ABSTRACT
162 The section of the study reported in this chapter, explored the possibility of modifying the physico-chemical properties of bauxite, a locally available material in many countries including Ghana, by thermal treatment and an aluminum coating, for water defluoridation. The synthesis of granular aluminum-coated bauxite (GACB) as a fluoride adsorbent is described. The study mainly focused on investigating the effects of varying synthesis process conditions on the defluoridation efficiency of GACB, using series of batch adsorption experiments. GACB performed better than the raw bauxite (RB) base material, and was able to reducefluoride concentration in modelfrom 5 ± 0.2 mg/L to < 1.5 mg/L (WorldHealth Organization (WHO) guideline). Based on nonlinear Chi-square (χ 2 ) analysis, the best-fitting isotherm model for the fluoride-GACB system was in the order: Freundlich > Redlich-Perterson ~ Langmuir > Temkin. The fluoride adsorption capacity of GACB (q^ax = 12.29 mg/g) based on the Langmuir model, was found to be either comparable or higher than the capacities of some reported fluoride adsorbents. XRD characterization suggested aluminum was incorporated in GACB in the form of gibbsite by the applied coating process. The results also suggested that the gibbsite incorporated by the Al coating procedure, was more effective forfluoride uptake compared to the intrinsic (natural) gibbsite content of the raw bauxite base material. Presumably the crystal structure of gibbsite incorporated by the Al coating possesses a higher percentage of specific surface area of edge faces, hence more reactive sites for anion adsorption, than the intrinsic gibbsite. Thermal pre-treatment of raw bauxite (RB) prior to the Al coating, contributed significantly to an increase of the fluoride adsorption efficiency of the produced GACB. The applied procedures in this study could therefore be a useful approach for synthesizing an effective fluoride adsorbent using bauxite, a locally available material. Kinetic and isotherm analysis as well as FTIR and Raman analysis suggested the mechanism of fluoride adsorption onto GACB was complex, and involved both physical adsorption and chemisorption processes.
