Insights into the retention behaviour of europium(III) and uranium(VI) onto Opalinus Clay influenced by pore water composition, temperature, pH and organic compounds

Abstract In the scientific community, a consensus exists to store high-level radioactive waste (HLW) in deep geological formations for several hundred thousand years to protect humanity and the environment. After a container damage followed by the release of HLW components, the retention of the radionuclides can be affected by diverse chemical reactions in the geosphere. In this study, geochemical effects possibly influencing the retention of the radionuclides were investigated. Therefore, the retention behaviour of U(VI) and Eu(III) (as homologue for trivalent actinides) onto Opalinus Clay (OPA) as a potential host rock for a HLW disposal was analysed at different geochemical conditions. For these experiments, a wide range of metal concentrations in presence and absence of natural clay organic matter (NOM) (lactate or humic acid) in different types of background electrolytes (0.01 mol L−1 sodium perchlorate and 0.4 mol L−1 synthetic OPA pore water) were investigated. Inductively coupled plasma mass spectrometry (ICP-MS) was used as a high specific and very sensitive method for elemental trace analysis. For the speciation of Eu3+-NOM-complexes, a method with high separation performance, capillary electrophoresis, was hyphenated with ICP-MS. The performed batch experiments simulate a potential water intrusion into the waste disposal. The retention (determined as log Kd values) of the metal ions strongly depends on the pH value, the presence of competing cations, temperature and NOM. At a pH value of pH 7.6 in synthetic OPA pore water, which is relevant for a disposal in OPA formations, the sorption of Eu(III) onto OPA (4 g L−1) is significantly higher (log Kd = 3.35 ± 0.04 at 25 °C and 4.85 ± 0.08 at 60 °C) than the retention of uranyl (log Kd = 2.17 ± 0.21 at 25 °C and 2.86 ± 0.01 at 60 °C) due to the formation of an uncharged aqueous calcium-uranyl-carbonate-complex (Ca2UO2(CO3)3(aq)) which does not sorb onto OPA. Modelling metal retention data onto OPA with PHREEQC reveals a very good agreement with most measurement results. At elevated temperatures, PHREEQC modelling could not be performed due to lack of data, especially for possible secondary minerals. The observed increased Kd values at elevated temperature revealed in this study might be assumed advantageous for the safe storage of HLW in a future repository.