The Use of Microfluidic Platforms with Raman Spectroscopy for Investigating the Co-Precipitation of Metals and Radionuclides in Carbonates

In the context of long-term safety assessments of deep geological repositories for radio-active wastes, a rigorous understanding of the retention of radionuclides such as Ra-226 due to co-precipitation with carbonate and sulphate minerals is important for a realistic prediction of radionuclide migration behaviour in the repository near and far field. The co-precipitation of Ra-226 in sulphate minerals, in particular barite, has been studied experimentally and numerically in detail throughout the last decade to establish the thermodynamic properties and mixing behaviour of its solid solutions over a wide range of temperatures. However, so far, few studies have been dedicated to the incorporation of Ra-226 into carbonates, and little is known about the mixing behaviour of Ra-226 and calcium carbonate phases such as calcite, aragonite, or vaterite. The aim of the work presented here was to develop and explore innovative microfluidic experiments in combination with in situ Raman spectroscopy that can be used to investigate co-precipitation processes of radionuclides in carbonate minerals, using stable Ba as a chemical analogue for Ra-226 in the first step, due to their similar ionic radii. Different microfluidic set-ups were developed to address co-precipitation in bulk solution as well as in confinement or under diffusive flow regimes. It could be shown by XRD and SEM-EDX analyses that high amounts of Ba can be incorporated into the lattice of calcite when formed via an amorphous precursor phase, suggesting that the formation of calcium carbonates can contribute to the retention of Ra-226 in the repository environment, which will be verified in future studies using the presented microfluidic platforms.