Gunnar Buckau

Humic colloid mediated transport of tetravalent actinides and technetium

Abstract The humic colloid mediated transport of tetravalent actinide and technetium ions is discussed. Numerous published and new results are used from comparable experimental systems and conditions. The results originate from investigations on two humic rich Gorleben groundwater/sediment systems under near-natural conditions. The conditioning time between groundwater and radionuclides vary from few minutes to four years. The migration time (residence time in the column) is varied from four hours to three days. The concentration of radionuclides added is varied between about 10-12 and 10-4mol/L. Humic colloids in natural groundwater contain trace metal ions in varying concentrations. In addition to studying the behavior of radionuclides added to the groundwater in the laboratory, the dissociation behavior of these natural tetravalent trace metal ions is studied by scavenging dissociating ions with a cation exchanger. The results show a kinetic behavior of the trace metal ion interaction as frequently reported in the literature, i.e. with increasing contact time, slow dissociation modes are progressively populated resulting in an enhanced radionuclide transport. The strong dependency of the radionuclide transport on the radionuclide concentration shows that humic colloid mediated transport may be overestimated if radionuclide concentrations used in laboratory studies are too high. Dissociation of the natural inventory of humic associated tetravalent trace metal ions proceeds with a kinetics comparable to the radionuclides added. This, however, is only observed for a small portion of the inventory and extrapolation beyond the observation range is not permissible. Therefore, the key question for application to predictive modeling of the mobility of tetravalent radionuclides in natural groundwater is if the behavior of the natural trace element inventory is applicable or the behavior of radionuclides added in laboratory studies.

Interaction of europium with humic acid covalently bound to silica beads

The uptake of Eu3+ (a trivalent actinide homolog) by Aldrich humic acid covalently bonded to an inorganic support is examined. Two types of covalent linkages are used and the synthetic routes to produce the resins are discussed. The use of these resins excludes having to account for humic acid desorption from the surface, yields a well characterized system, and allows the experiment to focus on and account for the role of humic acid in the sorption of Eu. The proton exchange capacity of the resins is examined by titration and the differences observed are traced to the resin synthesis. Europium sorption experiments are performed at pH 4 and pH 6 in 0.1 M NaClO4. Kinetic experiments show equilibrium is reached in 24 hours. The kinetic data are used to evaluate the loading capacity, with results similar to equilibrium experiments. The complexation results are evaluated based on the metal ion charge neutralization model. For the resins an effect of pH and resin synthesis route on the Eu uptake is observed. The uptake increases with pH for both resins. The resin HA-Epo (Epoxy linkage) has a higher metal binding at pH 4, while the resin HA-HAB (2-hydroxylazobenzene linkage) had more proton exchange sites occupied by metal ions at pH 6. Overall, more Eu is bound to HA-Epo at pH 6 since its proton exchange capacity is higher. The evaluated stability constants vary slightly and show a dependence on the linkage group but are similar to literature values that examined complexation by aquatic humic acid analyzed with the same model. This result supports the utility of the metal ion charge neutralization model and the applicability of the resulting stability constants.

Characterization of a humic gel synthesized from an activated epoxy silica gel

Summary Purified humic acid has been covalently bound on activated epoxy silica gel particles. Determination of physical properties and chemical properties was conducted in order to characterize the material at different stages of the preparation. FTIR spectra and the PEC of the surface bound humic acid is very similar to that of humic acid starting material. This shows that the humic acid was not deteriorated during the surface binding process. This humic gel can be used as an analogue for sediment associated humic acid, with the advantage that covalently bound humic acid does not desorb, and thus allows for simple species separation between non-complexed and humic bound metal ions in batch and column experiments.

The Collaborative EC Project ReCosy

The Collaborative Project (CP) ReCosy (Redox Controlling Systems) was established in 2008. It falls within the EURATOM program and is implemented within the European Commission’s 7th Framework Program. Main objectives of CP ReCosy are the sound understanding of redox phenomena controlling the long-term release/retention of radionuclides in nuclear waste disposal and providing tools to apply the results to Performance Assessment/Safety Case. Although redox is not a new geochemical problem, different questions are still not resolved and thus raised by implementers and scientists. Radionuclide redox transformations on minerals are considered to be decisive scenarios in various FEP lists. 32 institutions from 13 European countries contribute to the 4-years CP by i) development of advanced analytical tools, ii) investigations of processes responsible for redox control (thermodynamically and kinetically controlled processes, surface reactions and microbial processes), iii) provision of required data on redox controlling processes, and iv) response of the disposal systems to external/internal disturbances. Systems which are investigated comprise simple very well defined ones under controlled laboratory conditions, complex laboratory systems, near-natural systems in the laboratory, real systems, and near-field systems controlled spent nuclear fuel. The scientific results of the CP ReCosy are reported during annual workshops (AW). The first AW took place in February 10–12, 2009. The scientific contributions underwent a peerreview and are published in the form of workshop proceedings together with scientific-technical overview papers of the different project workpackages. The present paper summarizes the main results obtained after the first year of ReCosy. The status during the project preparation phase of the CP ReCosy is published elsewhere [1].© 2009 ASME