Page 101 - Adsorbents fundamentals and applications

86 ACTIVATED CARBON

interaction commences, leading to cluster formation and eventually pore filling
or capillary condensation in the micropores. Activated carbon is used, nonethe-
less, in processes dealing with humid gas mixtures and water solutions because
the organic and nonpolar or weakly polar compounds adsorb more strongly, and
hence preferentially, on its surface than water does.
Adsorption of water vapor on activated carbon has been studied extensively
because of its scientific as well as practical importance. Chemical modifica-
tion can significantly alter the adsorption behavior. It has long been known
that oxidation and reduction affect the “hydrophobicity” of carbon. As men-
tioned, the water isotherm generally follows an S-shaped curve, with little or
no adsorption at P/P 0 below 0.3 or 0.4. In this region, water molecules are
bonded to certain oxygen complexes, likely by hydrogen bonding and elec-
trostatic forces (the nonspecific interactions by Lennard–Jones 6–12 potential
are insignificant). At higher P/P 0 , clusters and eventually pore filling occur
through hydrogen bonding. Pore structure comes into play only in the latter
stage. Oxidation of the surface increases the oxygen complexes, hence shifting
the threshold P/P 0 for water adsorption. The extensive literature on this subject
has been discussed elsewhere (Jankowska et al., 1991; Rouquerol et al., 1999;
Leon y Leon and Radovic, 1994; Rodriquez–Reinoso et al., 1992; Carrasco-
Marin et al., 1997; Salame and Bandosz, 1999; and others to be discussed
specifically later).

5.4. SURFACE CHEMISTRY AND ITS EFFECTS ON ADSORPTION
As described above, activated carbon can be represented by a model of a twisted
network of defective hexagonal carbon layer planes (typically 5 nm wide), which
are cross-linked by aliphatic bridging groups. Heteroatoms are incorporated into
the network and are also bound to the periphery of the planes. The heteroatoms
bound to the surfaces assume the character of the functional groups typically
found in aromatic compounds, and react in similar ways with many reagents.
These surface groups play a key role in the surface chemistry of activated car-
bon. They are particularly important for adsorption in aqueous solutions and the
catalytic properties of carbon. The surface chemistry of activated carbon has
been a subject of long-standing scientific interest, and many reviews of the sub-
ject are available (Mattson and Mark, 1971; Puri, 1970; Cookson, 1978; Bansal
et al., 1988; Zawadzki, 1989; Jankowska et al., 1991; Leon y Leon et al., 1994;
Radovic and Rodriquez–Reinoso, 1996; Boehm, 2002). Chemical modifications,
particularly oxidation, have been used to effectively tailor the adsorption and
catalytic properties. Some typical surface groups are given in Figure 5.4 (Puri,
1970; Zawadzki, 1989; Radovic and Rodriquez–Reinoso, 1996; Boehm, 2002).
The surface groups shown in Figure 5.4 are acidic groups. Concentration of
these groups can be created or increased by oxidation with oxygen at elevated
temperatures (or by aging at mild temperatures) or with liquid oxidants, typically
nitric acid (Noh and Schwarz, 1990). The acidic surface shows cation exchange
properties in aqueous solutions. If the carbon is de-gassed at a high temperature,