Enhanced removal of fluoride from groundwater using biosynthetic hydroxyapatite modified by bimetallic (La–Fe or La–Al) hydroxides

Long-term exposure to groundwater contaminated with excessive fluoride (F−) can cause irreversible damage to ecosystems and human health. It is necessary for searching economical and reliable solutions to eliminate excess F− within aquatic environments. Herein, the doping technique was applied to modify the biosynthetic hydroxyapatite, and the maximum defluoridation efficiency (90.37%) was obtained in the La@HAP system. The introduction of Fe and Al species prominently reduced the dosage of La species while ensuring the ideal defluoridation effect. At optimal conditions (preparation parameters: biosynthetic time 12 h, pH 7.0, and dosage of La–Fe and La–Al 2 mM; operation parameters: pH 7.0, dosage 0.3 g L−1, and F− content 3.0 mg L−1), the average defluoridation efficiencies of La–Fe@HAP and La–Al@HAP reached 63.42% and 79.79%, respectively. Kinetic results indicated that the F− adsorption on both bimetallic composite adsorbents was better fitted with the pseudo-second-order model, and a higher initial F− content inhibited the reaction rate. Compared with La–Fe@HAP, La–Al@HAP exhibited lower sensitivity to coexisting anions and humic acid, and higher selectivity to F−. Electrostatic interaction and ligand exchange were the main mechanisms for enhancing the adsorption properties of biosynthetic hydroxyapatite, and F− removal was principally depended on the La species (La > Al > Fe). This study provides inspiration for saving the usage of rare earth metals and designing high-performance and low-cost adsorbents with low pH dependence.