**6. Conclusion**

three steps, there are two adsorption processes that take place during the transfer of adsorbate ions into the adsorbent, namely, physisorption and chemisorption. Physisorption occurs when the ions are adsorbed to the surface through the weak intermolecular interactions such as Van der Waal forces, hydrogen bonding and dipole–dipole interactions [45]. Conversely, chemisorption occurs when adsorbate ions forms chemical bond through electron exchange. Lee et al. [46] evaluated the adsorption of As(III) and As(V) onto HDTMA modified clays and they concluded that the adsorption of As(III) was via physisorption while the adsorption of As(V) was via chemisorption. They further indicated that the adsorption of As(III) occurred on the outer layer of the adsorbent while the adsorption of As(V) occurred in the inner layer of the adsorbent.

Solution pH is the main factor that influences the adsorption of arsenic and fluoride onto clay minerals since it determines the surface charges [37, 47]. Several authors observed that adsorption of fluoride and arsenic by clay minerals is low at alkaline pH values due to the abundance

According to Ren et al. [5] the adsorption of As(V) and As(III) onto modified clay minerals at

*AsO*<sup>4</sup> ↔ *FeH*<sup>2</sup> *AsO*<sup>4</sup> + *H*<sup>2</sup>

*AsO*<sup>3</sup> ↔ *FeH*<sup>2</sup> *AsO*<sup>3</sup> + *H*<sup>2</sup>

<sup>2</sup><sup>−</sup> + *H*<sup>2</sup>

*AsO*<sup>2</sup> ↔ *FeAsO*<sup>4</sup>

*AsO*<sup>3</sup> ↔ *FeHAsO*<sup>3</sup> + *H*<sup>2</sup>

 ions on the adsorbent surfaces that causes electrostatic repulsion. Furthermore, the adsorption of arsenic and fluoride has been observed to be favored by low pH values. This is mainly attributed to the fact that at low pH the clay surfaces are positively charges and as such ions will be removed easily through electrostatic attraction. Thakre et al. [42] used the Eqs. 1 and 2 to hypothesize the adsorption of fluoride onto magnesium incorporated benton-

*O* (1)

*O* (3)

*O* (4)

*O* + 2*H*<sup>+</sup> (5)

*O* + 2*H*<sup>+</sup> (6)

<sup>2</sup> + 2*F*<sup>−</sup> ↔ *MF*<sup>2</sup> + 2*OH*<sup>−</sup> (2)

of OH−

Acidic pH:

Alkaline pH:

Acidic:

Alkaline:

ite under different pH conditions.

*M*(*OH*)

*FeOH* + *H*<sup>3</sup>

*FeOH* + *H*<sup>3</sup>

*FeOH* + *H*<sup>3</sup>

*FeOH* + *H*<sup>3</sup>

acidic and alkaline may be elucidated by Eq. 3–6:

where M represents metals.

*MOH* + *H*<sup>+</sup> + *F*<sup>−</sup> ↔ *MF* + *H*<sup>2</sup>

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

This chapter presented a summary of mineralogical and chemical composition of raw and surface modified clay minerals and their application in arsenic and fluoride removal from drinking water. From the review, it is noted that properties of the clay minerals such as higher cation exchange capacity and specific surface areas as well as the chemical composition of clay minerals enables them to be used as arsenic and fluoride adsorbents. Furthermore, these physicochemical properties can be improved through intercalation and pillaring which consequently improves their applicability in arsenic and fluoride removal. Modification of clay minerals does not alter their phyllosilicate structure. However, it improves the basal spacing and specific surface areas of the clay minerals. The modification is mainly through ion exchange between the basic exchangeable cations such Na+ , K+ , Mg2+ and Ca+ with the guest species. The effectiveness of the clay minerals in adsorption of arsenic and fluoride is mainly influenced by the surface charges which are determined by the solution pH. At acidic pH the sorption is higher due to the fact that the surface is positively charged which influences the sorption of anions through electrostatic attraction. Conversely, at alkaline pH the sorption is low due to electrostatic repulsion. Recently, the attention is mainly on modification of clay minerals to enhance their application in various fields. However, little has been done on evaluating the chemical stability and the cost effectiveness of modifying clay minerals. As such future studies should be directed toward evaluating the chemical stability of modified clay minerals especially those applied for drinking water treatment. Also, studies should elaborate the cost effectiveness for modifying clay minerals with chemical species.
