**5. Adsorption**

In adsorption, raw water is passed through a bed containing a material that is able to retain F by physical, chemical, or ion exchange mechanisms. Adsorption of F onto solid adsorbents occurs through [74]: external mass transfer; surface adsorption; and, intra-particle diffusion processes. The technique has gained popularity in water defluoridation because it offers satisfactory results; it is simple, affordable, and ecofriendly. Many adsorbents, including alumina, clays, polymeric ion-exchange resin, activated carbons, biosorbents, and layered double hydroxides have been studied for water defluoridation [48].

#### **5.1 Activated alumina**

Activated alumina, Al2O3, is dehydrated Al(OH)3, which is prepared by heating Al(OH)3 at 300–600°C. It was first used in water defluoridation in the US in 1952, later in many other countries including China, Thailand, India [75], South Africa, and Ethiopia [76] by the 1980s. It is now widely used in many other countries of the world [77]. Alumina has one of the highest water defluoridation efficacies. In one study, aluminum hydroxide, which is a form of hydrated alumina was reported with an exceptional F adsorption capacity of 116.75 mg/g [78]. Recently hydrated alumina modified NaA zeolite was reported to have high F adsorption capacities of 104 mg/g [79]. The use of alumina in water defluoridation is, thus, widespread [39] but it is costly, requires frequent adsorbent regeneration, and the adsorbent gets fouled easily from the dissolved solids in the water [25].
