Fluoride Removal from Water by a La/Al Bimetallic Coordination Composite: Insights into Efficiency and Mechanisms

Elevated fluoride levels in aquatic systems have emerged as a critical public health concern, with chronic exposure linked to dental fluorosis, skeletal deformities, and compromised neurological function. This study introduces a La/Al bimetallic coordination composite adsorbent, synthesized by coordinating lanthanum and aluminum ions with terephthalate (BDC) linkers, and denoted as La/Al-BDC-(3:1). By meticulously adjusting the molar ratio between these two metals, the research aimed to improve the efficiency of fluoride ion (F−) removal from water. Batch adsorption experiments were conducted using various initial concentrations of F−, varying dosages of adsorbent, and under multiple pH conditions. The results demonstrated that La/Al-BDC-(3:1) showed a high adsorption capacity of 278.50 mg/g, slightly higher than that of the single-metal La-BDC (258.40 mg/g), while offering improved pH robustness, selectivity, and reduced lanthanum consumption through Al incorporation. Additionally, the material demonstrated strong performance across a broad pH range (ranging from 5 to 9) and significant affinity for F− ions, even when competing anions were present (except PO43− and HCO3−). After five adsorption–desorption cycles, La/Al-BDC-(3:1) retained a high removal efficiency, confirming its reusability. The adsorption process was well described by the Langmuir isotherm model (R2 > 0.99) and pseudo-second-order kinetics (R2 > 0.99), consistent with monolayer adsorption. XPS and FTIR analyses indicated that F− uptake involved ligand exchange and inner-sphere La–F coordination, with electrostatic interactions also contributing to the adsorption process. This research introduces a new methodology aimed at optimizing La/Al coordination composite adsorbents for the removal of fluoride from aqueous solution.