Identifying the causes of oil sands coke leachate toxicity to aquatic invertebrates

Geochemical conditions influence vanadium, nickel, and molybdenum release from oil sands fluid petroleum coke

Petroleum coke is a potential source of vanadium (V), nickel (Ni), and molybdenum (Mo) to water resources in Athabasca Oil Sands Region (AOSR) of northern Alberta, Canada. Large stockpiles of this bitumen upgrading byproduct will be incorporated into mine closure landscapes and understanding the processes and conditions controlling the release and transport of these transition metals is critical for effective reclamation. We performed a series of laboratory column experiments to quantify V, Ni, and Mo release from fluid petroleum coke receiving meteoric water (MW), oil sands process-affected water (OSPW), and acid rock drainage (ARD) influents. We found that influent water chemistry strongly influences metal release, with variations among metals largely attributed to pH-dependent aqueous speciation and surface reactions. Cumulative V, Ni, and Mo mass release was greatest for columns receiving the low-pH ARD influent. Additionally, cumulative V and Mo mass release were greater in columns receiving OSPW compared to MW influent, whereas cumulative Ni mass release was greater in columns receiving MW compared to OSPW influent. Nevertheless, only a small proportion of total V, Ni, and Mo was released during the experiments, with the majority occurring during the first 10 pore volumes (PVs). This study offers insight into geochemical controls on V, Ni, and Mo release from fluid petroleum coke that supports ongoing development of oil sands mine reclamation strategies for landscapes that contain petroleum coke.

Treatment of oil sands process water using petroleum coke: Field pilot

This is the first large-scale field pilot study that examined the feasibility and effectiveness of petroleum coke (PC), produced by a Fluid Coking Process, as an adsorbent for oil sands process water (OSPW) treatment. The pilot program consisted of an inline series of two reactors (pipeline reactor 1, and batch reactor 2) and lasted for approximately 4 months. The quality of treated OSPW as a function of residence time in the PC deposit under natural climatic conditions was assessed by looking at changes in organic compounds (acid extractable fraction (AEF), dissolved organic carbon (DOC), etc.), vanadium, and other trace element concentrations, major ions, conductivity, total suspended solids (TSS), pH and toxicity. The results indicated that the AEF adsorption by PC followed pseudo-second order kinetics and the overall combined removal efficiency of AEF was greater than 80%. Reactor 1 showed higher AEF removal than Reactor 2. DOC decreased about 50% after 4 weeks of retention in the PC deposit. An increase of vanadium concentration after PC contact indicated that vanadium leaching occurred. However, with increased residence time in the PC deposit, vanadium concentration decreased in the cells and tanks by 42% and 98%, respectively. Filtration through the PC deposit reduced the TSS in OSPW to less than laboratory detectable limits. Unlike untreated OSPW, treated OSPW did not show an acute toxic response based on whole effluent toxicity testing using trout, zooplankton, and bacteria. This study demonstrated that PC adsorption is a potentially commercially viable technology for highly efficient treatment of OSPW.

Reduction of Vanadium(V) by Iron(II)-Bearing Minerals

Fe(II)-bearing minerals (magnetite, siderite, green rust, etc.) are common products of microbial Fe(III) reduction, and they provide a reservoir of reducing capacity in many subsurface environments that may contribute to the reduction of redox active elements such as vanadium; which can exist as V(V), V(IV), and V(III) under conditions typical of near-surface aquatic and terrestrial environments. To better understand the redox behavior of V under ferrugenic/sulfidogenic conditions, we examined the interactions of V(V) (1 mM) in aqueous suspensions containing 50 mM Fe(II) as magnetite, siderite, vivianite, green rust, or mackinawite, using X-ray absorption spectroscopy at the V K-edge to determine the valence state of V. Two additional systems of increased complexity were also examined, containing either 60 mM Fe(II) as biogenic green rust (BioGR) or 40 mM Fe(II) as a mixture of biogenic siderite, mackinawite, and magnetite (BioSMM). Within 48 h, total solution-phase V concentrations decreased to <20 µM in all but the vivianite and the biogenic BiSMM systems; however, >99.5% of V was removed from solution in the BioSMM and vivianite systems within 7 and 20 months, respectively. The most rapid reduction was observed in the mackinawite system, where V(V) was reduced to V(III) within 48 h. Complete reduction of V(V) to V(III) occurred within 4 months in the green rust system, 7 months in the siderite system, and 20 months in the BioGR system. Vanadium(V) was only partially reduced in the magnetite, vivianite, and BioSMM systems, where within 7 months the average V valence state stabilized at 3.7, 3.7, and 3.4, respectively. The reduction of V(V) in soils and sediments has been largely attributed to microbial activity, presumably involving direct enzymatic reduction of V(V); however the reduction of V(V) by Fe(II)-bearing minerals suggests that abiotic or coupled biotic–abiotic processes may also play a critical role in V redox chemistry, and thus need to be considered in modeling the global biogeochemical cycling of V.

Ecological causal assessment of benthic condition in the oil sands region, Athabasca River, Canada

Contaminant loads to rivers of the Canadian oil sands region are linked to industrial and natural sources. To date, biomonitoring studies have been unable to unequivocally assess potential environmental impacts associated with this development. As part of the Joint Alberta-Canada Oil Sands Monitoring initiative, we aimed to assess cumulative effects of anthropogenic activities and exposure to natural bitumen geology on benthic macroinvertebrate assemblages in the lower Athabasca River. We examined associations among macroinvertebrates and environmental correlates, such as nutrients, ions, metals, polycyclic aromatic compounds, and total suspended solids. The study design included sites within and outside the mineable bitumen deposits, within and outside of the active mining and extraction area, and above and below municipal sewage effluents. We predicted observing a negative association between ecological condition of the river and exposure to natural bitumen and oil sands activity. However, contaminant concentrations in water and sediment were far below known toxicity thresholds, and benthic macroinvertebrate assemblages in sites exposed to oil sands mining activities appeared more affected by nutrient enrichment from the MSE than contaminants from mining or natural bitumen. Although sites within the area of intense oil sands activity showed signs of mild environmental stress, assemblage pattern was more strongly associated with MSE nutrient enrichment than to diffuse contamination from either natural bitumen or oil sands mining. Enrichment likely increases food resources available to consumers, thereby potentially masking toxic responses of consumers to contaminants. Current regulations prohibit the direct release of oil sands contaminants to waterways, with diffuse atmospheric deposition of aerial emissions and fugitive dust the main contaminant pathways to freshwaters. As the storage capacity of tailings ponds is reached, this nutrient-contaminant pattern could change if the river receives the proposed direct release of treated oil sands process water. Focused investigation-of-cause studies are required to better assess the consequences of cumulative interactions and ecological effects of nutrients and contaminant exposure in this system.

Multiple Linear Regression Modeling Predicts the Effects of Surface Water Chemistry on Acute Vanadium Toxicity to Model Freshwater Organisms

Multiple linear regression (MLR) modeling has been successfully used to predict how water chemistry variables influence the toxicity of cationic metals to aquatic organisms, but no MLR model exists for vanadium (V). Recent research has indicated that an increase in pH (from 6 to 9), or high concentrations of sodium (473 mg Na⁺ /L), increase V toxicity to Daphnia pulex. In contrast, increases in alkalinity (>100 mg as CaCO₃ ) and sulfate (>100 mg SO₄^2- /L) reduce V toxicity. How these variables influence V toxicity to Oncorhynchus mykiss (rainbow trout) was still unknown. Our results show that increasing pH from 6.2 to 8.9 tended to decrease the 96-h median lethal concentration (LC50) for V toxicity to O. mykiss by 9.6 mg V/L. An alkalinity increase from 71 to 330 mg/L as CaCO₃ tended to increase the 96-h LC50 by 3.3 mg V/L, whereas when SO₄^2- rose from 150 to 250 mg/L, the LC50 significantly increased by 0.3 mg V/L followed by a significant decrease of 1 mg V/L when SO₄^2- was >250 mg/L. Sodium (between 100 and 336 mg/L) showed no effect on V toxicity to O. mykiss. The toxicity patterns for O. mykiss were similar to those observed for D. pulex, except for that of SO₄^2- , potentially indicating different mechanisms of V uptake or regulation in the 2 species. The LC50s and associated water chemistry were combined to develop an MLR model for O. mykiss and D. pulex. Alkalinity and pH modified V toxicity to both species, whereas SO₄^2- influenced V toxicity to D. pulex. Overall, MLR models should be considered for creating new local benchmarks or water quality guidelines for V. Environ Toxicol Chem 2020;39:1737-1745. © 2020 SETAC.

Aqueous Vanadate Removal by Iron(II)-Bearing Phases under Anoxic Conditions

Vanadium contamination is a growing environmental hazard worldwide. Aqueous vanadate (H_xV^VO₄^(3-x)-_(aq)) concentrations are often controlled by surface complexation with metal (oxyhydr)oxides in oxic environments. However, the geochemical behavior of this toxic redox-sensitive oxyanion in anoxic environments is poorly constrained. Here, we describe results of batch experiments to determine kinetics and mechanisms of aqueous H₂V^VO₄^- (100 μM) removal under anoxic conditions in suspensions (2.0 g L^-1) of magnetite, siderite, pyrite, and mackinawite. We present results of parallel experiments using ferrihydrite (2.0 g L^-1) and Fe²⁺_(aq) (200 μM) for comparison. Siderite and mackinawite reached near complete removal (46 μmol g^-1) of aqueous vanadate after 3 h and rates were generally consistent with ferrihydrite, whereas magnetite removed 18 μmol g^-1 of aqueous vanadate after 48 h and uptake by pyrite was limited. Removal during reaction with Fe²⁺_(aq) was observed after 8 h, concomitant with precipitation of secondary Fe phases. X-ray absorption spectroscopy revealed V(V) reduction to V(IV) and formation of bidentate corner-sharing surface complexes on magnetite and siderite, and with Fe²⁺_(aq) reaction products. These data also suggest that V(IV) is incorporated into the mackinawite structure. Overall, we demonstrate that Fe(II)-bearing phases can promote aqueous vanadate attenuation and, therefore, limit dissolved V concentrations in anoxic environments.

Models for the acute and chronic aqueous toxicity of vanadium to Daphnia pulex under a range of surface water chemistry conditions

Alberta's oil sands petroleum coke (PC) generation has in recent years surpassed 10 million tonnes. Petroleum coke has been proposed as an industrial-scale sorbent to reduce concentrations of organic chemicals in oil sands process-affected water (OSPW). However, PC contains up to 1000 mg of vanadium (V) per kg of PC, and during the treatment it leaches from coke reaching levels of up to 7 mg/L in "treated" OSPW. Little information is available on how common water quality variables affect the toxicity of V to aquatic organisms. Here descriptive relationships are presented to describe how site-specific surface water characteristics representative of the Alberta oil sands region influence the toxicity of V to Daphnia pulex. Results revealed that when D. pulex was exposed to an increase in pH, a threshold relationship was found where acute V toxicity increased from a lethal median concentration (LC₅₀) of 1.7 to 1.2 mg V/L between pH 6 and 7 and then levelled off at around 1 mg V/L. When alkalinity (from 75 to 541 mg/L as CaCO₃) and sulphate (from 54 to 394 mg/L) increased, the acute toxicity of V decreased slightly with LC₅₀s changing from 0.6 to 1.6, and from 0.9 to 1.4, respectively. When the length of V exposure was extended (from 2 to 21 d), only an increase of sulphate from 135 to 480 mg/L caused a slight increase in V toxicity from a LC₅₀ of 0.6 to 0.4 mg V/L, the opposite trend seen in the acute exposures. In addition, the influence of two OSPW representative mixtures of increasing sodium and sulphate, and increasing alkalinity and sulphate on V acute toxicity to D. pulex were evaluated; only the mixture of increasing sodium (from 18 to 536 mg/L) and sulphate (from 55 to 242 mg/L) caused a slight decrease in V acute toxicity (LC₅₀ 1.0-2.1 mg V/L). Evidence is presented that variations in surface water chemistry can affect V toxicity to daphnids, although only to a small degree (i.e. within a maximum factor of 2 in all cases evaluated here). These relationships should be considered when creating new water quality guidelines or local benchmarks for V.

Aqueous- and solid-phase molybdenum geochemistry of oil sands fluid petroleum coke deposits, Alberta, Canada

Fluid petroleum coke generated at oil sands operations in the Athabasca Oil Sands Region of northern Alberta, Canada, contains elevated concentrations of molybdenum (Mo) and other metals including nickel (Ni) and vanadium (V). Solid-phase Mo concentrations in fluid petroleum coke are typically 10 to 100 times lower than V and Ni, yet dissolved Mo concentrations in associated pore waters are often comparable with these metals. We collected pore water and solids from fluid petroleum coke deposits in the AOSR to examine geochemical controls on Mo mobility. Dissolved Mo concentrations increased with depth below the water table, reaching maxima of 1.4-2.2 mg L^-1, within a mixing zone between slightly acidic and oxic meteoric water and mildly alkaline and anoxic oil sands process-affected water (OSPW). Dissolved Mo concentrations decreased slightly with depth below the mixing zone. X-ray absorption spectroscopy revealed that Mo(VI) and Mo(IV) species were present in coke solids. The Mo(VI) occurred as tetrahedrally coordinated MoO₄^2- adsorbed via inner- and outer-sphere complexation, and was coordinated in an environment similar to Fe-(hydr)oxide surface complexes. The OSPW likely promoted desorption of outer-sphere Mo(VI) complexes, resulting in higher dissolved Mo concentrations in the mixing zone. The principal Mo(IV) species was MoS₂, which originated as a catalyst added upstream of the fluid coking process. Although MoS₂ is likely stable under anoxic conditions below the mixing zone, oxidative weathering in the presence of meteoric water may promote long-term Mo release.

Nickel geochemistry of oil sands fluid petroleum coke deposits, Alberta, Canada

Nickel (Ni) leaching from oil sands petroleum coke can have toxicological effects on aquatic organisms. However, geochemical controls on Ni release, transport, and attenuation within coke deposits remains limited. We examined the geochemistry of fluid coke and associated pore waters from two deposits at an oil sands mine near Fort McMurray, Alberta, Canada. Synchrotron-based micro-X-ray fluorescence (μXRF) and micro-X-ray absorption near edge structure (μXANES) spectroscopy show that Ni(II)-porphyrin complexes dominate, but inorganic phases including Ni(II)-sulfide and Ni(II)-oxide comprise a minor component of fluid coke. Sequential chemical extractions suggested that sorption–desorption reactions may influence Ni mobility within fluid coke deposits. Although only a small proportion of total Ni (<4%) is susceptible to leaching under environmentally relevant concentrations, dissolved Ni concentrations ( n = 65) range from 2 to 120 μg·L−1 (median 7.8 μg·L−1) within the two deposits and generally decrease with depth below the water table. Pore water Ni concentrations are negatively correlated with pH, but not with dissolved sulfate, bicarbonate, or chloride. Overall, our findings suggest that pore water pH and sorption–desorption reactions are principal controls on dissolved Ni concentrations within oil sands fluid petroleum coke deposits.

Vanadium Geochemistry of Oil Sands Fluid Petroleum Coke

Vanadium has previously been linked to elevated toxicity of leachates derived from oil sands petroleum coke. However, geochemical controls on V mobility within coke deposits remain poorly constrained. Detailed examinations of porewater and solid-phase V geochemistry were therefore performed on oil sands fluid petroleum coke deposits in Alberta, Canada. Sample collection focused on both active and reclaimed deposits, which contained more than 3 × 10⁷ m³ of fluid petroleum coke. Dissolved V concentrations were highest (up to 3.0 mg L^-1) immediately below the water table but decreased rapidly with increasing depth. This trend corresponded to a transition from mildly acidic (pH 6-7) and oxic conditions to mildly alkaline (pH 7-8.5) and anoxic conditions. Scanning electron microscopy (SEM), electron microprobe analysis (EMPA), and micro-X-ray fluorescence (μXRF) mapping revealed coke particles exhibited an internal structure characterized by successive concentric layers. The outer margins of these layers were characterized by elevated V, Fe, Si, and Al concentrations, indicating the presence of inorganic phases. Micro-X-ray absorption near-edge structure (μXANES) spectroscopy revealed that V speciation was dominated by V(IV) porphyrins except at outer margins of layers, where octahedrally coordinated V(III) was a major component. Minor to trace V(V) was also detected within fluid petroleum coke particles.

Toxicity of aqueous vanadium to zooplankton and phytoplankton species of relevance to the athabasca oil sands region

Vanadium (V) is an abundant trace metal present in bitumen from the Athabasca Oil Sands (AOS) region in Alberta, Canada. The upgrading of bitumen can result in the production of large volumes of a carbonaceous material referred to as petroleum coke that contains V at elevated levels compared to the native bitumen. Previous studies have shown that coke has the capacity to leach ecotoxicologically relevant levels of V into water it contacts, yet limited data are available on the toxicity of aqueous V to planktonic organisms. Therefore, this study set out to evaluate the acute and chronic toxicity of V (as vanadate oxyanions) to freshwater zooplankton and phytoplankton species that are either commonly-used laboratory species, or species more regionally-representative of northern Alberta. Four cladoceran (2-d and 21-d tests) and two algal (3-d tests) species were exposed to V to obtain both acute and chronic toxicity estimates. Acute V toxicity (LC50s) ranged from 0.60mgV/L for Ceriodaphnia quadrangula to 2.17mgV/L for Daphnia pulex. Chronic toxicity estimates (EC50s) for cladoceran survival and reproduction were nearly identical within species and ranged from a low of 0.13 to a high of 0.46mgV/L for Daphnia dentifera and D. pulex, respectively. The lack of sublethal V toxicity in daphnia suggests a direct mechanism of toxicity through ion imbalance. Growth inhibition (EC50) of green algae occurred at concentrations of 3.24 and 4.12mgV/L for Pseudokirchneriella subcapitata and Scenedesmus quadricauda, respectively. Overall, cladocerans were more sensitive to V than green algae, with survival of the field-collected D. dentifera being approximately 2.5 to 3.5 times more sensitive to acute and chronic V exposure than the standard test species D. pulex. However, there were no significant differences in V toxicity between the field-collected cladocerans Simocephalus serrulatus and C. quadrangula, compared to the respective standard species D. pulex and Ceriodaphnia dubia. Similarly, there were no significant differences in sensitivity to V in the two algal species evaluated. Based on V concentrations reported in laboratory-generated coke leachates, zooplankton survival could be adversely impacted under conditions of chronic leachate exposure if V concentrations in the environment exceed 0.1mg/L. Furthermore, toxicity thresholds from commonly-used planktonic test species would likely have sufficed for derivation of a V water quality guideline (WQG) for protection of local aquatic communities near oil sands operations, but the new data presented here on V toxicity to more regionally-representative species will strengthen the database for WQG derivation.

Airborne Petcoke Dust is a Major Source of Polycyclic Aromatic Hydrocarbons in the Athabasca Oil Sands Region

Oil sands mining has been linked to increasing atmospheric deposition of polycyclic aromatic hydrocarbons (PAHs) in the Athabasca oil sands region (AOSR), but known sources cannot explain the quantity of PAHs in environmental samples. PAHs were measured in living Sphagnum moss (24 sites, n = 68), in sectioned peat cores (4 sites, n = 161), and snow (7 sites, n = 19) from ombrotrophic bogs in the AOSR. Prospective source samples were also analyzed, including petroleum coke (petcoke, from both delayed and fluid coking), fine tailings, oil sands ore, and naturally exposed bitumen. Average PAH concentrations in near-field moss (199 ng/g, n = 11) were significantly higher (p = 0.035) than in far-field moss (118 ng/g, n = 13), and increasing temporal trends were detected in three peat cores collected closest to industrial activity. A chemical mass-balance model estimated that delayed petcoke was the major source of PAHs to living moss, and among three peat core the contribution to PAHs from delayed petcoke increased over time, accounting for 45-95% of PAHs in contemporary layers. Petcoke was also estimated to be a major source of vanadium, nickel, and molybdenum. Scanning electron microscopy with energy-dispersive X-ray spectroscopy confirmed large petcoke particles (>10 μm) in snow at near-field sites. Petcoke dust has not previously been considered in environmental impact assessments of oil sands upgrading, and improved dust control from growing stockpiles may mitigate future risks.

Influence of inorganic anions on metals release from oil sands coke and on toxicity of nickel and vanadium to Ceriodaphnia dubia

In a previous study it was shown that pH significantly influences the release of metals from oil sands coke, particularly Ni and V which were identified as the cause of coke leachate toxicity. Coke comes in contact with oil sands process water (OSPW) during its transport to and long term storage in reclamation landscapes. However, the influence of dominant inorganic anions present in OSPW (i.e. HCO(3)(-), Cl(-) and SO(4)(2-)) on metals release from coke and on speciation and toxicity of Ni and V, has not been characterized before. Coke was subjected to a 15-d batch leaching process at four levels of HCO(3)(-), Cl(-) and SO(4)(2-) to determine the influence on metals release and speciation. Further, the effects of each of the three anions on Ni and V toxicity, as well as the mixture toxicity of Ni and V, were assessed using the three-brood Ceriodaphnia dubia test. Inorganic anions had a significant influence on the type and amount of metals released from coke. Specifically, sulfate increased the mobilization of cationic metals (e.g. Ni, Fe, Mn and Zn), whereas bicarbonate enhanced the release of oxyanion forming metals (e.g. Al, As, Mo and V) from coke. Chloride had no particular effect on the type and amount of metals released. With respect to toxicity, elevated bicarbonate levels decreased the 7-d Ni IC50 from 6.3 to 2.3 μg L(-1), whereas sulfate showed an ameliorative effect against V toxicity to C. dubia. In combination, Ni and V acted additively at their highest sub-lethal concentrations. Aqueous chemistry and toxicity of Ni and V are discussed with the goal of informing reclamation efforts at the Athabasca oil sands.