Abstract
iso-Alkanes, a major fraction of the solvents used in bitumen extraction from oil sand ores, are slow to biodegrade in anaerobic tailings ponds. We investigated methanogenic biodegradation of iso-alkane mixtures comprising either three (2-methylbutane, 2-methylpentane, 3-methylpentane) or five (2-methylbutane, 2-methylpentane, 2-methylhexane, 2-methylheptane, 2-methyloctane) iso-alkanes representing paraffinic and naphtha solvents, respectively. Mature fine tailings (MFT) collected from two tailings ponds, having different residual solvents (paraffinic solvent in Canadian Natural Upgrading Limited (CNUL) and naphtha in Canadian Natural Resources Limited (CNRL)), were amended separately with the two mixtures and incubated in microcosms for ~1600 d. The indigenous microbes in CNUL MFT produced methane from the three-iso-alkane mixture after a lag of ~200 d, completely depleting 2-methylpentane while partially depleting 2-methylbutane and 3-methylpentane. CNRL MFT exhibited a similar degradation pattern for the three iso-alkanes after a lag phase of ~700 d, but required 1200 d before beginning to produce methane from the five-iso-alkane mixture, preferentially depleting components in the order of decreasing carbon chain length. Peptococcaceae members were key iso-alkane-degraders in both CNUL and CNRL MFT but were associated with different archaeal partners. Co-dominance of acetoclastic (Methanosaeta) and hydrogenotrophic (Methanolinea and Methanoregula) methanogens was observed in CNUL MFT during biodegradation of three-iso-alkanes whereas CNRL MFT was enriched in Methanoregula during biodegradation of three-iso-alkanes and in Methanosaeta with five-iso-alkanes. This study highlights the different responses of indigenous methanogenic microbial communities in different oil sands tailings ponds to iso-alkanes.
Highlights
Alkanes are typically a major component of crude oils and petroleum products, and are among the least chemically reactive organic compounds [1]
Neither the heat-killed amended microcosms nor the unamended microcosms containing either Canadian Natural Upgrading Limited (CNUL) or Canadian Natural Resources Limited (CNRL) Mature fine tailings (MFT) produced significant CH4 (
Possible benefits for tailings pond management include methanogenic acceleration of geochemical processes that affect settling of colloidal MFT clays and enhance recovery of porewater for re-use [26,27,36,37]
Summary
Alkanes are typically a major component of crude oils and petroleum products, and are among the least chemically reactive organic compounds [1]. Despite the inert nature of alkanes, their utilization by microbes as sole carbon and energy sources under aerobic conditions has been well-known for a century [2]. Microbial metabolism of alkanes under anaerobic conditions was convincingly demonstrated only in the last three decades [3,4,5,6,7], and primarily has focused on n-alkanes. Biodegradation of iso- and cycloalkanes is much slower under anaerobic conditions than aerobic conditions where degradation occurs within a few days of incubation [12,13,14].
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