The Fluoride Adsorption Isothermal Studies of Activated Alumina Modified with Different Materials: A Critical Review

Fluoride in drinking water has become a global problem that has a profound effect on teeth and bones, fostering various health problems. Adsorption is a potential defluoridation technique because of flexibility, cost-effectiveness, environmental friendliness, simplicity in design, relative ease of operation, and capability of producing high water quality. Although activated alumina is an appropriate adsorbent, it has a narrow favorable pH range, a tendency to form toxic aluminum fluoride complexes, and the problem of aluminum metal leaching. This article critically reviews the applicability of activated alumina and its modification by metal oxides, rare earth elements, organic materials, alkaline earth metals, and acid treatment. The effect of process parameters like pH, contact time, adsorbent dose, initial fluoride concentration, and the presence of coexisting ions on the adsorption capacity of fluoride ions is discussed. The adsorption reaction rates were discussed by fitting various rate models into the experimental data and the model equations. The adsorption isotherm models like Langmuir, Freundlich, Temkin, and Dubinin-Radushkevich tested on the adsorption equilibrium data to identify the best fit model for adsorption isotherm are discussed in this chapter. The chapter finally discusses the advantages, disadvantages, and future prospects of all the adsorbents in order to improve their fluoride removal capacity.

Keywords: Adsorption, Activated alumina, alumina modification, Defluoridation, Isotherm models, purification, Kinetic models.