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The sorption of uranium(VI) and neptunium(V) onto surfaces of selected metal oxides and alumosilicates studied by in situ vibrational spectroscopy [Elektronische Ressource] / von Katharina Müller

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132 pages
The sorption of uranium(VI) and neptunium(V) onto surfaces of selected metal oxides and alumosilicates studied by in situ vibrational spectroscopy D I S S E R T A T I O N zur Erlangung des akademischen Grades Doctor rerum naturalium (Dr. rer. nat.) vorgelegt der Fakultät Mathematik und Naturwissenschaften der Technischen Universität Dresden von Dipl.-Ing. Katharina Müller geboren am 13.09.1979 in Gera Eingereicht am 10.12.2009 Die Dissertation wurde in der Zeit von 06 / 2006 bis12 / 2009 im Institut für Radiochemie angefertigt. Acknowledgements The research in this thesis was performed at the Institute of Radiochemistry at the Forschungszentrum Dresden-Rossendorf e.V. It was funded mainly by the Deutsche Forschungsgemeinschaft. I would like to express my deepest gratitude to all who have contributed to this work. At first, grateful thanks are due to Prof. Dr. G. Bernhard for the scientific supervision, for introducing me to the exciting field of radiochemistry and for his efforts during the completion process. I would like to express my sincere gratitude to my supervisor Dr. H. Foerstendorf for the patience, the practical advice and constant encouragement. I thank for valuable discussions and critical comments on the manuscripts. I could not have imagined a better scientific guidance. I gratefully acknowledge the efforts of Dr. V.
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The sorption of uranium(VI) and neptunium(V) onto surfaces of selected metal oxides and alumosilicates studied byin situvibrational spectroscopy D I S S E R T A T I O N zur Erlangung des akademischen Grades Doctor rerum naturalium (Dr. rer. nat.) vorgelegt der Fakultät Mathematik und Naturwissenschaften der Technischen Universität Dresden von Dipl.Ing. Katharina Müller geboren am13.09.1979inGera Eingereicht am 10.12.2009 Die Dissertation wurde in der Zeit von 06 / 2006 bis12 / 2009 im Institut für Radiochemie angefertigt.
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The research in this thesis was performed at the Institute of Radiochemistry at the Forschungszentrum DresdenRossendorf e.V. It was funded mainly by the Deutsche Forschungsgemeinschaft. I would like to express my deepest gratitude to all who have contributed to this work. At first, grateful thanks are due to Prof. Dr. G. Bernhard for the scientific supervision, for introducing me to the exciting field of radiochemistry and for his efforts during the completion process. I would like to express my sincere gratitude to my supervisor Dr. H. Foerstendorf for the patience, the practical advice and constant encouragement. I thank for valuable discussions and critical comments on the manuscripts. I could not have imagined a better scientific guidance. I gratefully acknowledge the efforts of Dr. V. Brendler for the introduction to thermodynamic modeling using EQ3/6, many fruitful discussions and his excellent support. Appreciation is due to R. Steudtner for TRLFS analysis, scientific discussion and a humorous PhD time. I highly appreciate the enormous experimental support from T. Meusel, K. Stolze and R. Husar. I wish to thank Dr. S. Tsushima for valuable discussions on quantum chemistry. I acknowledge the assistance of Dr. A. Ikeda and Dr. K. Takao and the advice of Dr. G. Geipel during the preparation of the Np stock solutions. I am pleased to thank K. Heim and B. Li for their technical assistance in FTIR spectroscopy, U. Schaefer and A. Ritter for ICPMS, C. Eckardt for BET measurements, S. Weiß for PCS, C. Nebelung forα,γspectrometry and the introduction to LSC, M. Eilzer for photoacoustic measurements and IT support, A. Rumpel, H. Heim, and S. Henke for introducing me to important aspects of radiation protection, B. Hiller and D. Falkenberg for the construction of the small N2 box and the technical assistance concerning glove box use, C. Kirmes for administrative support, Dr. N. Baumann for the preparation of the TiO2sample S5, Dr. K. Großmann for CLSM TRLFS measurements, T. Günther for AFM measurements, the PhD students of the Institute of Radiochemistry for the friendly atmosphere and their helpful attitude. Thanks are due to Prof. Dr. G. Lefèvre for the review of my PhD thesis and for valueable discussions. Furthermore, I thank to Dr. T. Payne and Dr. J. Comarmond for providing the TiO2samples. I thank the FZD for advanced vocational training opportunities. I send my warmest thanks to my friends for their understanding, amity and support during the last four exciting years. I thank Uwe for corrections on the manuscript. Finally, my heartfelt appreciation goes to Volker for his love and inspiration, and to my parents and my brother who have helped me to carry on for one decade in the academic and research field. Katharina Müller April 2010
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                 2.1..................................................................... 4Properties of uranium and neptunium 2.25Reactions in aqueous solution.................................................................................... 2.39Reactions at the solidwater interface....................................................................... 2.4............................................................. 12Migration of actinides in the environment
        !"   3.1Surface analytical techniques .................................................................................. 143.2Internal reflection spectroscopy .............................................................................. 153.2.1 Principles of attenuated total reflection spectroscopy........................................ 16 3.2.2 Internal reflection element materials and cell designs ....................................... 17 3.2.3 Infrared spectroscopy of water........................................................................... 18 3.2.4 Reactioninduced infrared difference spectroscopy........................................... 19 3.2.5 Interfacial spectroscopic studies of sorption processes...................................... 20 3.3ATR FTIR spectroscopy at the IRC...................................................................... 213.3.1 Instrumental setup............................................................................................. 21 3.3.2In situ23ATR FTIR spectroscopic sorption studies.............................................
#$   %&'( )&'  )&'   "  *  +4.1U(VI) speciation in air.............................................................................................. 284.1.1 Calculation of micromolar U(VI) speciation ..................................................... 28 4.1.2 U(VI) speciation at pH 4 .................................................................................... 29 4.1.3 U(VI) speciation in micromolar acidic solutions ............................................... 32 4.1.4 U(VI) speciation in micromolar neutral solutions ............................................. 35 4.239Np(VI) speciation in air............................................................................................ 4.2.1 Calculation of Np(VI) speciation in comparison to U(VI) ................................ 39 4.2.2 NIR spectroscopy of Np(VI) solutions in the acidic pH range .......................... 40 4.2.3 Isostructural complexes of Np and U................................................................. 41 4.2.4 Np(VI) and U(VI) speciation in aqueous solutions in the acidic pH range ....... 43 4.2.5 Colloidal species of Np(VI) and U(VI).............................................................. 48 4.350Np(VI) and U(VI) speciation in the absence of atmospheric carbonate.............. 4.3.1 Calculation of Np(VI) and U(VI) speciation at N2............................................ 50
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Contents
4.3.2 The species of micromolar Np(VI) and U(VI) solutions in the acidic pH at N251 4.4Np(V) speciation in the absence of atmospheric carbonate.................................. 544.4.1 Calculation of Np(V) speciation at N2............................................................... 54 4.4.2 Np(V) speciation in micromolar solutions at N2................................................ 55 4.5Conclusions and Outlook ......................................................................................... 56
,  %&'  )&'   -     ,. * 5.1........................... 58Introducing remarks on U(VI) sorption onto mineral surfaces 5.259U(VI) sorption on titanium dioxide......................................................................... 5.2.1 Monitoring the U(VI) sorption process onto TiO2............................................. 60 5.2.2 Identification of different U(VI) surface species on TiO2................................. 62 5.2.3 Influence of TiO2............................................................ 64crystallographic form 5.2.4 Influence of TiO2purity ..................................................................................... 67 5.2.5 Influence of the U(VI) solution properties on the sorption onto TiO2............... 70 5.3Photocatalytic effects during the U(VI) sorption on TiO2.................................... 755.4U(VI) sorption onto oxides of aluminium and silica.............................................. 785.4.1 Monitoring the U(VI) sorption process ontoγAl2O3........................................ 82 5.4.2 Influence of the U(VI) solution properties on the sorption ontoγAl2O3.......... 84 5.4.3 Influence of the aluminate mineral phase on U(VI) sorption ............................ 89 5.592U(VI) sorption onto alumosilicates ......................................................................... 5.695Introducing remarks on Np(V) sorption onto mineral surfaces .......................... 5.7........................................................................ 96Np(V) sorption on titanium dioxide 5.7.1 Monitoring the Np(V) sorption process onto TiO2............................................ 96 5.7.2 Influence of the Np(V) solution properties on the sorption onto TiO2.............. 99 5.8.Comparison of Np(V) sorption onto oxides of titanium, aluminum, silicon and zinc 1005.9....................................................................................... 103Conclusions and Outlook
/0   16.1Materials.................................................................................................................. 1046.2Methods ................................................................................................................... 1056.2.1 Thermodynamic data and speciation modeling ............................................... 105 6.2.2 Experiments at high actinide concentrations ................................................... 105 6.2.3 Preparation of actinide solutions...................................................................... 105 6.2.4 Preparation of diluted solutions ....................................................................... 106 6.2.5 Check for colloids in sample solutions ............................................................ 107 6.2.6 ATR FTIR spectroscopic measurements ........................................................ 107 6.2.7 NIR absorption spectroscopy ........................................................................... 107 6.2.8 Laserinduced fluorescence spectroscopy........................................................ 107 6.2.9 Analysis of uranium and neptunium concentration ......................................... 107
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Contents
6.2.10 Measurement of pH values ............................................................................ 108 6.2.11 TiO2108digestion analysis .................................................................................. 6.2.12 Determination of the Specific Surface Area .................................................. 108 6.2.13 Washing procedure of the TiO2samples........................................................ 108 6.2.14 Experiments at inert gas atmosphere ............................................................. 109 6.2.15 AFM measurements ....................................................................................... 109
+"1
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     Auger electron spectroscopy Atomic force microscopy Amorphous material which transmits IR radiation Attenuated total reflection Fourier transform – infrared BrunauerEmmetTeller Confocal laser scanning microscopy Cyclic voltammetry Density functional theory Electron spin resonance Extended xray absorption fine structureForschungszentrum DresdenRossendorf e.V.Grazing incidence xray absorption fine structureInternal reflection element Internal reflection spectroscopy Inductively coupled plasma mass spectroscopyIsoelectric pointInstitute of Radiochemistry at FZD Thallium bromoiodideLaserinduced photoacoustic spectroscopy Liquid scintillation countingMixed oxide fuel Milli Q water Microfocus option ofxray absorption fine structureMicrofocus option ofxray diffractionMicrofocus option of xray fluorescence Near infrared Nuclear magnetic resonance Nuclear Energy Agency Organization for Economic Cooperation and Development Optical density Photon correlation spectroscopy Pressurized water reactor Point of zero charge Surface complexation modelScanning electron microscopy
Abbreviations: AES AFM AMTIR ATR FTIR BET CLSM CV DFT ESREXAFSFZDGIXAFS IRE IRS ICPMSIEPIRC KRS5 LIPASLSCMOX MQwater µ XAFS XRD XRF NIR NMR NEA OECD OD PCS PWR PZC SCMSEM
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List of abbreviations and symbols
SITSNF TDBTMAOH TRLFSUVVisXASXANESXPSXRD
Specific ion interaction theorySpent nuclear fuel Thermochemical database (NEA)Tetramethylammonium hydroxideTimeresolved laserinduced fluorescence spectroscopyUltravioletVisibleXray absorption spectroscopyXray absorption nearedge structureXray photoelectron spectroscopyXray diffractionSymbols and their units: Absorptivity of the sample Effective thickness µ m Penetration depth µ m Distribution coefficient Wavelength nm Index of refraction RetardationInternal reflection angle degree −1 Wavenumber cm
αdedPKd λnRθυ
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The migration behavior of actinides and other radioactive contaminants in the environment is controlled by prominent molecular phenomena such as hydrolysis and complexation reactions in aqueous solutions as well as the diffusion and sorption onto minerals present along groundwater flow paths. These reactions significantly influence the mobility and bioavailability of the metal ions in the environment, in particular at liquidsolid interfaces. Hence, for the assessment of migration processes the knowledge of the mechanisms occurring at interfaces is crucial. The required structural information can be obtained using various spectroscopic techniques. In the present study, the speciation of uranium(VI) and neptunium(V) at environmentally relevant mineral – water interfaces of oxides of titania, alumina, silica, zinc, and alumosilicates has been investigated by the application of attenuated total reflection Fourier transform infrared (ATR FTIR) spectroscopy. Moreover, the distribution of the hydrolysis products in micromolar aqueous solutions of U(VI) and Np(V/VI) at ambient atmosphere has been characterized for the first time, by a combination of ATR FTIR spectroscopy, near infrared (NIR) absorption spectroscopy, and speciation modeling applying updated thermodynamic databases.
From the infrared spectra, a significant change of the U(VI) speciation is derived upon lowering the U(VI) concentration from the milli to the micromolar range, strongly suggesting the dominance of monomeric U(VI) hydrolysis products in the micromolar solutions. In contradiction to the predicted speciation, monomeric hydroxo species are already present at pH3. At higher pH levels (> 2.5 and become dominant at pH 6), a complex speciation is evidenced including carbonate containing complexes. For the first time, spectroscopic results of Np(VI) hydrolysis reactions are provided in the submillimolar concentration range and at pH values up to 5.3, and they are comparatively discussed with U(VI). For both actinides, the formation of similar species is suggested at pH4, whereas at higher pH, the infrared spectra evidence structurally different species. At pH 5, the formation of a carbonatecontaining dimeric complex, that is (NpO2)2CO3(OH)3, is strongly suggested, whereas carbonate complexation occurs only under more alkaline conditions in the U(VI) system.
The results from the experiments of the sorption processes clearly demonstrate the formation of stable U(VI) surface complexes at all investigated mineral phases. This includes several metal oxides, namely TiO2, Al2O3, and SiO2, serving as model systems for the elucidation of more complex mineral systems, and several alumosilicates, such as kaolinite, muscovite and biotite. From a multiplicity ofin situthe impact of sorbent experiments, characteristics and variations in the aqueous U(VI) system on the sorption processes was considered.
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Summary
A preferential formation of an innersphere complex is derived from the spectra of the TiO2and SiO2In addition, since the phases. in situ FTIR experiments provide an online monitoring of the absorption changes of the sorption processes, the course of the formation of the U(VI) surface complexes can be observed spectroscopically. It is shown that after prolonged sorption time on TiO2, resulting in a highly covered surface, outersphere complexation predominates the sorption processes. The prevailing crystallographic modification, namely anatase and rutile, does not significantly contribute to the spectra, whereas surface specific parameters, e.g. surface area or porosity are important. A significant different surface complexation is observed for Al2O3. The formation of inner spheric species is assumed at low U(VI) surface coverage which is fostered at low pH, high ionic strength and short contact times. At proceeded sorption the surface complexation changes. From the spectra, an outerspheric coordination followed by surface precipitation or polymerization is deduced. Moreover, in contrast to TiO2, the appearance of ternary U(VI) carbonate complexes on theγAl2O3surface is suggested. The first results of the surface reactions on more complex, naturally occurring minerals (kaolinite, muscovite and biotite) show the formation of U(VI) innersphere sorption complexes. These findings are supported by the spectral information of the metal oxide surfaces.
In this work, first spectroscopic results from sorption of aqueous Np(V) on solid mineral + phases are provided. It is shown that stable innersphere surface species of NpO2are formed on TiO2. Outersphere complexation is found to play a minor role due to the pH independence of the sorption species throughout the pH range 4 – 7.6. The comparative spectroscopic experiments of Np(V) sorption onto TiO2, SiO2, and ZnO indicate structurally similar bidentate surface complexes.
The multiplicity of IR spectroscopic experiments carried out within this study yields a profound collection of spectroscopic data which will be used as references for future investigations of more complex sorption systems in aqueous solution. Furthermore, from a methodological point of view, this study comprehensively extends the application of ATR FTIR spectroscopic experiments to a wide range in the field of radioecology. The results obtained in this work contribute to a better understanding of the geochemical interactions of actinides, in particular U(VI) and Np(V/VI), in the environment. Consequently, more reliable predictions of actinides migration which are essential for the safety assessment of nuclear waste repositories can be performed.
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1.Introduction
For decades, there has been an intense debate on the most appropriate approach to manage spent fuel from nuclear power reactors. On the one hand, the waste can be directly disposed in deep geologic repositories, on the other hand, it can be reprocessed to recover and recycle the plutonium and uranium and disposing only the residual waste from the recycling process. For disposal purposes, the waste produced is divided into two categories – heatgenerating waste and nonheatgenerating waste. Heatgenerating waste includes spent fuel from light water and other nuclear reactors, vitrified highlevel waste, and core instrumentation and residues from fuel element cladding after reprocessing. In 2040, Germany will possess 22,000 metric tons of heatgenerating waste for final disposal in deep geologic formations [1]. Uranium (U) is a constituent of highlevel nuclear waste arising from the nuclear spent fuel. Furthermore, it is found at elevated concentrations in the environment at facilities of former uranium mining and milling sites, e.g. in Saxony and Thuringia (Germany), and in subsurface dumps and sites with radioactive and/or heavy metal containing inventory [2, 3]. Neptunium (Np) is one of the most important components of nuclear waste to consider for the longterm safety assessment of nuclear waste repositories. Although its concentration in spent fuel is −1 relatively low at the beginning of storage (0.5 kgT spent nuclear fuel (SNF) in a pressurized 241 water reactor (PWR); Fig. 11, left), the radioactive decay of Am with a half life of 237 6 432.7 years causes enrichment of Np with time. Due to its long half life (2.14 × 10 years) 237 5 the hazardous isotope Np becomes the major contributor of the total radiation in 10 years. (cf. Fig. 11, right) [46]. For the longterm storage of nuclear waste, the assessment of water contamination, depending on retention and migration processes of radionuclides in the geosphere, is of primary environmental concern. The migration behavior of (radioactive) contaminants, i.e. their mobility and bioavailability in the environment is strongly affected by molecular reactions occurring in and among solid, aqueous, and gas phases [7]. Hence, in the safety assessment of potential underground disposals, great attention must be paid to the geochemistry and migration behavior of both elements. Geochemical reactions,
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