User:Joeho604
Introduction
The oil sands in Alberta, Canada, covering over 100000 km2, produce over 1.3 million barrels of bitumen per day [1]. Extraction of bitumen by surface mining of oil sands requires large amount of water and hydrocarbon solvents. The resulting byproduct creates large volumes of water, sands, clays, residual hydrocarbons, heavy metals, naphtha diluents, and naphthenic acids, which are termed as tailings. Due to operational policies preventing zero discharge of any liquids into the environment, these fine tailings are collected and confined into settling basins to create tailings ponds [1]. Despite the toxicity of tailings, microbial communities exist in tailings pond, which aid to accelerate tailings sedimentation while being able to degrade certain compounds in the tailings.
Physical & Chemical Environment
Composition of tailings pond
Along with the 20-30 wt% of solids (sands and clays) and slightly alkaline water (pH>7.5), mature tailings consist of 1-3 wt% of residual bitumen and naphtha, comprising of a mixture paraffins (n-alkanes), iso-paraffins (branched alkanes), olefins (alkenes), naphthenes (cycloalkanes), naphthenic acids (cyclopentyl and cyclohexyl carboxylic acids), and monoaromatics (benzene, toluene, ethylbenzene, and xylenes, or BTEX) [2][3]. Other minor elements include trace metals (Cr, Mn, Co, Ni, Cu, Zn, As, Sr, Mo, Ba), and ions (HCO3-, PO43-, NO3-, SO42-, Na+, K+, Mg2+, Ca2+, Cl-) [4]. Toxicity of tailings ponds to aquatic organisms is often associated with naphthenic acids, which have surfactant properties that penetrate the cell membrane [5].
Sedimentation & Gas Emissions
Closer to the inflow, sand settles quickly. However, tailings consolidation of fine clays occurs by gravity at a slow rate over 5-10 years [6]. Stratification therefore results with older and denser fine tailings settled to the bottom and fresh tailings deposited on top. Temperature also increases with depth due to lack of surface cooling and retention of heat at 30-60˚C by deposited tailings [7]. Densification remains an operational challenge, which prevents water from being recycled while delaying the land reclamation process. Anaerobic methanogenic activities by syntrophic bacteria and methanogenic archaea also enhance the rate of tailings sedimentation. Microbe-formed gas channels assist the escape of methane gas bubbles which also aid to drain pore water from deeper tailings. Each day, about 43000m3of methane (CH4) is released. Sulfate and nitrate reduction by sulfate and nitrate-reducing bacteria, however, impeded the actions of methanogenesis, thus slowing the rate of tailings densification. Densification is reached when the volumetric fraction of solids (Fs) increase to 85%(w/w) [8].
Active vs. Inactive pond
In active ponds, wastes from bitumen extraction are continually collected in the basin. This results in continual input of electron donors and acceptors of metal ions into the pond. In active ponds, gypsum is added to aid densification while microbial activities are dominated by anaerobes at lower depths. In inactive ponds, wastes from bitumen extractions are no longer collected. In this situation, there are no inputs of electron donor and acceptors while microbial activities are confined to the upper depths with lower anaerobic activities. Therefore, the status of the pond influences the microbial community found [9].
Microbial Processes
Hydrocarbon Biodegradation and Methanogenesis
Anaerobic methanogenic hydrocarbon degradation is the source of CH4 emissions in tailings ponds. Metabolism begins with beta-oxidation of naphtha (such as long and short n-alkanes) to acetate and H2 by Syntrophus/Smithella. The acetate produced is coupled to methanogenesis either by acetoclastic methanogens to generate CH4 gas directly from acetate; or by archaeal hydrogenotrophic methanogens to reduce CO2 into CH4 with the utilization of H2 as a reducing agent following acetate oxidation [6][10]. Biodegradation of short n-alkanes (C<12) are preferentially selected by order of decreasing length of C-chains: C10>C8>C7>C6. Long n-alkanes (C≥12), however, do not show any preferential order of biodegradation [3][10]. Preference for BTEX biodegradation follows the sequence of: toluene>o-xylene>m-plus p xylene>ethylbenzene> benzene [2].
Hydrocarbon Biodegradation and Sulfate/ Nitrate Reduction
Anaerobic biodegradation of hydrocarbons under sulfate reducing and denitrifying conditions, by sulfate-reducing bacteria (SRB) and nitrate-reducing bacteria (NRB) respectively, is known to impede with methanogenesis. The competition for electron donors ultimately determines the predominance of SRB or methanogens under anaerobic conditions. With a slight advantage in thermodynamic and kinetics over methanogens, SRB outcompete methanogens for byproducts from syntrophs following hydrocarbon oxidation. Under high sulfate concentrations, SRB form consortia with syntrophs which allows SRB to utilize its H2, acetate, and other electron donors for sulfate reduction [5][11].
Naphthenic acid degradation
Naphthenic acid degradation is performed by a wide range of aerobes through degradation of anteiso-alkyl substituted aliphatic chains by Acinetobacter cerificans, Pseudomonoas citronellonis, and Mycobacterium austrafri; or degradation of ternary substituted structures outside of the B-position to the carboxylic group by Mycobacterium austroafricanum, Mycobacterium fortuitum, and Brevibacterium erthyrogenes; or by methyl substitution on the cyclic ring of alicyclic acids. Modification of the branch and cyclic ring of naphthenic acids make naphthenic acids less refractory [5]. Generally, this involves aerobic α-oxidation and B-oxidation pathways [12].
