Naphthenic acid anaerobic biodegrading consortia enriched from pristine sediments underlying oil sands tailings ponds

Seepage from oil sands tailings ponds (OSTP), which contain toxic naphthenic acids (NAs), can infiltrate into groundwater. Clay sediment layer beneath is a critical barrier for reducing the infiltration of NAs into the sand sediment layer, where groundwater channels reside. Biodegradation has great potential as a strategy for NAs removal, but little is known about NAs biodegradability and potential functional microbes in these pristine sediments. This study investigated the potential for anaerobic biodegradation of NAs by microbial consortia enriched from clay and sand sediments underlying OSTP, amended with either acid extracted organics or Merichem NAs, under nitrate- and sulfate-reducing conditions. Degradation of NAs only be detected after DOC concentration reached to steady state after 163 days. Microbial community analysis shows that different electron acceptors, sediment types, and NAs sources associated with specific microbial taxa and can explain 14.8, 13.9 % and 5% of variation of microbial community structures, respectively. The DOC and methane were the most important geochemical properties for microbial community variations. This study approved the potential capability of indigenous microbial communities from the pristine sediments in NA degradation, demonstrating the barrier function of pristine clay sediments underlying OSTP in prohibiting organic contaminants from entering into groundwater.

Vertical gradients in carbon flow and methane production in a sulfate-rich oil sands tailings pond

Oil sands tailings ponds are primary storage basins for tailings produced during oil sands processing in Alberta (Canada). Due to microbial metabolism, methane production contributes to greenhouse gas emissions, but positively affects tailings densification, which is relevant for operational water re-use. Depending on the age and depth of tailings, the activity of sulfate-reducing bacteria (SRB) may control methanogenesis due to the competition for substrates. To assess the depth-related impact of sulfate reduction on CH₄ emissions, original tailings of two vicinal pond profiles were incubated in anoxic microcosms with/without molybdate as selective inhibitor of microbial sulfate reduction. Integrating methane production rates, considerable volumes of CH₄ emissions (∼5.37 million L d^-1) may be effectively prevented by the activity of SRB in sulfidic tailings between 3.5 and 7.5 m. To infer metabolic potentials controlling methanogenic pathways, a set of relevant organic acids (acetate, formate, propionate, butyrate, lactate) was added to part of the microcosms. Generally, organic acid transformation shifted with depth, with highest rates (305-446 μmol L^-1 d^-1) measured in fresh tailings at 5.5-7.5 m. In all depths, a transient accumulation of acetate revealed its importance as key intermediate during organic matter decomposition. SRB dominated the transformation of acetate, butyrate and propionate, but were not essential for lactate and formate turnover. Acetate as methanogenic substrate was important only at 13.5 m. At 1-7.5 m, methanogenesis significantly increased in presence of organic acids, most likely due to the syntrophic oxidation of acetate to CO₂ by SRB and subsequent conversion to CH₄.

Anaerobic BTEX degradation in oil sands tailings ponds: Impact of labile organic carbon and sulfate-reducing bacteria

The extraction of bitumen from oil sands in Alberta (Canada) produces volumes of tailings that are pumped into large anaerobic settling-basins. Beside bitumen, tailings comprise fractions of benzene, toluene, ethylbenzene and xylenes (BTEX) that derive from the application of industrial solvents. Due to their toxicity and volatility, BTEX pose a strong concern for gas- and water-phase environments in the vicinity of the ponds. The examination of two pond profiles showed that concentrations of indigenous BTEX decreased with depth, pointing at BTEX transformation in situ. With depth, the relative contribution of ethylbenzene and xylenes to total BTEX significantly decreased, while benzene increased relatively from 44% to 69%, indicating preferential hydrocarbon degradation. To predict BTEX turnover and residence time, we determined BTEX degradation rates in tailings of different depths in a 180-days microcosm study. In addition, we evaluated the impact of labile organic substrates (e.g. acetate) generally considered to stimulate hydrocarbon degradation and the contribution of sulfate-reducing bacteria (SRB) to BTEX turnover. In all depths, BTEX concentrations significantly decreased due to microbial activity, with degradation rates ranging between 4 and 9 μg kg(-1) d(-1). BTEX biodegradation decreased linearly in correlation with initial concentrations, suggesting a concentration-dependent BTEX transformation. SRB were not significantly involved in BTEX consumption, indicating the importance of methanogenic degradation. BTEX removal decreased to 70-90% in presence of organic substrates presumptively due to an accumulation of acetate that lowered BTEX turnover due to product inhibition. In those assays SRB slightly stimulated BTEX transformation by reducing inhibitory acetate levels.

Interaction of microbial sulphate reduction and methanogenesis in oil sands tailings ponds

Anaerobic turnover of organic compounds in oil sands tailings ponds is accomplished by a complex microbial consortium. We examined major electron accepting processes in mature fine tailings (MFT). Beside methanogenesis and sulphate reduction, microbial iron reduction was an important process of anaerobic respiration. Microbial numbers and activity were comparable to those reported for natural lakes. To understand metabolic interactions of indigenous methanogenic and sulphate-reducing communities, we conducted a 6 month microcosm experiment with MFT supplemented with easily available carbon sources and molybdate and/or 2-bromoethane sulphonate (BES) as specific inhibitors for sulphate reduction and methanogenesis. Methanogenesis increased when microcosms were supplemented with extra carbon, but was completely inhibited by the addition of BES. Molybdate not only inhibited sulphate reduction, but also methanogenesis, indicating a positive relation between the two processes. The turnover of extra carbon sources differed between microcosms treated with molybdate and BES. Acetate and propionate were not consumed in microcosms amended with molybdate, indicating that sulphate-reducing bacteria (SRB) were responsible for their metabolisation, and that methane was rather produced by hydrogenotrophic methanogens. In microcosms without molybdate, acetate transiently accumulated, indicating the activity of both incomplete and complete oxidizing SRB. Ethanol and lactate were also consumed in the simultaneous presence of BES and molybdate, demonstrating the occurrence of other anaerobic processes. Biomass increased by the addition of extra carbon, mainly due to a relative increase in the proportion of SRB. The addition of extra carbon lowered the degradation of BTEX compounds.