(PVDF) [61], while inorganic membranes including silica, carbon, metal-organic
frameworks (MOFs) and various metal oxides [62]. Polymeric membranes are the
dominant material of desalination technology today, essentially owing to their high
processability and low cost. These membranes are categorized into phase inversion
membranes, track etching membranes and thin-film composite (TFC) polyamide
membranes [15]. Due to the stochastic nature of the phase inversion process, a wide
range of membranes formed by this method, have polydisperse pore size distributions, that adversely influence the selectivity of the resulting active layer [15]. Tracketched membranes, porous systems, consisting of a thin polymer foil with channels
from surface to the surface, have a major limitation. Their porosity must be maintained below 5% to prevent pore overlap and consequently, the low porosity results
in low water permeability. TFC polyamide membranes consist of a non-porous,
highly crosslinked polyamide selective layer and an underlying porous support layer,
usually made of PSF. For porous membranes, flow transpires through the active layer
and selectivity is inherently linked with active layer pore size [15]. For non-porous
and dense membranes, water and solute transports are dominated by the solution–
diffusion model [20]. Water and solute molecules separate into the active layer of
the membrane, diffuse through the polymer matrix down their chemical potential
gradients and desorb into the permeate solution [15].
TFC membranes possess much higher water permeability and salt rejection
compared to the first-generation RO membranes (asymmetric cellulose acetate-based
membranes) which were developed more than 5 decades ago [63]. Furthermore, TFC
membranes proved to be stable over a wide pH range (pH 2–11) in continuous operation [15, 26]. The combined performances of the TFCs has led to their widespread
utilization in desalination applications to date, however, commercial membranes
still suffer from fairly low permeability and low stability to cleaning chemicals that
are inevitable for fouling mitigation [4, 64]. Membrane fouling is the build-up of
substances on the membrane surface or within the membrane pores. While inorganic
membranes solve most of the challenges involved with polymeric membranes, they
endure low scalability and low mechanical strength, which obstructs their large-scale
applications [65].