**5.2 Clay adsorbents**

Soils and clays present high prospects of application in water defluoridation. This is mainly because they are almost always: (1) available in natural abundance; (2) stable and usable in different water conditions; (3) have high adsorption capacities; (4) easy to prepare; and, (5) are eco-friendly [80]. The specific reactions F adsorptions at the soil surfaces are heterogeneous and the particular choice of soil adsorbent for water defluoridation is controlled by its known adsorption capacities, availability, and the desired physicochemical properties. Consequently, soils are among the most studied matrices for water defluoridation.

#### *Water Defluoridation Methods Applied in Rural Areas over the World DOI: http://dx.doi.org/10.5772/intechopen.105102*

Nonetheless, minerals, which have attracted the highest attention for water defluoridation research include: apatite, calcareous minerals, diatomite, attapulgites, and ferric minerals. The apatite minerals because are known to control the natural exchange of F in soil-water solutions in the environment [81]. Fan et al. [82] evaluated the capacity of hydroxyapatite, fluorspar, calcite, and quartz for water defluoridation. They found that F adsorption capacities for the minerals decreased from the apatite to quartz thus: hydroxyapatite > fluorspar > quartz activated using ferric ions > calcite > quartz. o, many workers have studied the capacity of apatite to enhance limestone, for example, and reported a maximum F adsorption capacity of 3.83 mg/g [83]. Else, it is often found that many calcareous minerals exhibit limited adsorption capacities for F [84]. In a study conducted by Kumar and Gupta [85], the authors investigated fluoride adsorption onto activated diatomite and found that the maximum defluoridation capacity of the mineral was 71.97 mg/kg. Other researchers have, however, reported a more enhanced defluoridation capacity of 51.1 mg/g for the [86]. These have also been collaborated most recently by Taabu et al. [87].

The adsorption of F onto modified attapulgite has been studied widely [88, 89]. The F adsorption capacity for the mineral approximates 24.55 mg/g. Hamdi and Srasra [90] found that water defluoridation capacities for some Tunisian soils was 55.8071.94 mg/g. However, other soils including ferrihydrite and kaolinite-ferrihydrite associate [91], ferric polymineral [92], lateritic minerals [93], clays [94], zeolites [95] and siliceous minerals [96] have been evaluated. Clearly, the capacity of clays to sorb F is greatly varied between the minerals and it is controlled mainly by their mineralogy and the operative conditions [80].

### **5.3 Ion-exchange resins**

Defluoridation protocols based on the ion-exchange technique use charged anion resins that substitute anions in the substrate structure (normally chlorides) for F ions in the water [97]. The resin exchange sites are made of adsorbed cations (usually calcium) [98]. The natural polymeric organic resins, chitin/cellulose composites, are among the adsorbents with the greatest potential for water defluoridation. The use of natural polymeric materials has additional advantages because they are readily available in nature. Subsequently, natural polymers have been studied with varying adsorption efficiencies such as for chitosan (8.10 mg/g) [99], nanocellulose/polyvinyl alcohol composite, agglutinin derived from *Strychnos potatorum L.* seed (11.363 mg/g) [100] and chitosan-zirconia-ferrosoferric oxide composites (17.81 mg/g) [101]. Much higher adsorption capacities of 45.45 mg/g and 52.63 mg/g have, however, been reported for gamma degraded chitosan-Fe(III) beads [102] and for zirconia modified chitosan beads [103], respectively. Furthermore, a sorption potential of 48.78 mg/g has been reported for β-cyclodextrin grafted upon nanoscale titania surfaces [104].

The main challenge of the use of natural polymers in water purification is their liability to chemical and biological degradation. Also, the F ions tend to bind irreversibly into the exchange sites of the resins degrading the membranes. Then it is found that F removal using ion-exchange resins is often limited by low ionic selectivity [105]. Plus, commercial resins are expensive and require continuous regeneration and the spent adsorbents are non-biodegradable, they persist in the environment and must be disposed of very carefully.
