Broad-spectrum insight into precise removal of immobilized hard acids for fluoride: DFT simulations, experiments, and QICAR analysis.

Fluoride, a typical hard-base ligand in chemical processes, presents a significant ecological threat due to its enrichment in aquatic environment. In this work, the specific affinities of hard-acid metal cations toward F were evaluated from a broad-spectrum perspective with the assistance of density functional theory (DFT), and six metal cations were screened and immobilized on D412 carrier. Batch experiments and mechanism analysis validated the DFT predictions, demonstrating that the ionic size, electrostatic interaction, and electronic hardness of hard-acid cations are related to F removal efficiency. The quantitative ionic character-activity relationship (QICAR) models, incorporating fourteen cationic characteristics, identified ionic charge (Z), vertical electron affinity (VEA), and the first hydrolysis constant (|logK|) as the predominant factors governing precise F adsorption. Z and VEA, which govern the electrostatic interaction of the hard acids, are more significant in the interference system of non-oxygen or high-electronegative anionic ligands, while the contribution of |logK| increases in oxygen-anionic competitive system and is related to pH resistance of precise F removal by metal cations. Besides, these immobilized metal cations exhibited consistent trends in application assessment. This study provides a novel optimization strategy for high-precision adsorbent design and an evaluation method for metal-ligand complexation strength.