**2.4. Specific surface area**

isomorphic substitutions are observed, for example, possible substitutions of Si4+ in tetrahedron by Al3+ or those of Al3+ in octahedron by Fe2+ [17]. Such substitution leads to permanent negative surface charge at the sheet by the presence of exchangeable cations [18]. The

group minerals have higher charge density, higher surface area and higher swelling capacity [3]. The swelling capacity of these type of clay arise from their structural features that enables water to overcome the electrostatic and van der Waals interactions keeping the layers together and penetrate into surface interlayers leading to hydrolization of Al and Si atoms to aluminol (AlOH) and silanol (SiOH) resulting in expansion. **Figure 3** presents the schematic

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

and NH<sup>4</sup>

+ [16].

, Na+ , H<sup>+</sup> , Na+

, Ca2+, Mg2+ and H<sup>+</sup>

. The 2:1

most common exchangeable cations in the interlayers are K+

48 Current Topics in the Utilization of Clay in Industrial and Medical Applications

diagram of 2:1 clay minerals.

**2.3. Cation exchange capacity**

clay mineral includes; Mg2+, Ca2+, K+

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

One of the essential properties of clay minerals is their larger surface area. This characteristic allows clay minerals to adsorb water and other environmental contaminants [26]. Type 2:1 clay minerals such as smectite and vermiculite possess higher specific surface area as compared type 1:1 clay minerals such as kaolinite and halloysite because of their ability to swell [10]. The total specific surface area of the clay is denoted by the sum of external surface area and the internal surface area corresponding to the interlayer spaces [8].

Several authors have indicated that total specific area of the clay mineral can be increased through modification to increase their functionality in different areas of application. Hua [27] reported an increase in surface area of Na-bentonite from 34.1 to 77.2 m2 /g after modification with Mn oxides. Bentonite modified with the combination of Mn oxides and poly(diallyldimethylammonium chloride) showed a sharp increase in surface area to 128.9 m2 /g. The increase in total specific


**Table 1.** Cation exchange capacity of raw and modified clay minerals.

area could be attributed to swelling of bentonite clay during modification. Mishra and Paride [28] also reported increased specific surface area for bentonite pillared with manganese oxides at temperature of 500°C. This was attributed to decomposition of the complex with increasing temperature to form the oxide pillar which generated the void micropores inside the clay layers. This phenomenon was also emphasized by Bertella and Pergher [29] who also observed an increased in specific surface area of bentonite clay after pillaring using Al and Co from 58 to 304 m<sup>2</sup> /g.

concentration. It was further believed that adsorption of fluoride by kaolinite clay is accompanied by slight expansion in the kaolin sheet lattice. After this study, Tor [35] evaluated the efficiency of montmorillonite clay mineral in adsorption of fluoride removal from groundwater. A maximum fluoride adsorption capacity of 0.263 mg/g was reported at initial pH of 6.

Mineralogical and Chemical Characteristics of Raw and Modified Clays and Their Application…

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51

Mudzielwana et al. [23] reported the efficiency of Mukondeni smectite rich clay in fluoride removal. They observed that percentage fluoride removal decreases with increasing pH of the solution with about 92% fluoride removal noted at acidic pH of 2. Ngulube et al. [24] also observed the same trend in the adsorption of fluoride by mixed Mukondeni clays. The decrease in percentage of fluoride with increasing pH during adsorption by raw clay minerals

 at alkaline pH. The fluoride adsorption capacity of selected South African clay soils was reported by Coetzee et al. [36]. They observed that kaolinite type clay has the lowest adsorption capacity while the gibbsite clay has the highest adsorption capacity toward fluoride ions. The adsorption capacity of South African clays can be summarized in the following increasing order: kaolinite> smectite> palygorskite> goethite> gibbsite. This was attributed to the structure of the clay,

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 exchange-

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

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].

. This was confirmed by the subsequent decrease

is often attributed to abundance of OH−

**removal**

able cations such as Mg2+, Na+

for guest species.

surface charges and also the chemical composition of the clay [36].

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

, Ca2+ and K+
