Complexation of europium and uranium with natural organic matter (NOM) in highly saline water matrices analysed by ultrafiltration and inductively coupled plasma mass spectrometry (ICP-MS)☆

Abstract Along with general scientific developments for the safe long-term storage as well as for disposal of high level radioactive waste (HLW) an onward improvement in the geochemical process understanding is crucial. Natural organic matter (NOM) can play an important role in the immobilisation or mobilisation of these metal ions due to their complexation and colloid formation tendency. In this study, the complexation behavior of humic acids (HA) and other NOM from different sources and its influence on the mobilisation of europium as homologues of the trivalent actinides like americium and uranium as main component of HLW have been analysed in highly saline solutions of CaCl 2 and NaCl solutions up to 1 moL L −1 ion strength. Ultrafiltration (UF) in combination with inductively coupled plasma mass spectrometry (ICP-MS) has been used for the evaluation of complex stability constants log β. In selected cases capillary electrophoresis (CE) coupled to ICP-MS is used as complementary speciation technique to verify the UF results. To determine the complex stability constants a simple single site model is used. Depending on the source of the analysed NOM log β values in the range of 6.8–4.5 for Eu(III) and 6.3–4.5 for U(VI) (UO 2 2+ ) can be estimated by UF and CE-ICP-MS experiments. Increasing ionic strength (1 moL L −1 NaCl) reduces the available complexation sites of NOM resulting in lower amounts of NOM-complexed Eu(III) and U(VI) (UO 2 2+ ), respectively. The additional presence of calcium (0.5–5 mmoL L −1 CaCl 2 added to 0.1 moL L −1 NaCl) as a bivalent competing cation leads to lower levels of binding to NOM particularly for europium and Aldrich HA as well as for Eu and HA extracted from Gorleben site. In the case of uranyl, the results are different in comparison to europium. Higher amounts of Ca (5 mmoL L −1 ) in solution increase the levels of NOM complexation for uranyl. In contrast to the results for HA the used Suwannee river NOM reveals log β values in the range of nearly two orders of magnitude lower (4.5–4.1 for Eu 3+ and 4.8–4.6 for UO 2 2+ ). Moreover, the three examined HA from different sources (soil and river HA extracts) show significant differences in their complexation behavior under the geochemical conditions applied in this study.