Oral Session 5a. Retention-Sorption. Chairman : J. CUADROS

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1 Oral Session 5a Retention-Sorption Chairman : J. CUADROS

2 O-5a-1 THERMODYNAMIC MODELLING OF RADIONUCLIDE RETENTION IN CLAYS : SURFACE COMPLEXATION OR SOLID SOLUTION EQUILIBRIA? Dmitrii Kulik, Enzo Curti, Urs Berner Waste Management Laboratory, Paul Scherrer Institute, CH-5232 Villigen PSI, Switzerland The GEM-Selektor Gibbs energy minimization code offers two alternative approaches for the thermodynamic modelling of metal uptake by mineral solids from aqueous solutions: surface complexation models (SCM) and solid solution - aqueous solution (SSAS) models. Yet, SCM and SSAS lead to divergent predictions when applied to radionuclide sorption on clays under conditions typical for nuclear waste repositories. Two main goals of this discussion paper are to identify the reasons for these discrepancies, and to comment on the suitability of both approaches depending on the system boundary conditions. SCMs [1] treat clay as a mineral sorbent with known surface parameters (specific surface area, fractional and maximum site density of surface patches, CEC, capacitance density, etc.). The sorbed metal is assigned exclusively to a surface monolayer located on the amphoteric particle edge- or permanent charge ion-exchange basal surfaces. In some cases (as Ni on smectite [2]), such binding could be confirmed by spectro/microscopic techniques. In the laboratory, adsorption and desorption seem to reach equilibrium within timescales of minutes to weeks. However, at longer timescales, the sorption of specific metals is irreversible [3], and phenomenological distribution coefficients (R d ) increase. For instance, sorption of Cs on illitic clays becomes essentially irreversible even after relatively short reaction times [4]. High R d values may also indicate that more than a surface monolayer is involved in sorption. Such facts raise doubts whether SCMs alone can predict the fate of radionuclides over time scales from 10 1 to 10 6 years in repositories hosted in clay formations or sealed with bentonite. SSAS models [5] quantify metal uptake into a variable-composition solid phase using thermodynamic stability constants of two or more end members. Knowledge of surface parameters is not required. Instead, the amount of solid equilibrated with the aqueous solution must be known independently (at best from measurements of recrystallization rates via radiotracer); considerable analytical effort (XRD, microscopy, spectroscopy) is often needed to characterize solid solutions unequivocally. Partial equilibration of a host mineral grain results in a growing rim containing the "sorbed" trace metal ions mixed within the hostmineral structure, while the particle core remains unchanged. This calls for defining an effective solid/ water ratio (s/w) for composition of such rims that can be represented thermodynamically as an ideal or non-ideal solid solution between the trace- and the hostmineral end-members. For each end-member, stoichiometry and standard molar Gibbs energy G o T must be determined, plus additional mixing parameters if the solid solution is non-ideal. Smectites and illites form complex solid solutions where cation substitutions occur at minimum three structural positions. Although end-member stoichiometries can be easily determined [6], the corresponding G o T values and mixing properties [7] are still largely unknown. To overcome such difficulties, new dual-thermodynamic techniques [8] may help greatly. Clays In Natural And Engineered Barriers For Radioactive Waste Confinement Page 63

3 O-5a-1 Solid solutions form slowly, over weeks to many years (depending on temperature and kinetics). With time, the extent of recrystallization increases while reducing the mobility of the captured metals a quite favourable situation in the context of waste repository performance assessment. Solubility of trace radionuclides incorporated in a mixed solid phase may be orders of magnitude below that of the pure phase, especially in the state of stoichiometric saturation. Such effects of solid solution formation, well known for carbonate [9] or sulfate minerals, may be misinterpreted for clay systems in terms of the surface binding processes. Why then the attempts to model trace metal retention in clays by using the SSAS approach were not very successful so far? We believe that, firstly, the common modeling tools (law of mass action, LMA- based codes) were not really efficient in solving multi-component solid solution equilibria; only recently, the appropriate GEM technique and software have been made available. Secondly, experiments with clay-radionuclide systems have been carried out so far over too short time scales, and usually in a way precluding the simultaneous determination of stoichiometries, G o T values of end-members, and excess mixing parameters. SCMs seem to be sufficient in quantifying metal adsorption on clays at laboratory times, despite that reversibility has rarely been checked. On the other side, when geologic times get involved, in our view, the solid solution equilibria are more appropriate. Since the prevalent incorporation mechanisms (sorption at the surface or structural incorporation) will change somewhere in between along the time scale, the question arises - how to decide which model (SCM or SSAS) is to be taken as the more appropriate. We believe that the answer will come from the side of basic scientific understanding integrating diffusion experiments and molecular dynamics simulations with studies of clay solubility and metal sorption, clay sedimentology, and thermodynamic data acquisition combined with GEM geochemical modelling. We would finally emphasize (to be shown in the paper) that surface adsorption reactions can also be represented in terms of SSAS models, while the opposite approach does not seem feasible. References 1. Kulik D.A.: Geoch.Cosmoch. Acta (2000) 64, 3161; Radioch. Acta (2002), in press. 2. Dähn R. Ph. D. Dissertation (2001) Nr , ETH Zurich. 3. Brümmer G.W., Gerth J., Tiller K.G.: J. Soil Sci. (1988) 39, Comans R.N.J., Hockley D.E.: Geoch. Cosmoch. Acta (1992) 56, Königsberger E., Gamsjäger H.: Amer. J. Sci. (1992) 292, Ransom B., Helgeson H.C.: Clays Clay Miner. (1993) 41, Stoessel R.K.: Geoch. Cosmoch. Acta (1981) 45, Kulik D.A., Kersten M., Heiser U., Neumann T.: Aquat. Geochem. (2000) 6, Curti E.: Appl. Geochem. (1999) 14, 433. Page 64 Clays In Natural And Engineered Barriers For Radioactive Waste Confinement

4 O-5a-2 THE ROLE OF NATURAL ORGANIC MATTER IN RADIONUCLIDETRANSPORT : THE MIGRATION BEHAVIOUR OF 241 Am - ORGANIC MATTER COMPLEXES IN BOOM CLAY N. Maes 1, T. Beauwens 1, L. Wang 1, H. Moors 1, P. De Cannière 1, G. Delécaut 1, M. Put 1, A. Dierckx 2, R. Gens 2, P. Warwick 3, A. Hall 3, J. Van Der Lee 4, A. Maes 5 1. SCK-CEN, Waste and Disposal R&D Geological Disposal, Mol, Belgium 2. NIRAS/ONDRAF, Brussels, Belgium 3. University of Loughborough, Loughborough, UK 4. Ecole des Mines de Paris, Fontainebleau, France 5. KULeuven, Leuven, Belgium Humic substances play an important role in the transport behaviour of a wide range of cations, and especially trivalent actinides form strong complexes with humic substances. The complexation of these actinides, with organic matter (OM) may have two opposite effects. If complexed by immobile organic matter, their migration will be retarded. On the other hand if a stable complex is formed with mobile organic matter, the transport of the radionuclides can be enhanced. Boom Clay (BC) contains 1-4 % OM from which only a small part, less than 1%, is considered as mobile. Mobile OM was concentrated from Boom Clay water sampled in the HADES underground research laboratory at SCK-CEN and 14 C-labelled at the University of Loughborough. Three size fractions were used for migration experiments: full size range (Full), fraction <1000 MWCO (Low) and the fraction > MWCO (High). The double labelled complexes are prepared by contacting 241 Am with these different 14 C-labelled OM fractions. Speciation calculations, using the CHESS code, with Am-OM complexation constants derived from short term complexation experiments predicted that Am should be completely associated to OM. Two different types of migration experiments were performed on undisturbed BC cores (cored at the HADES underground research laboratory at SCK-CEN): pulse-injection percolation experiments (with a hydraulic gradient), in which the breakthrough curve is monitored, and electromigration experiments (with an electrical gradient), where the distribution pattern in the clay is measured. The pulse injection experiments were performed with freshly prepared Am- 14 C-OM complex. The following conclusions could be made: the majority of the Am- 14 C-OM complex dissociates and the major part (99%) of the Am remains strongly sorbed onto the clay while the 14 C-OM is recovered. However a very small fraction of Am exhibits a first fast breakthrough (concentration < mol/l), and elutes at the same time as the OM. This fast breakthrough cannot be explained in terms of thermodynamic equilibrium between the different phases present in the system. The same 241 Am- 14 C-OM solutions were used for electromigration experiments after an ageing period of 4 years. We observed that in the OM fraction <1000 MWCO, less than 1% of 241 Am was associated to OM. Only in the high MW fraction (>100000) we found that 30% of the 241 Am was associated to OM. After electromigration, the distribution of the 241 Am and 14 C activity is determined in the clay core. The 241 Am distribution profile did not follow the Clays In Natural And Engineered Barriers For Radioactive Waste Confinement Page 65

5 O-5a-2 14 C distribution pattern. Within the limit of detection (10-10 mol/l) we could not observe any 241 Am moving away from the source position. To translate these results in parameters for performance assessment (PA), we considered a two-fold behaviour of Am. The major part of Am is strongly retained either by a strong complexation with immobile OM or sorption onto minerals present in the Boom Clay. The migration parameters are: retardation factor (R) = 1000, apparent diffusion coefficient (D app ) = m²/s and a diffusion accessible porosity (η) = 0.3. The second, much smaller part is a mobile Am fraction, not in equilibrium with the solid phase. The transfer of 241 Am between the strongly sorbed phase and the mobile phase cannot be estimated. Experiments demonstrated a rather constant concentration of 241 Am in the percolate, which can be attributed to a constant leaching of 241 Am from the retained part to the mobile part. This idea has been translated in term of an operational solubility for Am (S op = mol/l) which is much lower than the solubility of Am in Boom Clay water. Because the mobile Am-species is predicted to be an organic complex, the migration parameters of organic matter are used to describe the transport of the mobile Americium fraction. The parameters are: effective diffusion coefficient (D eff ) = m²/s, R=1 and η = Acknowledgement This study is financially supported by the European Commission through the R&D programme Nuclear Fission Safety contract No. FI4W-CT (Trancom-Clay) and FIKW-CT (Trancom-II) and by the Belgian Agency for Radioactive Waste and Enriched Fissile Materials (NIRAS/ONDRAF Belgium). The latter also ensures the scientific coordination of the conducted research. Page 66 Clays In Natural And Engineered Barriers For Radioactive Waste Confinement

6 O-5a-3 RETENTION OF ANIONIC RADIONUCLIDES BY NATURAL AND ORGANOPHILIC CLAYS IN HIGH - MOLAR SALINE SOLUTIONS B. Riebe, C. Bunnenberg Center for Radiation Protection and Radioecology (ZSR), University of Hannover, Herrenhaeuser Str. 2, D Hannover, Germany The retention of radionuclides by engineered clay barriers in waste repositories is primarily controlled by the low hydraulic conductivity of the employed material and its high sorption capacity for ions. Natural clays generally show excellent sorption capabilities for radionuclides in cationic forms, but have a weak affinity to anionic contaminants. The weak sorptive capabilities for anionic radionuclides (like iodide and pertechnetate) can be improved substantially when replacing the natural inorganic interlayer cations by organic cations, like quaternary alkylammonium. The resulting organo-clays are capable of sorbing non-ionic organic compounds [1, 2] as well as iodide and pertechnetate [3, 4]. In our investigations, organo-clays were prepared by adding hexadecylpyridinium (HDPy + ) to MX-80 bentonite and Russian vermiculite in amounts corresponding to CEC fractions of the clay between 50 and 200 %. The sorption behaviour of these organo-clays for iodide was studied in batch experiments. It was found that with increasing organophilicity, the clays exhibited increasing sorption of iodide, characterized by the distribution ratio, K d. This anion sorption can be explained by the uptake of HDPy + in excess of the CEC and/or by the uptake as HDPyCl molecules as well as by the change of the particle surface charge from negative to positive values. As different geological formations have been proposed as host rocks for waste repositories, a number of parameters have to be taken into account, which are typical for the formation and the type of waste under consideration. In earlier experiments, the effect of the electrolytes employed was investigated. Iodide and pertechnetate sorption on MX-80 bentonite in bidistilled water, in synthetic ground water and in sea water with half of the original ionic strength were compared [5]. Our present work deals with the sorption of anionic radionuclides on natural and organophilic clays as influenced by high-molar saline solutions, which are adapted to naturally occurring brines from rock salt formations. First results confirm the effect found in the previous tests mentioned above: The sorption of radionuclides decreases gradually with increasing ionic strength of the solutions. However, organophilic clays remain capable of sorbing iodide even in the presence of high amounts of competing anions. In order to simulate near-field conditions, a further set of experiments is in the planning stage, in which the organo-clays will be treated with saline solutions at elevated ambient temperatures. In tests with synthetic ground water at temperatures up to 90 C, a slight decrease in sorption K d -values was observed for MX-80-bentonite and Russian Vermiculite with rising temperature [6]. Also those results will be presented. [1] Boyd, S. A. and Janes, W. F. (1993): Role of Layer Charge in Organic Contaminant Sorption by Organo-Clays. In: A: R. Mermut (Editor). Layer Charge Characteristics of 2:1 Silicate Clay Minerals, CMS Workshop Lectures, Vol. 6, The Clay Minerals Society, Boulder, CO. U, S. A. [2] Stockmeyer, M. R. (1991): Adsorption of organic compounds on organophilic bentonites. Appl. Clay Sci., 6: Clays In Natural And Engineered Barriers For Radioactive Waste Confinement Page 67

7 O-5a-3 [3] Bors, J. (1990): Sorption of radioiodine in organo-clays and soils. Radiochim. Acta, 51: [4] Bors, J., Dultz, St., Riebe, B. (2000): Organophilic bentonites as adsorbents for radionuclides. I. Adsorption of anionic and cationic fission products. Appl. Clay Sci. 16, [5] Riebe, B., Bors, J., Dultz, S. (2001): Retardation capacity of organophilic bentonite for anionic fission products. J. Contaminant Hydrology 47, [6] Riebe, B., Bunnenberg, C., Erten, H.N. (2001): Influence of temperature and composition of electrolytes on iodide sorption by organophilic clays. 8 th Int. Conf. on Chemistry and Migration Behaviour of Actinides and Fission Products in the Geosphere, Bregenz, Österreich, Poster. Page 68 Clays In Natural And Engineered Barriers For Radioactive Waste Confinement

8 O-5a-4 SURFACE COMPLEXATION MODEL OF URANYL SORPTION ON GEORGIA KAOLINITE T. E. Payne 1, J.A. Davis 2, G.R. Lumpkin 1, R. Chisari 1, T.D. Waite 3 1. Australian Nuclear Science and Technology Organisation, PMB 1, Menai, NSW 2234, Australia 2. US Geological Survey, 345 Middlefield Rd, Menlo Park, CA School of Civil and Environmental Engineering, University of New South Wales, Sydney, NSW 2052, Australia Kaolinite is a significant component of many soils and rock fracture infill materials; and may also be present in engineered barriers for the confinement of radioactive waste. Therefore, a study of the adsorption of uranyl on Georgia standard kaolinites (KGa-1 and KGa-1B) has been undertaken. The effects of experimental variables such as ph, total U, mass loading and the presence of sulfate were examined. As previously observed, the uptake of uranyl in airequilibrated systems increased with increasing ph and reached a maximum in the near-neutral ph range. At higher ph values, the sorption decreased due to the presence of aqueous uranyl carbonate complexes. This decrease does not occur when carbonate is excluded from the experiments. Evidence that sorption is the dominant process in the experimental systems with low total uranium (ΣU) was provided by the sensitivity to kaolinite mass loadings of several of the U uptake curves. At higher ΣU, precipitation of a U-rich phase occurred in some experiments. Sulfate had a small effect on U sorption, which may be explained by uranyl complexation in the aqueous phase. The kaolinite samples were examined after the uranyl adsorption experiments using Analytical Electron Microscopy (AEM) and energy dispersive X-ray spectroscopy (EDS) to determine the U content. This indicated that uranium is preferentially adsorbed by Ti-rich impurity phases (predominantly anatase) which are present in the kaolinite samples. In modelling uranyl sorption on the Georgia kaolinites, the presence of the TiO 2 phases was taken into account. The system was modelled as a combination of titanol and aluminol sites, using a simple non-electrostatic surface complexation model. The relative amounts of U- binding >TiOH and >AlOH sites were estimated from the AEM results. Precipitation was considered in modelling the experimental data obtained with high U concentrations. The inclusion of a ternary uranyl carbonato complex on the titanol site improved the fit of the model to the experimental data in the higher ph range. The final model contained only three optimised log K values, and no other optimised parameters, and was able to simulate adsorption data across a range of experimental conditions. This is believed to be the first time that the AEM technique has been applied in interpreting the results of a study of metal sorption on a mixed phase. The modelling results show that the >TiOH (anatase) sites play an important role in retaining U at trace uranyl concentrations, which are relevant to many natural and engineered kaolinitic systems. As anatase is often present in kaolinitic materials, its presence may need to be considered when modelling systems involving kaolinite. The presence of abundant TiO 2 grains in the Georgia kaolinites should be taken into account when these materials are employed as model minerals in sorption experiments with radionuclides or trace metals. Clays In Natural And Engineered Barriers For Radioactive Waste Confinement Page 69

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