Established and potential adsorption media for fluoride

Adsorption technology is frequently used as a robust technique to remove water-soluble ions that are detrimental to human health from aqueous solutions, especially when these ions exist in low concentrations. Thus, a lot of studies have been reported in literature on the use of various adsorbents for fluoride removal from drinking water. The studies have mainly been motivated by the need to have alternative adsorbents that are low in cost, have local availability, require little processing and are superior in performance. Synthetic adsorbents have good capacities for fluoride but are always expensive, while natural materials that are available in large quantities or certain wastes from agricultural or industrial concerns may potentially be low-cost materials. An overview of some of the adsorbents that have been reported in literature over the last two decades are given below. 

Ghorai and Pant [39] Locally (India) available AA 2.41 Batch 

HF is weakly ionized (pH 3.2), and soluble alumino-fluoro complexes are formed resulting in the presence of aluminum ions in the treated water and lowering of the active sites. At near neutral pHs, the uptake of fluoride is maximum. Assuming that the pHpzc of AA is about 8-9 as reported in several literatures, then at near neutral pHs the active sites consist of = AI-OH (protonated) and = AI-OH (non-protonated) aluminol sites. The interaction between fluoride and the protonated aluminol sites leads to the formation of inner-sphere complexes and elimination of water. The reaction can be represented by 

The protonated aluminol sites are the most effective fluoride sorption sites and are usually responsible for the rapid kinetics due to coulombic attraction between the positively charged sites and the negatively charged fluoride species. The reaction with non-protonated sites involves ligand exchange, leads also to the formation of inner-sphere complexes, releases hydroxyl ions, is slow and characterized by a higher activation energy. 

Further increase in pH beyond pHpzc is expected to enhance electrostatic shielding of the active sites and to reduce their number and activity. This argument explains why AA is reported to perform poorly at pH pHpzc. 

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