Novel hydroxyapatite-biomass nanocomposites for fluoride adsorption

Hydroxyapatite-biomass nanocomposite (HAp-C) was successfully synthesized through a combined ultrasonic/ hydrothermal one-pot method and used as a novel adsorbent to remove fluoride ions from groundwater. The obtained HAp-C composite was entirely characterized by TG, FTIR, XRD, SEM, and TEM. The point of zero charge was determined by the drift method. It was found that the HAp partially coats the lignocellulosic matrix. XRD data suggested that biomass incorporation did not affect the crystalline structure of the HAp. FTIR analysis suggests that the bond between hydroxyapatite and biomass is given by coordinating links. The fluoride adsorption on HAp-C was tested through batch assays using model solutions. The effect of contact time, initial fluoride concentration, sorbent dose, pH, and co-existing species on the adsorption capacity was investigated. The results showed that fluoride adsorption capacity increased with contact time and initial fluoride concentration, decreasing with increasing sorbent dose. The maximum experimental adsorption capacity was 10.9 mg F- g(-1 )at the maximum concentration tested in this work (80 mg L-1). The pH variation (4.5-8.5) did not significantly affect the adsorption efficiency of the nanocomposite (similar to 90%), while the presence of co-existing species significantly decreased its removal efficiency (similar to 80%). The fluoride adsorption could be fitted by the Freundlich isotherm, and the adsorption kinetic data followed the pseudo-second-order model. Besides, adsorption assays were replicated using natural groundwater samples from a rural region of the southern ChacoPampean plain (Villarino, Buenos Aires, Argentina), registering up to 77.4% removal, reaching a fluoride concentration value (1.16 mg L-1) within the permitted limits.