*4.2.1. Size of molecule*

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

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

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

Many researchers observe decrease in adsorption of phenol with the increase in surface acid-



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

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

Adsorption of small organic molecules, especially containing functional groups, is affected by surface chemistry. The bigger is the molecule, the weaker is the effect due to the steric

surface available for sorption resulting in decrease of phenol adsorbability [23, 48].

ity and with surface oxidation [33, 46, 47]. This phenomenon can be explained by

resulting in the improvement in phenol adsorption [23].

16 Phenolic Compounds - Natural Sources, Importance and Applications

micropores [33].

formation of hydrogen bonds.

phic forms of phenol [33],

pH of sorbent slurry [48], are determined.

ones of ion exchangers or porous carbons [32].

**4.2. Adsorbate properties**

The molecular size of the adsorbate has a crucial meaning for adsorption process. The rate of mass transfer is controlled by diffusion of adsorbates in porous structure of the adsorbent. However, availability of the pores strictly depends on size of the molecule. The smallest molecules penetrate almost all pores, while for the larger ones, not all of them are accessible. This elimination of large molecules from entering the pores describes size exclusion mechanism. As a result, porous materials act as a molecular sieve. An increase in adsorption resulting from the rise of adsorption energy is supposed if the pore size is about twice the size of kinetic diameter of adsorbate molecule [46].

The species with branched or bulky substituents, which are too large to reach interior of micropores, are retained in mesopores [31]. Desorption of such solutes can be hindered due to being trapped inside pores.
