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

摘要

This study uncovers bifluoride-type (difluorohydrogenate(I); [HF2](-)) species formed at mineral/water interfaces. Bifluoride forms at equivalent to Al-F surface sites resulting from the partial fluoridation of gibbsite (gamma-Al(OH3)) and bayerite (alpha-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 degrees 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 F-19 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.