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An Examination of the Effect of Diluent on Microbial Dynamics in Oil Sands Tailings and the Mechanistic Insight on Carbon Dioxide-mediated Turbidity Reduction in Oil Sands Surface Water
- Author / Creator
- Poon, Ho Yin
Oil sands fluid fine tailings (FFT) are comprised of sand, silt, clay, residual organics, salt, trace metals and process-affected water (OSPW). In order to accommodate and remediate large amounts of FFT, a full-scale demonstration pit lake, where FFT was capped with a mixture of OSPW and fresh water, was constructed and named Base Mine Lake (BML). This strategy allows FFT to dewater and the water cap to gradually develop into a permanent wet-landscape with the expectation of an aquatic ecosystem development. However, high BML surface water turbidity has been observed, which hinders aquatic ecosystem development. The residual organics in the tailings have been shown to stimulate microbial activity thus generating CH4 and H2S in tailings. Ebullition of these biogenic gases could contribute to turbidity observed in BML, as well as transporting the residual organics to the surface water.
The impact of naphtha diluent (at 0.2, 0.8 and 1.5%) on microbial activity resulting in biogenic gases generation were investigated. In our anaerobic mesocosm study, H2S production preceded CH4 generation, and the amount gases producted increased linearly with increasing naphtha diluent concentration. The SRB communities were stimulated week 0-5, as an increase of relative abundance for Desulfobulbaceae (580%) and Desulfomicrobiaceae (505%) observed in MFT-PW-0.8% mesocosms. Consistent with the chemical data, an increase for Methanoregulaceae (380%) and Methanotrichaceae (360%) were observed in MFT-PW-0.8% mesocosms at week 11. Furthermore, Anaerolinealeaceae and Synergistaceae families were detected, suggesting competitive and syntrophic relationships. Organics from the naphtha diluent were fermented by members of Anaerolinealeaceae and Synergistaceae to generate fatty acids and H2, which serve as substrates for SRB and methanogens.
The ebullition of these biogenic gases from the FFT could transport BTEX compounds, which are a component of naphtha diluent. Thus, the changes of benzene and toluene concentrations in the headspace was monitored in these mesocosms. The highest benzene release rate was 1.1 g/mL MFT/d (week 4-6), coincided with the H2S emission in week 6. The highest toluene release rate was 0.35 g/mL MFT/d (week 11-15), corresponded with the CH4 emission in week 11. These data suggest the correlation between biogenic gas emissions and the transport of benzene and toluene from tailings to the surface water. Therefore, removing the BTEX compounds from the surface water may reduce biogenic gases production. Expanded graphite (EG) has large surface area, strong adsorption and electrical properties. As a result, the adsorption capacity of each BTEX compound was tested on EG and BioLargo’s advanced oxidation system (AOS). EG and AOS showed adsorption preference for BTEX compounds with higher hydrophobicity. In a batch experiment, ~37% of p-xylene and ~29% of ethylbenzene compared to 12% benzene and 17% were adsorbed using 500 mg EG. Electricity application further enhanced removal in AOS system for p-xylene (99%) and ethylbenzene (38%). These results showed that AOS system can only effectively removal p-xylene amongst BTEX compounds.
Several previous studies showed dissolved CO2 improves FFT dewatering/densification and reduces surface water turbidity, but the mechanism is not well understood. The effect of CO2-mediated carbonate dissolution on BML water turbidity reduction was examined. Greater than 98% turbidity reduction was observed in columns receiving CO2-treatment and calcite compared to ~95% turbidity reduction with CO2 treatment alone. Despite turbidity reduction, the Ca2+ concentration was (4X) below the theoretically equilibrium concentration. Since OSPW contains high sulfate concentration, the inhibitory effect on calcite dissolution was tested on synthetic BML water. A 23% reduction on calcite dissolution rate was calculated for the 400 mg/L SO42- column compared to the 50 mg/L SO42- column. These results demonstrated the high SO42- concentration reduce the calcite dissolution rate. Dolomite and siderite minerals were identified from previous oil sands tailings studies. As a result, CO2-mediated dissolution of calcite and dolomite mixture (100%, 50%, 25%, 0% dolomite) was tested. A mathematical (DLVO) model was constructed to determine suspended FFT stability. A reduction in energy barrier below 20 kT was calculated for all columns received carbonate minerals (100% to 0% dolomite) and CO2-treatment. Interestingly, an inverse correlation was observed between the amount of dolomite present and energy barrier values at day 42 of the experiment, with a difference of 3.2 kT for 100% dolomite and 100% calcite columns. These results suggest CO2-mediated calcite dissolution provides an optimal condition for suspended FFT to settle.
- Graduation date
- Fall 2019
- Type of Item
- Doctor of Philosophy
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