Environmental Engineering Reference
In-Depth Information
immiscible liquids were fed through the inner and outer capillaries at a speciic low rate
(say mineral oil through the inner and a metal oxide precursor, titanium [IV] isopropox-
ide dissolved in polyvinyl pyrrolidone, through the outer). Upon electrospinning, a iber
mat was collected on the collector electrode. Figure 23.4b shows the SEM images of the
two-, three-, four-, and ive-channel ibers fabricated by the multichannel coaxial electros-
pinning. Such core-shell and hollow nanoibers may open up interesting applications for
water puriication processes, although experimentally these have not been tried because
of technical issues.
23.3 Electrospun Nanofibers for Water Purification
The major challenges of using membranes for water iltration lie in the development of
new materials with high durability, low cost, and new structures that can produce high
permeation lux while maintaining a high selectivity or rejection rate. 22 In separation pro-
cesses, different types of electrospun ibrous media have been used. This can be of woven
or nonwoven type, depending on the conditions. Nonwoven membranes are more com-
monly used in iltration technology. 23 Although the random ibrous membrane forms the
backbone of the ilter, other components such as antimicrobial agents, particulate illers,
plasticizers, wetting agents, and softening agents can be incorporated by blending, coaxial
spinning, or by coating of ibers. 24-27 Most of the iltration membranes are made of syn-
thetic organic polymers since they are less costly and are available in large quantities. The
polymers currently in use include, polysulfone, poly(acrylonitrile) (PAN), poly(vinyl alco-
hol) (PVA), poly(vinylidene luoride) (PVDF), poly(ether sulfone) (PES), cellulose acetate-
cellulose nitrate blends, nylons, poly(vinyl chloride) (PVC), and many others. 8 Table 23.1
summarizes the list of polymers used for making electrospun iber mats for iltration pur-
poses. Use of electrospun membranes for water puriication can be generally classiied as
those used for (i) the removal of microbes, (ii) removal of organic compounds, (iii) removal
of particulate contaminants, (iv) heavy metal ion removal, and (v) membrane distillation.
The following sections detail the use of nanoibers for each of the applications mentioned
above.
23.3.1 Removal of Microbes
For water-based iltration/puriication applications, the membranes should be made hydro-
philic. Hydrophilicity to the polymer ibers can be rendered by the choice of hydrophilic
polymers or by suitable surface modiication. 28 Surface modiication techniques include
plasma treatment and surface graft polymerization. 29, 30 These treatments change the wet-
ting and surface adhesion properties by altering the surface chemical composition.
Plasma treatment and surface graft polymerizations involve the treatment of the mem-
branes with oxygen, air, or ammonia, which generates free radicals and free electrons on the
surface. Surface graft polymerization is initiated by the treatment with plasma followed by
UV irradiation to generate the free radicals for polymerization. 31 After plasma treatment,
other functional compounds such as quaternary ammonium salts or their derivatives are
covalently bound to the membranes by chemical reactions. Yao and co-workers reported
the development of modiied electrospun polyurethane (PU) nanoibers and tested the
antimicrobial activities against Staphylococcus aureus and gram-negative Escherichia coli . 32
Search WWH ::




Custom Search