Browsing by Subject "Methane"

Now showing 1 - 7 of 7
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    Carbon dioxide sequestration and heterotrophy in shallow lakes.
    (2009-10) Kenning, Jon M.
    Research has recently begun to show the importance of lakes in controlling global CO2 budgets, but this work has only been done on a few large lakes. Small, shallow lakes and wetlands are the most plentiful lake ecosystems in world, but the most ignored. Here, I explore their ability to sequester CO2 and in some cases release the greenhouse gas to atmosphere. I found that pristine shallow lakes where macrophytes (aquatic vegetation) dominated, the lakes sequestered much more CO2 than disturbed lakes where phytoplankton dominated. Furthermore, I found that heterotrophs in shallow lakes respired tremendous amounts of carbon of terrestrial origin, thus calling into question the net ability of terrestrial ecosystems to sequester carbon. Finally, I found that some of the underlying mechanisms, including the productivity of different autotrophs and growth efficiencies of bacteria, favor greater carbon sequestration by macrophyte-rich shallow lakes. All of my observations form a basis for future work into the ability of shallow lakes to sequester CO2 and stresses the importance of not only saving shallow lakes and wetlands, but preserving them in a macrophyte-rich state.
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    Kinetic and spectroscopic characterization of intermediates in the soluble methane monooxygenase catalytic cycle: the old and the new
    (2013-02) Banerjee, Rahul
    Soluble methane monooxygenase (sMMO) catalyzes the difficult reaction of oxidative hydroxylation of methane to methanol, thereby allowing the host methanotroph to grow on methane as the sole source of carbon and energy. As methane possesses the strongest aliphatic C-H bond, the chemical mechanism of this enzyme is the epitome of oxygen activation in metalloenzymes. Exhaustive research of the catalytic mechanism of sMMO in recent years has elucidated many aspects of the chemistry occurring at the dinuclear iron active site center. These studies include the three dimensional structures of the protein components of sMMO and fairly detailed outlines of both the chemical mechanism and the method of regulatory control. The discovery of a diferric peroxo intermediate P, and more crucially, a high valent bis-μ-oxo diiron(IV) intermediate Q have provided the greatest advances in the understanding of oxygen activation in diiron oxygenases. The research described here has extended this understanding in several aspects. A long standing issue of low accumulation of enzyme reaction intermediates for spectroscopic studies has been addressed and in part solved. A new intermediate P* that had been proposed to exist based on kinetic studies has been trapped and characterized through spectroscopic techniques. P* appears to be a diferrous cluster intermediate that binds O2 weakly with little transfer of charge density onto the oxygen atoms. This result suggests a general theme for heme and non-heme oxygen activating enzymes in which ferrous centers initially form a weak complex with O2, which is strengthened in following steps by interactions such as stabilizing hydrogen bonds and charge donation from trans ligands. The long sought after goal of characterization of the vibrational spectrum of compound Q has been achieved through a time resolved resonance Raman technique. The preliminary results corroborate the diamond core structure that has been proposed for Q. Another putative high valent intermediate Q' has been discovered to arise from compound Q in the catalytic cycle. This is potentially a very significant finding as it seem likely that Q' rather than Q reacts directly with substrate. Following from a precedent from synthetic diiron model compound studies, it is possible that Q' is an open core form of the Q intermediate in which a reactive, terminal Fe(IV)=O moiety replaces the more stable bis-μ-oxo bridging structure.
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    Kinetics and mechanism of deoxygenation reactions over proton-form and molybdenum-modified zeolite catalysts
    (2014-07) Bedard, Jeremy William
    The depletion of fossil fuel resources and the environmental consequences of their use have dictated the development of new sources of energy that are both sustainable and economical. Biomass has emerged as a renewable carbon feedstock that can be used to produce chemicals and fuels traditionally obtained from petroleum. The oxygen content of biomass prohibits its use without modification because oxygenated hydrocarbons are non-volatile and have lower energy content. Chemical processes that eliminate oxygen and keep the carbon backbone intact are required for the development of biomass as a viable chemical feedstock. This dissertation reports on the kinetic and mechanistic studies conducted on high and low temperature catalytic processes for deoxygenation of biomass precursors to produce high-value chemicals and fuels. Low temperature, steady state reaction studies of acetic acid and ethanol were used to identify co-adsorbed acetic acid/ethanol dimers as surface intermediates within specific elementary steps involved in the esterification of acetic acid with ethanol on zeolites. A reaction mechanism involving two dominating surface species, an inactive ethanol dimeric species adsorbed on Brønsted sites inhibiting ester formation and a co-adsorbed complex of acetic acid and ethanol on the active site reacting to produce ethyl acetate, is shown to describe the reaction rate as a function of temperature (323 - 383 K), acetic acid (0.5 - 6.0 kPa), and ethanol (5.0 - 13.0 kPa) partial pressure on proton-form BEA, FER, MFI, and MOR zeolites. Measured differences in rates as a function of zeolite structure and the rigorous interpretation of these differences in terms of esterification rate and equilibrium constants is presented to show that the intrinsic rate constant for the activation of the co-adsorbed complex increases in the order FER < MOR < MFI < BEA. High temperature co-processing of acetic acid, formic acid, or carbon dioxide with methane (CH3COOH/CH4 = 0.04-0.10, HCOOH/CH4 = 0.01-0.03, CO2/CH4 = 0.01-0.03) on Mo/H-ZSM-5 formulations at 950 K and atmospheric pressure in an effort to couple deoxygenation and dehydrogenation reaction sequences results instead in a two-zone, stratified bed reactor configuration consisting of upstream oxygenate/CH4 reforming and downstream CH4 dehydroaromatization. X-ray absorption spectroscopy and chemical transient experiments show that molybdenum carbide is formed inside zeolite micropores during CH4 reactions. The addition of an oxygenate co-feed causes oxidation of the active molybdenum carbide catalyst while producing CO and H2 until completely converted. Forward rates of C6H6 synthesis are unperturbed by the introduction of an oxygenate co-feed after rigorously accounting for the thermodynamic reversibility caused by the H2 produced in oxygenate reforming reactions and the fraction of the active catalyst deemed unavailable for CH4 dehydroaromatization. All effects of co-processing C1-2 oxygenates and molecular H2 with CH4 can be interpreted in terms of an approach to equilibrium. Co-processing H2O, CO2, or light (C1-2, C/Heff < 0.25) oxygenates with CH4 at 950 K over Mo/H-ZSM-5 catalysts results in complete fragmentation of the oxygenate and CO as the sole oxygen-containing product. The C/Heff accounts for removal of O as CO and describes the net C6H6 and total hydrocarbon synthesis rates at varying (0.0-0.10) C1-2 oxygenate and H2 to CH4 co-feed ratios. Co-processing larger (C3-5, C/Heff ≥ 0.25) oxygenates with CH4 results in incomplete fragmentation of the co-fed oxygenate and preferential pathways of C6H6 synthesis that exclude CH4 incorporation. This results in greater net C6H6 synthesis rates than would be predicted from observations made when co-processing C1-2 oxygenates. Catalytic technologies have served a crucial role in processing petroleum feedstocks and are faced with new challenges as the feedstock shifts to chemically diverse but renewable biomass sources. This research addresses these challenges at fundamental and applied levels as it offers the potential to convert readily available biomass to commodity chemicals and fuels while simultaneously examining the elementary concepts of deoxygenation reactions on catalytic surfaces.
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    Prospecting Fungi For Methane Biofiltration Reveals High-Efficiency Capture By Dried Mycelia (Necromass)
    (2017-12) Liew, Feng Jin
    Fungi can improve biofiltration of hydrophobic pollutants by improving capture, a rate-limiting step in bioreactors. We prospected fungi alongside native biofilm preparations and relevant controls for their efficacy capturing methane using gas-phase biofilters. Using a batch incubation system, we found that Ganoderma lucidum performed best in single-strain trials. Building on this, we tested other Ganoderma species and found comparable efficacies. The advantages of Ganoderma and Pleurotus isolates were lost and native colonizers wood substrates were deployed in the field, irrespective of where they were deployed. This relates to a stress-tolerant rather than competitive life history strategy, where Ganoderma species are outcompeted in less stressful environments. We also tested an alternative way to present Ganoderma for filtration. Using protocols for culinary and biomaterial applications, we re-tested several fungi, including Lentinus edodes ‘shiitake.’ In these trials, we found surprisingly high efficacy with Ganoderma mycelia (84%) relative to activated carbon. These results suggest that Ganoderma species might best be utilized for biofiltration in dried form, effective in field conditions and potentially more amenable for biofiltration indoors.
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    Role of Fungi in the Biofiltration of Livestock Housing and Manure Storage Emissions
    (2015-08) Oliver, Jason
    Biofilters use porous media colonized by microbial biofilms to capture and degrade odorous, hazardous and greenhouse gases making them well-suited for livestock housing and manure storage emissions. Fungi are abundant in these biofilters though their dynamics, degradation of media, community shifts, and functional roles have not been well-investigated. To explore spatial and temporal fungal dynamics in full-scale woodchip biofilters treating swine barn emissions, a novel monitoring approach was developed. Using wooden baits and microbial measures optimized to target biofilms biofilter fungi were characterized and shown to tolerate media desiccation. Additionally, successional patterns at the taxa and guild level were studied, and the development of a dominant fungal community was identified. To address the practical question of media longevity, a litter bag study was deployed in the same full-scale biofilters. Decay rates of various media types were identified, and microbial decay was dependent on media quality, nitrogen, and emissions levels. Using a lab-scale biofilter system, fungi were shown to improve the capture of methane, particularly after periods of low-concentration inlet emissions. Using a chromatographic isotherm the ability of fungi to sorb methane gas was verified for the first time. Collectively, this work showcases dynamics and potential abilities of fungi in biofilters treating livestock production emission and may be used to guide subsequent efforts to connect fungi to biofilter function. If these processes can be understood and controlled, there is the potential to improving biofilter performance, better protect air quality and improve farming system sustainability.
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    Small Size, Huge Impact: Disproportionate Effects of Ponds on Aquatic Carbon Cycling and Atmospheric Greenhouse Gases
    (2023-05) Rabaey, Joseph
    The carbon cycle is essential for all life and is a major driver of Earth’s climate. Freshwater ecosystems (such as lakes, ponds, rivers, wetlands, etc.) play an outsized role in the global carbon cycle, acting in the transport, storage, and emission of carbon to the atmosphere. Freshwaters are significant sources of the greenhouse gases carbon dioxide (CO2) and methane (CH4) to the atmosphere despite only covering 1% of Earth’s surface area, and they have become critical a piece in global greenhouse gas models. Surprisingly, the freshwater ecosystems that may contribute the most to global emissions are small ponds, though causes of emissions and variation across ponds are not well understood. My dissertation aims to more fully understand carbon cycling in ponds and identify key factors that influence greenhouse gas emissions. Through measurements of ecosystem metabolism (i.e., the balance of photosynthesis and aerobic respiration), I found that production and respiration rates in ponds are some of the highest across all freshwater ecosystems, with shallow depths influencing many factors that lead to high rates. By measuring greenhouse gas emissions from a wide range of ponds, I found that the absence of oxygen and high phosphorus concentrations can combine to lead to elevated CH4 emissions. In addition, duckweed on the surface of ponds can exacerbate these conditions and is potentially a target for management strategies. Finally, I monitored greenhouse gas emissions in four ponds throughout an entire year and found that water column mixing and stratification greatly impacts the seasonal timing and magnitude of greenhouse gas emissions. Overall, this work emphasizes that ponds are dynamic ecosystems with high rates of carbon cycling and greenhouse gas emissions, with implications for management and global greenhouse gas dynamics.
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    Warming and stratification changes in Lake Kivu, East Africa
    (2013-08) Aaberg, Arthur Allen
    To investigate changes in the temperature and stratification structure in Lake Kivu, we have installed a string of temperature recorders and performed CTD casts. The obtained data have been compared to historical profiles and the heat budget for the lake was analyzed. Lake Kivu is a meromictic lake characterized by an anomalous temperature distribution with a temperature minimum close to the base of the seasonally mixed layer. Warming rate at the depth of the temperature inversion is consistent with the historical warming rate of the surface layer of ∼0.14 ±0.02 °C per decade. Atmospheric warming rates since the 1970's in East Africa are between 0.20 and 0.25 °C per decade. Reported warming in surface waters of other East-African rift lakes is ∼0.13 °C per decade. Deep waters (greater than 350 m) in Lake Kivu exhibit variability in temperature and are currently warming at a rate of &sim0.06±0.02 °C per decade based on the increase in heat content since the 1970's and the increase in temperature seen in the deepest measurements between our 2011 and 2012 profiles. The monimolimnion of Lake Kivu cannot be considered to be in a steady state. The depth of wind-induced surface mixing during the dry season varies significantly between years. Mixing to 80 m (the present depth of the temperature inversion) requires continuous winds blowing from the south at 9–10 m s-1, whereas typical wind speed maxima are around 5–6 m s-1 and capable of mixing to around 65 m depth. Occasional stronger winds cause episodic mixing closer to the inversion which removes heat, but this does not happen on a regular basis. As the temperature inversion in recent historical profiles has been as shallow as 65 m, mixing to the temperature inversion depth is possible during years with stronger than average winds. With heat diffusing towards the temperature inversion from both above and below, the temperature at the inversion depth will continue to rise, resulting in a reduced transport of heat out of the deep waters that may increase the rate at which the water column is warming.