A study of the interaction of strontium ions in aqueous solution with the surfaces of calcite and kaolinite

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The interaction of Sr 2+ ions in aqueous solution with the surfaces of calcite and kaolinite have been studied by exposing powdered samples of these minerals to solutions containing a range of dissolved Sr 2+ concentrations. These experiments involved measuring the amounts of Sr 2+ taken up from solution as a function of initial Sr 2+ concentration and studying the reacted solids using scanning electron microscopy/energy dispersive X-ray spectroscopy (SEM/EDS) and, in particular, X-ray absorption spectroscopy (XANES and EXAFS). For calcite, very little Sr 2+ is removed from solutions where initial concentrations are less than 0.3 mmol/L, but at higher concentrations, uptake increases linearly with initial Sr 2+ concentration. Such behaviour, along with thermodynamic considerations, suggests precipitation of strontianite at these higher concentrations, and this was confirmed by X-ray absorption spectroscopy. The Sr K-edge X-ray absorption near edge structure (XANES) spectrum closely resembles that for strontianite, and the extended X-ray absorption fine structure (EXAFS) spectrum yields a Sr-O bond distance of 2.58 Å, very close to the value in pure strontianite (2.59 Å). Studies of such samples by SEM/EDS also suggest that a strontianite phase precipitates as a coating on the surface of the calcite. At lower concentrations of initial Sr 2+ (<0.3 mmol/L), the Sr K-edge XANES spectrum does not resemble the strontianite spectrum but is similar to the Ca K-edge of Ca 2+ in calcite, suggesting that Sr 2+ substitutes for Ca 2+ in sites at the calcite surface. The EXAFS data for such samples indicate a coordination number slightly higher than 6, possibly because Sr 2+ is sorbed as a partially aquated species. The uptake of Sr 2+ by kaolinite contrasts with calcite in that greatest uptake occurs when initial Sr 2+ concentrations are low. The EXAFS spectra for samples with the lowest initial Sr 2+ concentrations suggest that the Sr 2+ is taken up as a partially aquated species into octahedral sites on the kaolinite surface. At higher concentrations, the EXAFS data show longer average Sr-O bond distances suggesting weaker binding of Sr 2+ to the kaolinite surface.