Performance and Mechanism of Lanthanum-Modified Zeolite as a Highly Efficient Adsorbent for Fluoride Removal from Water

Defluoridation of water is still challenging due to the fluoride pollution of both groundwater and surface water worldwide. In this study, lanthanum-modified zeolite (LMZ) was synthesized from coal fly ash and was investigated for fluoride removal from water by conducting batch and column experiments. Our results indicated that LMZ exhibited a high maximum fluoride adsorption capacity of 141.5 mg/g, with 4.1 F − ions around each La. The process of F − adsorption kinetics on LMZ followed the pseudo-second-order model, and a higher temperature increased both the capacity and the rate of adsorption. We determined that fluoride adsorption greatly depended on pH, with optimal performance being achieved within ~5.0 to ~7.0. The point of zero charge of LMZ was pH 8.8, at which only bicarbonate ions greatly affected fluoride removal. However, no competing effect was observed at pH 6.3 for all tested anions including chloride, sulfate, nitrate, bicarbonate and acetate. The adsorption mechanism was the ligand exchange of fluoride with hydroxyls on LMZ, as illustrated by the rise in pH due to fluoride adsorption and by the molecular scale spectroscopic FTIR, Raman and XPS studies. Fluoride adsorbed on LMZ was successfully desorbed using NaOH solution, and regenerated LMZ could be reused. The results of column studies showed that LMZ granulated with alginate performed well in treating F − -containing water. In conclusion, LMZ is a promising material for efficient defluoridation from water