Christophe Bruggeman

Evidence for the existence of Tc(IV) - humic substance species by X-ray absorption near-edge spectroscopy

Summary The redox–sensitive fission product technetium–99 has been investigated in systems containing different reducing solid phases (pyrite, magnetite, ironsulphide and Gorleben sand) on the one hand and Gorleben groundwater, which contains a high amount of humic substances, on the other hand. Initially, technetium–99 was added to these systems as pertechnetate (Tc(VII)), which was reduced in presence and absence of humic substances with the aid of the different reducing surfaces (neutral to alkaline pH). Both in absence and presence of humic substances, Tc concentrations were observed which exceeded the TcO2 solubility limit, whereby the presence of humic substances had a 100 fold higher Tc concentration compared to its absence. Using the La–precipitation method, it was shown that Tc(IV) inorganic colloids and organic colloids were quantitatively precipitated. It was demonstrated for the first time by a combination of chemical speciation methods (La–precipitation method and gel permeation chromatography) and XANES spectroscopy of the humic substance containing supernatant solutions, that Tc(IV) species were formed in these systems, indicating an association between Tc(IV) and humic substances.

The influence of natural organic matter on the speciation and solubility of Eu in Boom Clay porewater

Abstract The influence of natural organic matter (NOM) on the speciation and solubility of europium (Eu) was studied under geochemical conditions representative for the Boom Clay. Different organic matter types were used, and analysis was performed both after 0.45 μm microfiltration and after 30000 MWCO ultrafiltration to distinguish between larger colloids (assumed to be immobile under in situ conditions) and small dissolved species. Equilibrium was approached from undersaturation starting from synthesised Eu(OH)3(s), which, during the experiment, transformed into EuOHCO3(s), in agreement with thermodynamic considerations. In the absence of NOM, the Eu solution concentrations after 0.45 μm filtration exceeded the thermodynamic solubility of EuOHCO3(s) by several orders of magnitude, indicating the presence of inorganic Eu colloids. In the presence of NOM, the Eu solubility increased with increasing NOM concentration as was expected, but, surprisingly, was dependent on the operational size cut-off: at an identical NOM concentration in the filtrate, the Eu solution concentration after 0.45 μm filtration was consistently higher compared to the Eu concentration after 30000 MWCO filtration. This latter observation necessitates detailed knowledge concerning the pore size cut-off of Boom Clay under in situ conditions in order to use the correct Eu-NOM complexation constant and/or maximum solubility in transport calculations. At higher NOM concentrations (TOC>30 mg/L) the Eu solubility after 0.45 μm filtration was seemingly independent of the NOM concentration. In contrast, after 30000 MWCO ultrafiltration, the Eu solution increased linearly with increasing DOC, from the expected thermodynamic solubility (∼5×10−7 mol L−1) at 0 mg L−1 DOC to ∼3×10−5 mol L−1 at 80 mg L−1 DOC. All of the data sets were modelled using the Nagra/PSI database [8] for solubility, hydrolysis and inorganic aqueous complexation reactions, and fitted organic complexation reactions between Eu3+ and NOM functional groups. Both a free ligand approach (with electrostatic correction) and the humic ion-binding model VI [23], which was for the first time successfully introduced into Phreeqc geochemical code, were tested and provided equally good fits to the data.

New selenium solution speciation method by ion chromatography plus gamma counting and its application to FeS2-controlled reducing conditions

Summary Selenium is a redox sensitive element. In reducing conditions its solubility is controlled by the formation of metallic Se and in the presence of Fe2+ also by the precipitation of FeSe or FeSe2. However very few data concerning this species in geochemical reducing environments is found in literature, particularly due to insufficient measuring methods. The assessment to what extent 79Se is a critical radionuclide for the geological disposal of High-Level Radioactive Waste, depends on its actual speciation in storage conditions. Therefore a new method based on ion chromatography of radiolabelled 75Se solutions followed by gamma-ray counting was developed to accurately measure selenium species with different degrees of oxidation (selenate (SeO42−) and selenite (SeO32−)) in solution. This method was then tested in laboratory conditions which mimic the reducing environment in Boom Clay. Different amounts of ground pyrite (< 125 µm) were contacted with Synthetic Boom Clay Water (essentially 10−2 M NaHCO3) and spiked with different amounts of 75SeO32− and SeO42−. The batches were allowed to equilibrate over different time periods (up to two months) before analysing. The experiments were carried out in an oxygen-depleted glove box (99.6% N2, 0.4% CO2). The kinetics of the redox reactions in the pyrite systems prevented the complete reduction of selenite (SeO32−) and especially selenate (SeO42−) on a limited time scale, probably due to the limited redox capacity of the studied systems.

Influence of Boom Clay organic matter on the adsorption of Eu3+ by illite - geochemical modelling using the component additivity approach

Abstract The solid–liquid distribution of europium (Eu) between an adsorptive surface and a solution phase containing a competitive colloid is the result of a delicate balance between several individual chemical reactions. In this study, adsorption isotherms of Eu in presence of dissolved Boom Clay natural organic matter were experimentally determined under conditions relevant for a geological repository (trace Eu concentrations, anoxic conditions, ∼0.014 mol l−1 NaHCO3 background electrolyte). It was found that both the concentration and size distribution (or operational cut-off used to discriminate between “mobile” and “immobile” colloids) of natural organic matter has a strong influence on the observed solid–liquid distribution. The experimental data were subsequently modelled using a component additive approach with two well-established sorption/interaction models: the 2 SPNE SC/CE model for describing Eu adsorption on illite, and Humic Ion-Binding Model VI for describing Eu complexation to natural organic matter. Model parameters were gathered from dedicated measurements in batch systems containing only Eu and the interacting phase under study, under similar conditions as in the ternary isotherm experiments. Mutual interactions between illite and natural organic matter were studied and quantified. Under the experimental conditions of this study, it was found that these interactions were only of minor importance. The two models were subsequently combined to blind predict the Eu solid–liquid distribution in the ternary batch experiments. Within an error margin of 0.5logߙKd units, the additivity approach succeeded well in predicting Eu uptake in all experimental systems studied. A sensitivity analysis was performed to select the most important model parameters influencing the Eu uptake, and the robustness of the model. This study has shown that the component additivity approach for describing and predicting uptake of trivalent lanthanides/actinides under Boom Clay conditions, is promising, and may help in unraveling the complex behaviour of these radionuclides witnessed in migration experiments.

Solubility study of Tc(IV) in a granitic water

The deep geological disposal of the high level radioactive wastes is expected to be a safe disposal method in most countries. The long-lived fission product 99Tc is present in large quantities in nuclear wastes and its chemical behavior in aqueous solution is of considerable interest. Under oxidizing conditions technetium exists as the anionic species TcO4− whereas under the reducing conditions, expected to exist in a deep geological repository, it is generally predicted that technetium will be present as TcO2·nH2O. Hence, the mobility of Tc(IV) in reducing groundwater may be limited by the solubility of TcO2·nH2O under these conditions. Due to this fact it is important to investigate the solubility of TcO2·nH2O. The solubility determines the release of radionuclides from waste form and is used as a source term in radionuclide migration analysis in performance assessment of radioactive waste repository. Technetium(IV) was prepared by reduction of a technetate solution with Sn2+. The solubility of Tc(IV) has been determined in simulated groundwater and redistilled water under aerobic and anaerobic conditions. The effects of pH and CO32− concentration of solution on solubility of Tc(IV) were studied. The concentration of total technetium and Tc(IV) species in the solutions were periodically determined by separating the oxidized and reduced technetium species using a solvent extraction procedure and counting the beta activity of the 99Tc with a liquid scintillation counter. The experimental results show that the rate of oxidation of Tc(IV) in simulated groundwater and redistilled water is about (1.49∼1.86)×10−9mol L−1d−1 under aerobic conditions, while no Tc(IV) oxidation was detected in simulated groundwater and redistilled water under anaerobic conditions. Under aerobic or anaerobic conditions the solubility of Tc(IV) in simulated groundwater and redistilled water is equal on the whole after centrifugation or ultrafiltration. The solubility of Tc(IV) increases with the decrease of pH at pH<2, increases with the increase of pH at pH>11 and is pH independent in the range 2<pH<11. The concentrations of Tc(IV) species were in the range of 10−8 to 10−9mol L−1 at 2<pH<11. The solubility of Tc(IV) slightly increases with increasing the increase of CO32− concentration. Geochemical modelling showed a good agreement between our experimental results and thermodynamic constants from the NEA TDB review. These data could be used to estimate the Tc(IV) solubility for cases where solubility limits transport of technetium in reducing environments of high-level waste repositories.