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Sorption and Direct Speciation of Neptunium(V) on Aluminium Oxide and Montmorillonite

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Sorption and Direct Speciation of Neptunium(V) on Aluminium Oxide and Montmorillonite Sorption and Direct Speciation of Neptunium(V) on Aluminium Oxide and Montmorillonite Thesis submitted for obtaining the degree of Doktor der Naturwissenschaften doctor rerum naturalium in Nuclear Chemistry at the Department of Chemistry, Pharmacy, and Geosciences of the Johannes Gutenberg - University Mainz Sonja Wendt born in Langenhagen, Germany Mainz, 2009 D77 Contents Abbreviations and Terms 3 1 Introduction and Motivation 7 2 Fundamentals 11 2.1 Neptunium – History and Current Situation ........... 11 2.1.1 Production and Global Inventory ............. 12 2.1.2 Industrial Usage and Application ............. 14 2.1.3 Releases into the Environment ............... 15 2.2 Chemical and Radiochemical Characteristics of Neptunium ... 16 2.2.1 Aqueous Chemistry ..................... 17 2.2.2 Radioactivity ........................ 21 2.2.3 Aspects of Toxicity – Health Effects ............ 22 2.2.4 Neptunium in the Environment .............. 24 2.3 Radioactive Waste ......................... 30 2.3.1 Waste Treatment ...................... 31 2.3.2 Disposal and Repository Safeguarding .......... 33 2.3.3 Safety Assessment and Public Perception ......... 36 2.4 Clay – Merits for Radionuclide Retention ............. 36 2.4.1 Structure and Features of Montmorillonite ........ 37 2.4.2 Aluminium Oxide as Reference Substance ........ 43 2.5 Radionuclide Reactions with Natural Materials .......... 47 2.5.1 Complexation ........................ 47 2.5.2 Sorption ........................... 49 2.5.3 Ion Exchange Processes .................. 53 3 Practical Approach and Techniques 55 3.1 Laboratory System ......................... 56 3.1.1 Neptunium Concentration ................. 57 3.1.2 pH-range ........................... 58 3.1.3 Ambient Air versus CO2-free Conditions ......... 59 iii iv Contents 3.1.4 Ionic Strength ........................ 60 3.1.5 Solid-to-liquid Ratio .................... 61 3.1.6 Thermodynamics and Reaction Kinetics ......... 63 3.2 Analytical Methods ......................... 64 3.2.1 γ Spectroscopy ....................... 64 3.2.2 Liquid Scintillation Counting ............... 66 3.2.3 Direct Speciation via EXAFS ............... 66 3.3 Modelling .............................. 80 3.3.1 The 2 SPNE SC/CE Model ................. 81 4 Experimental 87 4.1 Radionuclides ............................ 87 4.1.1 Production of the Isotopes 237Np and 239Np ....... 87 4.1.2 Separation and Purification ................ 88 4.1.3 Assurance of the Fifth Oxidation State .......... 90 4.2 Materials and Chemicals ...................... 90 4.2.1 Montmorillonite STx-1 ................... 90 4.2.2 Aluminium Oxide γ-Al2O3 ................. 91 4.2.3 Sodium Perchlorate NaClO4 and its Purification ..... 92 4.2.4 pH Adjustment and Buffer Solutions ........... 93 4.2.5 Carbonate and Bicarbonate ................ 94 4.3 Procedure of Batch Experiments .................. 95 4.3.1 Preparation: Conditioning of Montmorillonite ...... 95 4.3.2 Assemblage, Pre-conditioning and pH-adjustment .... 97 4.3.3 Addition of Np(V) and Contact Time ........... 98 4.3.4 Centrifugation and γ Counting .............. 100 4.3.5 Data Treatment ....................... 101 4.3.6 Batch Experiments ..................... 103 4.3.7 Sorption Isotherms ..................... 103 4.4 EXAFS Studies ........................... 105 4.4.1 Sample Preparation ..................... 106 4.4.2 Beamline Setup and Measurement ............. 108 4.4.3 Data Processing ....................... 111 4.5 Surface Complexation Modelling .................. 115 5 Results and Discussion 119 5.1 Np(V) Sorption on Montmorillonite and Aluminium Oxide ... 119 5.1.1 pH-Dependency: Edge and Maximum ........... 119 5.1.2 Effect of Variations in Ionic Strength ........... 124 5.1.3 Isotherms .......................... 127 5.1.4 Neptunium Complexation by Carbonates ......... 132 v 5.1.5 Distribution Coefficients .................. 134 5.2 Speciation .............................. 141 5.2.1 Theoretical Implications .................. 141 5.2.2 Indications from the Sorption Behaviour ......... 142 5.2.3 EXAFS-data and Curve-fitting .............. 142 5.2.4 Structure of the Neptunium Carbonate Surface Complexes 149 5.2.5 Insights into the Sorption Mechanism ........... 151 5.2.6 EXAFS on γ-Al2O3 – Reliability Test of the Results ... 151 5.2.7 Compilation of Speciation Results ............. 157 5.3 Modelling Np(V) Sorption on Montmorillonite and γ-Al2O3 ... 158 5.3.1 Fixed Programme Parameters ............... 158 5.3.2 Adjusted Constants ..................... 159 5.4 Consolidation of Results ...................... 165 6 Conclusion and Outlook 169 6.1 Summary of the Experiments ................... 170 6.1.1 Sorption Processes ..................... 170 6.1.2 Surface Species ....................... 171 6.2 Evaluation and Usability of the Results .............. 172 6.3 Future Progression ......................... 173 Bibliography 175 ooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooo189 APPENDIXooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooo189 A Chronology of Nuclear Research and Development 191 B Nuclear Properties of Np 199 B.1 Electronic Configuration of 93Np .................. 199 B.2 Isotopes and Decay Modes ..................... 200 B.3 Colour of Np-Ions in Aqueous Solution .............. 201 C Radioactivity 203 C.1 Types of Radioactive Decay .................... 203 C.2 Nuclear Data Sources ........................ 204 C.3 Decay Series ............................. 205 D Disposal of Radioactive Material 207 D.1 Treatment of Nuclear Waste .................... 207 vi Contents E Definition and Classification of Clay Minerals 209 E.1 Definition by Particle Size ..................... 209 E.2 Classification According to Mineral Composition and Structure . 210 F Instructions for Experimentation 211 F.1 The 237Np(V) Stock Solution .................... 211 F.2 The 239Np(V) Stock Solution .................... 213 F.3 Conditioning of Montmorillonite .................. 215 F.3.1 Adjustment of Ionic Strength from 0.1 to 0.01 M NaClO4 218 G Experimental Overview 219 H Periodic Table of the Elements 221 ooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooo221 I Curriculum Vitae 225 J Declaration 227 List of Tables 3.1 2 SPNE SC/CE model – fixed parameters ............. 84 4.1 Characteristics of the montmorillonite STx-1 ........... 91 4.2 Buffer substances .......................... 93 4.3 Volumes of NaHCO3 and Na2CO3 ................. 94 4.4 Experimental conditions for the EXAFS samples ......... 107 4.5 Specifications of the beamlines at ANKA and ESRF ....... 109 5.1 Bond distances determined for montmorillonite ......... 146 5.2 Literature data on bond distances ................. 147 5.3 Reproducibility of EXAFS analysis – example γ-Al2O3 ..... 153 5.4 Bond distances determined for γ-Al2O3 .............. 154 5.5 Fit parameter for the soddyite model ............... 154 5.6 Model parameters for Np(V) sorption on STx-1 and γ-Al2O3 .. 162 G.1 Batch experiments of this study .................. 219 G.2 pH and Np(V) concentration range of the sorption isotherms .. 220 G.3 Summary of the EXAFS samples prepared ............ 220 vii List of Figures 2.1 Edwin Mattison McMillan and Philip Hauge Abelson ...... 13 2.2 Solubility of Np(V) ......................... 21 2.3 Radiotoxicity based on ingestion .................. 23 2.4 Redox diagram ........................... 25 2.5 Eh/pH-diagram ........................... 27 2.6 Activity of nuclear waste ...................... 31 2.7 Multiple-barrier waste disposal concept .............. 32 2.8 Sheet silicate layers ......................... 38 2.9 Illustration of triple-layered silicates ................ 39 2.10 Empirical dissolution rate of montmorillonite ........... 40 2.11 Silicon and aluminium release from montmorillonite ....... 41 2.12 Cubic models of γ-Al2O3 ...................... 45 2.13 Dissolution of γ-Al2O3 ....................... 47 2.14 Molecular structure of surface complexes ............. 51 2.15 Electric double layer model ..................... 54 3.1 Carbonate equilibrium ....................... 60 3.2 Solid-to-liquid ratio and curve shape ............... 62 3.3 Interactions between X-rays and atom .............. 68 3.4 EXAFS – probability of absorption ................ 69 3.5 Possible scattering paths ...................... 70 3.6 Actinide XANES spectra ...................... 73 3.7 LIII -edge XANES spectra of neptunium ............. 74 4.1 Research reactor TRIGA Mark II, Mainz ............. 88 4.2 Cation exchange column and sand bath with infrared lamp ... 89 4.3 Fuming of the acid ......................... 90 4.4 Purification of NaClO4 ....................... 92 4.5 Collection of the particle size fraction <0.5 ¹m .......... 96 4.6 Adjustment of ionic strength .................... 96 4.7 Scales and rotator .......................... 98 4.8 pH adjustment ........................... 99 ix x List of Figures 4.9 Centrifuges used for phase separation ............... 100 4.10 γ detector .............................. 101 4.11 Aerial view of the ESRF ...................... 105 4.12 Glove box experiments ....................... 106 4.13 Preparation of the EXAFS samples ................ 108 4.14 Rossendorf Beamline at the ESRF ................. 109 4.15 Experimental hutch of the INE-beamline at ANKA ....... 110 4.16 Pre-edge subtraction ........................ 112 4.17 Spline removal ............................ 112 4.18 EXAFS data ............................ 112 4.19 Fourier transform .........................
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