Eco-Friendly Synthesis of Spherical Lanthanum-Doped Hydroxyapatite from Phosphogypsum Waste: A High-Efficiency Strategy for Fluoride Removal in Aqueous Solutions

ABSTRACT: To address the environmental challenges posed by massive phosphogypsum (PG) stockpiles and groundwater fluoride contamination, this study developed an eco-friendly strategy for synthesizing lanthanum-doped hydroxyapatite (La-PGHAP) from PG waste via an acid precipitation-hydrothermal method. The synthesized La-PGHAP exhibited a spherical morphology, high crystallinity, and a significantly enhanced specific surface area of 53.11 m²/g. Batch adsorption experiments revealed that pH critically influenced fluoride (F⁻) removal, with maximum adsorption capacities of 8.20 mg/g (PGHAP) and 31.98 mg/g (La-PGHAP) at pH 4. The adsorption process followed pseudo-second-order kinetics and the Langmuir isotherm model, indicating chemisorption-dominated monolayer adsorption. La doping introduced Lewis acid-base interactions through La³⁺–F⁻ coordination, improving both adsorption capacity and stability across a wide pH range (2–10). Reusability tests demonstrated that La-PGHAP retained 85.4% of its initial capacity after 4 cycles. This “waste-to-waste” approach not only repurposes PG into a high-efficiency adsorbent but also provides a sustainable solution for mitigating fluoride pollution, showcasing significant potential for industrial-scale water treatment applications.