Calcite interactions with selenite and neptunyl, from surface complexation to solid-solution formation

Several surface complexation models for calcite have been published in the past. We will present a review, showing that models from different developers converge towards a common picture with comparable protonation and ion adsorption constants. Newest theoretical findings on interfacial water permittivity can be included into this picture, resulting in an updated electrostatic surface complexation model for calcite.
Selenite has been shown to adsorb on calcite predominantly via an ion-exchange like process:
>CaCO3 + X SeO32-(aq) ↔ >Ca(CO3)(1-X)(SeO3)(X) + X CO32-(aq) ,
Leading to the formation of a surface solid-solution. Over a broad range of conditions this surface solid-solution is characterized by a constant partition coefficient, D = 0.02. In a long-term experiment we contacted Aragonite with various concentrations of Selenite (1 - 100 µM). In a Selenite free system, metastable Aragonite transforms spontaneously to Calcite (SI(Calcite) = 0.14). The experiments allowed us to refine a free energy threshold above which Selenite effectively blocks Calcite formation. At low Selenite concentrations (< 60 µM), Calcite growth buries the surface solid solution, leading to entrapment of Selenite in the Calcite bulk. ... mehrDue to the very slow growth rate at low supersaturation the partition coefficient decreases (D = 0.005). Growth inhibition effects lead to elongated Calcite particles with surface morphologies including (101) and (10 -1) faces, and pronounced sector zoning. The results can be described in the context of the Unified Uptake Kinetic Model (UUKM).
It has been shown that Neptunyl may coprecipitate with Calcite and may be structurally incorporated. In long-term experiments, similar to those described above but using Neptunyl instead of Selenite, we observed that Neptunyl is even more effective in inhibiting Calcite growth compared to Selenite. In contact with 1 µM Neptunyl only minor amounts of Calcite grow during 7 years contact time.
The results of the long-term coprecipitation experiments indicate that structural incorporation can be a very effective sequestration mechanism for radionuclides in the environment. However, the energetics and growth kinetics of the respective trace element – host mineral systems have to be carefully investigated, as growth inhibition effects induced by the trace elements, as well as changing partition coefficients at low growth rates may lead to negligible uptake even over extended periods of time.