Uranium(VI) and neptunium(V) retention by clay minerals and natural clay rock – Influence of clay organics, temperature and pore water salinity

The long-term disposal of high-level nuclear waste in deep geological formations is discussed worldwide as main strategy for nuclear waste management. In addition to salt and crystalline rock, argillaceous rock has been proposed as potential host rock and backfill material for nuclear waste repositories. This is due to favorable characteristics of clay minerals such as their swelling properties and their large surface areas leading to a low permeability and high retention efficiency toward radionuclides. Sorption and diffusion of safety relevant radionuclides on/in clay rock are important physicochemical processes that have to be studied to contribute to a reliable long-term safety assessment for future nuclear waste repositories.

Clay rock is closely associated with natural organic matter. Low molecular weight organic acids such as acetate, lactate, propionate and formate as well as fulvic and humic acid-like substances can be released from the clay under certain conditions [1]. These organics can influence the migration of radionuclides in the environment by forming soluble complexes or stable colloids. In addition, for the disposal of high-level nuclear waste in clay formations, elevated temperatures of up to 100 °C are expected close to the waste containers resulting from the radioactive decay of the stored radionuclides. Besides that, pore waters of North German clay deposits, considered for underground nuclear waste repositories, show relatively high salinities (2 to 3.5 M). Thus, effects of organic matter, temperature and salinity have to be taken into account for a reliable prediction of radionuclide migration in the geosphere.

Batch sorption studies with clay minerals revealed that the U(VI) sorption onto kaolinite [2] is much higher than the Np(V) sorption [3] in 0.1 M NaClO4. In the presence of humic acid, the actinide sorption is increased in the acidic pH range and decreased in the near-neutral pH range. The influence of ionic strength was studied for U(VI) sorption onto montmorillonite by applying 0.1 to 3 M NaCl and CaCl2 as background electrolytes. It was observed that the ionic strength effect on U(VI) sorption on montmorillonite is very small. In NaCl, a decrease of U(VI) sorption with ionic strength is only observed below pH 6 and up to an ionic strength of 2 M. In CaCl2, U(VI) sorption is at least partly governed by secondary phase formation.

The retention behavior of the natural clay rock Opalinus Clay (Mont Terri, Switzerland) toward U(VI) was studied by means of batch sorption and diffusion experiments. Opalinus Clay is considered as representative host rock of a potential nuclear waste repository in argillaceous rock. For the U(VI) sorption onto Opalinus Clay in synthetic Opalinus Clay pore water (pH = 7.6, I = 0.36 M) at 25 °C, the distribution coefficient, Kd, was determined with 22.2 ± 0.4 L/kg [4]. This shows that the U(VI) sorption is relatively weak under pore water conditions. It is comparable to the Np(V) sorption onto Opalinus Clay (25 ± 5 L/kg [5]). This can be attributed to the predominance of the weakly sorbing Ca2UO2(CO3)3(aq) complex in the Opalinus Clay pore water.

The U(VI) sorption onto Opalinus Clay was also studied in 0.1 M NaClO4 in the pH range 3 to 10. Results of surface complexation modelling [6], applied for blind prediction of pH-dependent U(VI) sorption onto Opalinus Clay, showed that U(VI) may predominantly sorb onto illite and montmorillonite. In addition, U(VI) sorption onto Fe(III) minerals was predicted. U(VI) sorption onto further minerals of the clay (kaolinite, chlorite, quartz, feldspars) is negligible.

With increasing concentration of low molecular weight organic acids (10-5 to 10-2 M), the U(VI) sorption onto Opalinus Clay in pore water decreases due to complex formation in solution. The mobilizing effect of the organics on U(VI) increases in the following sequence: formate < lactate ~ acetate ≤ propionate < tartrate < citrate. In the presence of 1×10-2 M citrate, which has been identified as important ligand in radioactive waste problems, the Kd value for U(VI) amounts to only (1.1 ± 0.3) L/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 affect U(VI) sorption [4]. With increasing temperature up to 60 °C, the U(VI) sorption increases both in the absence and in the presence of clay organics.

The U(VI) diffusion in compacted Opalinus Clay was studied in the absence and presence of humic acid at 25 and 60 °C under anaerobic conditions using Opalinus Clay pore water [7]. The effective diffusion and distribution coefficients (De and Kd) determined for U(VI) and humic acid at 25 and 60 °C show that humic acid has no significant influence on the U(VI) diffusion. The diffusion profiles obtained for humic acid in Opalinus Clay at 25 and 60 °C show contributions of at least two different humic acid particle size fractions (< 1 kDa and 10−100 kDa). The smaller humic acid fraction diffused through the whole clay samples at both temperatures within three months whereas the larger humic acid fraction diffused only about 500 µm into the clay. This shows the filtration effect of the compacted clay and also a different sorption affinity of the humic acid size fractions toward Opalinus Clay. At 60 °C, the diffusion profiles of two U(VI) species were observed, which were attributed to a colloidal and an aquatic U(VI) species. The De and Kd value of the aquatic U(VI) species increased with increasing temperature. However, these changes compensated each other, thus, an increase of the temperature to 60 °C did not accelerate the migration of U(VI) through Opalinus Clay.

Under environmentally relevant conditions (pH > 7), U(VI) is only weakly sorbed by the natural clay rock Opalinus Clay. However, since molecular diffusion is the decisive retardation process in clay rock, Opalinus Clay has a good retention potential toward U(VI).

[1] Courdouan, A. et al. (2007) Appl. Geochem. 22, 2926-2939.
[2] Křepelová, A. et al. (2006) Radiochim. Acta 94, 825-833.
[3] Schmeide, K. and Bernhard, G. (2010) Appl. Geochem. 25, 1238-1247.
[4] Joseph, C. et al. (2011) Chem. Geology 284, 240-250.
[5] Wu, T. et al. (2009) Environ. Sci. Technol. 43, 6567-6571.
[6] Joseph, C. et al. (2013) Appl. Geochem. 36, 104-117.
[7] Joseph, C. et al. (2013) Geochim. Cosmochim. Acta 109, 74-89.