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Some Processes Affecting the Mobility of Thorium in Natural Ground Waters

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££4*00/33 Some Processes Affecting the Mobility of Thorium in Natural Ground Waters Erik Östhols ill* VETENSKAP OCH KONST KTH l 1994 Erik Östhols: Some Processes Affecting the Mobility of Thorium in Natural Ground Waters Errata Page Line Reads: Should be: 12 14 specifics of the processes specifics of some processes 13 28 in the following sections in this section 20 14-15 excellent reviews on excellent reviews of 20 30 Reliable data often Reliable data are often 29 28 reveiwed reviewed 33 1 vaccuum vacuum 34 2 sorption data have sorption data in this work have 34 19 of a surface of surface 35 14 succesfully successfully 36 9 preferrably preferably 36 15 the type atoms the type of atoms 37 - Equation 6.18 should not be numbered. 44 10 in fig. 15 A likely in fig. 15. A likely 44 12 dashed cicles dashed circles 49 11 vigourously vigorously 49 32 Inversely Conversely 50 22 figs. 17 and 18 The figs. 17 and 18. The 50 _ Equations should be numbered. 58 25 Radiactive Radioactive 62 - Reference Schlesinger, 1991 is never used. 63 1 Radiii Radii 11:3 3 phoshate phosphate 11:5 1 pipet pipette 11:11 2 flourescence fluorescence 111:4 12 collodial colloidal 111:13 2 vaccuum vacuum 111:29 - log/?forSiO(OH)J should be -12.31 111:30-31 - — log[H+l values shown are not corrected for liquid junction potentials IV:12 25 Reference Östhols et al. 1993 should be 1994 IV: 11 21. incompatbie incompatible V:4 12 vigourously vigorously V:6 30 investigated ranges ranges where accurate measurements could be macie V:12 7 model of model for V:18 7 Reference Östhols et al. 1993 should be 1994 ROYAL INSTITUTE OF TECHNOLOGY Some Processes Affecting the Mobility of Thorium in Natural Ground Waters Erik Östhols Department of Chemistry Inorganic Chemistry i oyal Institute of Technology 5.-100 44 Stockholm, Sweden April, 1994 TRITA-OOK-1038 Akademisk avhandling, som med tillstånd av Kungliga Tekniska Högskolan framlägges till offentlig granskning för avläggande av teknologie doktorsexamen i oorganisk kemi, fredagen den 6 maj 1994 kl. 10.00 i hörsal K2, KTH, Teknikringen 28, Stockholm. Fakultetsopponent är Dr. John Westall, Dept. of Chem., Oregon State Univ., Corvallis, USA. Avhandlingen försvaras på engelska. ABSTRACT Erik Östhols: Some Processes Affecting the Mobility of Thorium in Natural Ground Waters. TRITA-OOK 1038. Department of Chemistry, Inorganic Chemistry, Royal Institute of Technology, Stockholm, Sweden. Thorium is a useful model element for tetravalent actinides such as U(IV), Pu(IV) and Np(IV) which are important constituents of spent nuclear fuel. Thorium is also an important tracer element for particle pathways in natural environments. In order to correctly model the transport of Th in the environment, it is impor- tant to have quantitative models for processes that affect its mobility. Some of these processes have been experimentally investigated in laboratory studies, and interpreted with quantitative models where possible. The carbonate complexation in aqueous solution of Th has been investigated through solubility studies of ThO2 in carbonate media. It is shown, that thorium carbonate complexes are likely to be predominant in many natural waters. They also increase the solubility of the oxide significantly, and hence the mobility of Th. Carbonate also increases the dissolution rate of thorium oxide. This has been shown in dissolution kinetics experiments performed in a continuous flow thin-film reactor. This effect will only be important in environments with a pH and total carbonate alkalinity higher than those of most natural aquatic environments. Solubility studies of thorium oxide in phosphate media show that phosphate does not significantly increase the mobility of Th in aqueous media. The presence of phosphate may cause the precipitation of sparingly soluble thorium phosphates which will decrease the mobility of Th. The pentahydroxo complex for Th is shown to be insignificant up to pH 13. Potentiometric studies of Th sorption on amorphous colloidal silica indicate, that pure aluminosilicates will probably not be efficient scavengers of tetravalent actinides above pH values of approximately 6. In neutral to alkaline solutions, iron (hydr)oxides are likely to be the predominant sorbents. The Th sorption on silica has also been studied with EXAFS spectroscopy. The results show that polynuclear surface complexes are probably not important foi this system. Th binds to the silica surface through corner-sharing bonds, where Th and Si share one, but not more oxygen atoms. The likely number of surface sites occupied by each sorbed Th is two to three. A tentative configuration for the surface complex is presented. KEYWORDS: Thorium, solubility, mobility, radionuclides, carbonate, phos- phate, sorption, silica, weathering, dissolution, kinetics, EXAFS, complex, surface. Preface This thesis is a summary of the five papers listed below, referred to in the text as paper I-V. The first sections of the thesis contain an introduction to and motivation for the work. The following sections explain some of the theoretical and experimental aspects of each paper, and also give summaries of the results. Finally, each of the papers is included in a separate section. Stockholm, April 1994 Erik Osthols I. On the Influence of CO2 on Mineral Dissolution III. The Solu- bility of Microcrystalline ThO2 in C02 - H20 media. E. Östhols, J. Bruno and I. Grenthe. Geochim. Cosmochim. Ada, 58, 613-623 (1994). II. The Solubility of Microcrystalline ThO2 in Phosphate Media. E. Östhols. Submitted to Radiochim. Ada. III. Thorium Sorption on Amorphous Silica. E. Osthols. Submitted to Geochim. Cosmochim. Ada. IV. Adsorption of Th on Amorphous Silica: an EXAFS Study. E. Osthols, L. Charlet and A. Manceau. Manuscript. V. Dissolution Kinetics of ThO2 in Acid and Bicarbonate Media. E. Osthols and M. Malmström. Submitted to Radiochim. Ada. Contents 1 Introduction 1 2 The General Chemistry of Thorium 2 3 Which Processes Affect the Mobility of Thorium in the En- vironment? 4 4 An Example Related to Ground Water: Spent Nuclear Fuel Disposal 8 5 Solution and Solubility Equilibria 13 5.1 Background and Theory 13 5.2 Experimental techniques 20 5.3 The Solubility of Microcrystalline ThO2 in CO2 - H20 media (paper I) 21 5.4 The Solubility of Microcrystalline ThO2 in Phosphate Media (paper II) 24 6 Sorption Studies 28 6.1 Background and Theory 28 6.2 Experimental techniques 34 6.2.1 Wet chemistry methods 34 6.2.2 EXAFS 35 6.3 Potentiometric Studies of Th sorption on Amorphous Silica (paper III) 37 6.4 An EXAFS Investigation of the Sorption of Th on Silica (paper IV) 42 7 Dissolution Rates of Minerals 46 7.1 Background and Theory 46 7.2 Experimental techniques 48 7.3 Dissolution Kinetics of ThO2 in Acid and Bicarbonate Media (paper V) 50 8 Conclusions: What Can We Learn? 54 9 Acknowledgements 56 1 Introduction This thesis is mainly concerned with the behaviour of the actinide element thorium (Th) in granitic ground waters. Although this may seem like a narrow subject of research, many of the processes investigated are of general importance for the transport of metal cations in natural water systems. This, in turn, is interesting because human activities, industrial and others, cause the release of metals into the environment. In a time when concern about the environment is becoming more apparent with every passing day, it is extremely important to possess accurate knowledge about the environmental behaviour of substances released by human activities. Once the metals are released to our environment, we want to know how and to what extent they will be transported, and in what forms. If we know this, we can also under- stand to what extent we will be exposed to them in our daily lives, and what precautions to take in order to minimize environmental damage and health risks. One of the most important modes of transport of metals is with water, whether in rivers, lakes, oceans, vapours or ground waters. If the released metals are toxic, it is important to know to what extent and how fast the metals will be transported. It is necessary to know the speciation in the aqueous phase of the metal, i. e. how and with what the metal ion will be coordinated in the water, since this will significantly affect the reactivity and hence the toxicity and general behaviour of the metal. The main reasons to study thorium, rather than some more common element, are two. First, thorium is an important model element for other tetravalent actinides such as U(IV), Pu(IV) and Np(IV). U(IV), in the form of UO2(s). is the main constituent of spent nuclear fuel, and Pu and Np are important constituents in radioactive waste [SKB91, 1992, KBS-3, 1983]. Whereas U, Pu and Np have several oxidation states, making it difficult to control the oxidation state of these elements in the course of experimental work, Th is almost always present as Th(IV). This greatly simplifies much of the experimental work compared with other actinides. Although there are no stable Th nuclides, 232Th is an a-emitter with a very long half life (1.4 - 1O10 years), which makes it easy to work with in laboratory experiments without difficult and time-consuming safety measures. This sets it apart from e. g. Pu. The second reason for studying the behaviour of Th in natural environ- ments is its importance as a tracer element. Tracer elements are elements that can be used to investigate characteristic rates of fluxes of e. g. water and particles between natural water reservoirs such as oceans, lakes and the atmosphere. Radioactive elements are particularly suitable for tracer appli- cations, since their concentrations are often relatively easy to measure and because the radioactive decay may help to measure the timescales of the transport processes.
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