Biogeochemical pathways that influence de-watering and consolidation of fluid fine tailings

Approximately 1 million m3 of fluid fine tailings are produced every day in northern Alberta, Canada from processing of surface-mined oil sands resources. The tailings, comprising an aqueous suspension of fines dominated by clay particles that remain dispersed for decades, are deposited into tailings ponds for containment, de-watering and consolidation. Slow consolidation of clays retained in tailings ponds hinders recovery and re-use of water, retards volume reduction and presents a technical challenge for effective tailings ponds management. Here, we reveal that microorganisms indigenous to oil sands tailings ponds change the surface chemistry of clay particles and accelerate tailings consolidation by two biogeochemical pathways: one pathway shows that microorganisms metabolize organic substrates and produce carbon dioxide (CO2) and methane (CH4) in oil sands tailings. Dissolution of biogenic CO2 increases bicarbonate (HCO3-) concentrations in the pore water and reduces pore water pH, which dissolves carbonate minerals and releases cations from tailings minerals. The higher ionic strength of the pore water resulting from increased concentrations of ions shrinks the double diffuse layers of clay particles and hence increases consolidation. In addition, biogenic CH4 ebullition creates transient physical channels for recovery of pore water. The second pathway exerts a more direct microbial effect on consolidation through transformation of iron (Fe) minerals in oil sands tailings. Microbial reduction and dissolution of FeIII minerals forms amorphous FeII minerals that entrap and mask electronegative clay surfaces. These biogeochemical processes provide essential information for construction of geotechnical models to predict settling of clay particles for effective reclamation and management of fluid fine tailings.