Carbon Nanotubes for Sorbents

Carbon is well known as one of the better adsorbents for gases. This property is due to (i) the ability of this material to exist in a very fine powdered form with highly porous structure and (ii) the existence of particular interactions between carbon atoms and gas molecules. During recent decades, many improvements have been accomplished to obtain micro porous and ultra micro porous carbonaceous materials having very high adsorbing properties for the most current gases

Graphite has a hexagonal structure with an in plane lattice parameter a=0.24612 and c = 0.6709 nm and assuming that all the atoms are in one single plane the maximum specific surface area is S1 = 1,315 m2 g-1 or if both the sides of plane are considered S2 = 2,630 m2 g-1. For a nanotubes sample, the specific surface area exposed to the gas depends on the numbers of shells.

The adsorption of a gas on a surface is a consequence of the field force at the surface of the solid called the adsorbent, which attracts the molecules of the gas or vapor, called adsorbate. The forces of attraction emanating from a solid may be of two main kinds, physical or chemical and they give rise to physical i.e. vanderwaals or chemisorption respectively.

Carbon nanotubes have been proposed as superior sorbents for dioxin removal (Long and Yang 2001). Due to the extreme toxicity of dioxins, it is desirable to have a more efficient sorbent than activated carbon so that the dioxin emissions can be reduced to lower levels. Long et al. found the interactions of dioxins with carbon nanotubes are much stronger than that with activated carbon (Long and Yang 2001). The origin of these strong interactions is not clear but the authors suggest that it may be attributed to the unique structure and electronic properties of carbon nanotubes (Long and Yang 2001).

Because of the higher bond energy between dioxin and carbon nanotubes compared to other sorbents (315 kJ/mol for nanotubes vs. 119 kJ/mol for ZX-4 carbon), the removal efficiency for dioxin by carbon nanotubes is much higher than other sorbents. Nanotubes can also be used as aqueous phase adsorbents. Peng et al. used as-grown nanotubes and graphitized nanotubes as adsorbents to remove toxic 1, 2-dichlorobenzene (DCB) from water (Peng et al. 2003). Nanotubes can be used as adsorbents in a wide pH range (3-10) (Peng et al. 2003). The experiments demonstrated that it takes only 40 min for nanotubes to attain equilibrium and that the adsorption capacity is relatively high, thereby making carbon nanotubes good adsorbents for the removal of DCB from water. Nanotubes were also evaluated as adsorbents for trapping volatile organic compounds from environmental samples. For example, Li and co workers found that nanotubes have much higher breakthrough volumes for toluene, xylene, dichloromethane, 1, 2-dichloroethane and so on than graphi-tized carbon black with the same surface area (Li et al. 2004). This was ascribed to the more porous structure of SWNTs. The results indicate that nanotubes are potential useful adsorbents for direct trapping VOCs from air samples.

The adsorption of heavy metals by nanotubes has been studied by Li group (Li et al. 2002). They found that nanotubes show exceptional adsorption capability and high adsorption efficiency for lead removal from water. The adsorption is significantly influenced by the pH value of the solution and the nanotubes surface status. Their work also showed that the oxidation dramatically increase the cadmium adsorption capacity. The Cd(II) adsorption capacity of the as grown nanotubes is much smaller than H2O2, HNO3 and KMnO4 oxidized nanotubes

Using carbon nanotubes in computer displays may further diminish environmental impacts by eliminating toxic heavy metals and dramatically reducing material and energy use requirement. Moreover, multi-walled nanotubes demonstrated good properties as solid phase extraction adsorbents for the extraction and quantification of several chemicals in water including bis phenol A, 4-n-nonylphenol, 4- tert-octylphenol and phthalate esters (Cai et al. 2005) .

It is realized that this is very good material for the sorption purposes but it is also found that the cost of carbon nanotubes is very high. It may be one of the main flaws for the commercialization of the technologies related to sorption.

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