A review on fluoride adsorption using modified bauxite: Surface modification and sorption mechanisms perspectives

Fluoride has been reported as one of the major water pollutants with myriads of health implications when present in water beyond permissible limits (>1.5 mg L-1). While the adsorption technology of treating fluoridated water remains one of the best options, the choice of suitable adsorbent remains a problem. However, shortcomings of some adsorbents such as low adsorption capacity, slow adsorption rate, narrow pH range, and high-cost make them unsustainable. Modification of adsorbents has become the most attractive method for effectively improving the treatment of fluoridated water. The objective of this work was to bring to light the surface modifications and sorption mechanisms of bauxite as a sorbent for fluoride removal. Bauxite is robust with sufficient mechanical strength and naturally abundant making it a suitable candidate over other conventional adsorbents for fluoride removal. Four main types of surface modification of adsorbents were identified; thermal activation, impregnation, functionalization, and chemical grafting. Materials such as surfactants, oxidizing agents, acids, bases, and metals could be used for surface modification. Modified bauxite demonstrated higher adsorption capacity over their unmodified ones due to the significant improvement in their surface area, pore size, pore-volume, and some structural transformations such as ordered and stable interlinked mesopores which facilitated effective chemisorption. This guarantees a stable chemical structure with more active sites and requires a minimum dose to reach equilibrium at a wider pH range (3-11). Finally, the mechanisms of fluoride sorption onto bauxite is a complex process involving intra-particle diffusion, physisorption, and chemisorption processes.