Univ., Diss.--Strasbourg, 1998.

The objective of the work described in this thesis was to understand sorption reactions of uranium occurring at the water-clay mineral interfaces in the presence and absence of arsenic and other inorganic ligands. Uranium(VI) removal by clay minerals is influenced by a large number of factors including: type of clay mineral, pH, ionic strength, partial pressure of CO2, load of the sorbent, total amount of U present, and the presence of arsenate and other inorganic ligands such as sulfate, carbonate, and phosphate. Both sulfate and carbonate reduced uranium sorption onto IBECO bentonite due to the competition between SO42- or CO32- ions and the uranyl ion for sorption sites, or the formation of uranyl-sulfate or uranyl-carbonate complexes. Phosphate is a successful ligand to promote U(VI) removal from the aqueous solution through formation of ternary surface complexes with a surface site of bentonite. In terms of the type of clay mineral used, KGa-1b and KGa-2 kaolinites showed much greater uranium sorption than the other clay minerals (STx-1b, SWy-2, and IBECO montmorillonites) due to more aluminol sites available, which have higher affinity toward uranium than silanol sites. Sorption of uranium on montmorillonites showed a distinct dependency on sodium concentrations because of the effective competition between uranyl and sodium ions, whereas less significant differences in sorption were found for kaolinite. A multisite layer surface complexation model was able to account for U uptake on different clay minerals under a wide range of experimental conditions. The model involved eight surface reactions binding to aluminol and silanol edge sites of montmorillonite and to aluminol and titanol surface sites of kaolinite, respectively. The sorption constants were determined from the experimental data by using the parameter estimation code PEST together with PHREEQC. The PEST- PHREEQC approach indicated an extremely powerful tool compared to FITEQL. In column experiments, U(VI) was also significantly retarded due to adsorptive interaction with the porous media, requiring hundreds of pore volumes to achieve breakthrough. Concerning the U(VI) desorption, columns packed with STx-1b and SWy-2 exhibited irreversible sorption, whereas columns packed with KGa-1b and KGa-2 demonstrated slow, but complete desorption. Furthermore, most phenomena observed in batch experiments were recognized in the column experiments, too. The affinity of uranium to clay minerals was higher than that of arsenate. In systems containing uranium and arsenate, the period required to achieve the breakthrough in all columns was significantly longer when the solution was adjusted to pH 6, due to the formation of the uranyl-arsenate complex. In contrast, when pH was adjusted to 3, competitive sorption for U(VI) and As(V) accelerated the breakthrough for both elements. Finally, experiments without sorbing material conducted for higher concentrations of uranium and arsenic showed no loss of total arsenic and uranium in non-filtered samples. In contrast, significant loss was observed after filtration probably indicating the precipitation of a U/As 1:1 phase.

The objective of the work described in this thesis was to understand sorption reactions of uranium occurring at the water-clay mineral interfaces in the presence and absence of arsenic and other inorganic ligands. Uranium(VI) removal by clay minerals is influenced by a large number of factors including: type of clay mineral, pH, ionic strength, partial pressure of CO2, load of the sorbent, total amount of U present, and the presence of arsenate and other inorganic ligands such as sulfate, carbonate, and phosphate. Both sulfate and carbonate reduced uranium sorption onto IBECO bentonite due to the competition between SO42- or CO32- ions and the uranyl ion for sorption sites, or the formation of uranyl-sulfate or uranyl-carbonate complexes. Phosphate is a successful ligand to promote U(VI) removal from the aqueous solution through formation of ternary surface complexes with a surface site of bentonite. In terms of the type of clay mineral used, KGa-1b and KGa-2 kaolinites showed much greater uranium sorption than the other clay minerals (STx-1b, SWy-2, and IBECO montmorillonites) due to more aluminol sites available, which have higher affinity toward uranium than silanol sites. Sorption of uranium on montmorillonites showed a distinct dependency on sodium concentrations because of the effective competition between uranyl and sodium ions, whereas less significant differences in sorption were found for kaolinite. A multisite layer surface complexation model was able to account for U uptake on different clay minerals under a wide range of experimental conditions. The model involved eight surface reactions binding to aluminol and silanol edge sites of montmorillonite and to aluminol and titanol surface sites of kaolinite, respectively. The sorption constants were determined from the experimental data by using the parameter estimation code PEST together with PHREEQC. The PEST- PHREEQC approach indicated an extremely powerful tool compared to FITEQL. In column experiments, U(VI) was also significantly retarded due to adsorptive interaction with the porous media, requiring hundreds of pore volumes to achieve breakthrough. Concerning the U(VI) desorption, columns packed with STx-1b and SWy-2 exhibited irreversible sorption, whereas columns packed with KGa-1b and KGa-2 demonstrated slow, but complete desorption. Furthermore, most phenomena observed in batch experiments were recognized in the column experiments, too. The affinity of uranium to clay minerals was higher than that of arsenate. In systems containing uranium and arsenate, the period required to achieve the breakthrough in all columns was significantly longer when the solution was adjusted to pH 6, due to the formation of the uranyl-arsenate complex. In contrast, when pH was adjusted to 3, competitive sorption for U(VI) and As(V) accelerated the breakthrough for both elements. Finally, experiments without sorbing material conducted for higher concentrations of uranium and arsenic showed no loss of total arsenic and uranium in non-filtered samples. In contrast, significant loss was observed after filtration probably indicating the precipitation of a U/As 1:1 phase.

Microorganisms interact with different soil components, such as varying substrates and soil minerals, to drive soil processes and functionality. They can be influenced by the environment, but they themselves can influence their environment by their activities, for example through the production of extracellular enzymes and extracellular polymeric substances (EPS). The formation and stability of aggregates as the backbone of the soil structure, for instance, are thought to be largely influenced by soil microorganisms, or vice versa. There remain, however, open questions as to whether and how microorganisms can influence soil aggregation. While microbes are influencing their environment their interaction with the soil minerals could also change their responses upon adsorption - affecting their influence on soil aggregation. Therefore, the overarching goal of this thesis was to investigate the effect of soil minerals, in particular clay content, on the composition and activity of soil microbial community, with a specific focus on enzyme activities and EPS. Finally, the microbial control of soil aggregation through the influence of substrate availability was explored. In total, two adsorption experiments and two incubation experiments were conducted using soils manipulated experimentally with increasing clay content. The sandy soil was amended with different amounts of soil minerals (i.e. montmorillonite) to achieve a gradient in clay content. For the first incubation experiment, organic substrates differing in decomposability (i.e., starch and cellulose) were added to the soil to stimulate microbial activities and incubated for 80 days. Soil samples from the first incubation experiment were analysed after 0, 3, 10, 20, 40 and 80 days for enzyme activities, microbial community composition, biomass C, EPS-protein and polysaccharide. Additionally, the geometric mean diameter and mean weight diameter of the soil aggregates were determined as measures of aggregate formation and stability, respectively. The first adsorption experiment examined the effect of soil mineral phases on the activities of extracellular enzymes using commercially available extracellular enzymes (α-glucosidase) added to the soil. The second adsorption and incubation experiment investigated the persistence of extracellular enzyme activities (commercially available α-amylase and cellulase) affected by soil minerals. For further insight into how other soil minerals affect extracellular enzymes, kaolinite and goethite in addition to montmorillonite were included in the second adsorption and incubation experiment. The prepared complexes (enzyme + soil and/or soil minerals) from the second adsorption experiment were incubated for 100 days. Further analytical methods include the determination of enzyme activities, microbial biomass C, extraction and quantification of the soil EPS, protein analyses, DNA isolation, DGGE, qPCR and Illumina sequencing. The adsorption experiment showed that extracellular enzyme activities decreased with increasing clay contents. In contrast, such an inhibitory effect on microbial enzyme activity was only observed directly in the incubation experiment after the stimulation of in-situ microorganisms for extracellular enzyme production through substrate addition. Higher extracellular enzyme activities at later incubation days in soils with high clay content suggested an adaptation of the microbial community in response to soil clay content and/ or persistence of extracellular enzymes by adsorption to mineral surfaces. However, the second adsorption experiment showed that the high specific activity and persistence of the enzymes were constrained by the availability of sorption sites. It is therefore reasonable to assume that soil mineral phases support microorganisms in less-sorptive environments by sparing energy on enzyme production, since even a small enzyme release could already propel sufficient activities to degrade target carbon substrates. Starch amendment accelerated respiration and microbial biomass much more than cellulose. While microbial community differed depending on the C substrate (starch or cellulose) added, clay addition had a stronger influence on alpha diversity than substrate addition. Although the production of EPS-protein was closely linked to the provision of additional substrates, the addition of clay minerals resulted in more EPS production than when no additional clay was present. By correlating soil aggregation (stability and formation) with the recorded microbial parameters (that is biomass C, EPS-protein and EPS-polysaccharide), both EPS-protein and EPS-polysaccharide exhibited a significant control on aggregate formation and microbial processes, though, surprisingly, more strongly with high clay content. It was observed that EPS is only a transient compound, which initiates aggregate formation, but clay content plays a more significant role in long-term aggregate stabilization. Overall, this thesis contributed to our knowledge about the interaction of microorganisms with the soil mineral phase and their influence on soil structural stability. The findings established that soil minerals shape the composition and activity of microbial communities. In turn, the microbial production of EPS seems to be more significant for aggregate formation than stability. The results on the effect of soil minerals on extracellular activities provided a paradigm that the persistence of enzyme activities by adsorption does not always hold. Producing EPS might contribute to microbial adaptation that mitigates the negative effect of adsorption on extracellular enzymes. It might also be probable that the EPS become a substance of degradation for the extracellular enzymes. Overall, the results indicated that in clay-rich soils the process leading to extracellular enzyme persistence can be stochastic, depending on multiple factors including sorption sites and substrate availability. Labile organic C clearly plays a role in aggregate formation by supporting EPS production. However, increasing clay content enhanced aggregate stability, promoted the development of distinct microbial communities and increased EPS production. The discrepancy so observed in the contribution of the two EPS parameters, EPS polysaccharide and protein, on soil aggregation points to the need for inclusion of different EPS compositions in future studies relating to soil aggregation.

Im Rahmen dieser Doktorarbeit wurden die Hangkarbonate von zwei miozänen heterozoischen Karbonatsystemen näher untersucht: die Malta Inselgruppe (zentrales Mittelmeer) und das Marion Plateau (Nordost Australien, ODP Leg 194). Die Auswirkungen der mittelmiozänen Abkühlung (Mi3), die auf 13.6 Ma datiert wird und starken Einfluß auf die Sauerstoffisotopenkurve hatte, in den oben genannten Flachwassersystemen stellten das Ziel dieser Arbeit dar. Dieses Abkühlungsereignis beeinflußte außerdem sehr stark die ozeanographischen und klimatischen Muster, die im weiteren Verlauf zum modernen Eishausklima führten. So steht insbesondere die Vereisung von Ostantarktika mit diesem Ereignis in Verbindung. Diese Arbeit untersucht den Einfluß dieses Ereignisses auf Flachwassersysteme, um vorliegende Untersuchungen in Tiefwassersystemen zu ergänzen und so zum globalen Verständnis des miozänen Klimawechsels beizutragen.<br /> <br /> Die Profile auf der Maltainselgruppe wurden mit Hilfe von Kohlenstoff- und Sauerstoffisotopen Auswertungen im Gesamtgestein, Gesamtgesteinmineralogie, Tonmineralanalyse und organischer Geochemie untersucht. Durch einen Wechsel von karbonatischeren zu tonigeren Sedimenten beeinflußte das mittelmiozäne Abkühlungsereignis die Sedimentation in diesem Gebiet sehr stark. Weiterhin wurde beobachtet, daß jede Phase der antarktischen Vereisung, nicht nur das mittelmiozäne Hauptereignis, zu einem erhöhten terrigenen Eintrag in den Hangsedimenten der Maltainselgruppe führte. Akkumulationsraten zeigen, daß dieser erhöhte terrigene Eintrag den einzelnen Vereisungsperioden zusammenhängt und die karbonatischen Sedimente durch tonreiche Sedimente &ldquo;verunreinigt&rdquo; wurden. Das daraufhin entwickelte Modell erklärt diesen erhöhten terrigenen Eintrag mit einer nordwärtigen Verlagerung der innertropischen Konvergenzzone durch die Bildung von kalten, dichten Luftmassen, die zu verstärkten Niederschlägen in Nordafrika führten. Diese verstärkten Niederschläge (oder verstärkter afrikanischer Monsun) beeinflußten die kontinentale Verwitterung und den Eintrag, mit der Folge, daß verstärkt terrigene Sedimente im Bereich der Hangsedimente der Maltainselgruppe abgelagert wurden. Die tonreichen Intervalle weisen Ähnlichkeiten zu sapropelischen Ablagerungen auf, was mit Hilfe der Spektral analyse des Karbonatgehalts und der geochemischen Analyse des organischen Materials gezeigt wurde. <br /> <br /> Auf dem Marion Plateau wurden die Sauerstoff- und Kohlenstoffisotopenkurven anhand von Foraminiferen der Gattung Cibicidoides spp. rekonstruiert. Der Karbonatgehalt wurde mit Hilfe einer chemischen Methode (Coulometer) ermittelt. Genauso wie die Sedimente der Maltainselgruppe beeinflußte das mittelmiozäne Abkühlungsereignis (Mi3) auch die Sedimente auf dem Marion Plateau. So kam es bei 13,8 Ma, in etwa zur Zeit der Vereisung von Ostantarktika, zu einem Abfall der Karbonatakkumulationsraten. Weiterhin traten Änderungen in der Zusammensetzung der Sedimente auf, so nehmen neritische Karbonatfragmente ab, der planktische Foraminiferengehalt nimmt zu und es wurden verstärkt Quarz und Glaukonit abgelagert. Ein überraschendes Ergebnis ist die Tatsache, daß der große N12-N14 Meeresspiegelabfall um 11,5 Ma die Akkumulationsraten der Karbonate auf dem Hang nicht beeinflußte. Dieses Ergebnis ist umso erstaunlicher, da Karbonatplattformen normalerweise sehr sensitiv auf Meeresspiegeländerungen reagieren. Der Grund, warum sich die Karbonatakkumulationsraten schon um 13,6 Ma (Mi3) und nicht erst um 11,5 Ma (N12-N14) verringerten, liegt in der Tatsache, daß die ozeanischen Strömungen die Karbonatsedimentation auf dem Hang des Marion Plateau schon im Miozän kontrollierten. Das mittelmiozäne Ereignis (Mi3) erhöhte die Stärke diese Strömungen und als eine Ursache wurde die Karbonatakkumulation auf den Hängen reduziert. Die Amplitude des N12-N14 Meeresspiegelabfalls liegt bei 90 m unter der Berücksichtigung der Sauerstoffisotopendaten aus der Tiefsee und Berechnungen des Meeresspiegels anhand des &ldquo;coastal onlaps&rdquo;, die während Leg 194 gemacht wurden. Die Isotopendaten dieser Arbeit weisen hingegen auf einen verringerten Meeresspiegelabfall von 70 m hin.<br /> <br /> Als allgemeine Schlußfolgerung kann gesagt werden, daß der mittelmiozäne Klimaumschwung die Karbonatsysteme zumindest an den beiden untersuchten Lokalitäten beeinflußt hat. Allerdings waren die Auswirkungen sehr von den unterschiedlichen lokalen Gegebenheiten abhängig. Insbesondere wirkten sich die Anwesenheit einer Landmasse (Malta) und die Abwesenheit einer Barriere vor den Einflüssen des offenen Ozeans (Marion Plateau) stark auf die Ablagerung der Karbonate aus. / This study investigated the slope carbonates of two Miocene carbonate systems: the Maltese Islands (in the Central Mediterranean) and the Marion Plateau (Northeastern Australia, drilled during ODP Leg 194). The aim of the study was to trace the impact of the Miocene cooling steps (events Mi1-Mi6) in these carbonate systems, especially the Mi3 event, which took place around 13.6 Ma and deeply impacted the marine oxygen isotope record. This event also profoundly impacted oceanographic and climatic patterns, eventually leading to the establishment of the modern ice-house world. In particular, East Antarctica became ice covered at that period. The rational behind the present study was to investigate the impact that this event had on shallow water systems in order to complement the deep-sea record and hence acquire a more global perspective on Miocene climate change.<br /> <br /> The Maltese Islands were investigated for trends in bulk-rock carbon and oxygen isotopes, as well as bulk-rock mineralogy, clay minerals analysis and organic geochemisty. Results showed that the mid Miocene cooling event deeply impacted sedimentation at that location by changing sedimentation from carbonate to clay-rich sediments. Moreover, it was discovered that each phase of Antarctic glaciation, not just the major mid Miocene event, resulted in higher terrigenous input on Malta. Mass accumulation rates revealed that this was linked to increased runoff during periods when Antarctica was glaciated, and thus that the carbonate sediments were &ldquo;diluted&rdquo; by clay-rich sediments. The model subsequently developed to explain this implies feedback from Antarctic glaciations creating cold, dense air masses that push the ITCZ Northward, thus increasing precipitation on the North African subcontinent. Increased precipitation (or stronger African monsoon) accelerated continental weathering and runoff, thus bringing more terrigenous sediment to the paleo-location of the slope sediments of Malta. Spectral analysis of carbonate content and organic matter geochemical analysis furthermore suggest that the clay-rich intervals are similar to sapropelic deposits.<br /> <br /> On the Marion Plateau, trends in oxygen and carbon isotopes were obtained by measuring Cibicidoides spp foraminifers. Moreover, carbonate content was reconstructed using a chemical method (coulometer). Results show that the mid Miocene cooling step profoundly affected this system: a major drop in accumulation rates of carbonates occurs precisely at 13.8 Ma, around the time of the East Antarctic ice sheet formation. Moreover, sedimentation changes occurred at that time, carbonate fragments coming from neritic environments becoming less abundant, planktonic foraminifer content increasing and quartz and reworked glauconite being deposited. Conversely, a surprising result is that the major N12-N14 sea-level fall occurring around 11.5 Ma did not impact the accumulation of carbonates on the slope. This was unexpected since carbonate platform are very sensitive to sea-level changes. The model developed to explain that mass accumulation rates of carbonates diminished around 13.6 Ma (Mi3 Event) instead of 11.5 Ma (N12-N14 event), suggests that oceanic currents were controlling slope carbonate deposition on the Marion Plateau prior to the mid-Miocene, and that the mid Miocene event considerably increase their strength, hence reducing the amount of carbonate being deposited on slope sites. Moreover, by combining results from deep-sea oxygen isotopes with sea-level estimates based on coastal onlaps made during Leg 194, we constrain the amplitude of the N12-N14 sea-level fall to 90 meters. When integrating isotopic results from this study, this amplitude is lowered to 70 meters.<br /> <br /> A general conclusion of this work is that the mid Miocene climatic shift did impact carbonate systems, at least at the two locations studied. However, the nature of this response was highly dependant on the regional settings, in particular the presence of land mass (Malta) and the absence of a barrier to shelter from the effects of open ocean (Marion Plateau).