*4.2.3. Initial concentration and contact time*

Increase in the phenols concentration in solution leads to an increase in the amount of adsorbed compounds. This can be attributed to a rise of the driving force of the concentration gradient with the rise in the initial phenols concentration and different arrangements of solute molecules on the surface of adsorbent [30]. At low concentrations, phenol molecules are lying flat in parallel with the surface. As a result, the surface occupied per molecule corresponds to the largest of its dimensions [54, 55]. At higher concentrations, accessibility of the surface for the individual molecules is considerably less. Consequently, the molecules have to change their position for upright to allow closer packing in parallel to each other. Then the area occupied in conversion to a single molecule is smaller that is observed as rise in adsorption density [55].

Different studies show that the contact time required for phenols solution to reach the equilibrium is in the range of 1–24 h [30, 56, 57]. However, the uptake of phenols is the most rapid during the initial 30 min [56, 57]. It was also observed that the higher initial concentration, the longer equilibrium times were needed [30, 56]. Shaarani and Hameed explained the run of the process as follows: "initially the adsorbate molecules have to first encounter the boundary layer effect and then diffuse from the boundary layer film onto adsorbent surface and then finally, they have to diffuse into the porous structure of the adsorbent. This phenomenon takes relatively long contact time" [30].

Moreover, it was noted that at relatively large phenol adsorption values, when interactions between adsorbate molecules predominate, phenol uptake is higher for the more oxidized surfaces, whereas, in case of diluted phenol solutions, the competition between phenol molecules and water leads to substantial reduction in adsorption of phenol on oxidized surface. *"*Competition between water adsorbing in pores and phenol is mainly responsible for the changes in adsorption*"* [33].

### **4.3. Environmental conditions**

### *4.3.1. Water and other solvents*

Water molecules interact very weakly with the hydrophobic surface unless their concentration in adsorbed mixture is low (e.g. sorption from gas phase). If the contents of water rise like in case of sorption from aqueous solution or in head space techniques, the influence of water interaction with the surface is no longer negligible. Water begins to interfere with adsorption of organic compounds. Due to the better affinity to the surface, organic solutes are able to replace preadsorbed water. That impacts the kinetics and efficacy of the process. The effect is even stronger, if the surface is hydrophilic. The polar functional groups are predominantly adsorption centers capable of forming the hydrogen bonding. Interactions between water molecules lead to formation of clusters, which block entrances of pore as well as *"*the condensation of water in micropores at much lower humidity than that at which it happens on a fully hydrophobic surface*"* can take place [32, 33]. In consequence, adsorption of phenols diminishes.

From practical point of view both adsorption and desorption processes are equally important. Adsorption of phenols is usually carried out from aqueous solution. However, to desorb retained compounds, organic solvent is necessary. It should be chosen so that its properties (solvating power, good wettability of adsorbent, miscibility, chromophoric nature and purity) provided quantitative, quick and simple process of recovery.

For elution of adsorbed phenolic compounds, the most frequently used solvents are methanol [58], acetonitrile [59], acetone [18], ethyl acetate [18], tetramethylammonium hydroxide [17] and others [43]. If a single eluent is ineffective, a mixture of solvents can be considered. The volume of applied eluent depends on its eluting strength, polarity of the phenols, amount and surface chemistry of adsorbent, and type of device [18].

Bruzzoniti et al. in their paper presented efficiency of different eluents in desorption of phenolic compounds from graphitized carbon black. They noted the effectiveness of eluent was affected by the value of pKa phenolic solute. Phenols with pKa about 7 were effectively eluted with tetramethylammonium hydroxide, while for phenols with higher pKa, methanol was better [17].

For newly synthesized sorptive materials, studies on eluent selection and optimization of desorption process should be conducted [18, 59].
