Michael D. MacKinnon

Estimating the in situ biodegradation of naphthenic acids in oil sands process waters by HPLC/HRMS

The oil sands industry in Northern Alberta produces large volumes of oil sands process water (OSPW) containing high concentrations of persistent naphthenic acids (NAs; C(n)H(2n+Z)O(2)). Due to the growing volumes of OSPW that need to be reclaimed, it is important to understand the fate of NAs in aquatic systems. A recent laboratory study revealed several potential markers of microbial biodegradation for NAs; thus here we examined for these signatures in field-aged OSPW on the site of Syncrude Canada Ltd. (Fort McMurray, AB). NA concentrations were lower in older OSPW; however parent NA signatures were remarkably similar among all OSPW samples examined, with no discernible enrichment of the highly cyclic fraction as was observed in the laboratory. Comparison of NA signatures in fresh oil sands ore extracts to OSPW in active settling basins, however, suggested that the least cyclic fraction (i.e. Z=0 and Z=-2 homologues) may undergo relatively rapid biodegradation in active settling basins. Further evidence for biodegradation of NAs came from a significantly higher proportion of oxidized NAs (i.e. C(n)H(2n+Z)O(3)+C(n)H(2n+Z)O(4)) in the oldest OSPW from experimental reclamation ponds. Taken together, there is indirect evidence for rapid biodegradation of relatively labile Z=0 and Z=-2 NAs in active settling basins, but the remaining steady-state fraction of NAs in OSPW appear to be very recalcitrant, with half-lives on the order of 12.8-13.6 years. Alternative fate mechanisms to explain the slow disappearance of parent NAs from OSPW are discussed, including adsorption and atmospheric partitioning.

Isolation and characterization of naphthenic acids from Athabasca oil sands tailings pond water

A laboratory bench procedure was developed to efficiently extract naphthenic acids from bulk volumes of Athabasca oil sands tailings pond water (TPW) for use in mammalian oral toxicity testing. This solvent-based procedure involved low solvent losses and a good extraction yield with low levels of impurities. Importantly, labour-intensive centrifugation of source water to remove solids was avoided, allowing processing of much larger volumes of water compared with previous protocols. Naphthenic acids, present at an estimated concentration of 81 mg/l, were procured from 515.5 l of TPW at an overall extraction efficiency of approximately 85%. By using distillation to recover and recycle solvent, a high solvent:water ratio was maintained while actual solvent consumption was limited to 70 ml per liter of water processed. Electrospray ionization mass spectrometry suggested a highly heterogeneous naphthenic acid mixture that exhibited nearly identical proportions of monocyclic, polycyclic, and acyclic acids with molecular weights primarily between 220 and 360. Biphenyls, naphthalenes, and phenanthrene/anthracene were the most prominent impurities detected, but their levels were low (< or = 13 microg/l) even in a concentrated solution of the naphthenic acids (8549 mg/l). Naphthenic acids stored at 4 degrees C at this concentration were stable, exhibiting no significant change in concentration over a 10-month period. This bulk isolation procedure should be useful to others needing to process large volumes of tailings or other source water for the purpose of procuring moderate amounts of naphthenic acids.

Naphthenic acids and surrogate naphthenic acids in methanogenic microcosms.

Naphthenic acids (NAs) are a complex mixture of naturally occurring acyclic and cyclic aliphatic carboxylic acids in petroleum. In the Athabasca oil sands. NAs have been identified as the largest component of dissolved organic matter in the tailings waters from oils sands extraction processes. They are the major contributor to the acute toxicity of the fine tailings wastewaters at the oil sands extraction plants in northeastern Alberta, Canada. In this study, three sources of NAs were studied, including commercially available NAs, those extracted from oil sands process-affected waters, and individual naphthenic-like surrogate compounds. Analysis by gas chromatography-mass spectrometry demonstrated differences between the commercial and extracted NAs. The NAs derived from the process-affected waters showed a short-term inhibition of methanogenesis from H2 or acetate, but with time the populations resumed methane production. It has been postulated that microbial metabolism of the carboxylated side chains of NAs would lead to methane production. The two NA mixtures failed to stimulate methanogenesis in microcosms that contained either oil sands fine tailings or domestic sewage sludge. However, in microcosms with sewage sludge, methanogenesis was stimulated by some surrogate NAs including 3-cyclohexylpropanoic acid at 400-800 mg/L, 5-cyclohexylpentanoic acid at 200 mg/L or 6-phenylhexanoic acid at 200 and 400 mg/L. When added at 200 mg/L to methanogenic microcosms containing fine tailings, 3-cyclohexylpropanoic and 4-cyclohexylbutanoic acids produced methane yields that suggested mineralization of the side chain and the ring.

A statistical comparison of naphthenic acids characterized by gas chromatography–mass spectrometry

Naphthenic acids are complex mixtures of alkyl-substituted acyclic and cycloaliphatic carboxylic acids, with the general chemical formula C(n)H(2n+z)O(2), where n is the carbon number and Z specifies a homologous family. These acids have a variety of commercial uses, including being used as wood preservatives. They are found in conventional and heavy oils, and in the oil sands of northeastern Alberta, Canada. Naphthenic acids are major contributors to the toxicity of tailings waters that result from the oil sands extraction process. Eight naphthenic acids preparations (four from commercial sources and four from the oil sands operations) were derivatized and analyzed by gas chromatography-mass spectrometry. The composition of each mixture was summarized as a three-dimensional plot of the abundance of specific ions (corresponding to naphthenic acids) versus carbon number (ranging from 5 to 33) and Z family (ranging from 0 to -12). The data in these plots were divided into three groups according to carbon number (group 1 contained carbon numbers 5-14, group 2 contained carbon numbers 15-21, and group 3 contained carbon numbers 22-33). A t-test, using arcsine-transformed data, was applied to compare corresponding groups in samples from various sources. Results of the statistical analyses showed differences between various commercial naphthenic acids preparations, and between naphthenic acids from different oil sands ores and tailings ponds. This statistical approach can be applied to data collected by other mass spectrometry methods.

Fathead minnow (Pimephales promelas) reproduction is impaired in aged oil sands process-affected waters.

Large volumes of fluid tailings are generated during the extraction of bitumen from oil sands. As part of their reclamation plan, oil sands operators in Alberta propose to transfer these fluid tailings to end pit lakes and, over time, these are expected to develop lake habitats with productive capabilities comparable to natural lakes in the region. This study evaluates the potential impact of various oil sands process-affected waters (OSPW) on the reproduction of adult fathead minnow (Pimephales promelas) under laboratory conditions. Two separate assays with aged OPSW (>15 years) from the experimental ponds at Syncrude Canada Ltd. showed that water containing high concentrations of naphthenic acids (NAs; >25 mg/l) and elevated conductivity (>2000 μS/cm) completely inhibited spawning of fathead minnows and reduced male secondary sexual characteristics. Measurement of plasma sex steroid levels showed that male fathead minnows had lower concentrations of testosterone and 11-ketotestosterone whereas females had lower concentrations of 17β-estradiol. In a third assay, fathead minnows were first acclimated to the higher salinity conditions typical of OSPW for several weeks and then exposed to aged OSPW from Suncor Energy Inc. (NAs ∼40 mg/l and conductivity ∼2000 μS/cm). Spawning was significantly reduced in fathead minnows held in this effluent and male fathead minnows had lower concentrations of testosterone and 11-ketotestosterone. Collectively, these studies demonstrate that aged OSPW has the potential to negatively affect the reproductive physiology of fathead minnows and suggest that aquatic habitats with high NAs concentrations (>25 mg/l) and conductivities (>2000 μS/cm) would not be conducive for successful fish reproduction.

The ecological effects of naphthenic acids and salts on phytoplankton from the Athabasca oil sands region.

To better elucidate the ecological effects of naphthenic acids and major ions liberated in oil sands development, the summer-time composition of phytoplankton communities in ten water bodies near Fort McMurray (northeastern Alberta) was studied in 1997. The water bodies varied in degree of process water influence, and in age, size and ancillary chemical characteristics. Community biomass of phytoplankton was not systematically related to naphthenic acid or major ion concentrations, even though the higher naphthenate concentrations exceeded published EC50s for acute effects on several different aquatic species. Chlorophyta were frequently dominant, particularly where naphthenate and major ion concentrations were highest. Canonical Correspondence Analysis (CCA) revealed gradients in taxonomic composition at a finer (genus and species) taxonomic level. Despite the simultaneous and uncontrolled variation of other environmental factors, naphthenate and major ion concentrations (as indexed by conductivity) explained a highly-significant 40% of the variation in taxonomic composition. Systems with naphthenates <6.5 mg l(-1) and conductivity <800 PhiS cm(-1) were clustered together near the origin of the CCA plots, suggesting little ecological effect at such concentrations. Taxa associated with elevated naphthenate and/or major ion concentrations were derived from six different algal divisions and included many that were identified as tolerant in previous bioassay experiments. Over the range of concentrations encountered (1.5-45 and 100-3000 mg l(-1) for naphthenates and ions, respectively), CCA indicated that the ecological effect of major ions appeared to be at least as great as that of naphthenates.

Effect of High Salinity Tailings Waters Produced From Gypsum Treatment of Oil Sands Tailings on Plants of the Boreal Forest

Bitumen extraction methods currently in use in the operating oil sands plants produce large volumes of fluid tailings. Ions leached from the ore and added by process chemicals during the extraction process result in tailings waters containing elevated ionic content relative to the non-process-affected waters of the area, in particular the sodium, sulfate, and chloride ions. It is anticipated that the areas requiring reclamation will be affected by this high salinity of the process waters. The objectives of this study were to test the impact of a tailings alternative (consolidated tailings process, based on gypsum treatment of extraction tailings) on the viability of plant species of the northern boreal forest and to determine the relative salt tolerance and suitability of selected plant species for land reclamation. Seedlings were grown for 4 weeks in a greenhouse in solution culture containing mineral nutrients and various dilutions of consolidated tailings water and with Na2SO4 additions (1 g L−1 and 3 g L−1). Of all examined plant species, raspberry and strawberry seedlings were the most susceptible to damage, while the seedlings of white spruce, black spruce and lodgepole pine survived, but showed some effects. In the willow and aspen seedlings, there was a rapid loss of leaves, which were quickly replaced by new, morphologically different leaves. Dogwood and hybrid poplar showed high tolerance to all treatments.

Development of Composite Tailings Technology at Syncrude

During extraction of bitumen from the Athabasca Oil Sands, an aqueous fines suspension, called mature fine tails (MFT), is produced. The geotechnical characteristics of MFT demand long term storage in geotechnically secure containment areas. The composite tailings (CT) process involves mixing a coarse tailings stream with a MFT stream and adding a coagulant to form slurry that rapidly releases water when deposited and binds the MFT in a coarse tailings/MFT deposit. Thus, more of the fines can be stored in a geotechnical soil matrix, which reduces the inventory of fluid-fine tails and enables a wider range of reclamation alternatives. CT process optimization, coupled with public and regulatory consultation at key milestones, has led to a corporate commitment to implement this technology. This paper reviews key aspects of the evolution of the CT process at Syncrude, including segregation, depositional and geotechnical characteristics of CT mixes that are formed with various chemical aids. Some of the treatments discussed include those based on the use of acid, lime, gypsum, alum, and organic polymers as the coagulant aids.

A first approximation kinetic model to predict methane generation from an oil sands tailings settling basin.

A small fraction of the naphtha diluent used for oil sands processing escapes with tailings and supports methane (CH(4)) biogenesis in large anaerobic settling basins such as Mildred Lake Settling Basin (MLSB) in northern Alberta, Canada. Based on the rate of naphtha metabolism in tailings incubated in laboratory microcosms, a kinetic model comprising lag phase, rate of hydrocarbon metabolism and conversion to CH(4) was developed to predict CH(4) biogenesis and flux from MLSB. Zero- and first-order kinetic models, respectively predicted generation of 5.4 and 5.1 mmol CH(4) in naphtha-amended microcosms compared to 5.3 (+/-0.2) mmol CH(4) measured in microcosms during 46 weeks of incubation. These kinetic models also predicted well the CH(4) produced by tailings amended with either naphtha-range n-alkanes or BTEX compounds at concentrations similar to those expected in MLSB. Considering 25% of MLSBs 200 million m(3) tailings volume to be methanogenic, the zero- and first-order kinetic models applied over a wide range of naphtha concentrations (0.01-1.0 wt%) predicted production of 8.9-400 million l CH(4) day(-1) from MLSB, which exceeds the estimated production of 3-43 million l CH(4) day(-1). This discrepancy may result from heterogeneity and density of the tailings, presence of nutrients in the microcosms, and/or overestimation of the readily biodegradable fraction of the naphtha in MLSB tailings.

Degradation and aquatic toxicity of naphthenic acids in oil sands process-affected waters using simulated wetlands

Oil sands process-affected waters (OSPWs) produced during the extraction of bitumen at the Athabasca Oil Sands (AOS) located in northeastern Alberta, Canada, are toxic to many aquatic organisms. Much of this toxicity is related to a group of dissolved organic acids known as naphthenic acids (NAs). Naphthenic acids are a natural component of bitumen and are released into process water during the separation of bitumen from the oil sand ore by a caustic hot water extraction process. Using laboratory microcosms as an analogue of a proposed constructed wetland reclamation strategy for OSPW, we evaluated the effectiveness of these microcosms in degrading NAs and reducing the aquatic toxicity of OSPW over a 52-week test period. Experimental manipulations included two sources of OSPW (one from Syncrude Canada Ltd. and one from Suncor Energy Inc.), two different hydraulic retention times (HRTs; 40 and 400 d), and increased nutrient availability (added nitrate and phosphate). Microcosms with a longer HRT (for both OSPWs) showed higher reductions in total NAs concentrations (64-74% NAs reduction, p<0.05) over the test period, while nutrient enrichment appeared to have little effect. A 96 h static acute rainbow trout (Oncorhynchus mykiss) bioassay showed that the initial acute toxicity of Syncrude OSPW (LC50=67% v/v) was reduced (LC50>100% v/v) independent of HRT. However, EC20s from separate Microtox® bioassays were relatively unchanged when comparing the input and microcosm waters at both HRTs over the 52-week study period (p>0.05), indicating that some sub-lethal toxicity persisted under these experimental conditions. The present study demonstrated that given sufficiently long HRTs, simulated wetland microcosms containing OSPW significantly reduced total NAs concentrations and acute toxicity, but left behind a persistent component of the NAs mixture that appeared to be associated with residual chronic toxicity.