X-ray absorption spectroscopy of strontium(II) coordination - II. Sorptionand precipitation at kaolinite, amorphous silica, and goethite surfaces

Sorption of dissolved strontium on kaolinite, amorphous silica, and goethit e was studied as a function of pH, aqueous strontium concentration, the pre sence or absence of atmospheric CO2 or dissolved phosphate, and aging over a 57-day period. Selected sorption samples ([Sr(aq)](i) approximate to 0.5- 1 x 10(-3) m) were examined with synchrotron X-ray absorption spectroscopy (XAS) at low (13-23 K)and room temperatures to determine the local molecula r coordination around strontium, Quantitative analyses of the extended X-ra y absorption fine structure (EXAFS) of kaolinite, amorphous silica, and mos t goethite sorption samples showed a single first shell of 9-10 (+/-1) oxyg en atoms around strontium at an average Sr-O bond-distance of 2.61 (+/-0.02 ) Angstrom, indicating hydrated surface complexes. The EXAFS spectra were u nchanged after reaction for up to 57 days. Likewise, in kaolinite sorption samples prepared in 100% nitrogen atmosphere, the presence of dissolved pho sphate (0.5 x 10(-3) m) in addition to strontium did not change the local c oordination around strontium. In two goethite sorption samples reacted in a ir at pH similar to 8.5, the EXAFS spectra (collected at low and room tempe rature) 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 carbon ate may nucleate the precipitation of SrCO3 in the pH range in which carbon ate sorption on goethite is near a maximum. At higher pH, carbonate surface sorption decreases as dissolved CO2 decreases. For goethite sorption sampl es above pH 8.6, hydrated surface complexes, rather than a precipitate, wer e observed in the EXAFS spectra. (C) 2000 Academic Press.