Eu uptake by calcite: Preliminary results from coprecipitation experiments and observations with surface-sensitive techniques

A lack of information in databases for contamination risk assessment about the transport behaviour of the trivalent f-orbital elements in groundwater systems where calcite is at equilibrium motivated this study of Eu³⁺ uptake. The free drift technique was used to examine the effects of Eu³⁺ concentration, presence of Na⁺ or K⁺ and temperature, as well as calcite nucleation and precipitation kinetics, on the partitioning of calcite. Changes in surface composition and morphology resulting from exposure of single crystals of Iceland spar to Eu³⁺-bearing solutions were observed with X-ray photoelectron spectroscopy (XPS) and atomic force microscopy (AFM). First results confirm that calcite has high affinity for Eu³⁺. Rates of nucleation and precipitation strongly affect the extent of uptake but the presence of Na⁺ and K⁺ has no effect, suggesting formation of solid solution as CaCO₃–EuOHCO₃. Surface-sensitive techniques prove that Eu³⁺ is adsorbed to calcite even when the surface is dissolving and adsorption is not accompanied by precipitation of a separate Eu³⁺-solid phase. Adsorbed Eu modifies calcite's dissolution behaviour, roughening terraces and rounding step edges, and producing surface morphology where some surface sites appear blocked. Results imply that Eu³⁺ concentrations in natural calcites are limited by Eu³⁺ availability rather than by a lack of ability to fit into calcite's atomic structure. This behaviour can probably be expected for other trivalent rare Earth elements (REE), actinides and fission products whose behaviour is similar to that of Eu³⁺. These elements are likely to be incorporated within the calcite bulk in systems where it is precipitating and the demonstrated strong partitioning ensures some uptake even where calcite is at or under saturation.