Page 111 - Adsorbents fundamentals and applications

96 ACTIVATED CARBON

The interpretation for the pH effects is based on the dissociation of phenol into
phenolate anion and proton (Snoeyink et al., 1969). The pKa value for phenol
is 9.89, hence the principal adsorbing species above this pH is mostly anionic.
Because phenolate anion has more affinity for the aqueous phase and the surface
is acidic, repulsion between the surface layer and the anionic phenolate results in
reduced adsorption. The low pH value was obtained by using an acid solution,
which introduced additional protons in the solution. The infrared study of Mattson
et al. (1969) indicated that surface carbonyl was the adsorption site that bonded
phenol and nitrophenol by forming an acceptor-donor-type charge-transfer com-
plex. The added protons in the solution would compete for the carbonyl sites,
hence the reduced adsorption at low pH (Snoeyink et al., 1969). The presence of
an adsorption maximum with pH could be explained by a model given by Muller
et al. (1980). The model was based on the surface charge in equilibrium with the
solution pH (as characterized by equilibrium constant, K) and the electrostatic
interaction potential of the ionized solute with the charged surface (which yields
the value of K).
It has long been known that some of the phenol and its derivatives adsorb on
carbon irreversibly, that is, the irreversibly adsorbed phenol cannot be desorbed
◦
in water or by heating (to temperatures as high as 800 C) (Coughlin and Ezra,
1968; Mattson et al., 1969; Snoeyink et al., 1969; Pahl et al., 1973; Suzuki et al.,
1978; Cooney, 1983; Sutikno and Himmelstein, 1983; Mange and Walker, 1986;
Martin and Ng, 1987; Grant and King, 1990; Leng and Pinto, 1996). The nature
and mechanism of the irreversible adsorption of phenol have been clarified by a
definitive study by Grant and King (1990). Because the adsorption equilibrium
is reached slowly at room temperature, >5 days equilibration time was typically
used in all studies. The irreversible amounts could be measured by extraction with
acetone or as the difference between the total amount and the reversibly adsorbed
amount (Grant and King, 1990). The reversibly and irreversibly adsorbed amounts
are shown in Figure 5.13.
The effect of pH on the total amount adsorbed, by adding the reversibly and
irreversibly adsorbed amounts in Figure 5.13, agree well with that shown in
Figure 5.12. The irreversibly adsorbed phenol was extracted with acetone, and
the extracted solutions were analyzed by mass spectrometry. The mass spectra
are shown in Figure 5.14. Their results showed clearly that polymerization of
phenol occurred on the surface of activated carbon. The abundance of the poly-
mer decreased with molecular weight. No polymers were detected in the control
solution. This product distribution is consistent with that from oxidative coupling
reactions. Grant and King further showed that the oxidative coupling reaction was
not caused by any minerals in the carbon and did not require oxygen or water
(although oxygen increased the amount of the irreversibly adsorbed phenol).
Numerous substituted phenols have also been studied by Grant and King (1990),
and all showed irreversible adsorption. The ordering of the irreversible adsorp-
tion follows: p-methoxyphenol > 2,4-dimethylphenol = p-chorophenol > phe-
nol > aniline > p-nitrophenol = p-hydroxybenzaldehyde. Reactivities of these