Joint Project: Interaction and transport of actinides in natural clay rock with consideration of humic substances and clay organics - Characterization and quantification of the influence of clay organics on the interaction and diffusion of uranium and americium in the clay

The objective of this project was the study of basic interaction processes in the systems actinide - clay organics - aquifer and actinide - natural clay - clay organics - aquifer. Thus, complexation, redox, sorption and diffusion studies were performed. To evaluate the influence of nitrogen, phosphorus and sulfur containing functional groups of humic acid (HA) on the complexation of actinides in comparison to carboxylic groups, the Am(III) and U(VI) complexation by model ligands was studied by UV-Vis spectroscopy and TRLFS. The results show that Am(III) is mainly coordinated via carboxylic groups, however, probably stabilized by nitrogen groups. The U(VI) complexation is dominated by carboxylic groups, whereas nitrogen and sulfur containing groups play a minor role. Phosphorus containing groups may contribute to the U(VI) complexation by HA, however, due to their low concentration in HA they play only a subordinate role compared to carboxylic groups. Applying synthetic HA with varying sulfur contents (0 to 6.9 wt.%), the role of sulfur functionalities of HA for the U(VI) complexation and Np(V) reduction was studied. The results have shown that sulfur functionalities can be involved in U(VI) humate complexation and act as redox-active sites in HA for the Np(V) reduction. However, due to the low content of sulfur in natural HA, its influence is less pronounced. In the presence of carbonate, the U(VI) complexation by HA was studied in the alkaline pH range by means of cryo-TRLFS (-120°C) and ATR FT-IR spectroscopy. The formation of the ternary UO2(CO3)2HA(II)4− complex was detected. The complex formation constant was determined with log β0.1 M = 24.57 ± 0.17. For aqueous U(VI) citrate and oxalate species, luminescence emission properties were determined by cryo-TRLFS and used to determine stability constants. The existing data base could be validated. The U(VI) complexation by lactate, studied in the temperature range 7 to 65°C, was found to be endothermic and entropy-driven. In contrast, the complex stability constants determined for U(VI) humate complexation at 20 and 40°C are comparable, however, decrease at 60°C. For aqueous U(IV) citrate, succinate, mandelate and glycolate species stability constants were determined. These ligands, especially citrate, increase solubility and mobility of U(IV) in solution due to complexation. The U(VI) sorption onto crushed Opalinus Clay (OPA, Mont Terri, Switzerland) was studied in the absence and presence of HA or low molecular weight organic acids, in dependence on temperature and CO2 presence using OPA pore water as background electrolyte. Distribution coefficients (Kd) were determined for the sorption of U(VI) and HA onto OPA with (0.0222 ± 0.0004) m3/kg and (0.129 ± 0.006) m3/kg, respectively. The U(VI) sorption is not influenced by HA (50 mg/L), however, decreased by low molecular weight organic acids (> 1×10-5 M), especially by citrate and tartrate. With increasing temperature, the U(VI) sorption increases both in the absence and in the presence of clay organics. The U(VI) diffusion in compacted OPA is not influenced by HA at 25 and 60°C. Predictions of the U(VI) diffusion show that an increase of the temperature to 60°C does not accelerate the migration of U(VI). With regard to uranium-containing waste, it is concluded that OPA is suitable as host rock for a future nuclear waste repository since OPA has a good retardation potential for U(VI)

Measurement and modelling of reactive transport in geological barriers for nuclear waste containment

Unit cell illustrating potential diffusion paths (bonds, yellow and red) in the neighbourhood of central particle (green); these join neighbouring cell faces and show where elongated pores may be assigned to the experimental pore system information.</p

Abtrennung von Uran aus wässriger Lösung durch Calix[6]arene mittels Flüssig-Flüssig-Extraktion sowie Festphasen-Extraktion

Die Uranspeziation in ausgewählten Sicker- und Grubenwässern des ehemaligen Uranbergbaus wurde mittels spektroskopischer Methoden (TRLFS, LIPAS) untersucht. Deren Kenntnis in Abhängigkeit vom pH-Wert ermöglicht die Optimierung der Uranabtrennung mittels uranophiler Calixarene. Mittels Flüssig-Flüssig-Extraktion wurde gezeigt, dass COOH-derivatisierte Calix[6]arene als effektive Extraktionsmittel für die selektive Uranylabtrennung aus umweltrelevanten Wässern bei pH-Werten größer 4 geeignet sind und für Praxisanwendungen eingesetzt werden können. Extraktionskonstanten wurden bestimmt. Die durch Fixierung dieser Calixarenderivate auf Polyester dargestellten calixarenmodifizierten Vliese sind in der Lage Uranylionen aus synthetischen Grubenwässern in Anwesenheit von Konkurrenzionen abzutrennen. Die Untersuchungen zur Reversibilität der Uranbindung an calixarenausgerüsteten Polyestervliesen haben gezeigt, dass eine fast vollständige Regenerierung der calixarenmodifizierten Vliese mittels verdünnter Mineralsäuren möglich ist. Die regenerierten textilen Filtermaterialien können für weitere Uranabtrennungszyklen eingesetzt werden. Nach Auswahl geeigneter Calixarenderivate ist eine Übertragung des entwickelten Abtrennungsprinzipes auf weitere Actinide (z.B. Np, Pu) bzw. Schwermetall-Kontaminanten (z.B. As, Cd, Pb) möglich

Joint Project: Migration of Actinides in the System Clay, Humic Substance, Aquifer - Migration Behavior of Actinides (Uranium, Neptunium) in Clays: Characterization and Quantification of the Influence of Humic Substances (Final Report BMWi Project No.: 02 E 9673)

Objective of this project was the study of interaction processes between humic substances, U(VI), Np(V) and kaolinite KGa-1b. It contributed to the attainment of a better process understanding, the improvement of the knowledge on the interaction of humic substances and metal ions and the enhancement of the thermodynamic database. With a synthetic humic acid (HA), N-containing functional groups of HA were characterized by 15N-NMR spectroscopy. Based on these results, model studies of the influence of amino groups on the complexation behavior of HA were performed. Spectroscopic studies with amino acids show that the amino group do not contribute to the U(VI) complexation at pH 4. The impact of kaolinite on the formation of HA and humic substance-kaolinite-sorbates was studied in model syntheses. The results exhibit that the presence of kaolinite during the syntheses mainly influences the yields on HA and their elemental compositions. Synthetic humic substance-kaolinite-sorbates were isolated. Under exclusion of CO2, the U(VI) complexation by HA was investigated at pH 7 by means of the conventional time-resolved laser-induced fluorescence spectroscopy (TRLFS) and TRLFS with ultrafast pulses. Complexation parameters for the ternary complex UO2(OH)HA(I) were determined. Studies of the Np(V) reduction in presence of HA with different functionalities under anaerobic conditions have shown that Np(V) is reduced to Np(IV) by HA. The redox capacity depends on the HA functionality. Applying a modified HA it was verified that phenolic/acidic OH groups play a dominating role in the Np(V) reduction. The influence of HA on the U(VI) and Np(V) sorption onto kaolinite was investigated in batch experiments. In dependence on the experimental conditions, HA effects the sorption and consequently the mobility of U(VI) and Np(V). From studies of the U(VI) sorption onto synthetic humic substance-kaolinite-sorbates it was concluded that the structure and functionality of sorbed/associated humic substances considerably influence the sorption behavior of U(VI). The structure of U(VI)-kaolinite-surface complexes in presence of HA was studied by means of X-ray absorption spectroscopy and TRLFS and compared to those of U(VI)-kaolinite-complexes. Investigations of the migration of HA and U(VI) in the laboratory system kaolinite-water were carried out in diffusion experiments. The migration of HA in compacted clay is governed by diffusion and influenced by its colloidal properties. Humic substances exert an immobilizing effect on the U(VI) transport in compacted kaolinite

Joint project: Geochemical retention of radionuclides on cement alteration phases (GRaZ) - Subproject B

The report summarizes the results obtained by the Institute of Resource Ecology of the Helmholtz-Zentrum Dresden-Rossendorf within the BMWi-financed Joint Research Project “Geochemical retention of radionuclides on cement alteration phases (GRaZ)”. The project focused on the retention behavior of Ca-bentonite and cementitious material, both constituents of the geo-engineered barrier of deep geological repositories for high-level radioactive waste, towards radionuclides. Specifically, the influence of increased salinities and of hyperalkaline conditions on interaction processes in the system radionuclides – organics – clay/cementitious materials – aquifer was studied. For this purpose, complexation, sorption and desorption studies were performed at alkaline to hyperalkaline pH conditions (pH 8-13) and under variation of the ionic strength (0.1 to 4 M) applying complex solution compositions. For the U(VI) citrate system molecular structures dominating in the pH range 2-9 were studied spectroscopically (NMR, UV-Vis, FT-IR). As dominating species 2:2, 3:3, 3:2 and, above critical concentrations also 6:6 and 9:6 U(VI) citrate complexes were identified or confirmed and complex formation constants were determined. U(VI) sorption on Ca-bentonite at (hyper)alkaline conditions in mixed electrolyte solutions was studied by means of batch sorption experiments. The U(VI) retention on Ca-bentonite was shown to be very effective at pH>10, even in the presence of carbonate and despite the prevalence of anionic aqueous uranyl species. The presence of two independent U(VI) surface complexes on Ca-bentonite at pH 8-13 was shown by site-selective TRLFS and EXAFS spectroscopy. The sorption of anionic uranyl hydroxide complexes to the mineral surface was shown to be mediated by calcium cations. In further experiments, the effect of isosaccharinic acid (ISA) and polycarboxylate ether (PCE) on U(VI) and Eu(III) sorption, respectively, on Ca-bentonite was studied. An effect of ISA on U(VI) sorption on Ca-bentonite only occurs when ISA is present in very high excess to U(VI). The effect of PCE, as a commercial cement superplasticizer, on Eu(III) sorption onto Ca-bentonite was negligible already at moderate ionic strengths. The retention of U(VI) and Cm(III) by various C-(A-)S-H phases, representing different alteration stages of concrete, was studied by batch sorption experiments. Sorbed or incorporated actinide species were identified by TRLFS. The stability of U(VI) and Cm(III) doped C-(A-)S-H phases at high ionic strengths conditions was studied in solutions simulating the contact with North German claystone formation water. Potential changes of actinide speciation as well as formation of secondary phases due to leaching effects were followed spectroscopically. The results of this project show that both bentonite and cementitious material constitute an important retention barrier for actinides under hyperalkaline conditions and increased ionic strength

Joint project: Retention of radionuclides relevant for final disposal in natural clay rock and saline systems: Subproject 2: Geochemical behavior and transport of radionuclides in saline systems in the presence of repository-relevant organics

The objective of this project was to study the influence of increased salinities on interaction processes in the system radionuclide – organics – clay – aquifer. For this purpose, complexation, redox, sorption, and diffusion studies were performed under variation of the ionic strength (up to 4 mol/kg) and the background electrolyte. The U(VI) complexation by propionate was studied in dependence on ionic strength (up to 4 mol/kg NaClO4) by TRLFS, ATR FT-IR spectroscopy, and DFT calculations. An influence of ionic strength on stability constants was detected, depending on the charge of the respective complexes. The conditional stability constants, determined for 1:1, 1:2, and 1:3 complexes at specific ionic strengths, were extrapolated to zero ionic strength. The interaction of the bacteria Sporomusa sp. MT-2.99 and Paenibacillus sp. MT-2.2 cells, isolated from Opalinus Clay, with Pu was studied. The experiments can be divided into such without an electron donor where biosorption is favored and such with addition of Na-pyruvate as an electron donor stimulating also bioreduction processes. Moreover, experiments were performed to study the interactions of the halophilic archaeon Halobacterium noricense DSM-15987 with U(VI), Eu(III), and Cm(III) in 3 M NaCl solutions. Research for improving process understanding with respect to the mobility of multivalent metals in systems containing humic matter was focused on the reversibility of elementary processes and on their interaction. Kinetic stabilization processes in the dynamics of humate complexation equilibria were quantified in isotope exchange studies. The influence of high salinity on the mobilizing potential of humic-like clay organics was systematically investigated and was described by modeling. The sorption of Tc(VII)/Tc(IV) onto the iron(II)-containing minerals magnetite and siderite was studied by means of batch sorption experiments, ATR FT-IR and X-ray absorption spectroscopy. The strong Tc retention at these minerals could be attributed to surface-mediated reduction of Tc(VII) to Tc(IV). An influence of ionic strength was not observed. The influence of ionic strength (up to 3 mol/kg) and background electrolyte (NaCl, CaCl2, MgCl2) on U(VI) sorption onto montmorillonite was studied. The U(VI) sorption is influenced by the background electrolyte, the influence of ionic strength is small. Surface complexation modeling was performed applying the 2SPNE SC/CE model. Surface complexation constants were determined for the NaCl and CaCl2 system and were extrapolated to zero ionic strength. Surface complexation in mixed electrolytes can be modeled applying surface complexation constants derived for pure electrolytes. The influence of citrate on U(VI) diffusion in Opalinus Clay was studied using Opalinus Clay pore water as background electrolyte. The diffusion parameter values obtained for the HTO through-diffusion and the U(VI) in-diffusion in the absence of citric acid were in agreement with literature data. In the presence of citric acid, U(VI) diffusion was significantly retarded, which was attributed to a change in speciation, probably U(VI) was reduced to U(IV). Larger-scale heterogeneous material effects on diffusive transport were investigated with PET. Diffusion parameters were derived by optimum fit of a FEM-model to the measurement. These parameters are in accordance with the results from 1D-through-diffusion experiments. Deviations from the simple transversal-isotropic behavior, which are identified as residuals from the model, are indications for heterogeneous transport on the mm-scale. PET measurements were also conducted in order to display the improvement of the EDZ with waterglass injections. These experiments enable to draw conclusions on the complex reactive transport process and thus an estimation of the achieved improvement of the barrier function. The image reconstruction procedure was largely improved, mainly with the aid of Monte-Carlo simulations, and now allows quantitative analysis and error estimation

Investigation of the Complexation and the Migration Behavior of Actinides and Non-Radioactive Substances with Humic Acids under Geogenic Conditions - Complexation of Humic Acids with Actindies in the Oxidation State IV Th, U, Np

Objective of this project was the study of basic interaction and migration processes of actinides in the environment in presence of humic acids (HA). To obtain more basic knowledge on these interaction processes synthetic HA with specific functional properties as well as 14C-labeled HA were synthesized and applied in comparison to the natural HA Aldrich. One focus of the work was on the synthesis of HA with distinct redox functionalities. The obtained synthetic products that are characterized by significantly higher Fe(III) redox capacities than Aldrich HA were applied to study the redox properties of HA and the redox stability of U(VI) humate complexes. It was confirmed that phenolic OH groups play an important role for the redox properties of HA. However, the results indicate that there are also other processes than the single oxidation of phenolic OH groups and/or other functional groups contributing to the redox behavior of HA. A first direct-spectroscopic proof for the reduction of U(VI) by synthetic HA with distinct redox functionality was obtained. The complexation behavior of synthetic and natural HA with actinides (Th, Np, Pu) was studied. Structural parameters of Pu(III), Th(IV), Np(IV) and Np(V) humates were determined by X-ray absorption spectroscopy (XAS). The results show that carboxylate groups dominate the interaction between HA and actinide ions. These are predominant monodentately bound. The influence of phenolic OH groups on the Np(V) complexation by HA was studied with modified HA (blocked phenolic OH groups). The blocking of phenolic OH groups induces a decrease of the number of maximal available complexing sites of HA, whereas complex stability constant and Np(V) near-neighbor surrounding are not affected. The effects of HA on the sorption and migration behavior of actinides was studied in batch and column experiments. Th(IV) sorption onto quartz and Np(V) sorption onto granite and its mineral constituents are affected by the pH value and the presence of HA. HA exhibits a significant influence on the transport of U(IV) and U(VI) in a laboratory quartz sand system. In order to provide the basis for a more reliable modeling of the actinide transport, the metal ion complexation with HA has to be integrated into existing geochemical speciation codes. Within this project the metal ion charge neutralization model was embedded into the geochemical modeling code EQ3/6. In addition to that, a digital data base was developed which covers HA complexation data basing on the charge neutralization model

Signatures of technetium oxidation states: a new approach

A general strategy for the determination of Tc oxidation state by new approach involving X-ray absorption near edge spectroscopy (XANES) at the Tc L-3 edge is shown. A comprehensive series of Tc-99 compounds, ranging from oxidation states I to VII, was measured and subsequently simulated within the framework of crystal-field multiplet theory. The observable trends in the absorption edge energy shift in combination with the spectral shape allow for a deeper understanding of complicated Tc coordination chemistry. This approach can be extended to numerous studies of Tc systems as this method is one of the most sensitive methods for accurate Tc oxidation state and ligand characterization

Uranium mobility in organic matter-rich sediments: A review of geological and geochemical processes

Uranium (U) is of enormous global importance because of its use in energy generation, albeit with potential environmental legacies. While naturally occurring U is widespread in the Earth's crust at concentrations of ~1 to 3 ppm, higher concentrations can be found, includingwithin organicmatter (OM)-rich sediments, leading to economic extraction opportunities. The primary determinants of U behaviour in ore systems are pH, Eh, U oxidation state (U(IV), U(VI)) and the abundance of CO3 2– ions. The concentration/availability and interrelationships among such determinants vary, and the solubility and mobility of ions (e.g. OH-, CO3 2–, PO4 3-, SiO4 4-, SO4 2-) that compete for U (primarily as U(VI)) will also influence the mobility of U. In addition, the presence of OM can influence U mobility and fate by the degree of OMsorption to mineral surfaces (e.g. Fe- and Si- oxides and hydroxides). Within solid-phase OM, microbes can influence U oxidation state and U stability through direct enzymatic reduction, biosorption, biomineralisation and bioaccumulation. The biogenic UO2 product is, however, reported to be readily susceptible to reoxidation and therefore more likely remobilised over longer time periods. Thus several areas of uncertainty remain with respect to factors contributing to U accumulation, stability and/or (re)mobilisation. To address these uncertainties, this paper reviews U dynamics at both geological and molecular scales. Here we identify U-OMbond values that are in agreement, relatively strong, independent from ionic strength and which may facilitate either U mobilisation or immobilisation, depending on environmental conditions. We also examine knowledge gaps in the literature, with U-OM solubility data generally lacking in comparison to data for U sorption and dissolution, and little information available on multi-component relationships, such as UOM-V (V as vanadate). Furthermore, the capability ofOMto influence the oxidation state of U at near surface conditions remains unclear, as it can be postulated that electron shuttling by OM may contribute to changes in U redox state otherwise mediated by bacteria. Geochemical modelling of the environmental mobility of U will require incorporation of data from multi-corporation studies, as well as from studies of U-OM microbial interactions, all of which are considered in this review