Page 287 - Adsorbents fundamentals and applications

272 CARBON NANOTUBES, PILLARED CLAYS, AND POLYMERIC RESINS

capacities can exceed those of high-surface-area activated carbons (Kuo et al.,
1987). The mechanism of the interactions between carboxylic acid and basic
resins has been studied by Garcia and King (1989) by using acetic acid as pro-
totype solute. The sorption was modeled by a 1 : 1 complex formation between
the monofunctional acid and the basic functional groups on the resin. The com-
plexation reaction leads directly to a Langmuir-type isotherm. Using data from
a number of resins with different pKa (i.e., Gutmann donor number), Garcia
and King (1989) showed that the Langmuir constant (K) can be correlated with
pKa. They also showed that the same basic idea, that is, chemical complexation
equilibria, may be used for selecting solvent used in regenerating the resin.
Uptake of organic molecules in polymer gels brings about swelling. Cross-
linking builds macroporosity as well as rigidity. Solvent swelling occurs in resin
beads that contain 1–5% of cross-links (Frechet and Farrall, 1977). Thus, the
commercial resins are not swellable. However, large uptakes by resins of organic
molecules with high molecular weights are known. The role of absorption in the
uptake of organic molecules is not understood.
The polymeric resins are substantially more hydrophobic than activated car-
bon. The non-wetting types are subjected to the pre-wetting procedure described
above (i.e., wetting with a solvent, such as methanol, followed by aqueous
solution). The pores are presumably wetted (completely) upon this treatment.
Undoubtedly, the mesopores (i.e., voids between the microspheres within each
bead) are wetted. It is not clear, however, whether the micropores within each
microsphere are completely wetted by this procedure.
An interesting study of adsorption on unwetted resins was reported by Rixey
and King (1987; 1989a; 1989b). A number of polar organic compounds and
four hydrophobic resins (XAD-2, XAD-4, Porapak Q and XE-340 carbonaceous
resin) were used in their study. In using these compounds, it was observed that
significant amounts of adsorption occurred when the resins were not wetted. The
adsorption took place by the vapors of the solutes, and the rates could be modeled
as being controlled by Knudsen diffusion of the vapors in the macropores. Selec-
tivity within the pores for solute vs. water is enhanced by the lack of wetting.
Moreover, separation of solutes based on difference in volatility by unwetted
resins was proposed.
In a follow-up study, Rexwinkel et al. (1999) measured adsorption of seven
common chlorinated hydrocarbons from aqueous solutions on wetted and un-
wetted resins. These solutes are nonpolar, as opposed to the polar solutes stud-
ied by Rixey and King. Interestingly, the wetted and un-wetted resin (XAD-4)
showed identical isotherms except for the heaviest compound, 2,4-dichlorophenol.
For 2,4-dichlorophenol, adsorption on the wetted resin was slightly higher.
The gas-phase applications of polymeric resins are primarily used for gas
chromatography and purification of air by removal of contaminants. The work of
Hollis (1966) laid the foundation for the use of resins as packing materials for
GC analysis. Hollis reported relative retention times of over 50 gas molecules on
different EVB/styrene/DVB resins, using beads packed in a column. The same
beaded forms are used today. The water vapor isotherm on the styrene/DVB resin