Microbial transformation of solid phase impacts quality of recovered water during consolidation of bioreactor-treated oil sands tailings

Abstract Tailings produced from surface-mined oil sands ores are retained in ponds on-site, pending water recovery for re-use and reclamation of solids. To enhance dewatering of tailings, we previously demonstrated that incubation of mature fine tailings (MFT) amended with an agricultural byproduct as a substrate in an anaerobic stirred tank reactor (STR) stimulated microbial activity and accelerated porewater expression (water recovery) and MFT consolidation by >50%. Here, we report how microbial activity stimulated by STR treatment transformed MFT minerals and affected the quality of recovered porewater during subsequent MFT consolidation. STR treatment (methanogenesis) created strong reducing conditions with slight decrease in pH that induced transformation of the tailings solid phase. The concentrations of major ions (Ca2+, Mg2+, and HCO3-) increased in the recovered porewater presumably due to dissolution/precipitation of carbonate minerals. Iron (Fe) fractionation of solids revealed that FeIII-bearing minerals in STR-treated MFT were transformed into FeII minerals, predominantly amorphous in nature, such as iron sulfides (FeS). The concentrations of trace elements (V, As, Co, Ni, and Sr) also increased in the recovered porewater, potentially released from ionically-bound and strongly adsorbed fractions of metal(oid)s determined through sequential extraction. Zn, Cu and Cd were not detected in porewater after STR treatment; these metals might have precipitated as sulfide minerals. MFT biodensification is a promising approach for tailings management, however, the quality of recovered porewater warrants optimization of substrate and STR retention time. These results also inform environmental monitoring of submerged tailings reclamation sites, particularly end-pit lakes.