Page 259 - Adsorbents fundamentals and applications

244 CARBON NANOTUBES, PILLARED CLAYS, AND POLYMERIC RESINS

Adsorption of Dioxin and Benzene. Ultrapurification is becoming an increas-
ingly important topic due to environmental and health concerns. Dioxins are
highly toxic and quite stable. Like dioxins, many of the highly toxic chemi-
cals are low-volatile compounds. Removal of these compounds to the ppb or
ppt level are often necessary. It has been a challenge to develop an experimen-
tal technique for measuring adsorption isotherms of low-volatile organics at low
concentrations. A simple technique based on temperature programmed desorption
(TPD) has been developed recently (Yang et al., 1999). This technique involves
dosing the sorbate (as solution in a solvent such as DMF) at the inlet of the
column that is packed with the sorbent, followed with TPD in a flow of an
inert gas. From the peak desorption temperature as a function of heating rate,
the activation energy of desorption is obtained, which is taken as the heat of
adsorption. The heat of adsorption yields the Langmuir constant. The saturated
amount is estimated from the monolayer amount, based on the molecular area of
the sorbate. Thus, the Langmuir isotherm is obtained (Yang et al., 1999). Using
this technique, Long and Yang (2001a) have measured the bond energy or heat
of adsorption of dioxin on carbon nanotubes, and compared this value with that
on other sorbents.
Dioxins and related compounds (e.g., polychlorinated dibenzofurans and bi-
phenyls) are highly toxic and stable pollutants. Dibenzo-p-dioxins are a family
of compounds consisting of two benzene rings joined by two oxygen atoms and
have from zero to eight chlorine atoms attached around the rings. The dibenzo-
furans are a similar family, which differs only in the manner one of the bonds
between the two benzene rings is bridged by oxygen. The toxicity of dioxins
varies with the number of Cl atoms, with non- and monochloro dioxins being
nontoxic, while becoming increasingly toxic with more Cl atoms. Dioxins are
mainly generated from combustion of organic compounds in waste incinerators,
such as municipal waste, medical waste, hazardous waste, and army stockpile
(chemical agents). They are formed downstream the combustion zone with typ-
3
ical concentrations of 10–500 ng/Nm . Current regulations on dioxin emissions
are complex, depending on the toxic equivalency of the actual compounds and
O 2 concentration, and vary in different countries. Nonetheless, removal to well
3
below 1 ng/Nm is generally required (Harstenstein, 1993). Since 1991, activated
carbon adsorption has been widely adopted for dioxin removal from waste incin-
erators in Europe and Japan (Harstenstein, 1993). Because of the higher bond
energy between dioxin and activated carbon than other sorbents, the removal effi-
ciency for dioxin by activated carbon is much higher than other sorbents, such
as clays, pillared clays, g-Al 2 O 3 and zeolites (Yang et al., 1999).
As mentioned, the desorption activation energy can be obtained from TPD
by varying the heating rates. From the temperature dependence of the desorption
peak temperature, one can calculate the desorption activation energy. For physical
adsorption, the desorption activation energy is equal to the bond energy, or heat
of adsorption (Yang, 1987). The bond energies for dioxin on three sorbents are
compared in Table 9.3. The carbon nanotubes used in this work were MWNTs
prepared from methane decomposition on Ni-MgO catalyst by using the recipe