Environmental Engineering Reference
In-Depth Information
stationary phases in a wide temperature range (80-360°C), with good thermal stability and high selectivity for aromatic com-
pounds. Chemical functionalization of FUls is often carried out to alter their chemical properties and/or improve miscibility
with host polymers [7-9].
14.3
adsorption on Cnms
Adsorption of pollutants on CNMs is controlled by hydrophobic, dispersion, and weak dipolar forces. The higher equilibrium
rates of CNMs over conventional carbons are attributed to π-π electron donor-acceptor interactions with sorbates, reduced het-
erogeneity of adsorption energies, and the absence of pore diffusion as an intermediate mechanism in adsorption. Additionally,
CNMs demonstrate a virtual absence of hysteresis between adsorption and desorption isotherms for liquids and gases under
atmospheric pressure. various carbon-based sorbents have been utilized for the preconcentration of analytes. These include
activated carbon, carbon molecular sieves, graphitized carbon black, and porous carbon. These differ in their physicochemical
characteristics, such as pore size/shape, surface area, pore volume, and surface functionality. Kinetic and thermodynamic prop-
erties of carbon-based sorbents including breakthrough volumes, adsorption isotherms, intermolecular interaction mechanisms
at the adsorbate/carbon sorbent interface strongly influence preconcentration [6]. Table 14.1 provides the general characteristics
of some popular adsorbents and CNTs. The ad
s
orbent can affect adsorption by providing a large number of available sorption
sites and by facilitating specific interactions such as hydrogen bonding.
The sorption mechanism between CNTs and other conventional sorbents (Table 14.1) is shown in Figure 14.5. The diffusion
into the porous structure leads to slow mass transfer, and consequentially the quantitative release of large molecules becomes a
limiting factor. on the other hand, in CNTs, sorption occurs mainly on the outer surface and in the hollow spaces of the open
tubes (provided the tubes are uncapped). Therefore, their release by both thermal processes and solvents are relatively simpler
and are not limited by diffusion. As such, a wide range of compounds from small molecules to large semivolatile compounds
can be easily concentrated and desorbed from CNTs [6, 7].
There are other factors that come into play. The exceptionally high aspect ratio (in millions) of CNTs provides a special confine-
ment effect, which leads to completely different physical behavior when compared to that of more conventional sorbents. Moreover,
outer tube surfaces have a large number of intertubular spaces that provide specific adsorption sites. As a result, the sorption
capacity is higher than what one would expect based on brunauer-Emmett-Teller (bET) surface area measurement. various
approaches have been used to describe the adsorption of CNTs. The high sorption capacity of CNTs has been explained by the
presence of high-energy adsorption sites, such as CNT defects and interstitial and groove regions between CNT bundles, and the
phenomenon of multilayer adsorption during microsorption on CNTs. Capillary forces in nanotubes can also be strong, which may
draw molecules from vapor or liquid phases by van der Waals attractive forces and dipole-induced dipole interactions. Hydrophobicity
and capillarity can provide ordering and orientation of sorbents on the sorbate. Adsorption studies detail rapid equilibrium rates,
high adsorption capacity, low sensitivity to pH range, minimal hysteresis in dispersed CNTs, and consistency with traditional
langmuir, bET, or Freundlich isotherms. Moreover, increase in dispersion energy and the overlapping force of adjacent carbon
walls expand sorbent-sorbate and sorbent-sorbent interactions, resulting in condensation within the nanotube. The filling of nano-
tubes may explain why some models describe adsorption capacity above the physical surface area of a CNT [8-10].
table 14.1
properties of porous adsorbents and Cnts
surface
area (m
2
/g)
Maximum
T
(°C)
sorbent
strength
Analytes
drawback
Tenax TA
Weak
35
350
Nonpolar compounds and less
volatile polar compounds
Undergo chemical decomposition
in highly oxidizing atmospheres
(i.e., in the presence of reactive
gases such as 0
3
and No
2
),
Carbotrap
Medium-weak
100
>400
voCs including ketones, alcohols,
aldehydes and perfluorocarbon
tracer gases
No sorption for polar compounds
Carbopack
Medium
240
>400
Hydrocarbons, bTEX
No sorption for polar compounds
Carbosieve
very strong
800
>400
C2, C3, and C4 hydrocarbons
Not suited for higher organics
Carboxen 1000
very strong
>1200
>400
Ultravolatile hydrocarbons
Not suited for higher organics
CNTs
very strong
150-1500
>400
very volatile to semivolatile
organics (Methane to PAHs)
Need purification, relatively
expensive
Reproduced with permission from Ref. [6]. © springer.
