title: 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 [triple bond, length as m-dash]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 HF(0) 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 (19)F 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 [triple bond, length as m-dash]Al-F-H-F-Al[triple bond, length as m-dash] and [triple bond, length as m-dash]Al-F-Na-F-Al[triple bond, length as m-dash], 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.
authors.affiliation: Department of Chemistry, Umeå University, SE-901 87 Umeå, Sweden. jean-francois.boily@chem.umu.se and Physical and Theoretical Chemistry Laboratory, Department of Chemistry, University of Oxford, OX1 3QZ Oxford, UK.
authors.forename: Kenichi
authors.initials: K
authors.lastname: Shimizu
authors.affiliation: Department of Chemistry, Umeå University, SE-901 87 Umeå, Sweden. jean-francois.boily@chem.umu.se.
authors.forename: Gordon W
authors.initials: GW
authors.lastname: Driver
authors.affiliation: Department of Chemistry, Umeå University, SE-901 87 Umeå, Sweden. jean-francois.boily@chem.umu.se.
authors.forename: Marie
authors.initials: M
authors.lastname: Lucas
authors.affiliation: Department of Chemistry, Umeå University, SE-901 87 Umeå, Sweden. jean-francois.boily@chem.umu.se.
authors.forename: Tobias
authors.initials: T
authors.lastname: Sparrman
authors.affiliation: Department of Chemistry, Umeå University, SE-901 87 Umeå, Sweden. jean-francois.boily@chem.umu.se.
authors.forename: Andrey
authors.initials: A
authors.lastname: Shchukarev
authors.affiliation: Department of Chemistry, Umeå University, SE-901 87 Umeå, Sweden. jean-francois.boily@chem.umu.se.
authors.forename: Jean-François
authors.initials: JF
authors.lastname: Boily