58
A. S. Kazemi and M. A. Abdol
and angle deformations as well as changes in electronic structure) are not explicitly included in this model [98]. Cohen-Tanugi et al. [98], performed an auxiliary
set of simulations that added the component of polarizability to the orientational
component. This allowed for enhanced water polarizability within individual water
molecules by permitting flexible bonds and angles, using the SPC/F (flexible) force
field. The results of these flexible water simulations indicated that the rigid TIP4P
model provided similar dynamics at a lower computational cost. They concluded that
intramolecular vibrations and polarizability play a negligible role in the dynamics
of saltwater transport and desalination performance. Other studies, modeled water
molecules using the nonpolarizable, rigid point charge extended (SPC/E) model
[102, 104, 118] because of its simplicity, reliability in reproducing water structure
and dynamics, and the availability of ion—water potentials specifically parametrized
for SPC/E water [17, 113, 119, 120].
Since the main components of seawater are Na
+ and Cl
− ions [121], the salinity
of seawater is often expressed as 0.6 M NaCl or 0.6 mol·L
−1 which is ~35 g/L.
The focus of most MD simulations is on separating water and NaCl and usually,
they consider a higher salinity than seawater in order to increase the occurrence of
ion—pore interactions and obtain more precise results for given system size and
simulation time [98]. Golchoobi et al. [122] have investigated the effect of feed
water salinity on permeability and salt rejection of 7.4 Å diameter hydroxylated
pores in graphene membranes. Concentration polarization graphs and density map
plots for water molecules near the nanopores clearly displayed the orientation of
water molecules as a function of time and coordinate. Their results proved that
higher salinity causes lower congestion of water molecules near the pore as a result
of hydration effects or decreased water permeability. The effect of hydrated ions on
water transport and salt rejection revealed the importance of fine manipulation of the
feed composition.
The pressures employed in MD simulations are considerably greater than the
pressures applied in RO plants (<8 MPa). High simulated pressures on the order
of ~100 MPa have the advantage of obtaining more precise data for water flux and
salt rejection in a finite simulation time (i.e., order of 10 ns) [123]. This approach
is reasonable since water flux scales linearly with the net driving pressure, i.e., the
results obtained at hundreds of MPa can be extrapolated to calculate the water flux
that would result from lower net driving pressures in an RO system [103].
(c) Mechanical stability
Recent developments in the fabrication of nano-porous graphene are promising for
the future of water supply by RO desalination. Although previous studies have highlighted the potential of nano-porous graphene membranes, there are few reports
exploring their strength and their mechanical integrity under the high hydraulic
pressures essential in the RO desalination process. Graphene exhibits exceptional
mechanical properties in its pristine, defect-free state; however, the structural strength
of nano-porous realistic defected graphene has not been examined in the specific
context of water desalination. It is well-known that pores tend to weaken graphene
A. S. Kazemi and M. A. Abdol
and angle deformations as well as changes in electronic structure) are not explicitly included in this model [98]. Cohen-Tanugi et al. [98], performed an auxiliary
set of simulations that added the component of polarizability to the orientational
component. This allowed for enhanced water polarizability within individual water
molecules by permitting flexible bonds and angles, using the SPC/F (flexible) force
field. The results of these flexible water simulations indicated that the rigid TIP4P
model provided similar dynamics at a lower computational cost. They concluded that
intramolecular vibrations and polarizability play a negligible role in the dynamics
of saltwater transport and desalination performance. Other studies, modeled water
molecules using the nonpolarizable, rigid point charge extended (SPC/E) model
[102, 104, 118] because of its simplicity, reliability in reproducing water structure
and dynamics, and the availability of ion—water potentials specifically parametrized
for SPC/E water [17, 113, 119, 120].
Since the main components of seawater are Na
+ and Cl
− ions [121], the salinity
of seawater is often expressed as 0.6 M NaCl or 0.6 mol·L
−1 which is ~35 g/L.
The focus of most MD simulations is on separating water and NaCl and usually,
they consider a higher salinity than seawater in order to increase the occurrence of
ion—pore interactions and obtain more precise results for given system size and
simulation time [98]. Golchoobi et al. [122] have investigated the effect of feed
water salinity on permeability and salt rejection of 7.4 Å diameter hydroxylated
pores in graphene membranes. Concentration polarization graphs and density map
plots for water molecules near the nanopores clearly displayed the orientation of
water molecules as a function of time and coordinate. Their results proved that
higher salinity causes lower congestion of water molecules near the pore as a result
of hydration effects or decreased water permeability. The effect of hydrated ions on
water transport and salt rejection revealed the importance of fine manipulation of the
feed composition.
The pressures employed in MD simulations are considerably greater than the
pressures applied in RO plants (<8 MPa). High simulated pressures on the order
of ~100 MPa have the advantage of obtaining more precise data for water flux and
salt rejection in a finite simulation time (i.e., order of 10 ns) [123]. This approach
is reasonable since water flux scales linearly with the net driving pressure, i.e., the
results obtained at hundreds of MPa can be extrapolated to calculate the water flux
that would result from lower net driving pressures in an RO system [103].
(c) Mechanical stability
Recent developments in the fabrication of nano-porous graphene are promising for
the future of water supply by RO desalination. Although previous studies have highlighted the potential of nano-porous graphene membranes, there are few reports
exploring their strength and their mechanical integrity under the high hydraulic
pressures essential in the RO desalination process. Graphene exhibits exceptional
mechanical properties in its pristine, defect-free state; however, the structural strength
of nano-porous realistic defected graphene has not been examined in the specific
context of water desalination. It is well-known that pores tend to weaken graphene