Bifluoride ([HF2]−) formation at the fluoridated aluminium hydroxide/water interface

Abstract

This study uncovers bifluoride-type (difluorohydrogenate(I); [HF₂]⁻) species formed at mineral/water interfaces. Bifluoride forms at Al–F surface sites resulting from the partial fluoridation of gibbsite (γ-Al(OH₃)) and bayerite (α-Al(OH₃)) particles exposed to aqueous solutions of 50 mM NaF. Fluoride removal from these solutions is proton-promoted and results in a strongly self-buffered suspensions at circumneutral pH, proceeds at a F : H consumption ratio of 2 : 1, and with recorded losses of up to 17 mM fluoride (58 F nm⁻²). These loadings exceed crystallographic site densities by a factor of 3–4, yet the reactions have no resolvable impact on particle size, shape and mineralogy. X-ray photoelectron spectroscopy (XPS) of frozen (−155 °C) wet mineral pastes revealed coexisting surface F⁻ and HF⁰ species. Electron energy loss features pointed to multilayer distribution of these species at the mineral/water interface. XPS also uncovered a distinct form of Na⁺ involved in binding fluoride-bearing species. XPS and solid state magic angle spinning ¹⁹F nuclear magnetic resonance measurements showed that these fluoride species were highly comparable to a sodium-bifluoride (NaHF₂) reference. First layer surface species are represented as Al–F–H–F–Al and Al–F–Na–F–Al, and may form multi-layered species into the mineral/water interface. These results consequently point to a potentially overlooked inorganic fluorine species in a technologically relevant mineral/water interfacial systems.