**6.8. Adsorption and ion exchange**

In an ion exchange process, an interchange of ions between two phases, usually a solid and a liquid phase, occurs. The ion change resin forms the solid phase while the sample under consideration is normally in the liquid phase. A typical ion exchange resin comprises a cross‐ linked polymer network with its surface covered with a uniform distribution of ions. Ions with charges similar to that of the resin are exchanged once the solution containing the ions comes into contact with the ion exchanger. The degree to which the ions are exchanged is governed by the concentration of the ions in solution (ions to be exchanged with the resin) and their degree of affinity for the ion exchange resin. The ion exchange process is reversible and is deemed as one of the unique technologies for wastewater treatment because of its high degree of recyclability and environmentally friendly nature [96]. The use of polymer‐ based anionic resins can allow successful elimination of phenolic compounds from polluted water through adsorption in conjunction with ion exchange. The ions of the resin provide sites for ion exchange while the porous nature permits adsorption through hydrophobic interaction [97].

The efficiency of phenol exclusion from polluted water involving the use of Amberlyst A26 and Amberlite IRA‐67 as strong and weak base ion exchangers, respectively, has been con‐ ducted. As an observation, phenol removal efficiency of both the strong and weak base ion exchange resins decreased significantly with increased initial phenol concentration from 99.6% (1 mg/l phenol concentration) to 74.2% of 200 mg/l (strong base), and from 65.7% 1 mg/l phenol concentration to 22.1% of 200 mg/l phenol concentration (weak acid). The strong base ion exchange (Amberlyst A26) was considered to be a better ion exchange medium for phenol removal compared to Amberlite IRA‐67 [98].

The use of Amberlite IRA‐420, a strong base ion exchange resin, for the removal of phenol from water was also carried out by Carmona et al. [99]. Both theoretical and empirical approaches were used to identify the parameters that governed the rate at which the Amberlite IRA‐420 removed phenol from the solution. According to their results, the phenol elimination process with Amberlite IRA‐420 occurred in the acidic medium through adsorption, and in the alka‐ line medium through both adsorption and ion exchange. The process increased with increas‐ ing pH values from 9 to 14. Ahmed et al. [100] also compared the phenolic compound removal effectiveness of four different ion exchange resins, namely, Ameberlite XAD‐4, Ameberlite XAD‐7, Ameberlite IRA‐94 and Ionac AFP‐329, and observed that the effectiveness of the degradation process depended on the size of the resins' surface area and temperature. Ionac AFP‐329 was the most efficient sample. However, Ameberlite XAD‐4 demonstrated the high‐ est desorption ability with almost complete pollutant recovery at 60 °C.
