Substitution–Leaching–Deposition (SLD) Processes Drive Reversible Surface Layer Reconstruction of Metal Oxides for Fluoride Adsorption

Abstract

Surface complexation has long been recognized as the basic mode involved in fluoride adsorption onto metal oxides. However, such general recognition is challenged by the unusual pH dependence observed in fluoride adsorption. Here, we selected hydrated zirconium oxide (HZO) as a representative metal oxide to revisit the fluoride adsorption mechanism. Multiple in situ microscopic analyses and thermodynamic simulations suggest that, unlike the adsorption of other anions that proceed exclusively via substituting protonated terminal hydroxyl (η-OH₂⁺) groups of metal oxides, fluoride can displace both η-OH₂⁺ and protonated bridging hydroxyl (μ-OH⁺) groups of HZO (i.e., Substitution). This distinctive displacement drives the leaching of Zr from HZO, generating aqueous polyfluorozirconium complexes (i.e., Leaching) which subsequently deposit onto HZO via outer-sphere complexation (i.e., Deposition). The adsorbed polyfluorozirconium gradually converts into a fluorozirconate (Na₅Zr₂F₁₃) coating, resulting in a surface layer reconstruction of up to 100 nm in depth. The atypical pH dependency of fluoride adsorption can be explained by the processes of Substitution, Leaching, and Deposition (i.e., SLD processes). More attractively, the SLD-driven surface layer reconstruction is reversible in nature, ensuring the constant defluoridation capability of HZO during cyclic adsorption-desorption assays. This study advances our understanding of fluoride adsorption at water-metal oxide interfaces.