Microbially-accelerated consolidation of oil sands tailings. Pathway I: changes in porewater chemistry

Abstract

Dispersed clay particles in mine tailings and soft sediments remain suspended for decades, hindering consolidation and challenging effective management of these aqueous slurries. Current geotechnical engineering models of self-weight consolidation of tailings do not consider microbial contribution to sediment behavior, however, here we show that microorganisms indigenous to oil sands tailings change the porewater chemistry and accelerate consolidation of oil sands tailings. A companion paper describes the role of microbes in alteration of clay chemistry in tailings. Microbial metabolism in mature fine tailings (MFT) amended with an organic substrate (hydrolyzed canola meal) produced methane (CH4) and carbon dioxide (CO2). Dissolution of biogenic CO2 lowered the pH of amended MFT to pH 6.4 vs. unamended MFT (pH 7.7). About 12% more porewater was recovered from amended than unamended MFT during 2 months of active microbial metabolism, concomitant with consolidation of tailings. The lower pH in amended MFT dissolved carbonate minerals, thereby releasing divalent cations including calcium (Ca2+) and magnesium (Mg2+) and increasing bicarbonate (HCO−3) in porewater. The higher concentrations increased the ionic strength of the porewater, in turn reducing the thickness of the diffuse double layer (DDL) of clay particles by reducing the surface charge potential (repulsive forces) of the clay particles. The combination of these processes accelerated consolidation of oil sands tailings. In addition, ebullition of biogenic gases created transient physical channels for release of porewater. In contrast, saturating the MFT with non-biogenic CO2 had little effect on consolidation. These results have significant implications for management and reclamation of oil sands tailings ponds and broad importance in anaerobic environments such as contaminated harbors and estuaries containing soft sediments rich in clays and organics.

A Post-Genomic View of the Ecophysiology, Catabolism and Biotechnological Relevance of Sulphate-Reducing Prokaryotes

“…To accelerate the long-term consolidation of MFTs, microbial processes reducing the hydrocarbon load and positively affecting the porewater chemistry are investigated (e.g. Siddique, Kuznetsov, Kuznetsova, Arkell, et al, 2014;. Methanogens and SRB have been repeatedly detected as members of the microbial community of MFTs.…”

Protocols for Handling, Storing, and Cultivating Oil Sands Tailings Ponds Materials for Microbial and Molecular Biological Study

“…Microbial activities in situ have sparked interest because the diverse tailings pond communities can generate greenhouse gases like methane (CH 4 ) and carbon dioxide (CO 2 ) [9,10] or potentially produce toxic hydrogen sulfide (H 2 S) gas [8] by anaerobically metabolizing organic compounds present in the tailings, particularly petroleum hydrocarbons. However, the microbial communities may also help mitigate CH 4 emissions at the pond surface [11], anaerobically degrade solvent (diluent) components [12,13] to potentially reduce OSTP toxicity, and positively affect the behavior of mature fine tailings [14,15] by accelerating consolidation of fine clays and migration of pore water to the surface for reuse.…”

“…Nitrogen "curtain" for use during column filling to reduce exposure to air (see Note 7). (c) 50-L acrylic column fitted with three ports along the length to measure pore pressure and three ports to insert pH and redox probes or to remove samples; adapted from Siddique et al [14]. The column is sealed under a top plate that is attached to a gas trap and gas chromatograph to measure evolved gas volume and composition.…”

“…Tailings can be incubated in glass, acrylic, or polyvinylchloride columns (Fig. 4), without [14,15] or with an overlying water layer [24]. Graduated glass cylinders may be capped under an aerobic atmosphere by using foil secured with construction tape to reduce evaporative losses, whereas acrylic columns may be sealed under N 2 gas.…”

Microbial metagenomics of oil sands tailings ponds: small bugs, big data

“…Only recently have some details begun to emerge about these novel microbes (e.g., Abu Laban et al 2014;Tan et al 2015a). Identifying the key species and understanding their inter-connected metabolisms is important because hydrocarbon degradation in situ not only has deleterious environmental consequences (e.g., production of greenhouse gases), but also beneficial effects in the ponds (e.g., depleting solvent concentrations to reduce the toxicity of tailings prior to reclamation, and accelerating settling of the clay minerals; Siddique et al 2014aSiddique et al , 2014b.…”