Ion Exchange Mechanism Op Caesium Removal by Cyanoferrates
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ION EXCHANGE MECHANISM OP CAESIUM REMOVAL BY CYANOFERRATES A Thesis submitted for the Degree of Doctor of Philosophy of the University of London by Edward Fook Tin Lee ( Fuk Tin Li ), B.Sc., C.Chem., M.R.S.C. Department of Chemical Engineering and Chemical Technology, Imperial College of Science and Technology, London S.W.7. 2.A.Z. October, 1980. -2- i ABSTRACT Two different caesium sorption mechanisms have been established for potassium copper ferrocyanide, KCuFC H, depending upon solution phase caesium concentration. Experimental investigations indicated that liquid film diffusion controlled the sorption process at caesium concentrations less than 1.5x10 M. The corresponding inter-diffusion coefficient was —9 2 —1 found to be 1.465x10 m sec for an aqueous caesium concentration of —6 —1 3.8x10" M. An activation energy of 15-14 kJ mole was evaluated, which is in the expected range of a liquid film diffusion process. The ion c exchange mechanism at higher caesium concentration (>1.5x10~-^ M) is believed to be governed by chemical reaction between the caesium and potassium in the cyanoferrate. A shell progressive reaction rate model was used to interpret the experimental results. An Arrhenius plot gives good agreement over the entire experimental temperature range from 20 °C to 80 C. The calculated activation energy of 74-85 kJ mole"" supports the hypothesis of a rate process controlled by-chemical reaction. The —1 effective capacity was found to be 2.25 ± 0.06 meq gm and this reduced to 2.04 ± 0.06 meq gm in the presence of 0.82 M magnesium nitrate. It has been observed that 0.2 M nitric acid solution can catalyse the caesium-potassium ion exchange reaction and therefore enhances the ) sorption rate. The sorption of caesium by other cyanof errate s has also been studied. Most of the cyanoferrates possess ion exchange "behaviour except for copper ferrocyanide, CuPC P, which adsorbs caesium. Potassium cobalt ferrocyanide was found to have faster sorption kinetics than the potassium copper and copper cyanoferrates. However, the latter compounds have a higher effective capacity. -3- To my beloved Grandmother and Parents t t I & -t* -4- AGKNOVLEGEMENT I would like to take thi3 opportunity to express my sincere thanks to my supervisor, Dr. M. Streat, for his guidance, encouragement and support throughout the course of this work, I am also indebted to Mr. E.W. Hooper for numerous research discussions and helpful suggestions extended to me. It has been a wonderful time working in the friendly atmosphere of the Nuclear Technology Group. I am obliged to its members, especially to Messrs B. Bennett and T. Davy. I am pleased to express my deepest appreciation to my fiancee, Lean Fong, who typed this manuscript and for her constant encouragement. Finally, I wish to thank the United Kingdom Atomic Energy Authority, Harwell, for providing financial sponsorship of this work. Edward F.T. Lee October, 1980. -5- CONTENTS Title Page 1 Abstract 2 Acknowledgement 3 Contents 5 Nomenclature 9 List of Tables 11 List of Figures 12 CHAPTER 1 INTRODUCTION 15 1.1 Importance of Caesium Removal on Nuclear Waste Disposal 15 1.2 Radioactive Waste Treatment by Ion Exchange 17 1.3 Objective 18 CHAPTER 2 A REVIEW OF COMPLEX CYANOFERRATE COMPOUNDS 20 2.1 Introduction 2.2 Structural Studies 20 The Structural Model of Keggin and Miles 21 The Prussian Blue Analogues 24 The Modified Model 27 Non-cubic Polymeric Cyanides 30 Recent Development 31 2.3 Caesium Removal 34 2.4 Ion Exchange Sorption Mechanism 46 -6- CHAPTER 3 ION EXCHANGE THEORY 48 3.1 General Introduction 48 3.2 The Kinetics and Mechanism of Ion Exchange 49 Diffusion Processes 50 Chemical Reaction Kinetics 56 3.3 Contrast of Kinetic Models 60 CHAPTER 4 EXPERIMENTAL SECTION 63 4.1 Introduction 63 4.2 The Counting System 63 Tracer Technique 63 The Tracer Caesium-134 64 The Counting 65 Error Estimation 66 4.3 Experimental Methods 68 Preparation of Cyanoferrates 68 Analysis of Cyanoferrates 69 Density Measurement 70 Capacity Measurement 70 Ion Exchange Experiment 71 Kinetic Measurement 72 Interruption Tests 75 Isotopic Redistribution Experiment 77 Particle Size Analysis 80 Desorption Experiment 80 4.4 Experimental Error Estimation 81 -7- 4.5 Computing 81 Introduction 81 Computer Program 02 CHAPTER 5 RESULTS AND DISCUSSION 83 5.1 General Properties 83 Chemical Composition and Structural Formula 83 Density, Surface Area and Pore Size 84 Ion Exchange Behaviour 87 Capacity 90 Desorption 92 Stability 95 5.2 Kinetic Aspects 96 The Rate Determining Step 96 Mechanism of Exchange at Low Caesium Concentration 99 An Isotopic Redistribution Experiment 107 Ion Exchange Kinetics at High Caesium Concentration 107 5.3 Mechanism of Exchange at High Caesium Concentration 113 CHAPTER 6 A COMPARISON OF SCME SELECTED CYANOFERRATES 126 6.1 General Properties 126 Chemical Composition and Structural Formula 126 Stability 127 Density 127 Exchange Mechanism 128 Capacity 130 -8— 6.2 Kinetics 131 Low Caesium Concentration (3»8x10 m) 131 High Caesium Concentration (3.8x10~^ M) 131 6.3 Conclusion 134 CHAPTER 7 CONCLUSIONS 135 Conclusions 135 References 137 Appendices 145 -9- NCMENCLATURE a Stoichiometric coefficient or -unit cell dimension. C Concentration. C' Concentration of radioisotopically labelled form. C» Concentration of A at the ion exchanger - film interface. A C. Concentration of A at the surface of the bead's unreacted core. Ac C, Concentration of A in the bulk solution. Ao C, Concentration of A at the surface of the bead. As C Concentration of solid reactant in the bead's unreacted core, so D Diffusion coefficient. Dg Effective diffusivity coefficient. Dq Pre-e3q)onential factor, d Density. AE Activation energy. J Faraday constant. F(t) Fractional equilibrium attainment at time t. i Species i. J Flux. kg Second order reaction rate constant. k Reaction rate constant based on surface, s k mA. Mass transfer coefficient of A through the liquid film. R Gas constant or specified. R Reaction rate, r r Radius. r Radius of the bead's unreacted core, c r o Radius of the bead, T . Temperature. -10- t Time. V Volume. W Mass. w Ratio of CV : CV X Fixed ionic groups or degree of conversion. fCB[1 - F(t)] ~ Value of In < - F(t)]J CA Z^ ' Electrochemical valence of ion i (negative for anions), erf Error function. Roots of the quadratic equation (x + 3wx - 3w = 0). X Dimensionless time. 0 Electrical potential. 5 Liquid film thickness. OC Separation factor. - Overbar represents solid phase. -11- 9 LIST OF TABLES Table 1.1 atypical fission product activity in 1 kg total 16 fission product. Table 5.1 Analysis of KCuFC H. 83 Table 5.2 Specific surface area of KCuFC H. 85 Table 5.3 Physical and surface properties of KCuFC H. 87 Table 5*4 Results of ion exchange experiment on KCuFC H. 89 Table 5.5 Analysis of caesium sorbed form of KCuFC H. 90 Table 5.6 Effective capacity of KCuFC H. 92 Table 5.7 Caesium desorption from KCuFC H by nitric acid. 93 Table 5.8 Effect of particle size on film diffusion. 102 Table 5.9 Temperature effect on film diffusion coefficient. 104 Table 5.10 Temperature effect on relative sorption rate. 122 Table 6.1 Structural formulae of cyanoferrates. 127 Table 6.2 Densities of cyanoferrates. 128 Table 6.3 Ion exchange experimental results of cyanoferrates. 129 Table 6.4 Effective capacities of cyanoferrates. 130 Appendix A List of computer program " Program Test 11. 146 Appendix G Particle size effect on kinetics in 3.8x10~^ M Cs 154 solution. -12- LIST OF FIGURES Figure 2.1 Structural models of Keggin and Miles. 23 Figure 2.2 A model of Prussian blue analogue 26 A B (M )k[M (CN)6]1.xH20. Figure 2.3 Basic structural unit of of hexacyanoferrates: 32 RgMFeCCNjg. Figure 2.4 Diagram of the cross section of the 33 crystallites unit cell. Figure 3.1 Schematic diagram of a partially reacted ion 58 exchanger bead. Figure 4.1 Count rate - caesium concentration calibration 67 curve. Figure 4.2 Apparatus arrangement. 74 Figure 5.1 Pore size distribution of KCuFC H. 86 «3 Figure 5.2 (a) Interruption test in 7.5*10 M Cs solution. 97 -5 Figure 5.2 00 Interruption test in 7.5x10 M Cs solution. 97 Figure 5.2 (c) Interruption test in 3.8x10"*^ M Cs solution. 98 -6 Figure 5.3 98 Speed interruption test in 3.8x10 M Cs solution. Figure 5.4 (a) Particle size effect in 3.8x10*"^ M Cs 100 solutione -6 Figure 100 5.4 (b) Particle size effect in 3.8x10 M Cs solution. —6 Figure 5.5 (a) 101 Plot of processed data in 3.8x10~ M Cs solution. —6 Figure 5.5 00 101 Plot of processed data in 3.8x10"" M Cs solution. —6 Figure 5.6 (a) Temperature effect in 3.8x10~ M Cs solution. 103 -6 Figure 5.6 00 Temperature effect in 3.8x10 M Cs solution. 103 Figure 5.7 The Arrhenius plot for liquid film diffusion. 105 -13- Figure 5.8 (a) Isotopic exchange profile in 7*5x10 ^ M Cs 108 solution. Figure 5-8 (b) % - t plot. 108 Figure 5.9 Particle size effect in 3.8x10r* ^3 M Cs 110 solution. Figure 5-10 Temperature effect in 3.8x10 M Cs solution. 111 -3 Figure 5*10 Temperature effect in 3.8x10 M Cs solution. 111 Figure 5*11 Test of mathematical model - Particle 114 diffusion. (Particle size 0.355-0.420 mm) Figure 5*11 Test of mathematical model - Particle 114 diffusion. (Particle size 0.090-0.106 mm) Figure 5*12 Test of mathematical model - Homogeneous 116 chemical reaction.