Abstract
Consolidation of clay particles in aqueous tailings suspensions is a major obstacle to effective management of oil sands tailings ponds in northern Alberta, Canada. We have observed that microorganisms indigenous to the tailings ponds accelerate consolidation of mature fine tailings (MFT) during active metabolism by using two biogeochemical pathways. In Pathway I, microbes alter porewater chemistry to indirectly increase consolidation of MFT. Here, we describe Pathway II comprising significant, direct and complementary biogeochemical reactions with MFT mineral surfaces. An anaerobic microbial community comprising Bacteria (predominantly Clostridiales, Synergistaceae, and Desulfobulbaceae) and Archaea (Methanolinea/Methanoregula and Methanosaeta) transformed FeIII minerals in MFT to amorphous FeII minerals during methanogenic metabolism of an added organic substrate. Synchrotron analyses suggested that ferrihydrite (5Fe2O3. 9H2O) and goethite (α-FeOOH) were the dominant FeIII minerals in MFT. The formation of amorphous iron sulfide (FeS) and possibly green rust entrapped and masked electronegative clay surfaces in amended MFT. Both Pathways I and II reduced the surface charge potential (repulsive forces) of the clay particles in MFT, which aided aggregation of clays and formation of networks of pores, as visualized using cryo-scanning electron microscopy (SEM). These reactions facilitated the egress of porewater from MFT and increased consolidation of tailings solids. These results have large-scale implications for management and reclamation of oil sands tailings ponds, a burgeoning environmental issue for the public and government regulators.
Highlights
Aqueous slurries with appreciable clay content are generated worldwide by industrial activities, during ore processing
A parallel unamended mature fine tailings (MFT) control column showed the effects of microbial metabolism of endogenous carbon
Total Fe1 in solid phase; n = 2 FeII2 in solid phase; n = 3 Fe-DCB3 in solid phase; n = 2 Fe-AOD4 in solid phase; n = 2 AVS5 in whole MFT; n = 2 Cr-R sulfides6 in whole MFT; n = 3
Summary
Aqueous slurries with appreciable clay content (e.g., mine tailings and unconsolidated sediments) are generated worldwide by industrial activities, during ore processing. ∼1 million m3 of fluid fine tailings day−1 are generated during bitumen extraction from surface-mined oil sands ores in northern Alberta, Canada. These wastes are deposited into tailings ponds for containment under a policy of no release to the environment. Strategies include dewatering (recovery of pore water from tailings suspensions for re-use in bitumen extraction) and consolidation of solids (to decrease tailings inventory) after placement in tailings ponds This poses a major challenge to oil sands companies because gravitational settling of fine colloidal clay suspensions takes years or decades to achieve even 30–35% solids content as mature fine tailings (MFT). We have accelerated biodensification of MFT by adding fermentable organic carbon sources to stimulate the indigenous microbes: faster dewatering and consolidation were observed with MFT from all five oil sands tailings ponds tested from three different operators, indicating that this is a general response (Bressler et al, 2010)
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