Microbial transformation of solid phase impacts quality of recovered water during consolidation of bioreactor-treated oil sands tailings

Anaerobic Microbial Activity May Affect Development and Sustainability of End-Pit Lakes: A Laboratory Study of Biogeochemical Aspects of Oil Sands Mine Tailings

“…GC analyses revealed depletion of added hydrocarbons (data not shown) by indigenous microbial metabolism. Our previous studies have shown that labile hydrocarbons (monoaromatics, n -alkanes and iso -alkanes, which are components of diluents used in bitumen extraction) are metabolized by indigenous microorganisms to produce CH 4 in oil sands tailings. ,,− Importantly, other anaerobic processes beside methanogenesis (i.e., fermentation and anaerobic respiration) might also have contributed to CO 2 production but could not be assessed reliably by measuring total CO 2 production because of its water solubility and the various geochemical reactions in FFT that would decrease measurable CO 2 …”

“…Conspicuous changes were only observed in A20-M at the T1 sampling (during active methanogenesis) where carbonate minerals decreased from 4.8 to 2.3% (Figure S3G–I) with a similar trend observed for siderite content (Figure S3J–L). Variation in carbonate contents in amended columns is generally attributed to dissolution and precipitation of carbonate minerals, in this case microbially mediated. Dissolution of carbonates releases divalent cations that increase porewater ionic strength, support cation exchange, and shrink electric double layer of clays for enhanced FFT consolidation (Siddique et al, 2014a) …”

“…Overall, the greater FFT porewater concentrations of Ca 2+ , Mg 2+ , K + , and HCO 3 − (Table S5) (Section 3.4.1) and trace elements Sr and Ba (Table S6) observed during active methanogenesis in this study agree with our previous findings in FFT consolidation studies. 29,36,53 These higher concentrations developed a significant positive chemical flux (Tables 1 and 2) across the mud line to cap water in amended columns through the same forces of advection (FFT pore water expression during consolidation) and diffusion proposed to explain in situ measurements in BML, 16 including migration of porewater expressed from FFT through aqueous channels shown in Figure 2F. The resulting cap water concentrations of these ions and trace metals in amended columns (Tables S5 and S6) are greater than those measured in BML cap water for 3 consecutive years from 2014−2016; 15 this difference is likely due to the exchange of water (inputs and outputs) 14 and dilution due to atmospheric precipitation in situ.…”

Nutrient Impacts on Biogenic Chemical Flux in an End Pit Lake Reclamation Scenario

“…The potential for transportation of dissolved chemical constituents from FFT porewater to overlying cap water might be a concern for sustainability of EPL as a wet reclamation strategy to manage the growing inventory of FFT. Our previous studies have revealed that anaerobic microbial metabolism of hydrocarbons and other carbon substrates in FFT increases ionic strength of porewater , by transforming clay minerals and can mobilize/immobilize certain trace metals in porewater and cap water . These studies were conducted without assessing the role of added nutrients (N and/or P) in anaerobic microbial activities and associated geochemical changes in FFT.…”

“…The first commercial demonstration of EPL known as Base Mine Lake (∼8 km 2 surface area) was established in 2013 by capping FFT with a mixture of oil sands process-affected water plus freshwater from a nearby creek to evaluate its performance. , Field measurements and thermodynamic and transport modeling indicated that gas ebullition at Base Mine Lake enhanced chemical mass transport from the underlying tailings to the water cap. , FFT harbors indigenous microbial communities , that anaerobically metabolize residual diluent hydrocarbons to produce CH 4 . − Laboratory studies evidence three key geochemical, geotechnical, and hydrological changes in FFT: (1) methanogenesis produces CH 4 creating gas ebullition that physically disturbs the FFT and the solid-water interface (mud line), (2) it increases porewater ionic strength by transforming (dissolving and precipitating) minerals (iron oxides and carbonates) in FFT, and (3) it increases dewatering and consolidation of FFT by altering the porewater and FFT solid-phase chemistry. − Transformation of minerals under methanogenic conditions can mobilize mineral-associated trace elements such as vanadium­(V), arsenic (As), cobalt (Co), nickel (Ni), and strontium (Sr) in the FFT porewater . The major drivers of flux of chemical constituents from underlying FFT to cap water in Base Mine Lake are (1) advective transport due to upward movement of expressed porewater by settling of FFT, which is declining with the decreasing settling rate of FFT, (2) diffusion of dissolved constituents across the concentration gradient between the FFT and cap water interface, which will increase in importance as a mode of chemical mass transport with continued freshwater inputs, and (3) CH 4 ebullition enhancing chemical mass transport through continuous physical mixing, which is likely to decline over time due to microbial depletion of labile residual hydrocarbons in FFT.…”