## 4 Influential Factors in Fluoride Adsorption
### The Effect of Co-Existing Ions on the Removal of F\({}^{-}\)
[51] point out that the reduction in the adsorption capacity of F\({}^{-}\) with increasing bicarbonate concentration is due to the increase in pH in the solution. To [PERSON] et al. [62], the adsorption mechanisms of phosphate cause this ion to be strongly adsorbed on metal oxides and high valence hydroxides being therefore able to compete easily with F\({}^{-}\). However, [PERSON] et al. [83], [PERSON] et al. [84], and [PERSON] et al. [63] clarify that considering the typical range of natural phosphate concentration in groundwater (ranging from 0 to 5 mg L\({}^{-1}\)), is very low, hence the effect of this ion on F\({}^{-}\) adsorption should hardly be a limiting factor for groundwater defluoridation. According to [PERSON] et al. [19] and [PERSON] et al. [90] HCO\({}_{3}\)\({}^{-}\), CO\({}_{3}\)\({}^{2-}\), and PO\({}_{4}\)\({}^{3-}\) significantly affect F\({}^{-}\) removal because these ions are hydrolyzed, leading to an increase in pH value. As a result, more OH\({}^{-}\) ions can compete with F\({}^{-}\) for the active adsorption sites, which results in decreased adsorption capacity. On the other hand, the NO\({}_{3}\)\({}^{-}\), SO\({}_{4}\)\({}^{2-}\), and Cl\({}^{-}\) ions interfered the least with F\({}^{-}\) removal. [PERSON] et al. [62] clarifies that sulfate competes with F\({}^{-}\) better than Cl\({}^{-}\) and NO\({}_{3}\)\({}^{-}\). Nitrate and chloride, on the other hand, are unable to compete well with F\({}^{-}\) when they are at equal concentrations. According to the authors, this order of anion competition is like that reported in other studies. According to [PERSON] et al. [5], anions that are not specifically adsorbent, such as nitrate and chloride, do not interfere with the adsorption process. For [PERSON] et al. [92], SO\({}_{4}\)\({}^{2-}\) does not significantly reduce the adsorption of F\({}^{-}\), due to its larger ionic radius (2.30 A). Chloride and nitrate do not significantly reduce the adsorption of F\({}^{-}\) because these two ions have a lower binding affinity for the active sites. In addition, these anions have a larger ionic radius than F\({}^{-}\). [PERSON] et al. [93] investigated the role of ZnCl\({}_{2}\)/FeCl\({}_{3}\)-rice husk-modified biochar (Zn-BC and Zn/Fe-BC) in treating F\({}^{-}\) contaminated surface and groundwater under the influence of varying solution chemistry, co-existing ions, and biochar-amended through column transport experiments. Modified biochar showed maximum F\({}^{-}\) adsorption, 99.01% and 91.90% using Zn/Fe-BC and Zn-BC, respectively, compared to 85.87% using raw biochar (R-BC). They also found that the increased salt strengths led to reduced electrophoretic mobility of biochar particles, i.e., biochar-biochar particles attract each other and increase the hydrodynamic diameter, which ultimately reduces the active sites on biochar for F\({}^{-}\) adsorption. The authors argue that co-transport and deposition of biochar and F\({}^{-}\) in saturated porous media revealed lower mobility of biochar, and maximum F\({}^{-}\) adsorption was observed at 10 mM salt strength. Biochar transport is governed by electrostatic interactions, whereas F\({}^{-}\) transport mainly occurs through chemisorption. [PERSON] et al. [94] studied bovine bone meal (BBM) used as a green adsorbent for Cd\({}^{2+}\) and F\({}^{-}\) adsorption in single and binary solutions. The results showed that with the increase in pH, the uptake ability of BBM towards Cd\({}^{2+}\) and F\({}^{-}\) showed an upward and downward trend, respectively, suggesting that electrostatic interaction was an important mechanism. Cl\({}^{-}\) and NO\({}_{3}\)\({}^{-}\) had little effect on Cd\({}^{2+}\) and F\({}^{-}\) adsorption, while H\({}_{2}\)PO\({}_{4}\)\({}^{-}\) enhanced adsorption capacities. Upon analyzing the cited articles, a clear consensus emerges, highlighting that HCO\({}_{3}\)\({}^{-}\), CO\({}_{3}\)\({}^{2-}\), and PO\({}_{4}\)\({}^{3-}\) ions exert the most pronounced impact on the adsorption of F\({}^{-}\), leading to a reduction in adsorption capacity.