Mitigation of Methane Emissions from Oil Sands Tailings by Redox Amendment: Mathematical Modeling of Empirical Observations

Anaerobic biodegradation of fugitive diluent hydrocarbons in oil sands fine tailings (FT) sustains CH4 emissions from tailings facilities and potentially from pit lakes, which impact the climate and effective tailings reclamation. We investigated the effectiveness of sulfate as a redox amendment to mitigate CH4 production from FT containing ∼0.2% naphtha. FT were collected from four different locations (two methanogenically more active and two less active) in a tailings-containing pit lake. Microcosms incubated for ∼800 d suggested that labile hydrocarbons (∼35–38% of naphtha, supporting methanogenesis), including monoaromatics, n-alkanes, and iso-alkanes, were biodegraded under sulfate-reducing conditions in all FT with no significant CH4 production. Although the extent of hydrocarbon biodegradation was similar, iso-alkanes were biodegraded faster in FT from sampling locations that were methanogenically less active in situ. A phenomenological model developed using zero-order kinetics predicted well naphtha biodegradation and sulfate reduction in microcosms. Using reported unrecovered naphtha input to an active tailings facility (Mildred Lake Settling Basin), the model suggested that sulfate amendment could reduce predicted CH4 production from the labile naphtha fraction by ∼51–85%, potentially reaching 95–100% if sulfate reduction supported by other endogenous substrates was also considered. These findings can inform potential methane mitigation solutions for diluent (naphtha) affected tailings.