Highly efficient Al(OH)3/HAp composite adsorbent for the removal of fluoride from drinking water: batch and column studies

Abstract

Excess fluoride in water is a significant public health concern, causing illnesses such as arthritis, dental fluorosis and skeletal fluorosis. Hydroxyapatite (Ca10(PO4)6(OH)2) has shown a promise in fluoride removal through demineralization during which fluoride ions are effectively replaced by hydroxide ions (OH−). However, performance issues with applying hydroxyapatite (HAp) remain a challenge. This study assessed the efficacy of aluminum hydroxide/hydroxyapatite (Al(OH)3/HAp) composite for fluoride removal from drinking water through batch and column experiments. The composite’s functional groups, crystalline phases, surface area, and the point of zero charge (pzc) were characterized, and its performance was evaluated in both batch and continuous experiments. During batch experiments, a 30% Al (OH)3/HAp ratio exhibited an adsorption capacity of 2.30 mg/g with a fluoride removal efficiency of 92.02%. Kinetic analysis indicated that fluoride adsorption followed a pseudo-second-order model, suggesting chemisorption, while the Redlich–Peterson isotherm confirmed heterogeneous adsorption behavior. Among the tested competing anions, the presence of carbonate ions had the most adverse impact on fluoride removal efficiency. Column experiments using simulated water containing fluoride, bicarbonate and carbonates further demonstrated that an adsorption capacity of 1.28 mg/g was obtained at 15 mL/min flow rate, 10 cm bed depth, and 10 mg/L initial fluoride concentration, with the Clark model best describing the breakthrough data. Breakthrough time decreased with increasing flow rate and initial fluoride concentration but improved with greater bed depth. This study revealed Al(OH)3/HAp as a cost-effective, efficient adsorbent for fluoride removal, addressing critical challenges in water treatment and providing insights into the adsorption behavior and mechanism of Al(OH)3/HAp composite for fluoride removal.