*4.3.6. Irreversible sorption of phenols*

Irreversible adsorption of phenols is closely related to process of chemical adsorption involving chemical reaction between adsorbates and surface functional groups of adsorbent [55]. As it was described above, the hydrogen bonding arising in the system adsorbate-active centers of adsorbent strengthens interactions and sorptive capacity, but also can cause difficulties related to irreversible sorption [18]. It can be also considered that phenol molecules or phenoxide radicals can react with active sites on a carbon surface, which leads to creation of covalent bond between them [33].

Tessmer et al. investigated this problem and found that "grater structural ordering and delocalized electrons on the carbon surface may increase the basicity of the carbon but do not enhance its ability to promote irreversible adsorption. The presence of oxygen containing basic groups (chromene-type or pyrone-type) is likely the key factor promoting irreversible adsorption" [48].

Other studies demonstrate that lactone surface groups are responsible for both irreversibility of phenol adsorption due to chemical reaction and decrease in the difference between adsorption in oxic and anoxic conditions [33].

And another one proves that irreversible adsorption to carbon surfaces does not depend on oxidative coupling reaction, but oxidative coupling may enhance irreversible adsorption when experimental conditions are conductive (pH of solution high enough, microporosity of adsorbent) [63].

The discrepancies in this matter indicate that it is a topic still open for discussion and further research.
