In Belgium Boom Clay (BC) is investigated as a potential host rock for the geological disposal of radioactive waste. An important issue concerns the mobility of uranium, representing the most abundant waste relevant radionuclide. In the past, uranium complexation, sorption and migration under geochemical conditions representative for Boom Clay were studied within the frame of different projects, i.e. TRANCOM I and II (Dierckx et al. 1999; Maes et al., 2004) and thesis (Dierckx 1995; Delécaut, 2004). The obtained results allowed already a good perception of the uranium behavior in Boom Clay, but nevertheless some open questions are remaining and require more fundamental investigations and in-depth studies.
When studying the U-complexation behavior under environmental conditions, the effect of organic dissolved organic matter (DOM) besides inorganic complexants (e.g. carbonate, sulphate, phosphate, etc.) cannot be neglected, as it has been shown that organic ligands and/or colloids may play a crucial role in the U-speciation, as well as U-mobility through porous media (Mibus et al., 2007; Luo and Gu., 2009; Lesher et al., 2013). Besides this, alkaline earth elements, and especially calcium (Ca2+) was observed to form complexes with U(VI) (Joseph et al., 2011; Marang et al. 2009), but also to compete with U(VI) for organic matter binding sites. Therefore, mechanisms governing these phenomena need to be characterized in more detail.
Boom Clay pore water contains up to 250 mg C/L of dissolved organic matter (DOM). Dissolved organic matter is a complex mixture of entities, heterogeneous in both chemical structure and physical conformation. Dissolved organic species/particles are polyfunctional, i.e. characterized by the presence of several functional groups with labile protons such as carboxylic groups, phenolic groups and amines. Humic substances are also said to be polydisperse, meaning they exist in a wide range of sizes, which do not only depend of their molecular weight, but also on the conformation they adopt. The Boom Clay dissolved organic matter was indeed observed to be composed of species ranging from hundreds of dalton up to hundred thousand of dalton (Durce et al., 2013).
Due to the redox sensitivity of actinides on the one hand and the polydispersive character (dissolved versus colloidal) of soluble organic matter on the other hand, a mathematical or thermodynamic description of the metal/radionuclide-OM interaction mechanism is not straightforward.
Although organic compounds have been extensively studied in the last decades, due to their ubiquitous presence in natural waters, their composition and molecular structures are however not sufficiently determined, the nature and number of functional groups involved in metal binding are still not perfectly known, and the stereochemistry of the complexes poorly defined (Delécaut, 2004). However, common agreement exists that the complexation of metal ions/actinides by humic substances can mainly be attributed to carboxylic groups (Kim, 1986; Choppin, 1992; Schmeide et al., 2012) and phenolic/OH groups (Pompe et al., 2000; Sachs and Bernhard, 2005) as their dominant functionalities.
With respect to the interaction of U(VI) with organics, many literature and in-house obtained data are available (Leinhart 2000; Delécaut, 2004; Schmeide et al., 2012), but integrating the influence of the physical structure of the organics on the U-interaction behaviour with HS has been lacking so far.
It is however presumed that the structure (and size) of the organics have an important influence on the interaction mechanism and should be better "conceptualized" in order to enable also the modeling of the system. Depending on the geochemical conditions, rather "true complexation" or "pseudocolloid formation" is expected to be dominant. While the former process can be represented and quantified by complex formation constants, the latter is considered to be rather related to a sorption and/or coprecipitation process.
In order to examine the previously described processes at microscopic level, different analytical techniques, e.g. Time-Resolved Luminescence Spectroscopy (TRLFS), Nuclear Magnetic Resonance Spectroscopy (NMR), etc. have been shown to represent useful tools and are foreseen to be applied within the framework of the proposed collaborations (i.e. HZDR, KIT).
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