## 4 Influential Factors in Fluoride Adsorption
### The Effect of Adsorbent Mass on the Removal of F\({}^{-}\)
The authors obtained the maximum adsorption capacity of 1.44 mg g\({}^{-1}\) for the mass of 2.5 g L\({}^{-1}\), with which a percentage removal of F\({}^{-}\) of 90% was obtained. [PERSON] and [PERSON] [52] studied the effect of masses from 10 to 50 g L\({}^{-1}\) of the adsorbent crystalline titanium dioxide powder (TiO\({}_{2}\)) on the adsorption of F\({}^{-}\). The adsorption increased with increasing mass up to 25 g L\({}^{-1}\). For masses from 37.5 to 50 g L\({}^{-1}\), no increase in adsorption was observed because saturation was obtained. [PERSON] et al. [23] used masses of zirconium hydroxide impregnated in a cellulose matrix ranging from 6 to 12 g L\({}^{-1}\), with the removal of F\({}^{-}\) being effective in the range of 10 to 12 g L\({}^{-1}\). The authors attribute the increased adsorption to the strong electrostatic attraction between F\({}^{-}\) and the biopolymer adsorbent. Beyond 12 g L\({}^{-1}\), the active adsorption sites were saturated and there was no significant change in the percentage of adsorption. A further increase in mass (>2.5 g L\({}^{-1}\)) did not result in any increase in adsorption capacity. [PERSON] et al. [17] demonstrated that with increasing the mass of iron oxide and silver (3:1) from 0.1 to 1 g L\({}^{-1}\), the percent removal increased from 26.21 to 100%, and the adsorption capacity decreased from 26.21 to 10 mg g\({}^{-1}\). According to the authors, this happened because an increase in the amount of adsorbent provides a greater number of active sites available for binding of F\({}^{-}\). The highest adsorption efficiency of F\({}^{-}\) was obtained with the mass of 1 g L\({}^{-1}\); however, considering the adsorbent cost, the authors adopted the use of 0.5 g L\({}^{-1}\) as it was a more economical value. [PERSON] and [PERSON] [4] used banana peel powder for the adsorption of F\({}^{-}\), varying the mass from 0.01 to 1.0 g L\({}^{-1}\). The adsorption percentage increased with increasing adsorbent mass, and the highest adsorption percentages were 69.4, 81.3, and 82.6% for BPD-1 (untreated), BPD-2 (heat-treated), and BPD-3 (calcium-coated), respectively. It was observed that, up to a certain level, higher masses resulted in higher F\({}^{-}\) removals. This was attributed to the higher availability of surface area and pore volume. [PERSON] et al. [49] studied the mass achievement of five different types of alumina in defluoridation. To reduce the F\({}^{-}\) concentration of natural water below 1.0 mg L\({}^{-1}\), the required masses of A1, A2, A3, A4, and A5 were 0.2, 0.3, 0.5, 0.5, and 1 g L\({}^{-1}\), respectively. In contrast, for the commercial Al(OH)\({}_{3}\) and Al\({}_{2}\)O\({}_{3}\) adsorbents, the mass was 5 g L\({}^{-1}\) for both, with the F\({}^{-}\) being reduced to 1.86 and 1.28 mg L\({}^{-1}\), respectively. [PERSON] et al. [90] studied the effect of the mass of Fe\({}_{3}\)O\({}_{4}\)/chitosan/Al(OH)\({}_{3}\) magnetic beads in the range of 0.05-0.25 g L\({}^{-1}\). For the masses of 0.05 and 0.10, the adsorption capacities were 23.5 and 35.5 mg g\({}^{-1}\), respectively. Thus, the adsorption capacity increased with increasing adsorbent mass. According to the authors, this occurred because the adsorption sites increased as the mass increased. For the masses of 0.15, 0.20, and 0.25 g L\({}^{-1}\), the adsorption capacities were approximately 31, 23.5, and 18.5 mg g\({}^{-1}\), respectively. Thus, the adsorption capacity decreased with increasing mass. Therefore, an excess of adsorbent reduces the adsorption efficiency, with the best mass being 0.10 g L\({}^{-1}\). [PERSON] et al. [20] found that an increase in alumina mass by acid activation in H\({}_{2}\)SO\({}_{4}\) from 2 to 26 g L\({}^{-1}\) increases the F\({}^{-}\) removal efficiency from 50.2 to 99.2%, but reduces the adsorption capacity from 10.15 to 0.32 mg g\({}^{-1}\). The optimum mass found was 14 g L\({}^{-1}\).