Environmental Engineering Reference
In-Depth Information
diffusion processes. In nanoporous membranes, water low is fast through the well-deined
nanosized channels. Moreover, the pore size can be tuned to ilter out different-sized con-
taminants. In most nanoporous membranes, the dimensions of the pores are comparable
to the Debye screening length for electrostatic interactions and smaller than the mean free
path between molecular collisions. Hence, the pores can use charge or hydrophobicity
to screen ions or other molecular solutes from contaminated water. Nanocarbon materi-
als including CNTs have found applicability in constructing nanoporous membranes.
74,75
However, low salt rejection rates and the dificulty of producing highly aligned and high-
density CNT arrays limit their use. Recently, Cohen-Tanugi and Grossman,
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using clas-
sical molecular dynamics, suggested that nanoscale pores in single-layer graphene could
be used for desalination of salt water. Desalination eficiency of membrane constructed by
graphene as a function of pore size, chemical functionalization, and applied pressure was
investigated. Their study pointed out that the presence of hydrophilic hydroxyl groups on
graphene surface can double the water lux. The study concluded that the water perme-
ability of graphene is several orders of magnitude higher than conventional RO mem-
branes. Zhu et al.
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recently reported a direct production strategy for graphene-based
free-standing titanate and TiO
2
nanoiber composite membranes with selective permeabil-
ity for water while blocking bacteria and organic dyes. Membrane was tested against MB,
methyl orange (MO), RhB, and
Bacillus coli
for their selective permeability.
Forward osmosis (FO) desalination is a comparatively new technology being proclaimed
a less energy-consuming strategy for generating freshwater from seawater and other water
sources. FO-based desalination works in a different way to RO-based technologies. Here,
saline water is permeated through a semipermeable membrane by a draw agent having
a higher osmotic pressure than the feed solution. Later, by application of suitable stimuli
(depending on the nature of the material used as draw agent), pure water can be drawn
out. Since the natural osmotic low is used for the permeation process, the process is highly
energy eficient and is attracting a great deal of attention. Internal concentration polar-
ization caused by the differing solute concentrations at the transverse boundaries of the
supporting layer in the membrane leading to the reduction in water lux is a big problem
associated with the FO process. Recently, a graphene-based composite was used to dra-
matically increase water lux in the FO process.
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Here, RGO-based hydrogels were used as
draw agents. Different hydrogels with varying concentrations of RGO (0.3-3.0 wt%) were
prepared using two polymers, namely poly(sodium acrylate) and poly(sodium acrylate)-
poly(
N
-isopropylacrylamide). Signiicant enhancements in water lux were achieved for
hydrogels with small amounts of RGO. Incorporation of small amounts of RGO increased
the swelling ratios and softness of the hydrogels. It was proposed that addition of RGO
also improved interparticle and particle-membrane contact. These factors led to the dra-
matic improvement of water luxes. Moreover, the incorporation of RGO (having a high
light-absorbing property) facilitated heat-induced (from the adsorbed solar light) dewet-
ting, to draw pure water from the composite hydrogels. For the recovery process, the pres-
ence of 1.2% of RGO was found to be the optimum dose.
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A strategy to fabricate inorganic
nanoibrous membranes using GO as the cross-linker was reported by Zhang et al.
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In this
study, GO was sulfonated irst to anchor -SO
3
H groups on the GO surface. This sulfonated
GO was treated with K-OMS-2 nanoibers to form a hierarchical membrane-like structure.
The coordination interaction between the sulfonic acid group and carboxylic acid groups
on GO and Mn center of the K-OMS-2 nanowire resulted in cross-linking between the two.
This material was assembled into a membrane via a low-directed assembly by iltration.
This fabricated microiltration membrane exhibited excellent rejection capacity for particle
sizes >0.2 μm.
