Removal of fluoride in groundwater by adsorption using hydroxyapatite modified Corbula trigona shell powder

Corbula trigona shell powder (CTSP) was modified by a hydrothermal method using phosphoric acid to remove fluoride ions from groundwater. This method was performed by keeping the Ca/P molar ratio constant at 1.67. Fourier Transform Infrared Spectroscopy (FT-IR), X-Ray Diffraction (XRD) and scanning electronic microscopy (SEM) analysis confirmed the formation of hydroxyapatite (HAP) after CTSP pre-treatment. Batch adsorption experiments were conducted in beakers containing modified-CTSP. Moreover, the modified-CTSP dose variated from 0.1 to 0.8 g, in the presence of 100 mL of groundwater. This groundwater was contaminated by 2.20 mg/L of fluoride. Adsorption kinetics indicated that the adsorption process was governed by pseudo-second-order kinetics. Furthermore, the reaction rate constant for fluoride removal was 0.161 g mg(-1) min(-1). Also, adsorption isotherms showed that fluoride removal was mainly controlled by physical adsorption with 4.517 mg g(-1) maximum adsorption capacity. But the relatively low enthalpy value (Delta H-ad(0) < 200 kJ.mol(-1)) indicated that during the adsorption process, very low interactions occurred between fluoride and modified-CTSP. The adsorption process was spontaneous, endothermic, and irreversible in nature. The best results with 89% fluoride removal were recorded at pH 7.5 +/- 0.1 in 175 min with 5 g/L modified-CTSP. These results showed that HAP derived from Corbula trigona shell waste can be a promising sorbent for groundwater fluoride removal in developing countries.