Environmental Engineering Reference
In-Depth Information
demonstrated that fluids can flow through the cores of CNTs (e.g., hydrogen, hydrocarbon gases, water, and methane). dense
arrays of aligned MWNTs have been described in literature, which could potentially be used for solute transport. Atomistic
simulations have predicted CNTs have exceptional flux and selectivity properties compared to other known nanomaterials and
the transport of gases in CNTs with diameter 1 nm was predicted to be orders of magnitude faster than in zeolites. These excep-
tionally high transport rates have been attributed to the inherent molecular smoothness of the nanotubes. some of these theoret-
ical predictions have been verified experimentally with larger CNTs. Holt et al. have constructed nanotube−si
3
N
4
composite
membranes using Cvd [47]. In related work, Hinds and coworkers constructed polymer-nanotube composite membranes
using MWNTs having large diameters (6-7 nm) [48]. They verified that transport of liquids (alkanes, water) is orders of mag-
nitude faster than can be accounted for by conventional hydrodynamic flow [49].
In the analytical field, Kamilah et al. and o. sae-Khow et. al. Prepared a CNT-immobilized membrane (CNIM) where func-
tionalized CNTs were immobilized. one membrane image is shown in Figure 14.10 [50, 51] (Fig. 14.12).
both sWNTs and MWNTs were used to fabricate the CNIM hollow fiber. Immobilization was carried out such that the CNT
surface was fully accessible to adsorption/desorption. It was found that incorporation was quite rugged, and the membrane did
not lose the CNTs in spite of several washes with water and solvent. several organic compounds including trichloroacetic acid
and tribromoacetic acid, two important disinfection by-products in water treatment, were selected as the model solutes for
CNIM hollow fibers. CNT-mediated membrane extraction provided higher enrichment for all compounds, with improvements
of up to 240% over the polypropylene membrane. This is also attributed to the enhanced partitioning of the uncharged acids in
the CNTs prior to preconcentration into the basic acceptor (Fig. 14.10).
14.7
other appliCations of Cnms
In addition to sorbent properties, CNMs present excellent electric, magnetic, and thermal properties. In general, analytical tools
based on carbon nanostructures exploit two or more properties. In the previous sections, we described applications in which
sorption was the most important property. However, there are applications in which sorption plays a secondary role, especially
the participation of CNTs in developing biosensors and sensors based on field-effect transistors (FETs). due to their excellent
electrical properties, CNTs can be used to develop electrical sensors in which CNTs mediate the electron-transfer reaction with
electroactive species [52].
The sorption properties of CNTs facilitate immobilization of biomolecules (e.g., antibodies, cofactors, or enzymes) and the
redox process by immobilizing the analyte on the electrode surface. Although CNTs are inert structures, their electrical prop-
erties very much depend on the effects of charge transfer and chemical doping. CNT-FETs have been used to detect gases, so
the electronic structures of target molecules near semiconducting nanotubes produce changes in the electrical conductivity of
CNTs. CNT-FETs can be applied to detect NH
3
, Co, and Co
2
. However, we need to make it clear that the applicability of CNT-
FETs is limited when the analytes have low adsorption energy or poor charge transfer to CNTs [53, 54].
14.8
ConClusions and future vision
CNMs, especially in the form of CNTs and FUls, have a tremendous potential in environmental analysis as nanoadsorbents in
a wide variety of environmental analytical applications. However, this is only the starting point because there are large numbers
of other CNMs with exceptional properties that have not yet been explored from the environmental analytical point of view. one
of the main challenges and research is to find cost-effective, scalable production methods that retain the essential properties of
CNMs. Additionally, the combination of CNMs with other new materials (e.g., quantum dots or ionic liquids) can enhance sorp-
tion properties of these, which will help in the development of new analytical tools that will simplify the environmental analytical
process. Moreover, to bring CNM development a step forward, the sorption kinetics on CNMs need to be studied in more detail
for understanding the interaction of gases and liquids. Although CNMs have demonstrated their potential because of their unique
chemical and physical properties, the applications of CNMs in environmental analysis are still limited and at an early stage.
In conclusion, the applications of CNMs in the field of environmental analysis are very interesting and endless; however, the
commercial production of CNMs is still some way off, and there is a need for important breakthroughs. Along with the growth
of interest in CNMs, the potential effects on human health and the environment, both adverse and beneficial, need to be consid-
ered. Their short- and long-term effects on the human body, immunotoxicity, and phototoxicity will also require detailed explo-
ration. Increasingly, a study of their fate and environmental impact is becoming important due to the discharges already
occurring to the environment. The likely further increase in CNM discharges along with the dramatic industry growth, and the
immense knowledge gaps in risk assessment and management, would necessitate further studies in this area.
