May 11 – 16, 2014
Casino Conference Centre
Europe/Prague timezone

Speciation of technetium after sorption and diffusion in Opalinus Clay

May 13, 2014, 9:15 AM
Mirror Hall (Casino Conference Centre)

Mirror Hall

Casino Conference Centre

Reitenbergerova 4/95, Mari&#225;nsk&#233; L&#225;zn&#283;, Czech Republic <font color=white>
Verbal Chemistry of Nuclear Fuel Cycle / 1st ASGARD International Workshop Chemistry of Nuclear Fuel Cycle 2


Ms Maria Lübke (Institute of Nuclear Chemistry; Johannes Gutenberg University Mainz (Germany))


The long-lived radioactive isotope technetium-99 (99Tc, t1/2= 2.14•105 a) is a fission product of 235U and 239Pu and can be released to the environment from nuclear facilities, high-level radioactive waste repositories or as a result from nuclear weapon testing. Its geochemistry is dominated by the very mobile and soluble pertechnetate anion (TcO4-) under oxic, and a less mobile and less soluble TcO2•nH2O phase under anoxic conditions [1]. For the storage of high-level nuclear waste, argillaceous rocks are under investigation as potential host rocks for repositories in several European countries. Thus, detailed information about the interaction of Tc with these host rock formations is mandatory for the assessment of the long-term safety of high-level nuclear waste repositories. For our studies we selected Opalinus Clay (OPA) from Mont Terri, Switzerland, and its corresponding pore water. OPA is mainly composed of clay minerals (66 wt.%), quartz (14 wt.%), calcite (13 wt.%), siderite (3.0 wt.%), pyrite (1.1 wt.%), and organic carbon (0.8 wt.%) [2]. The sorption of 4-13 µM Tc(VII) on OPA powder was studied in batch experiments under aerobic and anaerobic conditions as a function of pH, clay concentration, and amount of dissolved Fe(II). Diffusion experiments were performed with intact OPA bore cores (20x10 mm and 25x11 mm) with diffusion parallel and perpendicular to the bedding. The experiments were performed under aerobic conditions with 7 µM TcO4- at neutral pH. Furthermore, different sorption samples on OPA thin sections (contact area 16 mm²) with 3-35 µM Tc(VII) were prepared under anaerobic conditions for spatially resolved, molecular-level investigations to determine the speciation of Tc on the mineral surface. The batch experiments showed that in the neutral pH range the sorption of Tc on OPA is very low, i.e. ≤ 1% under aerobic and 8% under anaerobic conditions. We also found that the sorption is independent of the clay concentration. Addition of dissolved Fe2+ increased the uptake of Tc by OPA up to 99%. The low sorption of Tc on OPA under aerobic conditions agrees well with our results from diffusion experiments, where Tc(VII) migrates quickly through an intact OPA bore core within about one week in case of diffusion parallel to the bedding. For diffusion perpendicular to the bedding, TcO4- migrates through the bore core within one month. µ-XRF mappings of the sorption samples showed a homogeneous distribution of Tc on the OPA surface with some local enrichments. In these spots the dominant oxidation state of Tc was found to be tetravalent as determined by Tc K-edge µ-XANES measurements. Our results indicate that OPA is able to reduce at least part of TcO4- to a less mobile and less soluble Tc(IV) species and thus may retard the migration of Tc from the repository to the environment. This work was financed by BMWi under contract no. 02E10981. The authors thank Diamond Light Source for access to beamline I18 (Proposal 8725) that contributed to the results presented here and Prof. Mosselmans for his support during the experiment. The research leading to these results has received funding from the European Community’s Seventh Framework Programme (FP7/2007-2013) under grant agreement nº 226716. References: [1] K. Lieser; Ch. Bauscher, Radiochim. Acta, 42, 1987,205 [2] NAGRA, Techn. Bericht NTB 02-03, NAGRA Wettingen/Schweiz, 2002

Primary author

Ms Maria Lübke (Institute of Nuclear Chemistry; Johannes Gutenberg University Mainz (Germany))


Mr Jakob Drebert (Institute of Nuclear Chemistry; Johannes Gutenberg University Mainz (Germany)) Dr Samer Amayri (Institute of Nuclear Chemistry; Johannes Gutenberg University Mainz (Germany)) Prof. Tobias Reich (Institute of Nuclear Chemistry; Johannes Gutenberg University Mainz (Germany))

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