An integrated study on defluorination using CaSO4·2H2O: performance, mechanism, and control of CaF2 crystallization

Fluoride pollution in water remains a global concern, and calcium-containing materials have been widely used for fluoride removal through CaF 2 precipitation. Although the precipitation pathway has been established in calcium sulfate dihydrate (CaSO 4 ·2H 2 O)-based defluoridation, the crystallization kinetics and growth mechanism of CaF 2 in heterogeneous systems have not been quantitatively studied. Here, we combined the chemical potential gradient model with crystal growth kinetics analysis to investigate the crystallization behavior of CaF 2 during defluoridation by CaSO 4 ·2H 2 O. The growth kinetics of CaF 2 was analyzed using the apparent driving force derived from the concentration of the bulk solution. The growth mode was analyzed through kinetic fitting and predictive validation, and the results were consistent with the two-dimensional growth mechanism. The CaSO 4 ·2H 2 O surface served as a heterogeneous nucleation substrate that promoted CaF 2 crystallization. Further analysis indicated that each condition had an impact on crystallization kinetics. The dosage of CaSO 4 ·2H 2 O and stirring speed mainly affected the kinetic rate constant, while the initial fluoride concentration and pH value simultaneously affected the thermodynamic driving force and kinetic rate constant. Complementary XRD analysis and thermodynamic simulation confirmed that CaF 2 was the only crystalline product. Comparative evaluation with other calcium-based materials highlighted the excellent performance and economic advantages of CaSO 4 ·2H 2 O. This work goes beyond empirical descriptions of fluoride removal efficiency and provides quantitative mechanistic insights into the crystallization pathway of CaF 2 in calcium-based defluoridation systems. The findings contribute a kinetic and thermodynamic basis for controlling crystallization and potential recovery of CaF 2 from fluoride-containing water.