The purpose of this work was to recover uranium from residue originating from the production of molybdenum -99. Carbonate solutions containing hydrogen peroxide or oxygen gas as oxidants were used as leachants. Experimental parameters included: different peroxide and carbonate concentrations, dissolution time and temperature. Results indicate complete dissolution of the residue at 60 oC, after thirty minutes, in ammonium carbonate solution enriched with hydrogen peroxide. Almost two hours were needed to achieve the same extent in the presence of oxygen. The yield and rate of uranium extraction were found to increase as a function of both temperature, in the range of 25-60 oC, and hydrogen peroxide concentration. The leaching kinetics were determined for various leaching conditions and the activation energy was found to be 45.5kJ/mol. The order of reaction with respect to uranium concentration was found to be unity. / Dissertation (MSc)--University of Pretoria, 2013. / gm2014 / Materials Science and Metallurgical Engineering / unrestricted

Clays are considered as potential host rocks and backfill material for deep geological repositories for radioactive waste. Therefore, profound understanding of radionuclide retention processes at clay mineral surfaces is essential for a long-term safety assessment. This understanding has already been generated in the past for simple chemical systems, in which experiments are easy to conduct and interpretation is straightforward. However, there is still a lack of molecular process understanding when considering complex natural systems (low radionuclide concentrations, high ionic strength, high pH values, multi-mineral solid phases, complex solution composition). This thesis aims to close some of these knowledge gaps, focusing on U(VI) and Np(VI) sorption on clays at (hyper)alkaline conditions. pH values between 10 and 13 can prevail in the near-field of a radioactive waste repository as a result of the degradation of concrete, which is part of the geo-engineered barrier. Existing studies on radionuclide sorption on clays do not exceed pH 10. Therefore, within this work, a comprehensive investigation in the pH range 8-13 was conducted. This included the quantification of radionuclide retention in batch sorption experiments as well as spectroscopic investigations to generate understanding about the underlying retention mechanisms on a molecular level. Beside the pH, additional focus was on the influence of dissolved carbonate and calcium on radionuclide sorption at (hyper)alkaline conditions. Next to two small chapters dealing with the stability and surface charge of Ca-bentonite at (hyper)alkaline conditions (chapter 4.1) and the influence of ISA on U(VI) sorption at high pH values (chapter 4.3), the thesis can be subdivided in two major parts. The first part (chapter 4.2) is a detailed investigation of U(VI) sorption on Ca-bentonite at (hyper)alkaline conditions in mixed electrolyte solutions. Batch sorption experiments were conducted, varying a number of experimental parameters (sorption time, S/L ratio, U(VI) concentration, pH value, carbonate concentration) and assessing their effect on U(VI) sorption. In order to be able to explain the observed sorption behavior, next to U(VI) solubility tests, spectroscopic techniques were applied. The aqueous speciation of U(VI) was investigated with TRLFS, while its surface speciation was probed with ATR FT-IR, site-selective TRLFS, EXAFS and CTR/RAXR. Since the results of this chapter indicated a great importance of the presence of calcium (see below), the second major part of the thesis (chapter 4.4) was dedicated to a careful evaluation of the influence of calcium on An(VI) sorption on clay minerals at (hyper)alkaline conditions. This encompasses the sorption of Ca(II) on Ca-bentonite and its effect on the bentonite surface charge. Furthermore, U(VI) batch sorption experiments with Na-montmorillonite, synthetic kaolinite and muscovite were conducted in 0.1 M NaCl as well as in 0.1 M NaCl + 0.02 M CaCl2 at pH 8-13, in order to quantify the influence of calcium on U(VI) sorption on supposedly Ca-free mineral phases. Site-selective TRLFS was applied with the aim to observe U(VI) sorption species involving calcium. Finally, complementary sorption experiments Np(VI) on muscovite were performed in order to check whether its sorption behavior is analogous to U(VI) under the given conditions. Batch sorption experiments demonstrate that U(VI) retention on Ca-bentonite can be very effective at pH > 10, even in the presence of carbonate and despite the prevalence of anionic aqueous species. Above a certain pH, depending on the concentration of carbonate in solution, carbonate does not play a role in the aqueous U(VI) speciation anymore due to the predominance of hydrolysis. TRLFS measurements revealed a clear correlation between sorption behavior and aqueous U(VI) speciation, showing that retention reaches a maximum at pH 10-12, where UO2(OH)3− is the predominant aqueous species. This raised the question whether the strong retention can be achieved by adsorption of an anionic species to the negatively charged mineral surface or rather by precipitation of uranates. By in situ ATR FT-IR and CTR/RAXR experiments the formation of U(VI) precipitates on the mineral surface was observed at U(VI) concentrations of 2×10-5 M and 5×10-5 M, respectively. However, solubility tests at sub-micromolar U(VI) concentrations, which were also applied in the batch sorption experiments, showed that the observed complete U(VI) removal at pH 10-12 cannot be attributed to precipitation of (earth) alkali-uranates from the solution. In order to unambiguously distinguish between surface precipitation and surface complexation, direct spectroscopic investigations of the U(VI) complexes on the Ca-bentonite surface were performed with site-selective TRLFS and EXAFS. The occurrence of luminescence line-narrowing and the frequency of the total symmetric stretch vibration obtained from the site-selective TRLFS emission spectra, indicate the presence of two U(VI) surface complexes. Also EXAFS spectroscopy confirmed the presence of two independent U(VI) sorption species on Ca-bentonite at pH 8-13. With increasing pH, the nature of the retained U(VI) complexes shifts from bidentate inner-sphere surface complexes with an overall equatorial coordination of five adsorbed on aluminol or silanol edge sites to surface complexes with a 4-fold equatorial coordination, resembling the aqueous species UO2(OH)42−. For the first time, a 4-fold coordination in the equatorial plane of U(VI) was univocally proven with the help of a multiple-scattering feature originating from the strong symmetry of the complexes, and without the need for error-prone shell fitting. The lack of scattering paths from the substrate and the comparatively high value for the total symmetric stretch vibration indicate that the high-pH-component is an outer-sphere complex. Concerning the character of the second sorption species at very high pH it was hypothesized that the anionic uranyl hydroxide complexes are mediated to the surface by calcium cations. It was found that calcium sorbs strongly on Ca-bentonite between pH 8 and 13. Also zeta potential measurements showed a partial compensation of the strongly negative surface charge of Ca-bentonite due to adsorption of calcium. U(VI) sorption on kaolinite and muscovite was strongly reduced in the absence of calcium at pH > 10. An increased retention upon addition of calcium proved the sorption enhancing effect of calcium at pH 10-12. Site-selective TRLFS allowed the spectroscopic observation and identification of calcium-induced U(VI) sorption complexes on muscovite. The obtained spectra correspond to the outer-sphere species found on Ca-bentonite. Combining the findings from batch sorption, zeta potential, TRLFS and EXAFS suggests that calcium adsorbs to the mineral surface in the first place, displaying locally positively charged sites which enable an electrostatically driven attachment of anionic uranyl hydroxides. The same effect could also be demonstrated for Np(VI) sorption on muscovite, which was also strongly enhanced in the presence of calcium at pH 9-12. ISA leads to a mobilization of U(VI) at (hyper)alkaline conditions only when present in very high excess of U(VI). A reduction of sorption on Ca-bentonite and the formation of aqueous U(VI)-ISA complexes, detected with TRLFS, occurred at an U:ISA ratio of 1:100,000. Such conditions are not likely to be found in deep geological repository environments. Based on these findings it can be concluded that under certain alkaline repository conditions, where precipitation does not occur (due to very low concentrations or kinetic restraints), U(VI) and Np(VI) are still effectively retained in argillaceous minerals and rocks by adsorption despite the anionic character of prevailing aqueous species. Repulsive forces between the actinide species and the mineral surfaces are overcome by mediating Ca2+. This finding is of great relevance, as also the migration of very small amounts of uranium or neptunium out of waste repositories could lead to a hazardous accumulation in the long term. The achieved knowledge gain concerning radionuclide retention at environmental conditions helps to take the next step towards realistic long-term safety assessment of nuclear waste repositories.

Roztoky Intrusive Center (33-28 Ma) is connected with tertiary volcanic activity in the territory of the Bohemian Massif (Czech Republic). It is located in the České středohoří volcanic complex on the tectonic line of the Ohře rift. The aim of this work was to petrologically describe local rocks and to verify the possible connection of the Roztoky Intrusive Center (RIC) with carbonatite magmatism, as could be suggested by the presence of alkaline rocks including their exotic types (essexites, monzodiorites, sodalitic syenites,…). It is assumed that there is a hidden carbonatite intrusion under the RIC caldera structure, which may also indicate geophysical manifestations of two hidden gravimetric minimas. In a detailed exploration of a drill core from the R-2 borehole from the 1960s, a petrographic rocks composition of a part of the caldera filling was described, including new find of xenolite of carbonatite in 286,5 m of R-2 borehole. By means of geochemical and micro-probe analyzes the well material including carbonatite was examined in detail. Carbonate mineralization with high REE contents was found in carbonatite, but also in the exocontact of the previously described silicocarbonatite (Rapprich et al. 2017). The discovery of carbonatite xenolite and the presence of minerals with high concentrations of...

The laboratory-made apparatus for electrophoretic separations in short capillaries has been tested. The basic functions of the apparatus (separation efficiency, resolution, migration time) have been tested using mixture of K+ and Na+ ions. The model mixture of basic amino-acids (arginine, lysine, histidine and its derivatives, 3-methylhistidine and 1-methylhistidine) has also been separated and detected using contactless conductometric detector. Histidine in real sample of urine has also been determined. Keywords: capillary electrophoresis, short capillary, alkaline aminoacids

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>Magister Scientiae - MSc / Mining has a lengthy history in South Africa, and subsequent beneficiation processes have been conducted with little regard for the environment, thus leaving the land with un-rehabilitated abandoned mines. Currently, most of these abandoned mine sites are no longer operational and they continuously contaminate soil, air and water resources in various areas where mining took place. This study looks at the treatment of contaminated mine water using the Dispersed Alkaline Substrates (DAS) which is a new South African technology that uses a variety of substrates to neutralize and raise the pH of mine water while lowering the solubility of potentially dangerous metals.

Philosophiae Doctor - PhD / The depletion of fossil fuel resources such as coal and the concern of climatic change arising from the emission of greenhouse gases (GHG) and global warming [1] lead to the identification of the 'hydrogen economy' as one of the renewable energy sources and possible futuristic energy conversion solution. Sources of hydrogen as fuel such as water through electrolysis and liquid organic fuel (Hydrogen carriers) have been found as potential game-changers and received increased attention, due to its low-carbon emission.

Philosophiae Doctor - PhD / Power generation in South Africa depends majorly on the combustion of coal and the increasing demand for energy due to industrialisation and population growth signifies the continuous consumption of coal. During this process, a by-product known as coal fly ash (CFA) is generated which poses several environmental issues. The common management of the waste involves the disposal of CFA in landfills or the direct disposal to open uncultured lands, thus contaminating water bodies by the leaching of constituent CFA metals and salts that render arable land uncultivable.

The master thesis discusses the alkali-activation of aluminosilicate material (metakaolin), specifies the main components of geopolymer mixture, evaluates the influence of geopolymer mixture composition and the influence of other factors on final properties of geopolymer. Results of the mechanical properties and microstructure of the geopolymer pastes made out of metakaolin activated by various amounts (50–110 % by weight of metakaolin) of liquid water glass (with varying modulus from 0,5 to 3,0) or NaOH as well as KOH solution are presented in the experimental part.

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