Environmental Engineering Reference
In-Depth Information
optimization factors in the sol-gel coating technique. The chemical and electrochemical methods of coating preparation
emerged to overcome this obstacle [23].
Recently, electrodeposition of CNTs and conductive polymers on metal wires was carried out. Aniline dissolved CNTs
through the donor-acceptor complex, and the polyaniline (PANI/MWNT) coating was electrochemically prepared. This fiber
showed higher EE than the PANI fiber because of its porous structure and the π-π interaction of CNTs with aromatic com-
pounds. The MWNT/PANI fiber also showed high thermal stability and excellent reusability due to the chemical bonding bet-
ween the Pt substrate and coating and the interaction of PANI with MWCNTs. The MWCNT/polypyrrole (PPy) sPME-coated
fiber showed higher EE for the pyrethroids than the PPy-coated fiber and selected commercial fibers. The mechanical strength
of the MWCNT/PPy-coated fiber and the resistance of the coating to different organic solvents and strong acidic and basic
aqueous solutions illustrated the high chemical stability of the fiber [24].
Nafion was also used as a binder to immobilize MWNTs on a ss wire to prepare the MWNT/Nafion fiber. This fiber was
used as a working electrode for the EE-sPME of basic drugs in urine samples and organic ionic compounds. The EE-sPME of
anionic (deprotonated carboxylic acids) and cationic (protonated amines) compounds in aqueous solutions showed more effec-
tive and selective extraction in comparison with dI-sPME, due to electrophoresis and complementary charge interaction [25].
14.4.3.2 FULs as SPME Material
FUls have a large surface to volume ratio and a thermal stability that make them ideal
for sPME application. The main problem in using FUls as an sPME coating is their poor solubility in solvents, which is crucial
for preparing sPME coatings; so, to use FUls in sPME, they have to be attached to a polymeric matrix. Xiao et al. used a
polysiloxane-FUl composite for sPME. synthesis of the coating involved the reaction between a polyazidosilicone backbone
and C60. They then used epoxy resin glue to coat the treated silica capillary with the resultant polymeric FUl [26].
sol-gel technology was used to develop the FUl-polysiloxane surface-bonded porous coating to the Fs fiber. The sol-gel
hydroxyfullerene coating exhibited high thermal stability, excellent solvent (organic and inorganic) resistance, and long lifes-
pan because of the unique properties of FUl and the chemical bonding between the coating and fiber surface. The charge
transfer and structural similarity between the analytes, polychlorinated biphenyls (PCbs), and hydroxyfullerene caused the high
extraction capabilities of coating for the planar PCb congeners [27].
14.4.4
gas-phase micropreconcentration on Cnt
The gas adsorption capacity of CNTs has been studied by computational methods as well as by experimental measurements.
Adsorption of Co and Co
2
and of Ar, N
2
, and CH
4
has shown that CNTs are a favorable sorbent for lighter gases. limited
studies have been carried out with organics, where adsorption isotherms for benzene, methanol, and methane as well as affinity
of dioxins have been studied. Adsorption kinetics of ethanol, isopropanol, cyclohexane, benzene, and hexane and sorption
capacities of a variety of analytes on sWNTs and MWNTs have also been studied. CNTs were used for the preconcentration of
organic vapors on sWNTs and MWNTs, and retention was found to be stronger than for Tenax [6, 7, 28-30].
The high capacity of CNTs makes them excellent sorbents for gas-phase microconcentration, where the amount of sorbent is
limited. Typical methods of release for the trapped analyte is thermal desorption, and the ease of desorption of large molecules
from the CNT surface makes them high-performance sorbents. The microconcentration device can be made by two methods.
The first approach is the packing of CNTs into the capillary tubing. The packing of a nano-scale sorbent requires special efforts
to ensure uniform packing and low pressure drop. Powdered CNTs as well as nanotube paper have been used as a packing
material in a metal preconcentrator tube for a vapor preconcentrator. The second approach is a self-assembled format, where the
CNTs are synthesized in situ within preconcentration devices like a steel tubing or a microfabricated device by Cvd. The self-
assembled format of CNTs provided a consistent ordered structure along the tubing that facilitated a compatible flow within a
device, especially for a serpentine microfabricated structure that could be difficult to pack. The self-assembly of CNTs in 250
and 500-µm i.d. capillaries has been reported using Cvd of Co, C
2
H
4
, and ethanol. The surface of stainless-steel tubing was
found to be effective because it could be made catalytically active for CNT growth. The self-assembled layer was then heated in
a stream of oxygen for 5 h at 300°C to oxidize any impurities formed during the growth process [6, 7, 31-33]. An online moni-
toring CNT microtrap and chromatogram generated by it with a microfabricated sWNT device is shown in Figure 14.8.
The trapping efficiency of a sorbent depends upon its surface area as well as physisorption/chemisorption on the surface. As
mentioned earlier, the sorption capacity in a flow system is usually estimated by studying the breakthrough time, which is
defined as the time required by an analyte to elute through, or the time for which the solute is retained on the sorbent.
breakthrough time is known to be a function of the capacity factor, length, and flow rate. CNTs have been used for the precon-
centration of volatile organic compounds (voCs) as well semivolatile organics (svoCs). breakthrough times of selected voCs
obtained on a microtrap using different types of CNTs are presented in Table 14.4. Comparative data using as-prepared MWNTs
with NTCs, purified MWNTs, and sWNTs are presented. The breakthrough volume on impure MWNT was similar to that of
