Environmental Engineering Reference
In-Depth Information
(a)
(b)
(i)
V
Gravity-fed
seawater
External
pressure
V
Desalted
V
V
GND
Nanochannel
Repulsion
zone
GND
SG desalter
GND
GND
V
V
Microchannels
(ii)
V
Any charged
species
Concentrate
Fresh
Desalted
water
V+
pressure
Seawater
V
Ion-depletion
boundary
∆V
Nanojunction
GND
GND
FIGURE 27.13
(See color insert.)
(a) Small-scale setup for concentration polarization desalination. (From Shannon, M.A.,
Nature
Nanotechnology
, 5, 248, 2010.) (b) Schematic of microchannel-nanochannel prototype tested for water desalina-
tion using concentration polarization. (From Kim, S.J., S.H. Ko, K.H. Kang, and J. Han,
Nature Nanotechnology
,
5, 297, 2010.)
27.3.3 Advanced Membranes
Recently, many new classes of membranes with functional nanoscale components or those
inspired by biological systems have been developed as discussed previously [91]. One kind
of these advanced membranes are the fouling-resistant membranes [92,93] with integrated
polymeric brushes and nanostructures that allow the same membrane to have both hydro-
philic and hydrophobic regions that mitigate fouling. A second kind of advanced mem-
brane, composed of vertically aligned carbon nanotubes (CNTs), exhibits super-high lux
for water transport [30,31]. The measured luxes were found to be higher by up to three
orders of magnitude than those predicted by conventional Hagen-Poiseuille low theory.
These aligned CNT membranes were shown to have average tube diameters of 1.6 nm
[30] and ultrasmooth, hydrophobic walls that do not follow the basic “no-slip” condition,
thereby letting luids slide along the wall in an essentially frictionless coniguration per-
mitting high lux through the CNTs. Water velocities at 1 bar ranging from 9.5 to 43.9 cm/s
were reported, signiicantly exceeding the expected velocities ranging from 0.00015 to
0.00057 cm/s [31]. While water lows nearly effortlessly through the aligned CNTs, the
energy cost is expended in the water molecules entering the CNTs, and in the modiication
of the mouths of the CNTs to reject small salt ions, which increases the energy needed.
The inal energy usage for these membranes is still to be determined for concentrated salt
solutions with a high rejection potential.
