## 2 Zeolites
### Ion Exchange Process  
The ion exchange process is established between a solid material and a fluid phase acting as the solvent. In the process, the exchange between fluid- and solid-phase ions occurs and vice versa. During this phenomenon, the ions are exchanged with other ions of the same nature of charge. In other words, positive ions exchange positive ions and negative ions exchange negative ions. The ion exchanger is linked to functional groups of different charges, for example, cation exchange resins usually have sulfonic groups attached to their structure. Less frequently, these groups can be carboxylic, phosphonic, phosphinic or others. Anionic resins, on the other hand, involve quaternary ammonium groups or amino groups [42] (p. 8). The ion exchange capacity of the structure is a function of the composition of the solid and fluid phases, as well as the concentration of the solution in which it is found. The exchange takes place between two or more phases, with the ions moving from one phase to the other and bonding relatively strongly. The quantity of ions exchangeable by the structure depends on the so-called ion exchange capacity, a property measured in \(\mathrm{meq}/\mathrm{g}\). This process depends on the electroneutrality of the medium and the ion concentration in both phases [42] (p. 8). The capacity is related to the presence of extra-network cations, located in channels. In each type of zeolite structure, there is a cation capable of being exchanged, but this capacity depends on its position in the network and on the binding energy. The ion exchange can be complete or partial, and ions capable of being exchanged need to be compatible with the pore size and specific hydrophilic character [77]. An alternative method to facilitate the ion exchange of the structure is to carry out solid-state ion exchange [78]. In this process, the zeolitic structure is mixed with a salt containing the exchangeable cation. This mixture takes place in a solid medium and is followed by thermal dehydration. This method offers the following advantages: there is no need to handle large volumes of saline solutions; there is no disposal of the solution in the environment after the process; and the process is not limited by the solvation sphere of the exchangeable cation [79]. Zeolitic structures have great ion exchange capacity due to their thermal stability, stability to ionizing radiation and high adaptability to the environment, as they have no toxicity on it. Thus, these are ideal structures for catalytic processes, competing with ion exchange resins used in water purification. Much of the ion exchanging property of zeolites is due to the nature of their surface, with high reactivity and the presence of micropores [69] (p. 11). The porous structure of zeolites allows the incorporation of water and cations due to adjusted diameters through inlet ports in internal structure. The proportion of Na, K and Ca is higher than Mg, Ba, Sr, among others, since they are exchangeable with species aggressive to the environment, such as copper, cadmium, ammonium, and some radioactive species. CEC and selectivity are specific according to the nature of zeolite [80], and the number of exchangeable positions and the Si/Al ratio are other factors involved. The induction of an electric field using an electrokinetic system allows verification of the dependence of copper ions in the framework on the geometry, cavity dimensions, structural properties, and cation positions. The medium resistance for ions mobility is crucial in this process, the dynamic condition leading to as high a capture efficiency of metallic ions in zeolite as the concentration of H\({}^{+}\) in medium [81]. Some structures, such as Linde type and Na-P1 type, are strongly pH-dependent, even under weak acidity variation [82]. The CEC is the most important parameter to verify the zeolitization process, e.g., the use of some raw materials in agglomerate form may release some ions useless from a CEC perspective, leading to a decrease in yield of zeolite formation [83].