Co(II) sorption at the calcite-water interface: I. X-ray photoelectron spectroscopic study

Abstract

The mechanisms of Co uptake from solution by calcite surfaces have been studied by X-ray photoelectron spectroscopy (XPS) and sputter depth profiling. Two types of calcite samples were examined (finely powdered and individual {104} cleavage surfaces) as a function of pH (7.3–8.9), reaction time (0.9–168 h), and starting aqueous Co concentration (0.01–0.1 mM). In all cases, solutions were saturated with respect to calcite. No evidence was found for oxidation of Co2+ upon sorption for either type of sample. Chemical shift and Auger parameter assessment of the sorbed Co2+ species indicate that it is most likely present in the form of a (Ca,Co) carbonate solid solution under most conditions tested, although there is some evidence from XPS depth profiling experiments and scanning electron microscopy on the high pH sorption sample that a Co-carbonate or hydroxy carbonate precipitate is also present in minor amounts. More detail on the identity of the sorption reaction products from these experiments can be found in our companion X-ray absorption spectroscopy (XAS) study (Xu et al., 1996). Results of XPS-depth profiling experiments suggest that the distribution of Co2+ is controlled by pH and solution saturation state. For samples where the pH was adjusted during Co/calcite interaction, Co2+ was found to be more concentrated in the near-surface region of sorption samples produced by slow pH adjustments (from 8.2 to 8.5, never exceeding 8.6) than in samples produced by rapid pH adjustment (from 8.2 to 8.9 after initial excursion to 10.9). When pH was held constant (7.3 or 8.5) during Co/calcite interaction, Co is more concentrated in the near-surface region of the sorption sample produced at higher pH (8.5) than in the sample produced at lower pH (7.3). Cobalt was found to decrease with depth in calcite for the higher pH sample, but was relatively constant with depth for lower pH samples. XPS measurements on Co/calcite sorption samples as a function of time after solution exposure indicate that solid-state diffusion of Co is not the dominant mechanism of Co incorporation into calcite in the short time frame of our experiments (up to one week). Nevertheless, our results point towards a dynamic calcite surface, even when there appears to be no net Ca-carbonate deposition, as Co is found down to and below the 50–100 Å sputtering range of our experiments even after solution exposure of only 0.9 h with an aqueous Co concentration of 0.02 mM at pH 7.4.