*4.1.2. Chemical properties of an adsorbent*

Chemical composition of the adsorbent surface determines the mechanism of phenols adsorption. As it is described above, the retention of phenols on the solid surface is the result of specific and nonspecific interactions. The main role is played by strong dipole-dipole interactions and hydrogen bonds between phenolic compounds and surface functional groups. Equally important are π electrons interactions of the aromatic systems of the adsorbate and adsorbent (if contains) [26, 32].

Thus, retention of the polar phenols is enhanced by the presence of polar moieties on the sorbent surface, while sorption of the hydrophobic molecules and those forming strong intramolecular hydrogen bonds is not influenced by the polarity of an adsorbent surface [25].

The studies show that functional groups being active in retention of phenolic species are mainly located in larger micropores, and the increase in their concentration on the surface leads to the favoring of adsorption in larger pores. Consequently, the competition takes place between filling of the smallest micropores and the adsorption on active sites located in larger micropores [33].

The moderately and slightly acidic oxygen groups are as considered as the most important ones influencing the mechanism of phenols adsorption. The vital surface groups for this process are bases and carbonyls [18, 33, 45]. According to Su et al., the increase in concentration of carbonyl groups on the surface provides more sites for the donor-acceptor interactions resulting in the improvement in phenol adsorption [23].

In addition, other polar moieties, for example nitrile [25], amine [24] or bicarbonate [27] on the sorbent surface, can positively influence the uptake of phenol and its derivatives by the formation of hydrogen bonds.

Many researchers observe decrease in adsorption of phenol with the increase in surface acidity and with surface oxidation [33, 46, 47]. This phenomenon can be explained by


In case of adsorption from solutions, the role of surface functionalities increases more and maybe even dominant [49]. The surface of adsorbent can be treated as a huge molecule covered with uniform (e.g. polymers) or various (e.g. active carbons) in chemical character functional groups. If they are able to dissociate, ions are released to the solution. Their presence affects the acid-base equilibrium of the solvent. In order to characterize this process, point of zero charge (PZC) [44, 50], contents of acidic and basic groups [47] and their pKa [45, 50], or pH of sorbent slurry [48], are determined.

The ions on surface, those in solution and adsorbate molecules interact with one another by electrostatic attraction or repulsion. Confirmation of this phenomenon was presented in Tamon and Ozaki's studies, who found that sorption characteristic of phenols in aqueous solution depends on electronic states of adsorbent surface and adsorbate [51]. The same phenomenon controls adsorption of acids on surface with basic groups and bases on the acidic ones of ion exchangers or porous carbons [32].
