## 4 Influential Factors in Fluoride Adsorption
### The Influence of pH on the Adsorption of F\({}^{-}\)
The authors further argue that AA can be negatively affected by hydroxyl ions at high pHs, which resulted in its rather narrow optimal pH range of 5 to 6. [PERSON] et al. [83] demonstrated that a sulfate-doped Fe\({}_{3}\)O\({}_{4}\)/Al\({}_{2}\)O\({}_{3}\)-based composite adsorbent is positively charged for pH values \(<\) pH\({}_{\rm PZC}\), which benefits the adsorption of F\({}^{-}\). Thus, the adsorbent exhibited a high adsorption efficiency (from 90 to 70%) over a wide pH range (4 to 10). For pH \(>\) pH\({}_{\rm PZC}\), the surface of the nanoparticles was negatively charged, which tends to repel the F\({}^{-}\) ions, reducing adsorption. Consequently, at pH \(>\) 11.2 the removal efficiency decreased considerably. The low adsorption efficiency observed at pH \(<\) 4 was attributed to the formation of HF, which reduced the electrostatic attraction between the F\({}^{-}\) and the adsorbent surface. [PERSON] et al. [20] studied the effect of pH on the removal of F\({}^{-}\) by alumina by acid activation in H\({}_{2}\)SO\({}_{4}\) (AAA). The authors observed that the extreme ranges of lower and higher pH were unfavorable for the adsorption of F\({}^{-}\) by AAA. Very low pH promotes the formation of weak hydrofluoric acid as well as AlF\({}^{2+}\) and AlF\({}^{+}\) complexes that lead to decreased F\({}^{-}\) adsorption. In the basic region the reason for reduced F\({}^{-}\) adsorption is the competitive environment between F\({}^{-}\) ions and OH\({}^{-}\) to adsorb onto AAA. Both adverse effects were negligible in the pH range of 6-7, which contributed to the maximum adsorption of F\({}^{-}\) on AAA, which was approximately 97%. Thus, pH 6.5 was the optimal pH. The analysis of 56 articles reveals a significant pH dependency in the adsorption process of F\({}^{-}\), with a stable adsorption capacity in acidic conditions favoring protonation. This results in a positively charged surface on adsorbents, enhancing attraction forces with F\({}^{-}\) ions. Notably, adsorbents for F\({}^{-}\) removal tend to be effective within a narrow pH range, with only 17 papers highlighting efficiency in alkaline conditions. Given that groundwater typically has a slightly alkaline pH, selecting an adsorbent efficient in this range is crucial for effective defluoridation. Additionally, both lower (pH \(<\) 3-4) and higher (pH \(>\) 9-10) pH ranges are unfavorable for F\({}^{-}\) adsorption. At lower pH, weakly ionized HF formation reduces adsorption, while at higher pH, negative solution charge leads to repulsion between negatively charged adsorbents and F\({}^{-}\) ions. Competition with OH\({}^{-}\) ions further diminishes adsorption. Hence, water pH must be considered when choosing an adsorbent for fluoride removal, and optimizing pH during treatment is vital for maximum efficiency. Hence, careful consideration of water pH is essential when choosing an adsorbent for fluoride removal. Optimizing the water's pH during treatment is crucial to ensure peak efficiency. Additionally, the awareness of pH variations over time and across different locations underscores the necessity for ongoing monitoring throughout the treatment process. This consistent oversight is vital to guarantee a reliable and effective fluoride removal process.