Abstract
The interaction of calcite with trivalent europium under recrystallization conditions was studied on the molecular level using site-selective time-resolved laser fluorescence spectroscopy (TRLFS). We conducted batch studies with a reaction time from seven days up to three years with three calcite powders, which differed in their specific surface area, recrystallization rates and impurities content. With increase of the recrystallization rate incorporation of Eu3+ occurs faster and its speciation comes to be dominated by one species with its excitation maximum at 578.8 nm, so far not identified during previous investigations of this process under growth and phase transformation conditions. A long lifetime of 3750 μs demonstrates complete loss of hydration, consequently Eu must have been incorporated into the bulk crystal. The results show a strong dependence of the incorporation kinetics on the recrystallization rate of the different calcites. Furthermore the investigation of the effect of different background electrolytes (NaCl and KCl) demonstrate that the incorporation process under recrystallization conditions strongly depends on the availability of Na+. These findings emphasize the different retention potential of calcite as a primary and secondary mineral e.g. in a nuclear waste disposal site.
Highlights
Total concentration impurities storage of atmospheric CO2 in geological formations calcite and other carbonates with their slow kinetics of mineral-fluid reactions play a significant role[28]
All solids were characterized by powder X-ray diffraction (XRD), BET, and scanning electron microscopy (SEM)
Our findings conclusively show that Eu3+ is incorporated into calcite under recrystallization conditions
Summary
Total concentration impurities (ppm) storage of atmospheric CO2 in geological formations calcite and other carbonates with their slow kinetics of mineral-fluid reactions play a significant role[28]. Species B (λexc(Eu) = 578.4 nm) was found to be incorporated within the strongly distorted calcite structure. Species C (λexc(Eu) = 579.6 nm) was found to be located on the Ca2+ site on the calcite lattice, with an almost undisturbed octahedral symmetry, identifiable by its characteristic splitting pattern. For both species B and C, Na+ was required for charge compensation during the incorporation, suggesting a mechanism where two Ca2+ ions were replaced by one Eu3+ atom and one Na+ atom. There are already clear hints in the literature highlighting the trend that speciation and interaction of Eu3+ and Cm3+ with calcite is strongly affected by experimental conditions
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