## 3 Adsorption Process for Defluoridation: General Principles  
Adsorption is a prominent technique among the various methodologies available for water defluoridation, being one of the most efficient in terms of cost, design, and operation. It is environmentally friendly and offers reliable alternatives with satisfactory results, and is therefore widely used [1; 3; 16; 19; 36; 37; 38; 39; 40; 41; 42]. Moreover, its additional benefits are the absence of secondary byproducts and hazardous secondary pollutants [43]. [PERSON] et al. [44] state that adsorption is the most widely used method for fluoride removal from drinking water, and [PERSON] et al. [3] further note that adsorption is applicable for fluoride removal even at low concentrations. Adsorbent surfaces are often physically and/or chemically heterogeneous and binding energies can vary greatly from one site to another. Thus, one seeks to promote physical adsorption or physisorption, which involves van der Waals forces, and is best suited for a process where the capacity of the adsorbent must be regenerated. Organic and inorganic constituents present in the aqueous phase are efficiently removed by adsorption technology [45]. The efficiency of the process is highly dependent on the physicochemical properties of the adsorbent materials [18]. Thus, although adsorption is a practical and low-cost method for fluoride removal, it needs a mechanically resistant, selective and high-capacity fluoride removal material [14]. Among the factors that influence adsorption, [PERSON] and [PERSON] [2] cite the surface area of the adsorbent, temperature, pH, nature of the adsorbent and solute, and the stirring speed. In studies based on the adsorption process to F\({}^{-}\) removal, it is desirable to have knowledge of these variables and their influence on the adsorption capacity in order to maximize the contaminant removal efficiency [4; 8; 46; 47; 48]. In addition to these factors, the effect of interfering ions should also be considered in F\({}^{-}\) removal studies. Naturally, water contains anions such as chloride (Cl\({}^{-}\)), nitrate (NO\({}_{3}\)\({}^{-}\)), sulfate (SO\({}_{4}\)\({}^{2-}\)), bicarbonate (HCO\({}_{3}\)\({}^{-}\)), carbonate (CO\({}_{3}\)\({}^{2-}\)), and phosphate (PO\({}_{4}\)\({}^{3-}\)), in addition to metal cations, which can compete in the adsorption process and form stable complexes with F\({}^{-}\). [PERSON] et al. [48] studied the response surface methodology (RSM) for the removal of fluoride on Brushite. The authors reported four important process parameters including initial fluoride concentration (40-50 mg L\({}^{-}\)), pH (4-11), temperature (10-40 \({}^{\circ}\)C) and Brushite dose (0.05-0.15 g). Finding the optimum conditions, a maximum fluoride removal of 88.78% was achieved. [PERSON] et al. [49] studied five alumina's synthesized at different pH and calcination temperatures, and the surface properties and defluoridation performance of alumina were investigated. The authors reported batch experiments and observed than the weak acidic pH favored the fluoride adsorption. [PERSON] et al. [50] investigated the adsorption of fluoride from an aqueous solution using eggshell pretreated with plasma technology and compared to untreated eggshell. The maximum fluoride removal efficiency increased from 59.88% with untreated eggshell to 90.34%, which was due to an increase in adsorption sites after the surface modification process of the eggshell.