Complexation/colloid formation of U(VI) with Boom Clay dissolved organic matter

Buchatskaya Yulia



SCK•CEN Mentor

Salah Sonia
+32 14 33 32 21

SCK•CEN Co-mentor

Durce Delphine
+32 14 33 32 32

Expert group

R&D Disposal

PhD started


Short project description

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).



Choppin G. R. (1992). "The role of natural organics in radionuclide migration in natural aquifer systems". Radiochimica Acta, 58, 113-120.

Delécaut G. (2004). "The geochemical behaviour of uranium in the Boom Clay". PhD thesis, Université Catholique de Louvain – SCK?CEN, Louvain-La-Neuve, Belgium.

Dierckx(1995): Complexation of europium with humic acids. Influence of cations and competing ligands. PhD thesis. 152 pages.

Dierckx et al. (1999): Trancom-Clay. Transport of radionuclides due to complexation with organic matter in clay formations. Final report. R-3388. 170 pages.

Durce D., Bruggeman C., and Maes N. (2013). " Dissolved organic matter transport in Boom Clay: size effects". Oral presentation at Migration conference 2013, Brighton UK.

Joseph C., Schmeide K., Sachs S., Brendler V., Geipel G., and Bernhard G. (2011). "Sorption of uranium(VI) onto Opalinus Clay in the absence and presence of humic acid in Opalinus Clay pore water." Chemical Geology, 291, 240-250.

Kim J. J. (1986). "Chemical behaviour of transuranic elements in natural aquatic systems. Handbook on the Physics and Chemistry of the Actinides". Elsevier.

Lenhart J. J., Cabaniss S.E., MacCarthy P., and Honeyman B. D. (2000). "Uranium(VI) complexation with citric, humic and fulvic acids". Radiochimica Acta, 88, 345–353.

Lesher E. K., Honeyman B. D., and Ranville J. F. (2013). "Detection and characterization of uranium–humic complexes during 1D transport studies." Geochimica et Cosmochimica Acta, 109(0), 127-142.

Luo W. and Gu B. (2009). "Dissolution and Mobilization of Uranium in a Reduced Sediment by Natural Humic Substances under Anaerobic Conditions". Environmental Science & Technology, 43(1), 152-156

Maes N., Wang L., Hicks T., Bennett D., Warwick P., Hall T., Walker G. and Dierckx A. (2006). "The role of natural organic matter in the migration behaviour of americium in the Boom Clay - Part I: Migration experiments." Physics and Chemistry of the Earth, Parts A/B/C 31(10-14), 541-547.

Marang L., Eidner S.; Kumke M. U.; Benedetti M.F. and reiller P. E. (2009). "Spectroscopic characterization of the competitive binding of Eu(III), Ca(II), and Cu(II) to a sedimentary originated humic acid". Chemical Geology, 264, 154-161.

Mibus J., Sachs S., Pfingsten W., Nebelung C., and Bernhard G. (2007). "Migration of uranium(IV)/(VI) in the presence of humic acids in quartz sand: A laboratory column study." Journal of Contaminant Hydrology 89(3–4), 199-217.

Pompe S., Schmeide K., Bubner M., geipel G., Heise K. H., Bernhard G., and Nitsche H. (2000). " Investigation of humic acid complexation behaviour with uranyl ions using modified synthetic and natural humic acids". Radiochimica Acta, 88, 553-558.

Sachs S. and Bernhard G. (2005). "NIR spectroscopic study on the complexation of neptunium (V) with humic acids: Influence of phenolic OH groups on the complex formation". Radiochimica Acta, 93, 141-145.

Schmeide K., Joseph C., Sachs S., Steudnter R., Raditzky B., Günther A., Berhnard G. (2012). "Characterization and quantification of the influence of clay oragnics on the interaction and diffusion of uranium and americium in the clay. Joint Projet: Interaction and transport of actinides in natural clay rock with consideration of humic substances and clay organics". Wissenschaftlich-technische Berichte, HZDR-017.


Therefore, we propose a systematic and multi-parametric study on the U(VI) complexation with BC DOM

The 1st  part of the study will comprise:

  • The selection of dissolved OM fraction to be studied (size and chemical properties);
  • The separation and characterization of the different DOM fractions (elemental composition, proton exchange capacity ….).

The  2nd part of the study will be devoted to the parametric study on U(VI) interaction with different fractions of DOM:

  • in NaCl, NaHCO3 and CaCl2;
  • at different pH (4-10); and
  • different ionic strengths (0.01-0.5 M).

The 3rd part will be focused on the:

  • Characterization of the formed U(VI)-DOM complexes and/or pseudocolloids by a set of analytical techniques available in-house (UV-Vis, ATR FT-IR, SEC) or via collaboration (TRLFS, cryo-TRLFS, NMR…);
  • Modeling of the experimental results by using geochemical codes (Geochemist's Workbench, PHREEQC).