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1 Introduction Transport of contaminants, e.g. radionuclides, in the environment depends strongly on their interactions with mineral phases. In a repository for radioactive waste, crystalline rock (e.g. granite) as one potential host rock in Germany and many other countries, may affect the mobility of radionuclides. Main constituents of granite are feldspars. In spent nuclear fuel, trivalent actinides (Am, Cm, but also Pu) contribute strongly to the radiotoxicity. Therefore, this work studies the retention of Am and Cm, as well as their rare earths element analogues (Eu, La, Lu, Nd, Y) on K-feldspar. By combining batch sorption experiments and time-resolved laser-induced fluorescence spectroscopy (TRLFS), a generic surface complexation model (SCM) was obtained that is valid for all investigated M3+. Thermodynamic sorption data were obtained and an understanding of sorption mechanisms on the molecular level was achieved. 2 Results Batch sorption experiments were performed over a broad range of environmental conditions (pH 4 – 10, [M3+] = 52 nM – 10 µM, 3 – 50 g/L K-feldspar (dp < 21 µm; 63 – 200 µm))[1]. Sorption is weak for pH < 5, strongly increases between pH 5 – 7 and reaches complete uptake at higher pH. By deconvolution of Cm emission spectra, an inner-sphere complex and its first two hydrolysis forms were found to be responsible for retention in this pH range. For determination of the deprotonation constant pKa of K-feldspar, as one important input parameter of the model, column titration experiments were conducted. Batch sorption results of all studied M3+ were used to develop two alternative SCMs. The experimental sorption data were used to determine surface complexation parameters by coupling the parameter estimation code UCODE with PHREEQC (SCM-A). In a second approach, spectroscopic data were also considered (SCM-B). A generic approach was used to develop the geochemical models that satisfactorily describe all of the derived M3+/K feldspar sorption edges as well as TRLFS-derived speciation. The model delivered respective stability constants of the sorption complexes, which were added to the data base of the Smart Kd-concept[2]. Therefore, this work improves the risk assessment of repositories for radioactive waste. Figure 1: Experimental batch sorption data (symbols) and calculation results using the two developed SCMs for different experimental conditions.[1] References [1] Neumann, J. et al., "A comprehensive study of the sorption mechanism and thermodynamics of f-element sorption onto K-feldspar", J. Colloid Interface Sci. (2020). https://doi.org/10.1016/j.jcis.2020.11.041. [2] Stockmann, M. et al., "Smart Kd-values, their uncertainties and sensitivities - Applying a new approach for realistic distribution coefficients in geochemical modeling of complex systems", Chemosphere., 187, 277–285 (2017).

Cellulose is a very common organic polymer and also present in considerable amounts in low and intermediate level wastes (LILW). Since it is considered to stabilize LILW with cementitious materials and to use cement-based materials as construction and backfilling material, the alkaline degradation of cellulosic material has to be taken into account. This process will lead to the formation of water-soluble carboxylates and hydroxycarboxylates, with isosaccharinic acid (isa) being the main degradation product. It was shown that the α-form of the polyhydroxy-carboxylic acid is a stronger complexant compared to the β-form and that the interaction with radionuclides affects the solubility and the sorption behavior adversely [1]. Only a limited number of studies focused on the interaction of isa with UO22+. Hence, there are gaps in thermodynamic databases for this system. The basis for reliable thermodynamic data is a detailed structural knowledge about the formed species in the system of interest. This issue prompted us to characterize the formed complexes in the UO22+-isa system on a molecular level. Our approach to elucidate the mechanisms in aqueous solution combines different spectroscopic techniques (UV-vis, ATR-FTIR, EXAFS, and luminescence) with DFT-calculations. The mutual influence of UO22+ and isa on their speciation as well as the three detected complexes will be discussed. The outcomes are on the one hand the basis for thermodynamic investigations as well as for studies under neutral and alkaline conditions. They provide on the other hand also important information concerning the behavior of α-isa as ligand, including dominant binding motifs. [1] Van Loon, L. R., et al., Radiochimica Acta, 1999, Vol. 86, https://doi.org/10.1524/ract.1999.86.34.183. This project has received funding from the Euratom research and training programme 2014-2018 under Grant Agreement no. 61880.

Low and intermediate level waste contains considerable amounts of cellulosic materials, which will be degraded relatively fast under alkaline conditions, with isosaccharinic acid (ISA), a polyhydroxy-carboxylic acid, being the main degradation product. It has been shown that the α-form is a stronger complexant for certain radionuclides compared to the β-form and that the complex formation affects the sorption as well as the solubility adversely.[1] In the particular case of U(VI) the number of studies concerning the speciation in the presence of ISA is small. The excellent spectroscopic properties of the uranyl-entity were used to determine the speciation by UV-vis, luminescence, ATR-FTIR and EXAFS spectroscopy properly under acidic conditions. To understand the complex formation mechanism on a molecular level, the behavior of the ligand was simultaneously investigated by ATR-FTIR and NMR spectroscopy and the results were compared to theoretical data from DFT-calculations, whereby a dominant chelate binding motif via the carboxylic and the α-hydroxy-group was identified. Whereas hydrolysis of U(VI) and carbonate-complexation can be neglected under acidic conditions, they have to be carefully considered under neutral and alkaline conditions as competitive reactions in addition to the complex formation with ISA. In this context the influence of ISA on the retention of U(VI) on bentonite was investigated. Sorption experiments were performed under anaerobic (carbonate-free) and aerobic (with carbonate) conditions between pH 8 and 13 in the presence of ISA. Time-resolved laser-induced fluorescence spectroscopy was used to determine the aqueous speciation of U(VI) and the results will be compared to measurements without ISA. [1] Van Loon, L. R., et al., Radiochimica Acta, 1999, Vol. 86, https://doi.org/10.1524/ract.1999.86.34.183.

The alkaline degradation of cellulosic material present in low and intermediate level waste leads to the formation of water soluble organic compounds. α-Isosaccharinic acid (ISA) as main degradation product can act as complexing agent for radionuclides. This may affect the mobility as well as the sorption behavior adversely. Studies regarding to the interaction of ISA with U(VI) are scarce. Hence, the aim of this study is to describe the U(VI)-ISA complexes formed under acidic conditions on a molecular level. Since U(VI) can coordinate to different binding sites of the polyhydroxy carboxylic acid, the number of possible complexes is large. To elucidate the number and structure of those U(VI)-ISA species, different spectroscopic methods were applied to obtain information from the metal and from the ligand. UV-vis measurements lead to the conclusion that three complexes are formed and that both, the carboxylic as well as hydroxylic groups, are involved. Attenuated total reflection-Fourier-transformation-infrared spectroscopy (ATR-FTIR) and extended X-ray absorption fine structure-spectroscopy (EXAFS) offer structural information of the formed complexes. DFT-calculations provide data of optimized structures (e.g. figure below: [UO2(ISA)2(H2O)2]), which were used to underpin experimental results in order to identify the formed U(VI)-ISA species. These results are the basis for further investigations under neutral and alkaline conditions. Moreover, this study helps to elucidate the U(VI) speciation in long term tissue degradation experiments in the presence of microorganisms.

The coordination chemistry of radionuclides (RN) with organic components is of great interest since latter can affect the mobility as well as the sorption behavior adversely. Acetate (AcO-) can be a biotic or abiotic degradation product of organic material present in nuclear waste. Additionally it is often declared as model compound for more complex organic structures. A large number of studies is available which investigated the speciation of uranium(VI) in the presence of acetate under acidic conditions. From potentiometric titrations it seems to be very clear that three uranyl-acetate species are formed under acidic conditions: UO2AcO+ (1:1), UO2(AcO)2 (1:2) and UO2(AcO)3- (1:3). Spectroscopic methods (e.g. IR and EXAFS) provide no certainty whether the 1:3 complex exists or not. TRLFS and UV-vis spectroscopy were mainly applied to characterize the formation of the 1:1 complex. In this study TRLFS, ATR-FT-IR and UV-vis spectroscopy were applied to determine the speciation of uranium(VI) in the presence of acetate and to fill the gap of missing spectroscopic data for the other formed species. Experimental conditions were chosen so that all three previous identified species should be formed. Data evaluation was performed on the basis of factor analysis resulting in single component spectra and stability constants. The findings led us to the conclusion that the 1:3 complex was not formed. In addition the detailed evaluation of ATR-FT-IR spectra show that an approximation used in potentiometric studies should be treated with caution. All these indications reinforce the doubts with respect to the existence of the UO2(AcO)3- species.

The interaction of acetic acid as the smallest carboxylic acid with a side chain with uranium (VI) is of importance in two respects: firstly, it occurs often as a degradation product of organic components of radioactive waste (e.g. cellulose, bitumen, PVC), and secondly it can serve as model for more complex organic compounds like humic acids. Three complexes can be formed in aqueous solutions: UO2(AcO)n2-n, where n is 1,2, or 3. These complexes are well characterized under various experimental conditions in terms of stability and structure, reported in a number of publications. Recently published articles focused also on spectroscopic properties of uranium (VI) acetate complexes using on the one hand time-resolved laser-induced fluorescence spectroscopy (TRLFS) and on the other hand UV-vis spectroscopy. The uranium concentrations used (between 0.1 and 0.2 M) are comparatively high. However, both works revealed only data for the 1:1 complex UO2(AcO)+. Therefore, the aim of this study was to provide spectroscopic data for all three UO2(AcO)n2-n complexes to fill in the gaps in the uranium-acetate system. Both the TRLFS and UV-vis experiments were performed with solutions containing only 50 µM uranium (VI). The parameters of the test series were varied in a way that the speciation changes from the free uranyl-ion, over the 1:1 and 1:2, to the 1:3 complex UO2(AcO)3-. Thereby the pH was fixed and the acetate concentration was varied or vice versa. In contrast to conventional UV-vis set ups, we used a Liquid Waveguide Capillary Cell with a path length of 250 cm to obtain spectra of high quality at this very low uranium concentration. The absorption spectra were evaluated on the basis of factor analysis and it was possible to calculate reproducible single component spectra for all the uranyl-acetate species (see Fig. 1). The TRLFS experiments at 25°C revealed the known quenching of the 1:1 complex (see Fig. 2 left) but also at pH > 3 a strong increase in intensity, band shifts, and a rise of a new band at 461 nm (see Fig. 2). These findings allow the assumption that the 1:2 and 1:3 complexes are luminescent. To examine whether the 1:1 complex is non luminescent TRLFS measurements at 152 K to avoid dynamic quenching of the acetate ion will be carried out. Our approach of using a combination of UV-vis, TRLFS, and cryo-TRLFS allows the spectroscopic characterization of all three uranyl-acetate complexes including the determination of the corresponding stability constants.

Organic compounds present in LILW in a deep geological disposal are exposed to different conditions affecting their structure and chemical properties. Ionizing radiation, high pH, and microbial activity may lead to the generation of small water soluble organic compounds. These degradation products can probably enhance the mobility of radionuclides due to complex formation. Therefore the interaction of selected degradation products with certain actinides/lanthanides has to be investigated. This is necessary to predict the safety and long term evolution of a nuclear waste repository. This poster presents the objectives of the doctoral thesis and first results.