Evaluating the Biogeochemical and Geotechnical Behavior of Oil Sands Tailings in Pit Lakes and Mitigating Pit Lake Turbidity with Biofilms
Creators: Cossey, Heidi L
Publisher: name: University of Alberta Library
Year: 2023

<p>Permanent storage and reclamation of oil sands mine waste (fluid fine tailings, FFT) in pit lakes has been proposed by industry. Pit lakes are considered best practice in terms of geotechnical stability, however, there are numerous challenges and knowledge gaps surrounding this proposed oil sands tailings reclamation practice, including water cap quality and biogeochemical cycling processes which can generate greenhouse gases and H2S(g). This research included three laboratory studies, the first of which was designed to improve pit lake water chemistry by mitigating turbidity generation. Two column studies were also designed to evaluate the biogeochemical and geotechnical behavior of untreated and treated FFT in pit lakes and to investigate the effects of pressure on these behaviors. Collectively, these studies provide valuable information regarding a promising biological turbidity mitigation mechanism, as well as key biogeochemical and geotechnical behaviors of untreated and treated FFT that will be important to mitigate, monitor, and/or manage as part of ensuring successful reclamation of oil sands tailings in pit lakes.</p>

<p>The first laboratory study investigated turbidity mitigation in oil sands pit lakes using mudline biofilms that were made up of diverse microbial communities indigenous to oil sands tailings. Mudline biofilms were grown on FFT that was capped with water in 1 L jars. Biofilms reduced turbidity generation by up to 99% during physical mixing experiments, depending on the biofilm age and mixing speed. This study demonstrated that biostabilization of FFT with mudline biofilms is a promising mechanism of turbidity mitigation in pit lakes. </p>

<p>The second laboratory study involved 64 columns (1 L or 19 L), each containing FFT and a water cap and stored under anaerobic conditions. FFT was either left untreated or treated with a coagulant (alum) and flocculant (polyacrylamide, PAM). Over the 540 d study, the tailings in all columns underwent self-weight consolidation, though FFT had a higher net water release (NWR) in 19 L columns than 1 L columns and, surprisingly, treated FFT had a lower NWR than untreated FFT. Water cap concentrations of pore water constituents generally increased over time due to consolidation-driven advection, diffusion (particularly in 1 L columns), and biogeochemical reactions in the FFT. The dominant microbial process in all columns was sulfate reduction, which subsequently influenced alkalinity, CO2(g), and pore water concentrations of divalent cations. Treated FFT and/or FFT amended with hydrocarbons had the most extensive sulfate reduction, which resulted in the generation of aqueous sulfide species throughout the pore water and water caps. </p>

<p>The third laboratory study involved 12 columns (5 L), each containing (alum and PAM) treated FFT and a water cap. All columns were stored under anaerobic conditions for 360 d. Pressure (0.3 to 5.1 kPa) was applied to the tailings in six columns using dead loads and a multi-step loading scheme. This study revealed that pressure (0.3 to 5.1 kPa) significantly influences microbial activity and biogeochemical cycling in treated FFT, presumably by increasing the solubility of microbial substrates and metabolites. Pressure enhanced methanogenesis, leading to increased generation of CH4(g) and CO2(g), and shifted the predominant methanogenic pathway from hydrogenotrophic to acetoclastic methanogenesis. Further, pressure led to increased ammonium concentrations, likely through PAM degradation and/or N2 fixation. Both sulfate reduction (which generated gaseous and aqueous H2S) and methanogenesis (which generated CH4(g) and CO2(g)) occurred in the treated FFT and, similar to the previous column study, hydrocarbon amendments enhanced sulfate reduction in the tailings. </p>