X-Ray Absorption Spectroscopy of Strontium(II) Coordination: II. Sorption and Precipitation at Kaolinite, Amorphous Silica, and Goethite Surfaces

Abstract

Sorption of dissolved strontium on kaolinite, amorphous silica, and goethite was studied as a function of pH, aqueous strontium concentration, the presence or absence of atmospheric CO2 or dissolved phosphate, and aging over a 57-day period. Selected sorption samples ([Sr(aq)]i ≈ 0.5–1×10−3m) were examined with synchrotron X-ray absorption spectroscopy (XAS) at low (13–23 K) and room temperatures to determine the local molecular coordination around strontium. Quantitative analyses of the extended X-ray absorption fine structure (EXAFS) of kaolinite, amorphous silica, and most goethite sorption samples showed a single first shell of 9–10 (±1) oxygen atoms around strontium at an average Sr–O bond-distance of 2.61 (±0.02) Å, indicating hydrated surface complexes. The EXAFS spectra were unchanged after reaction for up to 57 days. Likewise, in kaolinite sorption samples prepared in 100% nitrogen atmosphere, the presence of dissolved phosphate (0.5×10−3m) in addition to strontium did not change the local coordination around strontium. In two goethite sorption samples reacted in air at pH ∼8.5, the EXAFS spectra (collected at low and room temperature) clearly showed that the local structure around strontium is that of strontianite (SrCO3(s)). We also noted an increase in strontium uptake on goethite in the presence of atmospheric CO2 in batch experiments, relative to CO2-free experiments. These observations suggest that sorption of carbonate may nucleate the precipitation of SrCO3 in the pH range in which carbonate sorption on goethite is near a maximum. At higher pH, carbonate surface sorption decreases as dissolved CO2 decreases. For goethite sorption samples above pH 8.6, hydrated surface complexes, rather than a precipitate, were observed in the EXAFS spectra.