Synthesis of hydroxyapatite-iron oxides nanocomposite for fluoride adsorption in groundwater samples

Fluoride contamination in groundwater is a major environmental concern due to its serious health implications. Among various remediation strategies, hydroxyapatite-based adsorbents are promising due to their high affinity for fluoride ions. This study evaluates the performance of two hydroxyapatite-iron oxide nanocomposites, Hap-IONp-1 and Hap-IONp-2, synthesized via a hydrothermal method with varying iron oxide content. The materials were characterized by X-ray diffraction, scanning electron microscopy, and Fourier-transform infrared spectroscopy. Kinetic and equilibrium adsorption experiments were conducted in model solutions, with the pseudo-second-order and Freundlich models providing the best fit. The influence of pH on fluoride removal was also assessed. Further evaluation was performed using three groundwater samples from different regions of Argentina, all exceeding the World Health Organization limits for fluoride and arsenic. The samples differed in fluoride concentration, hardness, alkalinity, and other dissolved species. Adsorption experiments over six reuse cycles revealed that Hap-IONp-1, with a lower iron oxide content, consistently achieved higher fluoride removal, particularly in waters with moderate fluoride levels and high hardness. However, the efficiency of both materials decreased in samples with high alkalinity and fluoride concentrations, likely due to ion competition. These findings highlight the importance of water chemistry in adsorbent selection and support the development of tailored materials for effective fluoride remediation in natural waters.