Corbula trigona shell powder (CTSP) was modified by a hydrothermal method using phosphoric acid in order 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 (with a dose varying from 0.1 to 0.8 g) in the presence of 100 mL of groundwater contaminated by 2.20 mg/L of fluoride. Adsorption kinetics indicated that the adsorption process is governed by pseudo-second order kinetics. The reaction rate constant for fluoride removal was 0.161 g mg-1 min-1. Adsorption isotherms showed that fluoride removal was mainly controlled by physical adsorption with the maximum adsorption capacity of 4.517 mg g-1. The relatively low enthalpy value (∆adH0 < 200 kJ.mol-1) indicated that during adsorption process, very low interactions occurred between fluoride and modified-CTSP. The adsorption process was spontaneous, endothermic and irreversible in nature. The best results (89 % of fluoride removal) were recorded at pH 7.5 ± 0.1 in 175 min with a concentration of 5 g/L of modified-CTSP. From a cost-effectiveness point of view, the removal and desorption efficiency of fluoride were evaluated. The results of this study show a gradual decrease in the removal efficiency of fluoride on HAp during the different regeneration cycles. Indeed, the maximum adsorption capacity was 89.3 % during the first cycle and gradually decreased with the increase of the number of cycles. Thus, Hap3 could be used up to 5 times during fluoride removal with a loss of nearly 56 % of its adsorption efficiency. The results obtained showed that Hap derived from Corbula trigona shell waste can be a promising sorbent for groundwater defluoridation in developing countries.
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