Influence of organic ligands and temperature on U(VI) sorption and diffusion in the system Opalinus Clay/Opalinus Clay pore water

In addition to salt and crystalline rock, argillaceous rock is investigated as potential host rock and backfill material for nuclear waste repositories. For safety assessment, knowledge on the migration behavior of potentially released actinides in these environments is required. Clay rock contains natural organic matter. Low molecular weight organic acids such as acetate, lactate, propionate and formate as well as fulvic and humic acids can be released from clay under certain conditions [1, 2]. In continuation of our former study where the retention properties of the natural clay rock Opalinus Clay (Mont Terri, Switzerland) towards U(VI) were investigated [3], we studied the influence of various organic ligands on the sorption of U(VI) (1•10-6 M) onto Opalinus Clay under aerobic conditions applying synthetic Opalinus Clay pore water (I = 0.36 M, pH 7.6 [4]) as background electrolyte. It was found that the low U(VI) sorption onto Opalinus Clay in the absence of ligands (Kd = (0.0222 ± 0.0004) m³/kg [3]) further decreases with increasing concentration of low molecular weight organic acids (1•10-5 to 1•10-2 M) due to complex formation in aqueous solution. The mobilizing effect of the organic ligands on U(VI) increases in the following sequence: formate < lactate ~ acetate ~ propionate < tartrate < citrate. For instance, in the presence of citrate (1•10-2 M), which has been identified as important ligand in radioactive waste problems, the Kd value for U(VI) amounts to only (0.0011 ± 0.0003) m³/kg. The influence of the organic ligands on the U(VI) sorption onto Opalinus Clay correlates with the stability of the respective U(VI) complexes. In contrast, humic acid (< 50 mg/L) does not change U(VI) sorption [3]. A reduction of U(VI) to U(IV) was not detected.
Since elevated temperatures are expected for the disposal of high-level nuclear waste in clay formations, the influence of temperature on the U(VI) sorption onto Opalinus Clay was studied in the temperature range from 10 to 60°C. In the absence of organic ligands, the U(VI) sorption increases with increasing temperature. For U(VI), the apparent endothermic sorption enthalpy amounts to 34 ± 1 kJ/mol. The temperature dependence of the U(VI) sorption onto Opalinus Clay is almost not influenced by lactate and humic acid (50 mg/L), however, slightly changed by citrate, when present in the concentration range from 1•10-5 to 1•10-2 M.
For interpretation of the sorption results, the U(VI) speciation in aqueous solution has to be known. Therefore, we studied the U(VI) complexation by lactic acid (pH 3) and citric acid (pH 1-10) in the temperature range from 7 to 65°C. Species distribution and complex formation constants were determined by means of UV-Vis and time-resolved laser-induced fluorescence spectroscopy. The complex formation between U(VI) and these ligands was found to be endothermic and entropy-driven.
Finally, the diffusion of U(VI) (1•10-6 M) in intact bore core samples was studied under anaerobic conditions in the absence and presence of humic acid (10 mg/L) both at 25°C and at 60°C. After three months of diffusion time, the U(VI) and humic acid diffusion profiles were determined. Both an influence of the temperature and of humic acid on U(VI) diffusion was detected. Currently, diffusion and distribution coefficients are determined by fitting the U(VI) and humic acid diffusion profiles using the modeling software COMSOL Multiphysics 3.3 [5].
The results show that the sorption and diffusion behavior of U(VI) in the system Opalinus Clay/Opalinus Clay pore water is influenced by natural organic matter and temperature. Especially strongly complexing low molecular weight organic acids can enhance the mobility of U(VI) in clay.

[1] Courdouan, A., Christl, I., Meylan, S., Wersin, P., Kretschmar, R.: Characterization of dissolved organic matter in anoxic rock extracts and in situ pore water of the Opalinus Clay. Appl. Geochem. 22, 2926-2939 (2007).
[2] Claret, F., Schäfer, T., Bauer, A., Buckau, G.: Generation of humic and fulvic acid from Callovo-Oxfordian clay under high alkaline conditions. Sci. Total Environ. 317, 189-200 (2003).
[3] Joseph, C., Schmeide, K., Sachs, S., Brendler, V., Geipel, G., Bernhard, G.: Sorption of uranium(VI) onto Opalinus Clay in the absence and presence of humic acid in Opalinus Clay pore water. Chem. Geology 284, 240-250 (2011).
[4] Pearson, F.J.: Opalinus Clay experimental water: A1Type, Version 980318, PSI Internal Report TM-44-98-07. Paul Scherrer Institut, Villigen, Switzerland (1998).
[5] Finite-element software package. http://www.comsol.com.