Adsorption performance and mechanism of Y-La modified serpentine for F⁻ removal from water

In order to overcome the limited active sites and low adsorption capacity of natural serpentine (Srp), a bimetallic Y-La modified serpentine adsorbent (Y-La/Srp) was synthesized via a co-precipitation method for fluoride (F⁻) removal from aqueous solutions. The optimal adsorbent performance was achieved at a Y/La atomic molar ratio of 2 to 1 (Y₂-La₁/Srp). Using state-of-the-art characterization techniques, the surface properties of Y₂-La₁/Srp and the mechanisms governing F⁻ adsorption were elucidated, and its efficiency for fluoride removal from aqueous systems was systematically assessed. The results showed that under an initial F⁻ concentration of 10 mg·L⁻¹, an adsorbent dosage of 0.4 g·L⁻¹, pH 4, and a temperature of 298 K, Adsorption equilibrium was achieved within 2 h, delivering an adsorption capacity of 23.78 mg·g⁻¹ and a fluoride removal efficiency of 95.11%. At pH 4 and 318 K, the maximum adsorption capacity obtained from isotherm fitting reached 205.34 mg·g⁻¹. The adsorption process followed pseudo-second-order kinetics and was well described by the Freundlich isotherm model, indicating a spontaneous, endothermic process dominated by multilayer chemisorption. Among coexisting anions, PO₄³⁻ and SO₄²⁻ exerted the most pronounced inhibitory effects. Compared with pristine serpentine, Y₂-La₁/Srp exhibited a larger pore volume and average pore size. After four adsorption–desorption cycles, the material retained 71.84% of its initial fluoride removal efficiency. Fluoride adsorption on Y₂-La₁/Srp was governed by electrostatic attraction, ion exchange, and surface complexation. The above results indicate that Y₂-La₁/Srp has a strong affinity for F⁻ in water and is a promising adsorbent for fluoride removal.