Efficient defluoridation of high-fluoride water using rare earth-based adsorbents: adsorption performance, mechanism, and kinetics

This study investigated the adsorption behaviour of fluoride ions from fluorine-containing solutions at a defined concentration using Ce³⁺ and hydrogen peroxide. The influences of adsorption time, solution pH, adsorbent dosage, initial fluoride ion concentration, and coexisting ions on adsorption efficiency were systematically assessed. The results demonstrated that the adsorbent exhibited high adsorption performance at ambient temperature, achieving a saturated adsorption capacity between 114.47 and 118.43 mg/g. The synergistic action of Ce³⁺ and H₂O₂ provided dual adsorption-precipitation pathways, in which thermodynamically stable CeF₃ formed through ion exchange while surface Ce⁴⁺-OH sites produced by oxidation further captured fluoride via ligand exchange. The adsorption kinetics conform to the pseudo-second-order model, and the equilibrium data fit both the Langmuir and Freundlich isotherm models. Sulphate ions promoted fluoride removal, whereas chloride and nitrate ions had little effect; however, bicarbonate and carbonate ions strongly inhibited this process. These findings demonstrate that the Ce³⁺/H₂O₂ system offers a robust, cost-effective, and regenerable approach with good tolerance to coexisting anions. Overall, this work not only elucidates the dual mechanism of Ce-based defluoridation but also provides a practical strategy for developing rare-earth-based adsorbents for sustainable and high-efficiency water purification.