Europium(III)-Calcite study with site-selective TRLFS

Calcite is an important mineral that plays a significant role in nuclear waste disposal concerning the safety and performance in geological formations. At these sites it can be found in the near field as a secondary phase (weathering of the geochemical barrier) and as a rock-forming mineral in the surrounding rocks. Geochemically, calcite has the potential to adsorb as well as incorporate guest ions with a similar ionic radius, such as europium and curium, for calcium in the host lattice. Because of the long half-lives of actinides like curium and americium, they and their lanthanide homologues (i.e., europium) are the subject of recent research.
Calcite samples were doped with Eu(III) in batch experiments. Calcium carbonate powder was contacted with a Eu(III) solution (5 x 10-7 M) in a calcium carbonate saturated solution with a NaCl (10 mM) background electrolyte solution. Batch samples were analyzed at varying contact times to understand the step-by-step kinetic and mechanistic behavior of incorporation of Eu(III) into the solid phase. After the contact period, the supernatant was investigated with ICP-MS. The Eu(III) concentration in solution varies from 0.1 to 3.2 % of the initial concentration, which indicates that almost all Eu(III) is adsorbed.
The calcite powder was examined with site-selective TRLFS at temperatures below 20 K. The direct excitation of the ⁷F₀ →⁵D₀ transition in the range of 576-581 nm and the integration of the respective emission spectra yields a characteristic excitation spectrum. These excitation spectra show only one broad peak with a maximum at ~579.2 nm, independent of the sorption time (up to 31 days). This behavior is dissimilar to that determined by Stumpf and Fanghänel ^[1] who investigated Cm(III) sorption on calcite with NaClO₄ as background electrolyte and found 2 peaks, which change over time. Lifetime measurements of our samples exhibit biexponential decay indicative of two species. The first specie has a lifetime of 460 to 985 µs and the second 2155 to 4577 µs. Using Horrock´s equation^[2] the number of coordinating water molecules in the first sphere surrounding the Eu(III) can be determined. This value corresponds to its location (surface sorbed vs incorporated) on or within the calcite lattice. Therefore, calculated values of 0.5 to 1.7 indicate the formation of an inner sphere sorption species whereas a value of 0 is indicative of incorporation of the Eu(III) within the calcite. The emission spectrum shows a threefold splitting of the ⁷F₁ band.This indicates a ligand field with low symmetry. To better understand these surface species, future measurements with CTR and RAXR will be performed.

^[1] Stumpf, T. and T. Fanghanel (2002). J. of Colloid and Interface Science 249(1), 119-122.
^[2] Horrocks (1979) J. Am. Chem. Soc. 101, 334.