nature of membrane to allow the movement of water by restricting the passage of
ions highly depends on the pore diameter (Cohen-Tanugi and Grossman 2012)
which can be easily tuned by various techniques employed during the development
of nanopores in single-layer sheets. Movement of ions through nanoporous membrane is also governed by the radius of the ions and charge possessed by the ions and
pores. When radii are larger, ions show increased passage rates resulting in the
separation of ions from the water molecules (Sint et al. 2008). Nanopores is
negatively charged at short oxidation times, whereas at longer oxidation times, the
movement of salt is not disturbed, but the transport of large organic molecule is
prevented which is an indication of steric size exclusion (O’Hern et al. 2014). During
the process, water flux increases linearly with the osmotic pressure of the draw solute
resulting in increased rate of salt rejection (O’Hern et al. 2015). Increase in the
magnitude of electric field decreases the ion pore binding, but the transport of ions
through functionalized nanopores in graphene monolayers is highly selective even in
the presence of large electric field due to the existence of coulomb coupling between
the ion and the functional groups attached to the polar and charged rim of the
nanopores (Sint et al. 2008). Efficacy in mapping and eventual removal of ion shells
from water molecules are due to the close contact between the ion and pore rim
which leads to the selective removal of ions, as in channel proteins (Sint et al. 2008)
in biological membranes. Pore size increases with the increase in temperature,
whereas their shape and total porosity remain constant (Santos et al. 2000). An
efficient desalination system should maintain constant temperature throughout the
process in order to maintain the constant pore size. Movement of water by isothermal
evaporation and condensation across the nanopores with pure water on one side and
saline water on the other side under a pressure difference across vapor trapping pores
results in selective transport of water through the nanopores (Lee and Karnik 2010).
Complete removal of ions by nanoporous membrane can be achieved by focusing on
the optimization of various process parameters involved in the functioning of the
desalination plant (Fig. 4.11).
4.8 Nanoporous Zeolite Membranes
Zeolites are crystalline, hydrated aluminosilicate framework (Breck 1964; Davis
1991; Sklenak et al. 2007) that can act as an excellent absorbent in gas and liquid
separation due to their unique surface properties and uniform pore sizes
(Kazemimoghadam 2010). As stated in the previous sections, conventional technologies such as polymeric RO membranes are inefficient in desalination due to
material instability and membrane fouling (Kazemimoghadam 2010). The need to
develop efficient membrane process with superior properties for desalination
increased the interest of the researchers and industrial groups toward zeolite membranes since the beginning of the 1980s. From the literatures available on
nanoporous zeolite membranes, it has been considered as an economically effective
and suitable material for desalination without any substantial pretreatment (Liu and
4 Functional Properties of Nanoporous Membranes for the Desalination of Water
147
ions highly depends on the pore diameter (Cohen-Tanugi and Grossman 2012)
which can be easily tuned by various techniques employed during the development
of nanopores in single-layer sheets. Movement of ions through nanoporous membrane is also governed by the radius of the ions and charge possessed by the ions and
pores. When radii are larger, ions show increased passage rates resulting in the
separation of ions from the water molecules (Sint et al. 2008). Nanopores is
negatively charged at short oxidation times, whereas at longer oxidation times, the
movement of salt is not disturbed, but the transport of large organic molecule is
prevented which is an indication of steric size exclusion (O’Hern et al. 2014). During
the process, water flux increases linearly with the osmotic pressure of the draw solute
resulting in increased rate of salt rejection (O’Hern et al. 2015). Increase in the
magnitude of electric field decreases the ion pore binding, but the transport of ions
through functionalized nanopores in graphene monolayers is highly selective even in
the presence of large electric field due to the existence of coulomb coupling between
the ion and the functional groups attached to the polar and charged rim of the
nanopores (Sint et al. 2008). Efficacy in mapping and eventual removal of ion shells
from water molecules are due to the close contact between the ion and pore rim
which leads to the selective removal of ions, as in channel proteins (Sint et al. 2008)
in biological membranes. Pore size increases with the increase in temperature,
whereas their shape and total porosity remain constant (Santos et al. 2000). An
efficient desalination system should maintain constant temperature throughout the
process in order to maintain the constant pore size. Movement of water by isothermal
evaporation and condensation across the nanopores with pure water on one side and
saline water on the other side under a pressure difference across vapor trapping pores
results in selective transport of water through the nanopores (Lee and Karnik 2010).
Complete removal of ions by nanoporous membrane can be achieved by focusing on
the optimization of various process parameters involved in the functioning of the
desalination plant (Fig. 4.11).
4.8 Nanoporous Zeolite Membranes
Zeolites are crystalline, hydrated aluminosilicate framework (Breck 1964; Davis
1991; Sklenak et al. 2007) that can act as an excellent absorbent in gas and liquid
separation due to their unique surface properties and uniform pore sizes
(Kazemimoghadam 2010). As stated in the previous sections, conventional technologies such as polymeric RO membranes are inefficient in desalination due to
material instability and membrane fouling (Kazemimoghadam 2010). The need to
develop efficient membrane process with superior properties for desalination
increased the interest of the researchers and industrial groups toward zeolite membranes since the beginning of the 1980s. From the literatures available on
nanoporous zeolite membranes, it has been considered as an economically effective
and suitable material for desalination without any substantial pretreatment (Liu and
4 Functional Properties of Nanoporous Membranes for the Desalination of Water
147