Thèse de doctorat : Génie des procédés et des produits : INPL : 2008. / Titre provenant de l'écran-titre.

The isotopic and elemental composition of speleothem calcite precipitated from dripwaters in caves have been used to elucidate changes in karst groundwater flow paths and changes in climate such as shifts in vegetation and rainfall patterns. Given that recent studies have shown seasonal isotopic and elemental changes in cave dripwater chemistry, then these changes are most likely reflected in the precipitated calcite. This affirms the need to study the environmental controls on the dripwater isotopic and elemental chemistry on seasonal time scales, since ultimately these are environmental controls on the precipitated speleothem calcite. In this study, physical, chemical, and meteorological variations at 13 drip-water and pool sites in two cave systems in the Edwards aquifer of central Texas are used to delineate the controls on the evolution of cave waters. The chemical evolution of these cave waters can be accounted for by variations in calcite precipitation, soil geochemistry, flow paths, and degassing of CO₂ driven by seasonal overturn of the cave atmosphere.Calcite precipitated on glass substrates placed under three of the thirteen dripwaters in this study was analyzed for its C and O isotopic composition in order to assess 1) seasonal variations in the isotopic composition and 2) the extent to which precipitation occurs in isotopic equilibrium with its host dripwater. Each winter season, calcite growth rates increase, which corresponds to changes in the C and O isotopic composition of the calcite: higher ơ13C and ơ18O values during winter and lower ơ13C and ơ18O values during summer. Some calcite samples were found to precipitate out of C and O isotopic equilibrium, with maximum departures of 1.3% with respect to carbon and 1.4% with respect to oxygen. The results show that ơ13C and ơ18O in the modern speleothem calcite:1) can vary sub-annually without change in vegetation or climate, and 2) reflect in the majority of cases precipitation out of isotopic equilibrium with the host dripwater that precipitates the calcites. / text

A new three-angle energy-dispersive (ED) diffractometer has been successfully commissioned on station 16.4 SRS, Daresbury, England. The diffractometer facilitates the simultaneous collection of three spectra at three Bragg scattering angles. This enables the sampling of a far greater range of reciprocal space as compared to conventional single-angle diffractometers. Additionally the arrangement allows changes in sample density to be monitored. A protocol has been developed to align the diffractometer such that the origins of the diffracting volume are coincident on the diffractometer axis. Spectra obtained from the diffractometer were improved by the construction and placement of shielding. Experimental determination of components of the resolution function show that the resolution is close to the instrumental limit. The flux distribution of station 16.4 was determined experimentally. A novel whole pattern method has been developed for the quantitative analysis of synchrotron ED diffraction data. The method, which accounts for the differential absorption across the ED spectrum, was developed using spectra collected from a set of test binary phase mixtures and pure phases. Parameters relating to the proposed models were determined using linear and non-linear least-squares methods. Although the final model is the most physically complete it does not take account of certain non-diffraction derived events which appear as counts within the test spectra. A novel application of synchrotron ED diffraction, energy-dispersive diffraction tomography (EDD-T), is described. The method facilitates the non-destructive examination of the interior of crystalline and semi-crystalline objects. The resolution and limitations of this technique have been demonstrated using test objects. The method has been used to map the phase distributions of a variety of materials in a range of different samples. Quantitative EDD-T was used to determine the invasion of calcite into simulated oil-reservoir rocks.

Thesis (Ph.D.)--Murdoch University, 2008. / Thesis submitted to the Faculty of Sustainability Environmental and Life Sciences. Includes bibliographical references (leaves 237-251)

Thesis (M.S.)--University of California, Berkeley, 1961. / "Chemistry General" -t.p. "TID-4500 (16th Ed.)" -t.p. Includes bibliographical references (p. 53-55).

Thesis (MS)--Montana State University--Bozeman, 2009. / Typescript. Chairperson, Graduate Committee: Robin Gerlach. Includes bibliographical references (leaves 89-93).

Th. doct.-ing.--Toulouse 3, 1978. N°: 633.

abstract: Natural variations in 238U/235U of marine carbonates might provide a useful way of constraining redox conditions of ancient environments. In order to evaluate the reliability of this proxy, we conducted aragonite and calcite coprecipitation experiments at pH ~7.5 and ~ 8.5 to study possible U isotope fractionation during incorporation into these minerals. Small but significant U isotope fractionation was observed in aragonite experiments at pH ~ 8.5, with heavier U in the solid phase. 238U/235U of dissolved U in these experiments can be fit by Rayleigh fractionation curves with fractionation factors of 1.00007+0.00002/-0.00003, 1.00005 ± 0.00001, and 1.00003 ± 0.00001. In contrast, no resolvable U isotope fractionation was observed in an aragonite experiment at pH ~7.5 or in calcite experiments at either pH. Equilibrium isotope fractionation among different aqueous U species is the most likely explanation for these findings. Certain charged U species are preferentially incorporated into calcium carbonate relative to the uncharged U species Ca2UO2(CO3)3(aq), which we hypothesize has a lighter equilibrium U isotope composition than most of the charged species. According to this hypothesis, the magnitude of U isotope fractionation should scale with the fraction of dissolved U that is present as Ca2UO2(CO3)3 (aq). This expectation is confirmed by equilibrium speciation modeling of our experiments. Theoretical calculation of the U isotope fractionation factors between different U species could further test this hypothesis and our proposed fractionation mechanism. These findings suggest that U isotope variations in ancient carbonates could be controlled by changes in the aqueous speciation of seawater U, particularly changes in seawater pH, PCO2, [Ca], or [Mg] concentrations. In general, these effects are likely to be small (<0.13 ‰), but are nevertheless potentially significant because of the small natural range of variation of 238U/235U. / Dissertation/Thesis / Masters Thesis Chemistry 2015

In many countries the current plan for the management of intermediate and high level radioactive wastes is to dispose of the radioactive materials underground in a Geological Disposal Facility (GDF) to prevent release of radioactivity to the environment. In the UK, the repository for intermediate level waste may be backfilled with cementitious material and it is clear that grout and cement will be used during many disposal concepts. Upon saturation, the cement will react creating a region of hyperalkaline geochemical conditions extending away from the GDF, within which, significant changes in radionuclide behaviour are expected. Therefore, this thesis utilises a range of experimental and analytical techniques to try to gain a mechanistic understanding of the behaviour of some key radionuclides (U(VI), Np(V) and Eu(III) as an analogue for Cm(III)/Am(III)) in a range of high pH systems of direct relevance to any cementitious GDF. U(VI) interaction with calcite (calcium carbonate, a common component in high pH cements and the natural environment) surfaces was studied in the 'old' (Ca(OH)2 solution; pH 10.5) and 'young' (Na+, K+, Ca2+; pH 13.3) leachates. In the 'old' leachate, luminescence spectroscopy, batch experiments and kinetic modelling suggested that at low concentrations (smaller or equal to 0.42 µM) a Ca2UO2(CO3)3-like surface complex formed. At higher concentrations, batch experiments, extended X-ray absorption fine structure spectroscopy and luminescence suggested that a surface mediated precipitation mechanism was controlling U(VI) concentrations. Further TEM analysis confirmed that a calcium uranate (CaUO4) solid phase was forming on the calcite surfaces. In the 'young' leachate, batch experiments showed that U(VI) had little affinity for the calcite surface, with no statistically relevant removal from solution observed over a 18 month period. Small angle X-ray diffraction data demonstrated that the U(VI) was probably present in the form of U(VI) intrinsic colloids. Np(V) solubility and sorption to calcite under hyperalkaline conditions were studied using batch, X-ray absorption spectroscopy, and geochemical modelling techniques. It was determined that Np(V) solubility in 'old' cement leachates was consistent with the literature. However, in 'young' cement leachates, an unidentified calcium containing phase was controlling solubility. It was demonstrated that sorption to calcite in 'old' leachates was controlled by the formation of a >CO3NpO2 surface complex, whereas, in the 'young' leachates interaction with the calcite surface was controlled by a precipitation mechanism. Eu(III) sorption to a potential GDF backfill material, Nirex Reference Vault Backfill (NRVB) cement, was studied. The kinetics of removal were rapid with 98.5% Eu(III) removal within 24 hours. Ultrafiltration experiments indicated that all Eu(III) remaining in solution was associated with NRVB derived colloids. Additional experiments using ethylenediaminetetraacetic acid (EDTA) as a competing ligand show that removal from solution was significantly reduced at high concentrations (>0.01 M). These EDTA experiments also indicated some irreversibility in the systems, possibly caused by incorporation into the C-S-H or calcite structures.

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