**4. Modification of clays and application in arsenic and fluoride removal**

It has been observed that raw clays exhibit low adsorption capacities toward arsenic and fluoride adsorption from solutions. This is attributed to the permanent negative charges on the edges of clay sheets [37]. As such clay modification by higher density charge species and organic cationic surfactant is essential to improve their binding affinity. Common techniques that have are used for modification of clays for arsenic and fluoride removal includes intercalation, coating and pillaring. Intercalation includes insertion of guest species in the interlayers of the clay mineral with preservation of the clay layered structure [38]. Guest species may be the inorganic cations such as Mn2+, Fe3+ and Al3+ or organic cationic surfactants such as HDTMA and CTAB. Gitari et al. [20] intercalated Fe3+ ions onto South African bentonite clay, their results showed that the process involved the cation exchange between main exchangeable cations such as Mg2+, Na+ , Ca2+ and K+ . This was confirmed by the subsequent decrease of the content of these chemical species in the Fe3+ modified bentonite. Mudzielwana et al. [39] and Masindi et al. [22] intercalated Mn2+ and Al3+ onto the interlayers of bentonite respectively, and also observed decrease in the content of Mg, Na Ca and K oxides. These results confirm that during intercalation basic exchangeable cations in the interlayers are exchanged for guest species.

Pillaring is the most commonly used procedure to transform phyllosillicate materials into microporous and mesoporous materials. It involves the formation, intercalation and subsequent fixation of polynuclear cations between the clay interlayers [29]. Thus the lamellar spacing and specific area increases, making these materials attractive adsorbents for adsorption of various inorganic contaminants. **Figure 4** presents a schematic diagram of a pillared clay [40].

**Figure 4.** Schematic diagram of a pillared clay [40].

Lenoble et al. [21] pillared montmorillonite clay using titanium, iron and aluminum as pillaring solution. Their results confirmed that pillaring increases the basal spacing and the specific surface area by many folds. These results were confirmed by Mishra and Mahato [40] who also observed increased in both specific surface area for Mn and Fe pillared bentonite clay. Beside the side the change in basal spacing and chemical oxide content in the pillared clays, the whole process of pillaring does not change the mineralogical composition of the clay.
