Microbially-accelerated consolidation of oil sands tailings. Pathway II: solid phase biogeochemistry

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 Fe(III) minerals in MFT to amorphous Fe(II) minerals during methanogenic metabolism of an added organic substrate. Synchrotron analyses suggested that ferrihydrite (5Fe2O3. 9H2O) and goethite (α-FeOOH) were the dominant Fe(III) 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.

Keywords: FeIII reduction; aggregation of clay particles; consolidation; formation of FeII minerals; methanogenesis; oil sands tailings.

Figure 1

Proposed model for microbially-mediated geochemical pathways of clay consolidation, modified from Figure 7 in companion paper (Siddique et al., 2014). In Pathway I, microbial metabolism decreases pH and dissolves carbonate minerals in MFT, increasing bicarbonate (HCO⁻₃), calcium (Ca²⁺), and magnesium (Mg²⁺) ions in porewater. These ions increase ionic strength (I) of porewater, thus reducing the diffuse double layer (DDL) of clay particles and facilitating their consolidation. In Pathway II under anaerobic conditions, Fe^III minerals (goethite and ferrihydrite; FeOOH) in tailings are reduced to Fe^II as dissolved Fe²⁺ that may contribute to the cation exchange process (Pathway IIA) and/or to formation of mixed-valence Fe^II–Fe^III “green rust.” Dissolved Fe²⁺ and/or green rust react with H₂S (aqueous), HS⁻, PO³⁻₄, or HCO⁻₃ to form FeS, FeCO₃, and Fe₃(PO₄)₂¢erdot8H₂O minerals. Transformed minerals entrap clay particles and/or mask the reactive surfaces of clays, increasing clay particle consolidation (Pathway IIB). Competing reactions are shown by solid arrows while dashed arrows indicate pathways not considered significant in our study. CEC, cation exchange capacity.

Figure 2

Redox potential (E_h) in unamended (U) and amended (A) MFT. Measurements were taken from port 1 (cap water; see Figure 1 in Siddique et al., 2014) and ports 2 and 3 (solids) after 213 days incubation. Bars represent the mean values from analyses of triplicate samples taken from each column and error bars represent 1 standard deviation.

Figure 3

Iron (Fe) transformation observed in unamended (U) and amended (A) MFT at 213 days incubation. Labels A2 through U3 designate the columns and ports used to withdraw MFT samples below the mudline. (A) Fe fractionation (Table 2) calculated in % (oven dry basis): Fe (total) = Fe (acid digested Fe); Fe^III = Fe (dithionite-citrate-bicarbonate; DCB) – Fe^II (ferrozine); Fe^III (amorphous) = Fe (ammonium oxalate extraction; AOD) − Fe^II (siderite) − Fe (acid volatile sulfides; AVS) − Fe (ferrozine); Fe^III (crystalline) = Fe^III - Fe^III (amorphous); Fe^II = Fe (total) − Fe^III; Fe^II (crystalline) = Fe (pyrite) + Fe (vivianite) + Fe (siderite); and Fe^II (amorphous) = Fe^II − Fe^II (crystalline). Standard deviations for Fe (total) represent duplicate samples. (B) Fe associated with newly formed Fe^II minerals (amorphous sulfide [FeS], pyrite [FeS₂], and vivianite [Fe₃(PO₄)₂.8H₂O]). Bars represent means of triplicate samples and error bars represent 1 standard deviation.

Figure 4

X-ray Absorption Near Edge Structure (XANES) spectra of unamended and amended MFT from ports 2 and 3 after 213 days incubation (shown as inset in all panels) compared with K-edge XANES of dominant authentic Fe mineral standards (shown as main graphs).

Figure 5

Scanning electron microscope (SEM) micrographs of clay architecture in amended and unamended MFT at 213 days incubation. (A–D), cryo-SEM; (E,F), conventional SEM. (A) Unamended MFT with random clay particle structure; scale bar 10 μm. (B) Canola-amended MFT with flocculated clay particles (light) having card-house structure, producing a network of interstitial pores (dark); scale bar 10 μm. (C) Phyllosilicate particles in unamended MFT, lacking transformed mineral coating; scale bar 1 μm. (D) Phyllosilicate particles in amended MFT, forming aggregates with amorphous surface coating; scale bar 1 μm. (E) Weakly aggregated phyllosilicate particles in unamended MFT. (F) Aggregated phyllosilicate particles in amended MFT. (G) Energy dispersive spectrum (EDS) of (E) showing phyllosilicates with negligible iron mineral coating. (H) EDS of (F) revealing phyllosilicates coated with amorphous iron minerals.

Figure 6

Microbial community analysis. (A) Non-metric multidimensional scaling (NMDS) analysis of 16S rRNA gene pyrosequences comprising microbial communities in initial MFT (black diamond) and MFT samples taken from unamended (U; orange circles) and amended (A; blue squares) column ports. Numbers indicate the ports (Figure 1 in Siddique et al., 2014), with port 1 accessing cap water and ports 2 and 3 accessing MFT in both columns. (B) Microbial community composition in the initial bulk MFT, cap water (port 1) and tailings (ports 2 and 3) in the unamended and amended MFT after 213 days incubation, based on 16S rRNA gene pyrosequencing. Only OTUs present at an abundance of ≥1% in one or more communities were considered; hence the total community is <100%.

Figure 7

Proposed biogeochemical reduction of Fe^III and cycling of S in amended MFT, based on enrichment of Clostridiales, Synergistaceae, and Desulfobulbaceae in amended MFT as revealed by 16S rRNA gene pyrosequencing (Figure 6). (1) Clostridiales ferment organic substrates yielding fatty acids, alcohols, CO₂, and H₂. (2) Fermentative metabolism can divert a proportion of electron flow to Fe^III reduction without conserving energy for growth (Coleman et al., ; Dobbin et al., 1999). (3) Some Clostridia reduce Fe^III through respiration (dissimilatory reduction) to support growth (Slobodkin et al., ; Kunapuli et al., 2007). (4) Synergistaceae ferment organic substrates (Zavarzina et al., 2000). (5) H₂ produced by Clostridia and Synergistaceae during fermentation can be utilized syntrophically by sulfate-reducing bacteria (SRB; e.g., Desulfobulbaceae) (Kunapuli et al., 2007). (6) some SRB can indirectly reduce Fe^III via S cycling (Straub and Schink, 2004), biogenic S²⁻ can chemically reduce Fe^III minerals (Raiswell and Canfield, ; Poulton et al., 2004), and/or (7) SRB can directly transfer electrons to Fe^III (Knoblauch et al., 1999). (8) Methanogens oxidizing H₂ can reduce Fe^III by transferring electrons to Fe^III both directly and via electron shuttling (Bond and Lovley, ; Liu et al., 2012). (9) Hydrogenotrophic and acetoclastic methanogens produce CH₄ syntrophically with Bacteria in MFT (Siddique et al., 2011, 2012).