*4.3.3. pH*

The change in pH affects chemistry of both the adsorbate and the adsorbent by shifting the equilibrium dissociation process of solutes and surface functional groups of the sorbent towards ionized or unionized form. Thus, adsorption of phenols is limited by the acid-base characteristic of the adsorbent and its microporosity, which subsequently influences kinetics 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 ion takes advantage.

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 adsorbability of phenols.

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].

### *4.3.4. Temperature*

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 surface chemical composition [33].

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 capacity of the adsorbent for particular adsorbates [30].
