*4.3.5. Oxic and anoxic conditions*

Influence of oxygen for process of phenols sorption was at first perceived for carbon adsorbents; hence, the facts presented here have been developed only based on the results obtained for these materials.

When phenols are adsorbed under oxic and anoxic conditions, differences in the uptake can be observed. The sorption capacities of some sorbents are higher under oxic conditions that imply promoting action of molecular oxygen in this process [3, 49]. The increase in adsorption is attributed to the oxidative coupling reaction leading to formation of timers or more substituted derivatives of phenols (multimers) that are attached to the surface of adsorbent [33, 55].

The course of the process can be explained as follows: the molecules of phenols and those of oxygen diffuse to the surface of carbon, where they are preadsorbed. The adsorbed oxygen molecules form superoxo ions that yield rearrangements and further reactions with surface functional groups and adsorbed phenols leading to creation of various surface compounds. Thus, described reaction occurs only on the surface of carbon and is catalyzed by it [33].

Studies on this phenomenon revealed that

and effectiveness of the overall process. Higher values of uptake are observed for the compounds in undissociated form [30, 61]. Therefore, most of phenols such as weak acids are better adsorbed from neutral or acidic solutions. At lower pH values, oxonium ions are present in solution and they prevent dissociation of surface acidic groups. In these conditions, formation of hydrogen bonds and π-π interactions between phenolic adsorbates and sorbent surface is privileged. Accordingly, adsorption capacity is the highest [19]. The same applies to phenols, their transition point between acidic and basic form is associated with their pKa value. Below this value dominates acidic form and above it the conjugated base-phenoxide

The uptake of phenols decreases at higher pH, when the molecules turn into phenolate ions and the surface of the adsorbent is negatively charged. The presence of electrostatic repulsive forces impedes adsorption [16, 57]. Thus, the more acidic the surface of sorbent, the lower the

However, some researchers report the in certain pH range, adsorption for most phenols initially increases with the rise in pH to reach a certain value and then decreases with further increase in pH [16, 56]. This phenomenon can be explained by the fact that, in the presence of oxonium ions the surface functional groups of adsorbent gain a positive charge. As a result, adsorption of water increases and cluster formation takes place. Consequently, some of more active sites are not available. Second, already adsorbed molecules of phenols can block the entrances to fine pores and cause decrease in uptake. This effect is stronger at the lowest pH values [33].

Due to the fact that process of physical adsorption is exothermic in nature, increase of temperature causes decrease in phenol's uptake [56]. Moreover, the rise in temperature increases the rate of the sorption process and leads to the disappearance of the effects associated with

In contrast, chemisorption is an endothermic process. Therefore, Shaarani and Hameed observed the increase of temperature slightly increases the uptake of 2,4-dichlorophenol at a higher concentration. They also found that the change of temperature alters the equilibrium

Influence of oxygen for process of phenols sorption was at first perceived for carbon adsorbents; hence, the facts presented here have been developed only based on the results obtained

When phenols are adsorbed under oxic and anoxic conditions, differences in the uptake can be observed. The sorption capacities of some sorbents are higher under oxic conditions that imply promoting action of molecular oxygen in this process [3, 49]. The increase in adsorption is attributed to the oxidative coupling reaction leading to formation of timers or more substituted derivatives of phenols (multimers) that are attached to the surface of adsorbent [33, 55].

ion takes advantage.

20 Phenolic Compounds - Natural Sources, Importance and Applications

adsorbability of phenols.

*4.3.4. Temperature*

surface chemical composition [33].

*4.3.5. Oxic and anoxic conditions*

for these materials.

capacity of the adsorbent for particular adsorbates [30].

