Evidence of a new Incorporation Species of Eu(III) in Calcite and its dependence of the background electrolyte

Calcite plays a significant role in nuclear waste disposal sites, both as a constituent of geological formations and as a secondary mineral, e.g. upon weathering of concrete. As such it has a direct impact on a repository’s safety and performance. Geochemically, calcite has the potential to adsorb as well as incorporate guest ions with a similar ionic radius, e.g. Eu(III), Pu(III) and Am(III), for Ca(II) in the host lattice. For the safety assessment of nuclear waste disposal sites these trivalent actinides with long half-lives (especially Am) dominate its long-term radiotoxicity and are thus of particular interest.
Schmidt et al. investigated the influence of different dissolved cations on the incorporation process by [1] time-resolved laser fluorescence spectroscopy (TRLFS) with Eu(III)/Cm(III). They could show that there exists a coupled substitution mechanism [Cm(III)/Eu(III) + Na(I) ↔ 2 Ca(II)]. The experiments by Schmidt, et al. were performed under growth conditions, representative of the formation of a secondary phase. Calcite already present as a constituent of the host rock, however, would be more likely to interact with the contaminants at, or very close to equilibrium. Under these conditions its reactivity will be governed by its recrystallization rate, and different interaction mechanism – and consequently different contaminant speciation – may be expected.
For our experiments we used Eu as homologue because of its similar ionic charge and radius, as well as its desirable luminescence properties [2]. We conducted batch studies with calcite powder in calcite saturated solutions with NaCl or KCl as background electrolyte. The speciation of the incorporated Eu(III) was then investigated by site-selective time-resolved laser fluorescence spectroscopy (TRLFS). The speciation of both systems is dominated by a species with its excitation maximum at 578.9 nm, which had not been identified in previous investigations of this process under growth [1] and phase transformation conditions [3]. A long lifetime of ~ 4000 µs demonstrates complete loss of hydration [4], consequently Eu must have been incorporated into the bulk crystal. The corresponding emission spectrum shows the maximum splitting pattern implying a low symmetry of the ligand field surrounding Eu(III)[2]. After 1 month reaction time the excitation spectrum of the calcite in contact with NaCl shows a strongly blue-shifted excitation spectrum compared to the same calcite with KCl, demonstrating the effect of the background electrolyte on the Eu(III) speciation. As the peak at 579.3 nm belongs to a sorption species, this indicates enhanced incorporation in NaCl background relative to the KCl system. This may indicate that also under recrystallization conditions coupled substitution of Eu(III) and Na(I) for two Ca(II) is required for incorporation. Incorporation remains a significant interaction mechanism in the KCl system, likely due to a considerable amount of naturally occurring Na in the calcite
The results show, that the speciation of Eu(III) in calcite depends on the conditions of its incorporation, i.e. growth versus recrystallization. A hitherto unknown incorporation has been identified, and our results strongly suggest incorporation under recrystallization conditions strongly depends on the availability of Na(I).
[1] M. Schmidt, Angew. Chem., Int. Ed. 2008, 47, 5846-5850.
[2] K.Binnemans, Coord. Chem. Rev. 2015,295, 1-45.
[3] M. Schmidt, J. Colloid Interface Sci. 2010, 351, 50-56.
[4] W. DeW. Horrocks, Jr., J. Am. Chem. Soc. 1979, 101, 334-340.