Tanvir Arfin
[29][96] studied the efficiency of fluoride removal by using different adsorbent materials such as rice husk, eggshell, activated carbon, and rice husk ash, showing that the efficiency of fluoride removal was 40% by rice husk while the efficiencies for the other matrices were 61.8, 53.4, and 42.5% respectively. A wide range of treatment methods have been proposed and investigated in literature to improve the adsorption efficiency
[30][31][97,98]. Deshmukh et al.
[32][99] prepared rice husk by chemical impregnation with nitric acid, followed by physical activation to investigate the removal of fluoride in aqueous solution. They reported that the maximum fluoride removal was 75% at pH = 2, with the fluoride uptaking capacity decreasing by increasing pH in the range 2–10. This finding was confirmed by Ahmaruzzaman and Gupta
[33][37]. Therefore,
resea
rcheuthors emphasized that it is preferable to carry out the fluoride adsorption process at lower pH values. Moreover, fluoride adsorption increases with time, reaching equilibrium within 1–1.5 h. Some
researcheauthors
[33][34][37,100] investigated the removal of fluoride by modifying rice husk ash by coating with aluminum hydroxide. The
res
earchtudy showed promising results of fluoride removal capacity, and the adsorption was determined to be 910 mg/g. Other
researcheauthors investigated the fluoride removal capacity by using silica nano adsorbent modified by rice husk
[35][101]. The results illustrated that the fluoride adsorption capacity is 12 mg/g. By way of comparison with rice husk, according to the experiments of Tembhurkar and Dongre
[36][102], commercially available powdered activated carbon (2.0 g/100 mL) allowed a 94% removal of fluorides under optimal conditions (contact time: 120 min, pH: 2). As reported by Tomar and Kumar
[37][103], some coal-based adsorbents (lignite, fine coke, and bituminous coal) showed an adsorption capacity in the range of 6.9–7.4 mg/g for the removal of fluoride. A pH between 6 and 12 was more favorable in the case of lignite, while fine coke and bituminous coal showed a greater efficiency at acid pH.