Spectroscopic and modeling study of Ln³⁺ (Eu³⁺, Y³⁺) and An³⁺ (Cm³⁺, Am³⁺) on feldspars

Influence of the competition of Al on the retention of trivalent actinides and their homologues in orthoclase
J. Lessing,1 M. Schmidt,1 T. Stumpf1
1 Helmholtz-Zentrum Dresden-Rossendorf e.V., Institute of Resource Ecology, Bautzner Landstraße 400,
01328 Dresden, Germany, email: j.lessing@hzdr.de

Most countries worldwide consider disposal in a deep geological formation as the safest concept for nuclear waste disposal. For a realistic safety assessment of such a repository, understanding the mechanisms of the prevalent retention processes is of utmost importance. Sorption of radio-active elements on many minerals is well described in literature, but there is a lack of data re-garding the influence of other natural cations especially Al3+ [1]. These cations will be present in all scenarios as Al3+ is the third most common element (following O and Si) in the earth crust, and will occur locally e.g. due to the dissolution of minerals (especially alumino-silicates). Its concentration can be expected to exceed that of the actinides manifold. In addition to competi-tion for sorption site, Al3+ can then also re-precipitate on a primary mineral’s surface and form a secondary phase, which will impact the interaction of the radionuclides with these minerals.
Alumino-silicates, such as feldspars (orthoclase) and mica, together with quartz are the main components of crystalline rock, which is considered as possible host rock for radioactive waste repositories. The other common option are clay formations, which also consist of alumino-silicate minerals. The retention of trivalent actinides by feldspars was already investigated thor-oughly [2,3]. The minor actinides (Np, Am, and Cm) as well as plutonium dominate the radio-toxicity of spent nuclear fuel over geological time scales. Am and Cm are predominantly triva-lent in aqueous solution and Pu is also expected to occur at least partly in its trivalent state, due to the expected reducing conditions in deep geological formations. The less radiotoxic lantha-nide Eu3+ is often used as homologue for the trivalent actinides with excellent luminescence properties.
Here, we study the effect of dissolved Al3+ on the retention of trivalent actinides (Cm3+) and lan-thanides (Eu3+) on orthoclase. The quantitative effect of different [Al3+] on actinide retention was first evaluated in batch sorption experiments using Eu3+ as an analogue. For further analysis on a molecular level, time resolved laser spectroscopy (TRLFS) was applied, from which infor-mation about the formed surfaces complexes can be gained. We will discuss the results with re-spect to the impact of Al3+ on quantity and speciation of An3+ sorption on feldspars.
The derived speciation and quantitative retention data is foreseen to be implemented into a sur-face complexation model, with parameters available in thermodynamic databases. Ultimately this will provide a better understanding of the fundamental mechanisms of sorption process of the minor actinides Am and Cm on naturally occurring mineral phases under close to natural conditions.