Defluorination by ion exchange of SO42− on alumina surface: Adsorption mechanism and kinetics

Electrostatic and complexation effects have been considered as the primary adsorption mechanisms for defluorination using aluminum based materials, while the effect of ion exchange between anions and fluorine ion has been mostly ignored, although synthesized alumina materials usually contain a large amount of anions, such as SO42−, NO3−, and Cl−. In this study, the effect of anions exchanges and its key role on defluorination were systematically investigated for adsorption by aluminas loaded with various typical anions (SO42−, NO3− and Cl−). Experimental results showed that SO42-- loading alumina had the best defluorination performance (94.5 mg/g), much higher than NO3− (45.0 mg/g) and Cl− (19.1 mg/g). The contribution ratio of ion exchange between SO42− and F− was as high as 20–60% in all potential defluorination mechanisms. By using Density Functional Theory calculation, the detailed mechanism revealed that the ion exchange process was mainly driven by the tridentate chelation of SO42− which reduced the exchange energy ( Δ F − − SO 4 2 − 4.8 eV). Our study clearly demonstrated that ion exchange between SO42− and F− is a critical mechanism in defluorination using aluminum-based materials and provides a potential alternative method to enhance the adsorption performance of modified alumina.