**3.1. Arsenic removal**

Mohapatra etal. [32] reported the adsorption of arsenic from water by Kaolinite, Montmorillonite and illite and its dependence on solution pH and temperature. The results indicated that As(V) adsorption is dependent on pH and optimum adsorption capacities of 0.86, 0.64 and 0.52 mg/g were achieved at pH 5 for kaolinite, montmorillonite and illite, respectively. Furthermore, it was found that increasing temperature decreases the adsorption efficiency indicating that the interaction between As(V) was exothermic. This was observed in both evaluated clay soils. When evaluating the mechanistic aspect for As(V) adsorption onto kaolinite, montmorillonite and illite, authors ascribed it to inner sphere surface complexion and strong specific ion adsorption.

The adsorptive properties of five different clay minerals namely, R-clay, Y-clay, E-clay, A-clay and Rhassoul-clay found in Morocco for As(V) removal has been evaluated by Bentahar et al. [33]. The results showed that As(V) adsorption was more favorable at acidic pH and clays containing higher iron oxide content had higher arsenic binding affinity. The maximum adsorption capacity observed was 1.076 mg/g.

#### **3.2. Fluoride removal**

The effectiveness of raw clay minerals in fluoride removal has been reported by several authors. Kau et al. [34] evaluated the fluoride retention by kaolin clay. It was observed that the degree of fluoride sorption by kaolin is dependent on solution pH and available fluoride 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.

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>

The presence of arsenic and fluoride has attracted worldwide attention due to their toxicity in human health after a prolonged period of exposure [30]. Excessive intake of fluoride can lead to dental and skeletal fluorosis while the intake of arsenic can lead to various types of cancer. The World Health Organization (WHO) has set the standard of fluoride and arsenic in water for human consumption at 1.5 and 0.01 mg/L, respectively [31]. Recently much attention is being driven into developing a technique that can be used to remove excess of fluoride and arsenic in drinking water to acceptable levels. Adsorption using clay mineral has emerged to be the promising technique and several studies pertaining to removal of fluoride and arsenic using clay minerals has been reported in the literature. This is because clay minerals are abundantly available in nature at little or no cost. Furthermore clay minerals are more chemical and mechanical stable, poses larger

specific surface are and higher cation exchange capacity making them good adsorbents.

Mohapatra etal. [32] reported the adsorption of arsenic from water by Kaolinite, Montmorillonite and illite and its dependence on solution pH and temperature. The results indicated that As(V) adsorption is dependent on pH and optimum adsorption capacities of 0.86, 0.64 and 0.52 mg/g were achieved at pH 5 for kaolinite, montmorillonite and illite, respectively. Furthermore, it was found that increasing temperature decreases the adsorption efficiency indicating that the interaction between As(V) was exothermic. This was observed in both evaluated clay soils. When evaluating the mechanistic aspect for As(V) adsorption onto kaolinite, montmorillonite and illite, authors ascribed it to inner sphere surface complexion and strong

The adsorptive properties of five different clay minerals namely, R-clay, Y-clay, E-clay, A-clay and Rhassoul-clay found in Morocco for As(V) removal has been evaluated by Bentahar et al. [33]. The results showed that As(V) adsorption was more favorable at acidic pH and clays containing higher iron oxide content had higher arsenic binding affinity. The maximum

The effectiveness of raw clay minerals in fluoride removal has been reported by several authors. Kau et al. [34] evaluated the fluoride retention by kaolin clay. It was observed that the degree of fluoride sorption by kaolin is dependent on solution pH and available fluoride

**3. Application of clay in arsenic and fluoride removal**

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

**3.1. Arsenic removal**

specific ion adsorption.

**3.2. Fluoride removal**

adsorption capacity observed was 1.076 mg/g.

/g.

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 is often attributed to abundance of OH− 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, surface charges and also the chemical composition of the clay [36].
