**2.3. Cation exchange capacity**

One of the important properties of the clay minerals is that contains cations that can be exchanged for any guest species of anion or cation by treating clay mineral with such clay mineral. The exchangeable cations are held on the outside of silica-alumina clay mineral structural units and the exchange does not affect the layout of the silica-alumina units [11]. The ion exchange capacity phenomenon is measured in terms of milli-equivalents per gram or per 100 g. The commonly used cations used to evaluate the cation exchange capacity of the clay mineral includes; Mg2+, Ca2+, K+ , Na+ , H<sup>+</sup> and NH<sup>4</sup> + [16].

**Figure 3.** Typical structure of 2:1 clay [19].

Different clay minerals portray different cation capacity depending on the substitution within the lattice structure [11]. Type 1:1 clay minerals such as kaolinite have limited substitution between their lattice structure compared to type 2:1 clay minerals such as smectite, vermiculite and sepiolite and consequently they have lower cation exchange capacity [10]. Several studies have indicated that cation exchange capacity of the clay mineral decreases after modification by inorganic species [2, 20, 21]. This is because modification involves the ion exchange reaction between the exchangeable cations in the clay interlayer and the guest species. Low CEC of modified clays suggests the irreversibility of cationic exchange and thus intercalated metallic polycations are hardly exchanged [21]. **Table 1** summarizes the cation exchange capacity of different raw and modified clay minerals.
