Effect of naphtha diluent on greenhouse gases and reduced sulfur compounds emissions from oil sands tailings

•Indigenous microbes degraded residual diluent in tailings thus producing emissions.•Reduced sulfur compounds were produced first at a max. of 0.12 μmol RSCs/mL MFT.•H2S and 2-methylthiophene contributed 81% of the RSCs gas produced.•CH4 and CO2 production occurred after week 5.•A max. of 40.7 μmol CH4/mL MFT and 5.9 μmol CO2/mL MFT was produced.

The long-term storage of oil sands tailings has resulted in the evolution of greenhouse gases (CH4 and CO2) as a result of residual organics biodegradation. Recent studies have identified black, sulfidic zones below the tailings-water interface, which may be producing toxic sulfur-containing gases. An anaerobic mesocosm study was conducted over an 11-week period to characterize the evolution of CH4, CO2 and reduced sulfur compounds (RSCs) (including H2S) in tailings as it relates to naphtha-containing diluent concentrations (0.2, 0.8, and 1.5% w/v) and microbial activity. Our results showed that RSCs were produced first at 0.12 μmol°RSCs/mL MFT (1.5% w/v diluent treatment). RSCs contribution (from highest to lowest) was H2S and 2-methylthiophene > 2.5-dimethylthiophene > 3-methylthiophene > thiofuran > butyl mercaptan > carbonyl sulfide, where H2S and 2-methylthiophene contributed 81% of the gas produced. CH4 and CO2 production occurred after week 5 at 40.7 μmol CH4/mL MFT and 5.9 μmol CO2/mL MFT (1.5% w/v diluent treatment). The amount of H2S and CH4 generated is correlated to the amount of diluent present and to microbial activity as shown by corresponding increases in sulfate-reducers' Dissimilatory sulfite reductase (DsrAB) gene and methanogens' methyl-coenzyme M reductase (MCR) gene.

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