Kinetics and mechanistic interpretation of fluoride removal by nanocrystalline hydroxyapatite derived from Limacine artica shells

The present investigation describes the synthesis and characterization of nanocrystalline adsorbent, hydroxyapatite (HAp) from Limacine artica shells and its application towards fluoride removal from aqueous solution. The high content of calcium carbonate (92.9%) analyzed by inductively coupled plasma optical emission spectrometry (ICP-OES) confirmed the snail shell to be a good precursor for HAp synthesis. The X-ray diffraction (XRD) and Braunauer-Emmet-Teller (BET) analyses supported the formation of a nanocrystalline material with a surface area and pore diameter of 152.45m(2)/g and 4.2 nm, respectively. The observed decrease in crystallite size after fluoride ion adsorption was attributed to the dispersion of agglomerated HAp particles due to H-bonding between adsorbed fluoride ions with bulk water molecules. The good fitting of Lagergren pseudo second-order kinetics and Elovich model indicated the rate limiting step to be of chemisorptions type. The adsorption data followed Langmuir, Freundlich and Dubinin-Radushkevich (D-R) isotherms satisfactorily supporting both monolayer and multilayer physisorption at HAp-water interface. Thermodynamic study revealed the adsorption process to be spontaneous and endothermic in nature. Electrostatic attraction, ion exchange and hydrogen bonding are mainly responsible for the adsorption mechanism. The used HAp could be reused for fourth cycle without a significant loss of adsorption ability.