Applications of Lime and Enzymes for Oil Sands Tailings Management: Dewatering and Mitigation of Methane Emissions
Creators: Allam, Nesma M.E.E.
Publisher: name: University of Alberta Library
Year: 2023

<p>The remediation of oil sands tailings is a crucial environmental aspect of the Canadian oil sands industry. Oil sands tailings management involves the endless storage of massive volumes of fluid fine tailings (FFT). The slow gravity settling of tailings, the release of toxic compounds, and the release of greenhouse gases (GHGs) emissions are all synergistic reasons for the comprehensive tailings environmental concerns and challenges. These mandates finding a cost-effective tailings reclamation strategy. This thesis focused on the engineered non-natural novel strategies to improve the consolidation of tailings and the inhibition of GHGs emissions. This thesis was divided into three research components. <br>
           First, the effect of lime treatment on FFT dewatering and improving cap water quality under the simulated End pit lake (EPL) scenario was investigated. EPLs are being examined as one potential approach to reduce FFT inventories for the oil sands industry. Experiments were conducted with various doses of lime (650 to 4000 ppm). The results illustrated that a high lime dose of 3500 ppm achieved the highest FFT water recovery% (WR), decreased the cap water alkalinity after 90 days, and increased the possibility of cation exchange (at day 0). In contrast, the degradation of petroleum hydrocarbons was slightly enhanced at a low lime dose of 650 ppm. In addition, the 650 ppm dose resulted in minimal change in the microbial cell counts at day 90, compared with high lime doses that significantly reduced the cell counts. FFT pore water exhibited higher toxic effects for lime dosages &gt;1600 ppm. Nevertheless, at all lime doses, low cap water toxicity (i.e., &lt;1.0 Toxicity Unit) at day 90 was attained. The low water toxicity for the cap water can be ascribed to the reduction of cap water pH over time due to the dissolution of atmospheric carbon dioxide into cap water. <br>
            Second, the feasibility of enzymatic treatment (cellulase, protease, and lysozyme) was investigated for the first time to accelerate the dewatering of FFT. The findings illustrated that lysozyme (0.5% and 1%) significantly improved FFT dewatering by increasing the WR up to 20% compared with the other enzymes (up to 12%) or the control (2%). Moreover, lysozyme treatment resulted in the highest increase in ionic strength (0.038 to 0.1 mol/L), decrease in diffuse double layer (DDL) thickness (1.54 × 10−7 to 9.40 × 10−8 cm), and increase in zeta potential (−34.7 to −14.8 mV). Increased methane production was observed for cellulase, lysozyme, and protease (0.5%). The enhanced dewatering could be linked to the ebullition of methane gas resulting from the methanogenic activity, which created pathways for the trapped water release. In addition, the dissolution of carbonate minerals during the release of methane gas increased ionic strength and decreased the DDL of the FFT. Lysozyme 1% treatment was also the most effective in reducing naphthenic acid fractions (1934.6 to 243 ng/mL); however, the released water had high toxicity toward Vibrio fischeri and had a slight decrease in microbial populations.<br>
           The final investigation focused on the engineering strategies to reduce GHGs emissions through methanogenesis inhibition in tailings (methane inhibition) by chemical treatment (lime) and biological treatment using enzymes (lysozyme and protease). Overall, treatment with protease 3%, lysozyme 3%, and lime 5000 ppm inhibited CH4 production (by 52%, 28%, and 25%, respectively) and were weakly associated with the archaeal abundance. Enzyme treatment resulted in a higher reduction in CH4 production compared with lime treatment. A 3% protease suppressed CH4 production throughout the experiment (the change in methane was 0.78 mM), which could be attributed to the pH reduction to pH 4.9 at week 23 resulting from the formation of volatile fatty acids. The toxicity effect was greater with protease 3% and lysozyme 3% treatment than with lime treatment. Lime treatment resulted in the highest reduction in 16S rRNA gene copies. <br>
           In summary, the significant implications for the use of lime to improve water quality in EPLs and benefit the long-term success of FFT remediation within EPLs were highlighted in the first part of the thesis. The second and third parts of the study introduced a novel technique based on the enzymatic treatment that was applied for the first time in oil sand tailings. These parts provide fundamental insights into the dewatering and inhibition of GHGs emissions by biological treatment using enzymes.</p>