Uranium(VI) Diffusion in Opalinus Clay in the Absence and Presence of Humic Acid

As possible host rock and backfill material for a nuclear waste repository clay is discussed. The interaction of the clay with actinides, especially uranium (U), is the focus of our work. Batch sorption and diffusion experiments are performed in order to estimate the actinide migration in clay formations. Furthermore, the influence of humic acid (HA) on these processes is investigated. HA are ubiquitously found in natural environments. In a worst case scenario these polyelectrolytic macromolecules can be transported by aquifers through the clay formation, where they can interact with e.g. actinides of the nuclear waste. Natural clay contains also HA and fulvic acid like organic matter, which can be released from the clay. The extraction of small organic ligands from natural clay was shown in literature (Courdouan et al. 2007). HA has a multitude of functional groups, which are able to complex metal ions, like U(VI), very well. HA also forms stable colloids. Due to this behavior HA can influence the mobility of U(VI) in a nuclear waste repository.
Currently, we are studying the diffusion of U(VI) in opalinus clay (Mont Terri, Switzerland) in the absence and presence of HA by diffusion experiments using synthetic opalinus clay pore water (Pearson 1998; pH 7.6, I = 0.39 M) as mobile phase. In our studies the assembly of Van Loon et al. (2004) is used, where an intact opalinus clay bore core (diameter: 2.57 cm, thickness: 1 cm) is put into a diffusion cell, which is connected with a high and a low concentration reservoir containing the mobile phase with the tracer. All experiments are performed under anaerobic conditions. The direction of diffusion is perpendicular to bedding. The pressure exerted on the bore core samples is 5 MPa. The density of the bore core pieces is 1.85 g/cm3. For the experiments two diffusion cells are used for investigation of i) U(VI) diffusion in the absence and ii) presence of HA. At first the diffusion properties of the opalinus clay samples in the diffusion cells were determined by through- and out-diffusion experiments using HTO as tracer. As diffusion parameters for HTO in opalinus clay De = (5.31 ± 1.01) x 10-11 m2/s and De = (7.23 ± 1.36) x 10-11 m2/s were determined. The diffusion accessible porosities of HTO were ε = 0.44 ± 0.08 and ε = 0.55 ± 0.10. These two different results for two bore core parts show the heterogeneity of the clay. However, the data are in agreement with literature data (e.g. Tevissen et al. 2004). For the U(VI) diffusion experiments 233U and 14C-labeled synthetic HA (Sachs et al. 2004) are used as tracer. The concentrations are set to [U] = 1 x 10-6 M and [HA] = 10 mg/L in synthetic opalinus clay pore water as mobile phase. So far, the experiments are carried out for three months. During this time no activity of 233U was measured in the low concentration reservoirs. That points out the good retention behavior of opalinus clay towards U(VI). However, this result is in contrast to results of batch sorption experiments with opalinus clay powder. There, a low sorption affinity of U(VI) towards opalinus clay was determined in pore water, although a higher surface area was accessible for U(VI) (specific surface area (BET) = 41.6 m2/g). Opalinus clay contains a calcite fraction of 13 % (Nagra 2002). The leached out calcium ions determine the speciation of U(VI) in the pore water, because the neutral aquatic complex Ca2UO2(CO3)3 (Bernhard et al. 2001) is formed, which dominates the U(VI) speciation (99 %). This complex sorbs only weakly onto opalinus clay (0.05 ± 0.002 µg U/m2 clay). The good retention ability of opalinus clay during the diffusion experiments can be attributed to the lack of free length of paths for the present Ca2UO2(CO3)3(aq) in the intact bore core piece. In contrast to the U(VI) diffusion, diffused HA molecules were measured in the low concentration reservoir 13 days after start of the experiment...