*4.3.2. Electrolytes: ionic strength*

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

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

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

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.

takes relatively long contact time" [30].

18 Phenolic Compounds - Natural Sources, Importance and Applications

changes in adsorption*"* [33].

*4.3.1. Water and other solvents*

diminishes.

**4.3. Environmental conditions**

The presence of electrolytes, for example salts in the solution, affects the strength of electrostatic interaction between phenolic solutes and the surface of the adsorbent. If these interactions are attractive and surface concentration of adsorbates is low, then the rise in ionic strength of solution causes decrease in uptake. But if they are repulsive or concentration of adsorbates on the surface is high, increase in ionic strength of the solution enhances the adsorption [60].

The presence of salts can influence the adsorption of phenols also by the effect of salting out. The electrolytes dissolved in the solution reduce or even destroy the hydration layer of organic compound causing decrease in its solubility, which affects favorably on adsorption.

Kuśmiderek and Świątkowski have compared the influence of an electrolyte on the adsorption of 4-chlorophenol on active carbon and multiwalled carbon nanotubes (MWCNTs). They used three different salts: Na<sup>2</sup> SO4 , NaCl, NaNO3 at various concentrations. The presented results proved higher uptake of 4-chlorophenol on the active carbon in the presence of salts in all cases, whereas no significant difference was observed for adsorption on MWCNTs. It was also indicated that the desorption of the 4-chlorophenol increased with the increase in the ionic strength of the solution [60].

Dissimilar results were obtained by Mukherjee and De for sorption of catechol on mixed matrix membrane in the presence of NaCl. They justified their outcome by weakening forces between sorbent and phenols resulting in decrease of the uptake. They also defined this phenomenon as the shielding effect [22].
