Bioremediation of Persistent Contaminants of Concern in the Petroleum Industry

The petroleum industry is a major economic driver in Canada; however, toxic and persistent contaminants such as Naphthenic Acid Fraction Compounds (NAFCs) and benzene can be generated during upstream and downstream operations. NAFCs are acutely toxic organics which are solubilized and concentrated during bitumen extraction from Alberta’s oil sands. Coupling chemical oxidation with biodegradation may be a feasible remediation method that requires further investigation. This research utilized persulfate as a stable, persistent, and effective oxidant that has not previously been coupled with biodegradation for NAFCs. An initial oxidation phase was followed by a coupled treatment phase consisting of the residual persulfate with biodegradation by Pseudomonas fluorescens. The first project used unactivated persulfate and commercial Merichem NAs (100 mg/L), while the second project used an initial heat activated persulfate phase followed by continued reaction at room temperature for oil sands process-affected water (OSPW) NAFCs (61 mg/L). Using 100-1000 mg/L of unactivated persulfate, 30-99% of Merichem NAs were removed and toxicity towards Vibrio fischeri was reduced by 75-100% over 317 days. Coupled treatments improved Merichem NA and chemical oxygen demand (COD) removals by up to 1.8- and 6.7-fold, respectively, compared to oxidation alone, and cell counts were higher in bottles with 100-500 mg/L persulfate compared to bottles without persulfate. Activating 250-1000 mg/L of persulfate at 60oC for 8 hours decreased OSPW NAFCs by 45-89%, significantly reducing the acutely toxic O2 group and shifting to more oxidized (O3+) NAFCs. Toxicity towards V. fischeri increased from 18% to 28-61% inhibition effect after activated persulfate oxidation but was reduced to non-toxic with continued oxidation at room temperature. Recalcitrant NAFCs such as those with higher molecular weight (n>16) and unsaturation (DBE>6) were oxidized, highlighting the potential to be combined with biodegradation. However, the addition of P. fluorescens with the residual persulfate did not improve the treatment over 150 days but increased the toxicity up to 40%. The bacteria appeared to experience considerable stress, with cell counts decreasing over 3 orders of magnitude. This research demonstrated that persulfate oxidation is promising to reduce NAFC concentrations and coupling with biodegradation can increase mineralization and lower costs, however improving microbial viability is imperative for success. Accidental spills and releases at downstream facilities such as refineries and underground storage tanks is an additional concern for the oil and gas industry, with benzene often driving remediation efforts due to its carcinogenicity. Sites with anaerobic conditions are particularly challenging as benzene contamination persists, and thus may require specialized bioaugmentation cultures to treat. Salt co-contamination can also occur at these sites, however the impact of salinity on in situ anaerobic biodegradation is largely unknown. A highly enriched methanogenic benzene-degrading consortium (DGG-B) was tested under sudden osmotic stress by adding 2.5-100 g/L NaCl and by gradual acclimation to NaCl. The impact of salinity on benzene fermenting bacteria (ORM2) and methanogenic archaea was differentiated by feeding DGG-B benzene, acetate or H2/CO2(g). Benzene degradation rates were inhibited at 5 and 10 g/L NaCl, decreasing from 20 µM/day in control bottles to 6 µM/day. Benzene degradation stalled at 15 g/L NaCl and ORM2 cells exhibited predominately decay. Slowly acclimating DGG-B to salinity increased the benzene degradation rate to 29.5 µM/day at 10.5 g/L NaCl. Methanogenesis occurred at 25 g/L NaCl however shifted from Methanosaeta and Methanoregula to Methanosarcina and Methanobacterium. This is one of the first studies to show that methanogenic benzene biodegradation is sensitive to salt, however strategies such as a gradual acclimation process can improve bioaugmentation success at impacted sites.

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