Sorption and diffusion of U(VI) in the system Opalinus Clay/pore water in the absence and presence of organic ligands


Sorption and diffusion of U(VI) in the system Opalinus Clay/pore water in the absence and presence of organic ligands

Schmeide, K.; Joseph, C.; Bernhard, G.

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 this environment is required. Since clay rock contains natural organic matter, we studied the influence of various organic ligands on the sorption of U(VI) (1•10-6 M) onto the clay rock Opalinus Clay (Mont Terri, Switzerland) under aerobic conditions applying synthetic Opalinus Clay pore water (I = 0.36 M, pH 7.6 [2]) as background electrolyte at 25°C. It was found that the low U(VI) sorption onto Opalinus Clay in the absence of ligands (Kd = (0.0222 ± 0.0004) m3/kg [1]) 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.
The diffusion of U(VI) (1•10-6 M) in intact Opalinus Clay bore core samples was studied under anaerobic conditions in the absence and presence of humic acid (10 mg/L) at 25°C. From the U(VI) and humic acid diffusion profiles, obtained after three months of diffusion time, diffusion and distribution coefficients were determined using the modeling software COMSOL Multiphysics 3.3 [3]. The Kd value of the U(VI) sorption onto Opalinus Clay could be confirmed by the results of diffusion experiments. Humic acid does not have a significant influence on U(VI) diffusion in Opalinus Clay.
[1] 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).
[2] Pearson, F.J.: Opalinus Clay experimental water: A1Type, Version 980318, PSI Internal Report TM-44-98-07. Paul Scherrer Institut, Villigen, Switzerland (1998).
[3] Finite-element software package. http://www.comsol.com.

Keywords: Uranium; sorption; diffusion; citric acid; lactic acid; humate; opalinus

  • Poster
    NEA ClayClub Workshop - Clays under Nano- to Microscopic Resolution, 06.-08.09.2011, Karlsruhe, Germany

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Publ.-Id: 15831