Depth analysis of the mechanism for fluoride removal by adsorption of schwertmannite: Experiments and theoretical calculations

Due to fluoride pollution in the global water, the development of economical and efficient adsorption materials is urgent. In this study, schwertmannite were successfully synthesized by a low-cost chemical method, and the complex mechanism of fluoride adsorption process was analyzed in depth by combining experiments and theoretical calculations. The fluffy cauliflower-like morphology contributed to increasing the specific surface area of adsorbent (193.4 m2/g) and exposing active sites. The effect of pH on the adsorption process was discussed in conjunction with density functional theory. The experiment found that higher temperature improved fluoride removal, best performance at 318 K with 40.5 mg/g. In addition, fitting results showed that the Langmuir and pseudo-second-order models could better describe the adsorption behavior. Coexisting anions and adsorption regeneration experiments demonstrated the high selectivity and reusability of schwertmannite. Fourier transform infrared spectroscopy and X-ray photoelectron spectroscopy were used to determine the mechanism of adsorption and fluoride removal. Combined with theoretical calculations, three adsorption configurations and the corresponding binding energies were obtained. The stability of different adsorption modes specifically showed OH-F > SO4-HF > OH-HF. Density of states analysis revealed the formation of resonance peaks and stable bonding interactions, further confirming the chemical adsorption mechanism. It provides the foundation for future industrial applications by using schwertmannite to deal with fluoride.