Microbial-Induced Calcium Carbonate Precipitation to Accelerate Sedimentation of Fine Tailings

Reclaiming oil sand tailing impoundments presents substantial challenges. Bitumen extraction produces large quantities of fine tailings (FT). FT consist of fine-grained sediment, sand, and residual bitumen with high water content. The materials remain as a stable suspension within impoundments for decades (Jeeravipoolvarn et al. 2009). Methods to accelerate the sedimentation process have had limited success; thus, there is a need for additional innovative methods.

Microbial-induced carbonate precipitation (MICP) has been used to improve the engineering behavior of granular soil in a variety of applications. Ureolytic-driven MICP produces ammonium and bicarbonate, and triggers calcium carbonate precipitation. The innovative aspect advocated herein is the use of MICP to accelerate the sedimentation process of the FT materials. Elucidating the particle-scale mechanics of MICP within FT provides insight into the potential acceleration of the sedimentation process.

FT were acquired from Syncrude Canada (Fort McMurray, Alberta, Canada) at two distinct times. The average water content was 180%. The FT consisted of 76.3% fines content, 10.7% sand content, and 13% bitumen content, on average; the specific gravity was 1.94. The FT were augmented with Sporosarcina pasteurii [American Type Culture Collection (ATCC) 11859] following Safavizadeh et al. (2019). Self-weight sedimentation cylinders were used to assess the flocculation process of the FT over 1 week, in triplicate and including both distinct FT sources. In addition to testing baseline untreated FT, results were obtained for samples treated with MICP (0.25 M urea, 0.075 M ${\mathrm{CaCl}}_2$, and 50 mL of ${10}^7\mathrm{cells}/\mathrm{mL}$ of S. pasteurii), and for chemical controls with 0.075 M ${\mathrm{CaCl}}_2$ to discern the effect of ionic strength on the FT without the MICP process. Hydrometer tests were conducted to assess the change in particle size due to MICP. A microscale evaluation of the FT also was conducted using a cryoscanning electron micrograph (cryo-SEM).

The results indicated that the MICP-treated FT experienced more sedimentation and at a faster rate (Fig. 1) compared with the control (baseline). Within the first 2.5 h, the ${\mathrm{CaCl}}_2$- and MICP-treated specimens had a similar response, after which the MICP specimens demonstrated accelerated sedimentation. After 1 day, the MICP-treated FT had experienced 59% sedimentation, whereas the ${\mathrm{CaCl}}_2$-treated FT had experienced 51% and the baseline had 0% sedimentation. Additionally, the ${\mathrm{CaCl}}_2$-treated FT required $\sim 3$ times longer to reach the same level of sedimentation as the MICP-treated FT. The MICP process creates a coarser particle distribution through the precipitation process (Table 1). The cryo-SEM images also showed a denser arrangement of the card-house structure of the FT (Fig. 2).