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Uranium (VI) interactions with mineral surfaces: controlling factors and surface complexation modelling
(University of New South Wales, 1999-08) Payne, TE
The objective of the work described in this thesis was to improve the scientific basis for modelling the migration of U in the sub-surface environment. The project involved: · studying the sorption of U on model minerals (Georgia kaolinite and ferrihydrite) in laboratory experiments · carrying out experimental studies of U sorption on complex natural substrates · studying the mechanisms influencing U retardation in the natural environment, including transformation processes of iron oxides · identifying chemical factors which control U sorption on model and natural substrates · developing a mechanistic model for U sorption on ferrihydrite and kaolinite using the surface complexation adsorption model , and · assessing and modelling the effect of complexing ligands on uranyl adsorption. Uranium (VI) sorption on geological materials is influenced by a large number of factors including: pH, ionic strength, partial pressure of CO2, adsorbent loading, total amount of U present, and the presence of inorganic and organic ligands. The sorption of UO22+ typically increases with increasing pH (the 'low pH sorption edge') up to about pH 7. In systems equilibrated with air, there is a sharp decrease in sorption above this pH value (the 'high pH edge'), due to strong complexation between uranyl and carbonate. The adsorption model being used for ferrihydrite is a surface complexation model with a diffuse double layer, and both strong and weak sites for U sorption. Based on the analysis of EXAFS data, the U surface complexes were modelled as mononuclear bidentate surface complexes of the form (>FeO2)UO20. Ternary surface complexes involving carbonate with the form (>FeO2)UO2CO32- were also required for the best simulation of U sorption data. There was a slight decrease in U sorption on ferrihydrite in systems that contained sulfate. It was necessary to consider competition between UO22+ and SO42- for surface sites, as well as complexation between UO22+ and SO42- to model the data. The presence of citrate considerably reduced U sorption and caused dissolution of ferrihydrite. Complexation of citrate with both uranyl and ferric ions was taken into account in modelling this system. The model required the optimisation of the formation constant for a postulated mixed metal (U/Fe/citrate) aqueous complex. Humic acid increased U uptake at pH values below 7, with little effect at higher pH values. In terms of the amount of U sorbed per gram of adsorbent, U uptake on kaolinites KGa-1 and KGa-1B was much weaker than U uptake on ferrihydrite under similar experimental conditions. Electron microscope examination showed that titanium-rich impurity phases played a major role in the sorption of U by these standard kaolinites. A relatively simple model for uranyl sorption on the model kaolinites was able to account for U sorption under a wide range of experimental conditions. The model involved only three surface reactions on two sites (>TiOH and >AlOH), with a non-electrostatic surface complexation model. The relative amounts of the sites were estimated from AEM results. Precipitation was taken into account in modelling the experimental data obtained with high U concentrations. The effects of sulfate and citrate on U sorption by kaolinite were also assessed and modelled. Sulfate had a small effect on U sorption, which may be explained by aqueous complexation. Citrate had a greater effect, and this was not wholly explained by the formation of aqueous U-citrate complexes. The most likely explanation would also involve competition for surface sites between U and citrate. Uranyl uptake on ferrihydrite was greatly increased by the presence of phosphate. This was not due to precipitation, and was attributed to the formation of a ternary surface complex with a proposed structure of (>FeO2)UO2PO43-. The log K value for the formation of this complex was optimised using FITEQL. Phosphate also increased uptake of uranyl on kaolinite, and this was also attributed to the formation of ternary uranyl phosphate surface species. Uranium sorption on weathered schist samples from the vicinity of the Koongarra U deposit in northern Australia was generally similar to the model minerals (in terms of the effects of pH, ionic strength, total U, etc). Many experiments with the natural materials were spiked with an artificial U isotope (236U), which allowed adsorption (of 236U) and desorption (of 238U) to be distinguished, and provided a means of estimating the 'labile' or 'accessible' portion of the natural U content. A significant advantage of this method is that (unlike chemical extractions) it does not rely on the assumptions about the phases extracted by 'selective' reagents. Uranium sorption experiments were also carried out with Koongarra samples which had been treated with citrate / dithionite / bicarbonate (CDB) reagent to remove iron oxides. Uranium sorption was greatly decreased by the CDB extraction, which reduced the surface area of the samples by about 30-40%. To further elucidate the impact of iron minerals on U mobility in the natural environment, the transformation of synthetic ferrihydrite containing adsorbed natural uranium was studied. In these experiments, the ferrihydrite was partially converted to crystalline forms such as hematite and goethite. The uptake of an artificial uranium isotope (236U) and the leaching of 238U from the samples were then studied in adsorption / desorption experiments. The transformation of ferrihydrite to crystalline minerals substantially reduced the ability of the samples to adsorb 236U from solution. Some of the previously adsorbed 238U was irreversibly incorporated within the mineral structure during the transformation process. Therefore, transformation of iron minerals from amorphous to crystalline forms provides a possible mechanism for uranium immobilisation in the groundwater environment. In considering the overall effect on U migration, this must be balanced against the reduced ability of the transformed iron oxide to adsorb U. The experiments with the model and natural substrates demonstrated that trace impurities (such as Ti-oxides) and mineral coatings (such as ferrihydrite) can play a dominant role in U adsorption in both environmental and model systems. Although the various substrates had different affinities for adsorbing U, the effects of chemical factors, including pH, ionic strength, and carbonate complexation were similar for the different materials. This suggests that a mechanistic model for U sorption on model minerals may eventually be incorporated in geochemical transport models, and used to describe U sorption in the natural environment.
Optimisation of neutron capture radiography for analysis of boron-10 in tissues
(University of Technology, Sydney and Australian Nuclear Science and Technolog Organisation, 1991-01) Mrayati, H
In Boron Neutron Capture Therapy (BNCT), a non—toxic compound of boron is selectively taken up by tumour tissue. The tumour is then irradiated by thermal neutrons, inducing the 10B(n, α)7Li fission reaction. These ions have a range of about one cell diameter, high linear energy transfer (LET) and result in the energy deposition of up to 2.3 MeV in the cancer cell. Consequently, reproductive cell death results, leading to the regression of the tumour reproduction. It is of critical importance to determine the spatial distribution of boron uptake in the tumour and this is achieved by Neutron Capture Radiography (NCR). The NCR system was characterised prior to its utilisation for boron—10 visualisation. NCR utilises a Solid State Nuclear Track Detector (SSNTD) to register the products of the fission reaction. These products cause structural damage to the SSNTD that can be revealed as pits after chemical etching with 6.25M NaOH at 70°C for 60 minutes. The pits can be observed by a light microscope. Thus, the boron—10 (10B) distribution in a biological section can be mapped by the location and density of pits. A polymer type of SSNTD known as Colombia Resin—39 (CR—39) is an alpha track detector and it was theoretically characterised in terms of: pit diameter (d), critical angle (ƟC), etching rate ratios (V), track (VT) and bulk (VB) etching rates, etching efficiencies (η), etching times (tE) and etch—induction time(tind) of alpha, lithium, and hydrogen ions. V and VB parameters were calculated from semi—empirical equations for etching condition T=70°C and etchant 6.25M NaOH. Their values are listed in the table below. Theoretical values were then compared with the experimental results. The calculated parameters can provide an estimation of the track parameters and thereby the optimum etching conditions for a NCR experiment. Error analysis was applied to both approaches. The measured and the published values of pit diameters of alpha are comparable with 5.2% variance. The measured bulk etching rate (VB is 10.5% higher than the calculated one. The calculated pit diameter of proton is comparable with the published value within 0.6%. The effects of high fluence neutron beam on the track background were investigated. It found that the background is proportional with neutron fluence. Fluence of order of 1012 n cm- 2 gives high track background and changes the optical characteristics of the detector. On the other hand, fluence of order of l010 n cm -2 or less gave a good track to background ratio. The results of CR—39 characterisation are utilised to determine the optimum etching. These conditions are essential for preferential visualisation of alpha from proton tracks in tissue samples. They are found to be: etchant concentration=6.25M NaOH, etching temperature=70C° and 60 minutes etching time, combined with thermal neutron fluence of order of 1012 n cm-2. Pits from different concentrations of 10B sample were photomicrographed and later were counted by using a light microscope to determine the lower and upper detection limits. The limits were found to be 2.5 and 80 parts per million (ppm) of boron - l0 solution respectively. The pit densities decrease as the etching time is increased. The spatial resolution of the technique is considered.to be 20 μm because of uncertainty in the origin of particle in tissue. The theoretical approach leads to the optimum conditions which were applied to reveal the spatial distributions of boron-10 in tissue samples. The conditions give a satisfactory result and demonstrate a differential 10B uptake by tumour tissue of nude mice. The maximum ratio of tumour pits to the adjacent muscle=32.l. Copyright ©1991 The Author.
A study of uranium and thorium migration at the Koongarra uranium deposit with application to actinide transport from nuclear waste repositories
(Macquarie University and Australian Nuclear Science and Technology Organisation, 1991-01) Payne, TE
One way to gain confidence in modelling possible radionuclide releases is to study natural systems which are similar to components of the multibarrier waste repository. Several such analogues are currently under study and these provide useful data about radionuclide behaviour in the natural environment. One such system is the Koongarra uranium deposit in the Northern Territory. In this dissertation, the migration of actinides, primarily uranium and thorium, has been studied as an analogue for the behaviour of transuranics in the far-field of a waste repository. The major findings of this study are: 1. the main process retarding uranium migration in the dispersion fan at Koongarra is sorption, which suppresses dissolved uranium concentrations well below solubility limits, with ferrihydrite being a major sorbing phase; 2. thorium is extremely immobile, with very low dissolved concentrations and corresponding high distribution ratios for 230Th. Overall, it is estimated that colloids are relatively unimportant in Koongarra groundwater. Uranium migrates mostly as dissolved species, whereas thorium and actinium are mostly adsorbed to larger, relatively immobile particles and the stationary phase. However, of the small amount of 230Th that passes through a 1μm filter, a significant proportion is associated with colloidal particles. Actinium appears to be slightly more mobile than thorium and is associated with colloids to a greater extent, although generally present in low concentrations. These results support the possibility of colloidal transport of trivalent and tetravalent actinides in the vicinity of a nuclear waste repository.
Effect of indium and niobium segregation on the surface vs. bulk chemistry of titanium dioxide (rutile)
(University of Western Sydney, 2013-01-01) Atanacio, AJ
Since the landmark paper in 1972 by Fujishima and Honda [1], TiO2 has become one of the most promising candidates of a new generation of solar energy materials capable of generating clean hydrogen fuel using only sunlight (photo-electrochemically) to dissociate water. TiO2 has both bulk properties and surface properties which contribute to its functional performance. Considering that all of the electrochemical reactions induced by light occur at the surface of TiO2, it becomes clear that understanding the surface properties of TiO2 is of crucial importance for its performance; specifically the conversion of solar energy into chemical energy. The surface phase of TiO2 can be substantially different from that of the bulk phase as a result of a phenomenon known as segregation. Segregation involves the transport of certain lattice species from the bulk phase to the surface, driven by excess surface energy. To date, developments in the understanding of TiO2 solid solutions and related properties have mainly been centred on bulk properties. In comparison, relatively little work has been reported on segregation in TiO2 solid solutions and its influence on functional properties, such as reactivity and photoreactivity. The present work has studied the effect of indium (acceptor-type ion) and niobium (donor-type ion) segregation on the surface chemistry of well-defined In-doped and Nb-doped TiO2 solid solutions. Specifically, examining the relationship between imposed sample processing conditions, such as the gas phase oxygen activity, on segregation-induced surface enrichment. This was achieved using a range of complimentary analysis techniques including X-ray photoelectron spectroscopy (XPS), secondary ion mass spectrometry (SIMS), Rutherford backscattering (RBS) and proton-induced X-ray emission (PIXE). Copyright © 2013 Western Sydney University
New materials for selective separations at the back end of the nuclear fuel cycle
(University of Sydney, 2016) Veliscek-Carolan, J
Storage and recycling of nuclear waste are important issues that will increase in importance if nuclear power becomes more widely adopted worldwide. Recycling of used nuclear fuel is of benefit both in terms of increasing the nuclear lifetime (ie the number of years nuclear power will be a viable option for power generation) and decreasing the hazards (radiotoxicity, volume and longevity) of nuclear waste. Currently, most reprocessing of used nuclear fuel is performed using liquid-liquid extraction. However, use of solid sorbent materials has many advantages such the lack of organic solvent wastes. This research involves development of materials that are able to selectively remove specific target elements from solutions of used nuclear fuel. Once loaded with radionuclides, these materials may be utilised as transmutation matrices or wasteforms. Therefore, radiolytically and hydrolytically stable materials able to withstand the conditions of nuclear separations, such as titania and zirconia, have been targeted. Further, ordered porosity has been introduced into these titania and zirconia framework materials to improve their sorption capacity and kinetics. In order to impart selectivity to these materials, organic ligands are incorporated. Functional groups, including phosphonates, amines and peptides, have been chosen or designed based on their selectivity for elements relevant to the nuclear fuel cycle. Elements of interest include uranium, which constitutes >96% of used nuclear fuel and can be recycled; minor actinides, which contribute significantly to the radiotoxicity of nuclear waste and can also be recycled in fast neutron reactors; and lanthanides, which are targets for separation from the minor actinides as their high neutron absorption cross sections prevent transmutation of the minor actinides. Novel hybrid materials have been synthesized and their sorption characteristics, including selectivity, capacity and kinetics, evaluated. © 2016 The Author.

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