Environmental Engineering Reference
In-Depth Information
(a)
(b)
80
80
60
60
40
40
20
20
MWCNT-reinforced Fe-PAA/PVA mat
MWCNT-reinforced PAA/PVA mat
Fe-PAA/PVA mat
Pure PAA/PVA mat
0
0
0 0
20
30
Time (min)
40
50
60
0 0
20 30
Time (min)
40
50
60
FIGURE 23.11
Removal of Cu(II) ions from solutions at different contact times by nanoibrous mats with (a) or without (b) MWCNT
reinforcement ([Cu(II)] = 50 mg/L, [mat] = 0.5 mg/mL). (From Xiao, S. et al.,
Colloids Surf. A
, 381, 48, 2011.)
another study, silk ibroin and a blend of silk ibroin with wool keratose was electrospun
into membranes and tested for the removal of heavy metal ions (Cu
2+
ions as the model). The
adsorption capacity was signiicantly enhanced when nanoibers (1.65-2.88 mg/g) were
used, as compared with the conventional iber materials like wool silver (0.71 mg/g) and
ilter paper (0.23 mg/g). The higher adsorption capacity has been attributed to the larger
surface area of the nanoiber membrane.
59
It can be concluded that a suitable choice of
the polymer and surface modiier during/after the electrospinning process could achieve
higher removal eficiency for heavy metal ions.
23.3.5 Membrane Distillation
Membrane distillation using hydrophobic nanoiber membranes is a convenient way
for desalination.
60
This is different from conventional technologies and is an alternative
to RO especially when the solute concentration is high.
60
In a thermally driven process,
a nanoiber membrane with nano/submicron/micron pores acts as a separator between
a warm solution and a cooler chamber that contains a liquid/gas. Being a nonisother-
mal process, vapor molecules migrate through the membrane pores from the warmer
to the cooler side.
60
The surface of the distillation membrane should be hydrophobic
to prevent the entry of water to the pores, and polymer nanoibers are appropriate
for the purpose as their hydrophobicity is far more than the intrinsic hydrophobicity
of the polymer. Shih
61
fabricated two composite nanoibrous membranes of PVDF and
PVDF-HFP by electrospinning and used them in a direct contact membrane distillation
process.
The permeate lux of the PVDF-HFP composite membrane was 4.28 kg/m
2
h, which was
higher than the PVDF and commercial polytetraluoroethylene membranes. Feng et al.
62
used the above PVDF-nanoiber membrane for membrane distillation, and the membrane
could produce potable water (NaCl concentration <280 ppm) from saline water of NaCl
concentration 6 wt% by air gap membrane distillation. This implies the membrane dis-
tillation process could eventually compete with RO and conventional distillation for the
desalination purpose. Figure 23.12 shows the plot for lux versus temperature difference
for 1, 3.5, and 6 wt% NaCl solutions. It is obvious that as the NaCl concentration in the
