Environmental Engineering Reference
In-Depth Information
once the intensity of the electric field exceeds a critical value, a jet flow of charged solution is ejected from the tip of the Taylor
cone continuously, which undergoes stretching and elongation. Finally, the solvent evaporates, and the formed nanofibers deposit
on the collector in a random manner [51].
A variety of materials such as naturally occurring polymers, synthetic polymers, ceramics, metals, or their mixture have
been electrospun to produce ultrafine nanofibers. The sizes, composition, and morphology of the electrospun nanofibers are
able to be well-controlled through the regulation of the electrospinning parameters [52-55]. A number of electrospinning
methods have emerged. For instance, Chu et al. [56] designed an array of multiple needles for large-scale production of
electrospun nanofibers. In some other approaches, core-shell electrospinning, auxiliary electric fields, rotating tube, rotating
disk, needleless electrospinning, patterned electrodes, or magnetic electrospinning were also utilized to produce different
nanofibers [57-63]. The objectives of these explorations were to fabricate nanofibers with controlled alignment or orienta-
tion or to render nanofibers with special functionalities.
The electrospinning process is influenced by many parameters, including (i) solution properties such as polymer molecular
weight, viscosity, conductivity, elasticity, and surface tension; (ii) processing parameters such as applied voltage, flow rate,
distance between needle tip and collector, and needle inner diameter; and (iii) ambient conditions such as temperature,
humidity, and air velocity in the electrospinning chamber [64]. For example, the polymer solution should have an appropriate
concentration to maintain polymer entanglement. The distance between the needle and the collector can influence the
morphology of nanofibers, and incorrect distance may result in a rough or beaded fibrous mat, or significantly decreased
porosity of the fibers. Since the applied voltage can produce ionized solution and impact the elongation and splitting of the jet
flow, smooth and uniform fiber structures can be formed only at a proper applied voltage. Extremely high voltage often results
in unstable electrospinning [65].
6.4 fabricatioN of Hybrid Metal Np-coNtaiNiNg polyMer NaNofibers
6.4.1 Zero-valent iron Nanoparticles
ZVI NPs have been used as a cost-effective and environmentally friendly material for environmental remediation [66]. Much effort
has been devoted to the synthesis and application of ZVI NPs. To improve stability and mitigate aggregation, ZVI NPs have been
immobilized or incorporated within or onto different supporting materials for environmental applications [12-14, 19, 20, 30].
In our previous work, polyelectrolyte (PE) multilayers assembled onto electrospun cellulose acetate (CAc) nanofibrous
mats were used as nanoreactors to immobilize ZVI NPs [20, 67]. Figure 6.2 illustrates the whole experimental process. In
brief, smooth and continuous CAc nanofibers with a mean diameter of 295 ± 145 nm were first produced through electrospin-
ning. Then, polyacrylic acid (PAA)/poly(diallyldimethylammonium chloride) (PdAdMAC) multilayers were layer-by-layer
(lbl)-assembled onto the prepared electrospun CAc nanofibers via electrostatic interaction. SEM (scanning electron micros-
copy) morphology observation of the CAc nanofibers deposited with three, four, six, and nine bilayers of PAA/PdAdMAC
shows that when the number of bilayers is below six, the porous structure of the nanofibrous mats can be well maintained.
Therefore, six-bilayer-assembled CAc nanofibrous mats were selected to immobilize ZVI NPs. The six-bilayer-assembled
CAc nanofibrous mats were then immersed into an aqueous solution of FeCl
2
to allow Fe
2+
ions to complex with the free car-
boxyl groups of PAA through ionic exchange, followed by in situ reduction with NabH
4
to generate ZVI NPs-immobilized
polymer nanofibrous mats. Transmission electron microscopy (TEM) images (Figure 6.3) show that the formed ZVI NPs are
evenly distributed onto the CAc nanofibers with an average size of 1.4 ± 0.28 nm. We show that the loading capacity of ZVI
NPs can be tuned by changing the number of PE layers and the cycles in the binding/reduction process, and increasing the
number of binding/reduction cycles leads to slightly bigger ZVI NPs.
To generate hybrid nanofibers incorporated with ZVI NPs both on the fiber surface and inside the nanofibers, we utilized
PAA/poly(vinyl alcohol) (PVA) nanofibers as a nanoreactor for reductive formation of ZVI NPs [12, 14]. First, a mixture solu-
tion of PAA and PVA was electrospun to form nanofibers. To render them with water stability, freshly prepared PAA/PVA
nanofibers were cross-linked at 145°C for 30 min. Then, water-stable electrospun PAA/PVA nanofibrous mats were soaked in
an aqueous solution of ferric trichloride to allow Fe
3+
to complex with available free carboxyl groups of PAA through ionic
exchange, followed by NabH
4
reduction to form ZVI NPs. This process enables the generation of ZVI NPs with a diameter of
1.6 nm, uniformly distributed both within the nanofibers and on the fiber surface. It was possible to control the loading percentage
and size of the ZVI NPs simply by varying the number of ferric ion binding/reduction cycles [68].
To enhance the mechanical durability of the ZVI NP-containing PAA/PVA nanofibers, we fabricated multiwalled carbon
nanotube (MWCNT)-reinforced PAA/PVA composite nanofibrous mats by electrospinning a PAA/PVA mixture solution con-
taining well-dispersed MWCNTs. The so-formed MWCNT-containing PAA/PVA nanofibers were processed to immobilize
