Soil – Atmosphere Interaction: Modeling the fate of semi-volatile organic compounds and chemical weathering of marine mudrocks

In this dissertation, reactive transport modeling is applied to analyze coupled physical, biological, geochemical and hydrological processes at the soil-atmosphere interface. Focus is on mass transfer of compounds between different environmental compartments (e.g. groundwater, the unsaturated zone, soils, plants and the atmosphere). The influence of soil-atmosphere diffusive gas exchange and water infiltration is addressed with respect to environmental fate of semi-volatile organic compounds (SVOCs) as well as oxidation and carbon turnover during chemical weathering of pyrite- and kerogen-bearing mudrocks. Conceptual models considering soils, plants, and the atmospheric boundary layer were developed and solved numerically with a multicomponent reactive transport code (MIN3P). The models consider eddy diffusion and photochemical oxidation in the atmosphere, changes in the thickness of the atmospheric boundary layer, gas diffusion and heat transport in the soil, temperature dependence of sorption and partitioning in soils and plants as well as solute transport by seepage water. Besides these transport processes, biodegradation and geochemical changes in water and solids are considered. Model results on environmental fate of SVOCs show that on the long term (i.e. for centuries) soils are sinks for atmospheric pollutants because of strong sorption and thus limited bioavailability. Potential re-volatilization back into the atmosphere following declined anthropogenic emissions does not change this finding substantially. Modeling shows also that diurnal, temperature-driven volatilization of SVOCs from soils into the atmosphere cannot account for short-term concentration fluctuations often observed in the atmosphere. The latter can only be simulated if a rapidly-exchanging storage compartment is introduced into the model – a function that can be taken over e.g. by plants in conjuction with fast atmospheric mixing due to eddy diffusion. Numerical simulations of chemical weathering show that initially diffusion is the main physical control in the chemical weathering of pyrite- and kerogen-bearing mudrocks (e.g. Opalinus Clay of the Swabian Alb, Southwestern Germany). Pyrite and kerogen oxidation cause acidification of seepage water, which consequently leads to dissolution of carbonate minerals, i.e. calcite and siderite, and thus to an increase in porosity, release of CO2 into the atmosphere as well as elevated groundwater alkalinity. Seepage water chemistry highly depends on water infiltration rates (or fluid residence times). In case water infiltration rates are low, ions accumulate in the seepage water and finally gypsum precipitation starts. The latter has geotechnical consequences such as swelling of the ground, which is often observed for buildings founded in pyrite bearing mudrocks in Southern Germany. Overall this dissertation demonstrates the capability of reactive transport modeling to elucidate the controls on pollutant and gas exchange between different environmental compartments by considering various coupled physical, biological, geochemical and hydrological processes. This allows to investigate trends in long term soil and groundwater pollution as well as evolution of seepage water chemistry during chemical weathering of mudrocks including CO2 release into the atmosphere.In dieser Dissertation werden mit Hilfe eines numerischen Simulationsmodells für den reaktiven Transport von Wasserinhaltsstoffen physikalische, biologische, geochemische und hydrologische Prozesse im Übergangsbereich Boden - Atmosphäre untersucht. Der Fokus liegt dabei auf dem Stofftransfer und -austausch der Stoffe zwischen unterschiedlichen Umweltkompartimenten (z.B. Grundwasser/wassergesättigte Zone, vadose Zone, Boden, Pflanzen, Atmosphäre). Im Besonderen wird dabei auf den Einfluss des diffusiven Gasaustausches an der Grenzfläche Boden – Atmosphäre und der Infiltration von Niederschlagswasser in die Bodenzone auf das Verhalten und die Verteilung von sogenannten semi-volatilen organischen Verbindungen (SVOCs), sowie auf Oxidationsdynamik und Kohlenstoffumsatz während der Verwitterung von Pyrit- und Kerogen-haltigen Sedimentgesteinen, eingegangen. Dazu wurden konzeptionelle Modelle für die Verbindung bzw. Kopplung der Kompartimente Boden, Pflanze und atmosphärische Grenzschicht entwickelt und in Mehrkomponenten-Transportmodelle mit dem Modell-Code MIN3P umgesetzt. Die Modelle berücksichtigen Eddy-Diffusion und photochemische Oxidation in der Atmosphäre, Änderungen der Mächtigkeit der atmosphärische Grenzschicht, Gasdiffusion und Wärmeübertragung in Böden, Temperaturabhängigkeit von Stoffsorption und -verteilung in Böden und Pflanzen sowie den gelösten Stofftransport im Sickerwasser. Neben diesen Prozessen werden zudem der mikrobiologische Abbau der Stoffe und geochemische Änderungen in Wasser und Festphase betrachtet. Die Modellergebnisse zeigen, dass - langfristig betrachtet - Böden als Senken für SVOCs in der Atmosphäre fungieren. Die vergleichsweise starke Sorption und dementsprechend geringe Bioverfügbarkeit der SVOCs führt zu einer Anreichung im Oberboden. Eine denkbare Ausgasung der SVOCs in die Atmosphäre als Folge rückläufiger anthropogener Schadstoffemissionen ändert diesen Befund nur geringfügig. Die Simulationen zeigen auch, dass die in der Atmosphäre beobachteten kurzfristigen Konzentrationsschwankungen nicht durch die regelmäßige Ausgasung von SVOCs aus dem Boden aufgrund der täglichen Temperaturschwankungen erklärt werden können. In der Simulation ist dies nur dann möglich, wenn im Modell ein Speicherkompartiment eingeführt wird, welches einen schnellen Austausch ermöglicht - eine Funktion, die z.B. die Pflanzen in Verbindung mit einer schnellen Vermischung durch Eddy Diffusion übernehmen können. Numerische Simulationen zur chemischen Verwitterung von Pyrit- und Kerogen-haltigen Sedimentgesteinen (z.B. der Opalinuston der Schwäbischen Alb in Südwestdeutschland) belegen, dass die Diffusion der anfänglich kontrollierende physikalische Prozess ist. Pyrit- und Kerogen-Oxidation führen zu einer Versauerung des Sickerwassers, wodurch Karbonatmineralien (z.B. Kalzit und Siderit) gelöst werden und es schließlich zu einer Vergrößerung des Porenraums, einer Ausgasung von CO2 in die Atmosphäre sowie zu einer erhöhten Alkalinität des Grundwassers kommt. Der Chemismus des Grundwassers ist dabei abhängig von den relevanten Verweilzeiten, d.h. von der Höhe der Grundwasserneubildungsrate. Bei geringer Grundwasserneubildung kann es zur Akkumulation von Ionen im Sickerwasser und in der Folge zur Ausfällung von Gips kommen. Letzteres kann zu einer Volumenvergrößerung und schließlich zu Bodenhebungen führen, deren Folgen häufig an Gebäuden, die auf pyrithaltigem Sedimentgestein in Süddeutschland gebaut sind, zu beobachten sind. Insgesamt demonstriert diese Dissertation wie die reaktive Stofftransportmodellierung als Werkzeug genutzt werden kann, um die kontrollierenden Faktoren für den Austausch von Schadstoffen und Gasen zwischen verschiedenen Umweltkompartimenten zu ergründen, indem unterschiedliche physikalische, biologische, geochemische und hydrologische Prozesse gekoppelt betrachtet werden. Damit können langfristige Trends der Verunreinigung von Böden und Grundwasser untersucht und erklärt werden, ebenso wie die Entwicklung von Sickerwasserchemismus durch Gesteinsverwitterung und der ggf. resultierenden Freisetzung von CO2 in die Atmosphäre

Springback of hot stamping and die quenching with ultra-high-strength boron steel

Hot bending and die quenching for U-shaped parts with ultra-high-strength boron steel were experimented and simulated to study the effect of die geometric parameters on springback and its mechanism. The results indicate that through hot contact bending and die quenching, bending parts with higher strength than that of cold stamping can be achieved, the tensile strength of which can reach 1500MPa. The springback angle of hot bending part increases by increasing the die radius, by increasing the gap between the punch and the die. Springback is mainly negative caused by/due to different cooling rate and the impact of thermal restoring moments. This provides a basis for the control of the hot stamping process applied in the production of complicated shape parts

A cross scale investigation of galena oxidation and controls on mobilization of lead in mine waste rock.

Abstract Galena and Pb-bearing secondary phases are the main sources of Pb in the terrestrial environment. Oxidative dissolution of galena releases aqueous Pb and SO4 to the surficial environment and commonly causes the formation of anglesite (in acidic environments) or cerussite (in alkaline environments). However, conditions prevalent in weathering environments are diverse and different reaction mechanisms reflect this variability at various scales. Here we applied complementary techniques across a range of scales, from nanometers to 10 s of meters, to study the oxidation of galena and accumulation of secondary phases that influence the release and mobilization of Pb within a sulfide-bearing waste-rock pile. Within the neutral-pH pore-water environment, the oxidation of galena releases Pb ions resulting in the formation of secondary Pb-bearing carbonate precipitates. Cerussite is the dominant phase and shannonite is a possible minor phase. Dissolved Cu from the pore water reacts at the surface of galena, forming covellite at the interface. Nanometer scale characterization suggests that secondary covellite is intergrown with secondary Pb-bearing carbonates at the interface. A small amount of the S derived from galena is sequestered with the secondary covellite, but the majority of the S is oxidized to sulfate and released to the pore water

Modeling short-term concentration fluctuations of semi-volatile pollutants in the soil–plant–atmosphere system

Temperature changes can drive cycling of semi-volatile pollutants between different environmental compartments (e.g. atmosphere, soil, plants). To evaluate the impact of daily temperature changes on atmospheric concentration fluctuations we employed a physically based model coupling soil, plants and the atmosphere, which accounts for heat transport, effective gas diffusion, sorption and biodegradation in the soil as well as eddy diffusion and photochemical oxidation in the atmospheric boundary layer of varying heights. The model results suggest that temperature-driven re-volatilization and uptake in soils cannot fully explain significant diurnal concentration fluctuations of atmospheric pollutants as for example observed for polychlorinated biphenyls (PCBs). This holds even for relatively low water contents (high gas diffusivity) and high sorption capacity of the topsoil (high organic carbon content and high pollutant concentration in the topsoil). Observed concentration fluctuations, however, can be easily matched if a rapidly-exchanging environmental compartment, such as a plant layer, is introduced. At elevated temperatures, plants release organic pollutants, which are rapidly distributed in the atmosphere by eddy diffusion. For photosensitive compounds, e.g. some polycyclic aromatic hydrocarbons (PAHs), decreasing atmospheric concentrations would be expected during daytime for the bare soil scenario. This decline is buffered by a plant layer, which acts as a ground-level reservoir. The modeling results emphasize the importance of a rapidly-exchanging compartment above ground to explain short-term atmospheric concentration fluctuations

Modeling controls on the chemical weathering of marine mudrocks from the Middle Jurassic in Southern Germany

Chemical weathering of sedimentary rocks is of great importance in determining seepage water chemistry, carbon, iron, calcium and sulfur turnover, as well as mineral transformation. In this study, we used the numerical code MIN3P to investigate controls on seepage water chemistry during chemical weathering of marine mudrocks. In particular, we focused on the pyrite- and kerogen-bearing formation, Opalinus Clay (with outcrops in the area of the Swabian and Franconian Alb in Southern Germany), a typical fine-grained sedimentary mudrock that had been deposited during the Middle Jurassic in a shallow marine environment. In the geochemical model we considered four reactive minerals, i.e., pyrite, kerogen, calcite and siderite (assuming silicate minerals to be stable), and ran model scenarios over a time period of 10kyrs (since the last ice age). Our numerical results show that chemical weathering of Opalinus Clay is driven by oxygen ingress (which depends on effective gas diffusion, and thus on water saturation). Due to oxidation of pyrite and kerogen seepage water acidifies, which leads to dissolution of carbonate minerals, i.e., calcite and siderite. As a consequence, porosity and groundwater alkalinity increase, and CO2 is released into the atmosphere at early decades. Following the consumption of primary reactive minerals, iron oxides precipitate in the oxic zone. We compared our model results with field data of water saturation, porosity, and water chemistry. The overall reasonable fit between model results and field data demonstrates the applicability of the numerical code MIN3P to quantify chemical weathering of pyrite-bearing sedimentary mudrocks and to predict seepage water chemistry that is impacted by geochemical water-rock interactions