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Methane Production and Emission Mitigation in Oil Sands Tailings Concurrent with Hydrocarbon Degradation under Nitrogen Limited Conditions
- Author / Creator
- Collins, Catherine
Alberta’s oil sands generate large volumes of tailings from bitumen ore processing. These tailings ponds produce biogenic methane, which can be measured across 60-80% of the tailings surface. Based on current surface area data and emissions studies, tailings ponds could account for 8% of Canada’s methane emissions. With a government mandate to reduce methane emissions by 45% of 2012 levels, understanding the dynamics of methanogenesis in tailings is highly important. Methane production from oil sands tailings is driven by hydrocarbon metabolism by the microbial community. For community growth and metabolism to occur, sufficient nutrients such as nitrogen (N) must be available in the environment. Several tailings ponds as well as reclaimed wet landscapes that incorporate mature fine tailings (MFT) are deficient in bioavailable N (NH4+, NO2-/NO3-) and yet continue to produce methane. Understanding this process with hydrocarbon amendments is important as it is generally assumed methane production will halt in the absence of nutrients. Additionally, literature suggests that methanogenesis is inhibited in the presence of amorphous Fe(III). This implies amendments such as amorphous Fe(III) oxides (rust) may be effective at reducing methane emissions in tailings ponds and potentially in reclaimed wet landscapes.
In Chapter 2, we investigate methane production resulting from the degradation of n-alkanes and toluene in the presence and absence of fixed N in microcosms under an N2/CO2 headspace. Acetylene reduction assays indicated that N2-fixation was present in the absence of fixed N concurrent with n-alkane and toluene degradation, and methane production. Community gene analysis using 16S rRNA indicated that the bacterial community was dominated by fermentative bacteria and the archaeal community consisted primarily of Methanoseata in amended cultures. Functional gene analysis (DNA and mRNA) for the N2 fixing enzyme, nitrogenase (nifH) indicated genes can be expressed disproportionately to the coding DNA. In n-alkane cultures, nifH mRNA was predominantly expressed by Methanosaeta and in sequenced toluene cultures, nifH expression was detected in both Desulfovibrionales and Methanosaetaceae. These results suggested Archaea were responsible for most of the nitrogenase expression, and therefore N2-fixation in n-alkane amended N depleted treatments and in nearly equal proportions with Desulfovibrionales in toluene amended N depleted CNRL cultures. The presence of these genes in MFT suggest that anaerobic N transformations such as N2-fixation are possible in-situ. The results of this study support our hypothesis that N2-fixing microorganisms within the microbial communitycan support hydrocarbon degradation and methanogenesis in oil sands tailings communities.
In Chapter 3, we investigated the use of amorphous Fe(III) as a methane inhibitor in conjunction with hydrocarbon degradation under N depleted conditions. As in chapter 2, we established cultures using tailings from three operators active at the time of this work, Albian, CNRL, and Syncrude. Cultures were established with and without fixed N and amended with toluene or n-alkanes in the presence of Fe(OH)3. 16S rRNA and functional genes were sequenced to define the microbial community. While no n-alkane degradation was observed, toluene degradation was recorded. In all cases, methanogenesis was inhibited. As observed in Chapter 2, N2-fixation as determined using an acetylene reduction assay was present in N deficient cultures alongside nifH expression. These data support our hypothesis that amorphous Fe(III) can be effectively used as a treatment to mitigate methane emissions by oil sands tailings communities while maintaining detectable rates of hydrocarbon degrading activity under N deficient conditions.
In Chapter 4, we sought to determine if Fe(OH)3 could be used to inhibit methane production from the metabolism of citrate, a dispersant and labile methanogenic substrate found in some tailings ponds. Acetylene reduction assay revealed that N2-fixation was occurring in N deficient cultures, both with and without Fe(III). Methanogenesis was observed in the absence of Fe(III) as expected, however, methane was not inhibited in cultures treated with amorphous Fe(OH)3. We theorized this was due to abiotic reactions between citrate and Fe(OH)3 resulting in the formation of ferric citrate complexes. Unlike amorphous Fe(OH)3 ferric citrate complexes are not known to inhibit methanogenesis. These data suggest that Fe(OH)3 may not effectively treat methane emissions in ponds that contain a continuous source of citrate.
- Graduation date
- Fall 2020
- Type of Item
- Doctor of Philosophy
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