Adsorption of Fluoride and Chromium by Chitosan-clay Composite

This chapter describes the synthesis, characterization, and application of chitosan-montmorillonite microsphere composites for the removal of fluoride and chromium from aqueous solution. The basic structure of clays and composite clays is described and reviewed, with emphasis on their adsorption properties for anions present in water, such as fluoride and chromate. A practical case study is presented for the synthesis of low-molecular-weight chitosan and a montmorillonite-type clay for adsorption. using pH-variation gelation to obtain microspheres. The results of the physicochemical characterization of the clay show the interaction of chitosan between the nanoclay sheets, due to the electrostatic attraction between the hydroxyl groups of the clay and the protonated amine of the chitosan. The zero charge point for the composite clay was 7.3, indicating a neutral surface at that pH value. The adsorption equilibrium results at pH 5, 7 and 9 at T = 25 °C demonstrated the positive adsorption of fluoride and chromate ions on nanoclay-chitosan compounds by decreasing the pH of the solution. The maximum adsorption capacity obtained for an equilibrium concentration of 40 mg/L was 10 mg/g and 90 mg/g for fluoride and chromate, respectively. These results demonstrate a novel, simple, and easily applicable adsorbent. The results regarding the effect of temperature (15, 25, and 35 °C) on the adsorption equilibrium and adsorption capacity of the composite clay microspheres suggest a high chromate adsorption energy, achieved through surface complexation, creating ideal conditions for the removal of this element from aqueous solution.