Effect of temperature and humic acid on the U(VI) diffusion in compacted Opalinus Clay

Beside salt dome and granite rock, argillaceous rock is discussed as host rock for a nuclear waste repository. For safety assessment, knowledge about the migration behavior of the actinides is required. In this study, the interaction of U(VI) (110-6 M) with the natural clay rock Opalinus Clay (OPA) from Mont Terri, Switzerland was investigated by diffusion experiments under anaerobic conditions using synthetic OPA pore water [1] as background electrolyte and intact bore cores (diameter: 2.57 cm, thickness: 1.1 cm, density: 2.4 g/cm³).
Due to radioactive decay of the radionuclides, temperatures in the near-field of a nuclear waste repository in argillaceous rock are likely to be increased to ≤ 100°C [2], which can impact the migration behavior of actinides. Thus, the U(VI) diffusion in OPA was investigated in dependence on temperature (25 and 60°C).
Humic acids (HA) occur ubiquitously in nature. They are able to influence the migration of actinides due to their ability for complex and colloid formation and their redox properties. Thus, the influence of HA (10 mg/L) on the U(VI) diffusion was also studied.
At first, HTO diffusion experiments were performed for determination of the diffusion parameters of the OPA samples. The obtained HTO diffusion coefficients and porosities agreed with literature values.
After three months of diffusion time the U(VI) diffusion profiles in the OPA bore core samples were determined. The U(VI) diffusion profiles showed that, compared to the experiment at 25°C, the U(VI) diffusion is increased within the first 1.5 mm at 60°C. After 1.5 mm this effect levels off, which can be attributed to the increased sorption of U(VI) at 60°C. In the presence of HA, at 60°C and diffusion distances ≥ 800 µm, a lower amount of U(VI) was detected in the clay. That means, the U(VI) diffusion was hindered by the presence of HA.
Currently, diffusion and distribution coefficients are determined by fitting of the U(VI) and HA diffusion profiles using the modeling software COMSOL Multiphysics 3.3 [3]. The results will be compared to results of sorption experiments with OPA [4].

[1] F.J. Pearson (1998) PSI Internal report TM-44-98-07, PSI, Villigen, Switzerland.
[2] T. Brasser, et al. (2008), GRS-247, GRS, Braunschweig, Germany.
[3] Finite-element software package. http://www.comsol.com.
[4] C. Joseph, et al. (2011), Chem. Geol., accepted.