Retention Of An(III)/Ln(III) By Calcite: Influence Of Mineral Formation On Contaminant Speciation

Calcite has been intensively studied for its potential to structurally incorporate trivalent actinides and lanthanides. As calcite is both a ubiquitous mineral in many host rock formations, and an important weathering product of cementitious phases it may occur in a nuclear waste disposal site as both, a primary and a secondary phase. In addition, in the far-field of such a disposal site calcite can be formed by microbiota. In order to understand if the origin of the mineral will impact its capacity to incorporate trivalent cations, we will present and compare the speciation of Eu(III) and Cm(III) in calcite formed inorganically via co-precipitation[1] and co-precipitation/phase transformation[2] with calcite formed in a microbially-induced precipitation reaction[3] and calcite contacted with Eu(III) close to equilibrium.[4]
Investigations using site-selective TRLFS reveal that the speciation of M(III) co-precipitated with calcite formed inorganically consists of three species, labelled A, B, and C, respectively, independent of the formation mechanism. The species have been characterized as two incorporation species, differing in the degree of lattice distortion, and a sorption species in contact with the solution.[1] Contrary to this behaviour the speciation of Eu(III) in both, calcite formed in a microbially-induced reaction, and after reaction with calcite close to equilibrium conditions comes to be dominated by a different incorporation species, labelled γ.
Species γ is also Eu(III) incorporated into the crystal bulk, but shows a lower symmetry than observed previously, and does not match with any of the previous species. It can be shown that the rate of formation of γ depends heavily on the recrystallization rate of the calcite used in the reaction, and that its formation is accompanied by several other species, some of which had been identified previously.
The formation of species γ as the main mode of interaction in the microbial process is particularly interesting as the reaction proceeds via vaterite, the same metastable polymorph used in a previous abiotic study. The speciation in the intermediate vaterite phases does not differ, yet the speciation in the calcite product shows little similarity.
In summary, the results highlight the importance of molecular level understanding for accurately describing the interaction of dissolved cations with surrounding mineral phases.

*Present address: Department of Materials Science and Engineering, The University of Sheffield, Sheffield, United Kingdom, S10 2TN.
[1] Schmidt, M., T. Stumpf, M. Marques Fernandes, et al., Angew. Chem. Int. Ed., 47, 5846 (2008).
[2] Schmidt, M., T. Stumpf, C. Walther, et al., J. Colloid Int. Sci., 351, 50 (2010).
[3] Johnstone, E.V., S. Hofmann, A. Cherkouk, et al., Env. Sci. Tech., 50, 12411 (2016).
[4] Hellebrandt, S.E., S. Hofmann, N. Jordan, et al., Sci. Rep., 6, 33137 (2016).

Acknowledgments. This study is supported by the Helmholtz Gemeinschaft Deutscher Forschungszentren by funding the Helmholtz Young Investigator Group “Structures and reactivity at the aqueous/mineral interface” (VH-NG-942).