Effects of tailings treatment and hydrocarbons on sulfur biogeochemistry and consolidation of oil sands tailings in pit lake scenarios

Tailings reclamation is a significant challenge facing the oil sands industry. There are 1.4 billion m 3 of fluid fine tailings (FFT), a slurry waste stream generated from oil sands surface mining, awaiting reclamation. Pit lakes may be a suitable reclamation option, however further evaluation is required before pit lakes receive regulatory approval as an FFT reclamation method. FFT destined for pit lakes may be treated (with alum and polyacrylamide) and contain residual hydrocarbons. This study is the first publicly available comparison of the behavior of untreated versus treated FFT in pit lakes. Forty columns were monitored for 540 d under anaerobic conditions to evaluate the effects of tailings treatment, hydrocarbon amendments, and temperature (10 °C and 20 °C) on the biogeochemical and geotechnical behavior of FFT in pit lakes. Although tailings treatment aims to improve dewatering performance, this study found that treatment significantly reduced dewatering and consolidation. This highlights challenges in achieving optimal flocculation conditions, even in a controlled laboratory setting. Tailings treatment and hydrocarbon amendments significantly enhanced sulfate reduction, which was the dominant microbial process. Treated FFT had a greater abundance of known sulfate reducing bacteria and more extensive sulfate reduction than untreated FFT, while hydrocarbon amendments quickly led to >99% sulfate reduction. Sulfate reduction generated dissolved sulfide species throughout the columns and led to significant increases in alkalinity and CO 2(g) . Divalent cations were subsequently impacted and presumably precipitated as carbonate minerals. These pore water chemistry changes resulted in higher tailings solids contents, leading the authors to propose a new bioconsolidation pathway for FFT via sulfate reduction. • Coagulation/flocculation treatment decreased tailings dewatering and consolidation • Sulfate reduction was significantly enhanced by tailings treatment and hydrocarbons • Treated tailings had a greater abundance of sulfate reducing bacteria • Hydrocarbon amendments led to rapid and > 99% sulfate reduction • Sulfate reduction increased alkalinity and CO 2 and decreased cation concentrations