Detection of naphthenic acid uptake into root and shoot tissues indicates a direct role for plants in the remediation of oil sands process-affected water

The effect of rhizosphere pH on removal of naphthenic acid fraction compounds from oil sands process-affected water in a willow hydroponic system

The extraction and processing of bitumen from the oil sands in northern Alberta, Canada generates large volumes of oil sands process-affected water (OSPW). OSPW contains a complex mixture of inorganic and organic compounds, including naphthenic acid fraction compounds (NAFCs) that are of particular concern due to their toxicity to aquatic organisms. Phytoremediation is a cost-effective, scalable approach that has the potential to remove NAFCs from OSPW and reduce OSPW toxicity. Environmental pH influences the chemical form and bioavailability of NAFCs. However, little is known about the influence of pH on the uptake of NAFCs in plant systems. This study sought to elucidate the impact of rhizosphere pH on the uptake of NAFCs using a sandbar willow (Salix interior) hydroponic system. To mimic and maintain the naturally low pH conditions of the root, OSPW solutions in these systems were adjusted to a low pH level (pH 5.0) and their NAFC uptake from solution was compared to that of OSPW at native pH (pH 8.0). Our findings revealed that the lower pH hydroponic systems demonstrated enhanced NAFC removal from solution as determined by LC-MS analysis, where up to 26% of NAFCs were removed from OSPW over 72 h at pH 5.0 compared to 8% removed at pH 8.0. Similarly, analysis of spike-in 13C-labeled NAs demonstrated that the OSPW hydroponic system rapidly removed a relatively labile NA (13C-cyclohexane carboxylic acid) from solution at both pH levels, whereas near complete removal of a recalcitrant NA (13C-1-adamantane carboxylic acid) was observed in pH 5.0 solutions only. These results provide insight into the importance of rhizosphere pH on efficient NAFC uptake by plant root systems. Further research will determine whether OSPW phytoremediation efficiency can be enhanced using field treatment conditions that promote low rhizosphere pH levels.

Phytoremediation of metals in oil sands process affected water by native wetland species

Process affected water and other industrial wastewaters are a major environmental concern. During oil sands mining, large amounts of oil sands process affected water (OSPW) are generated and stored in ponds until reclaimed and ready for surface water discharge. While much research has focused on organics in process waters, trace metals at high concentrations may also pose environmental risks. Phytoremediation is a cost effective and sustainable approach that employs plants to extract and reduce contaminants in water. The research was undertaken in mesocosm scale constructed wetlands with plants exposed to OSPW for 60 days. The objective was to screen seven native emergent wetland species for their ability to tolerate high metal concentrations (arsenic, cadmium, copper, chromium, copper, nickel, selenium, zinc), and then to evaluate the best performing species for OSPW phytoremediation. All native plant species, except Glyceria grandis, tolerated and grew in OSPW. Carex aquatilis (water sedge), Juncus balticus (baltic rush), and Typha latifolia (cattail) had highest survival and growth, and had high metal removal efficiencies for arsenic (81-87 %), chromium (78-86 %), and cadmium (74-84 %), relative to other metals; and greater than 91 % of the dissolved portions were removed. The native plant species were efficient accumulators of all metals, as demonstrated by high root and shoot bioaccumulation factors; root accumulation was greater than shoot accumulation. Translocation factor values were greater than one for Juncus balticus (chromium, zinc) and Carex aquatilis (cadmium, chromium, cobalt, nickel). The results demonstrate the potential suitability of these species for phytoremediation of a number of metals of concern and could provide an effective and environmentally sound remediation approach for wastewaters.

Comprehensive characterization of organics in oil sands process water in constructed mesocosms utilizing multiple analytical methods

This study aims to provide a thorough characterization of dissolved organics in oil sands process water (OSPW) in field-based aquatic mesocosms at both molecular and bulk measurement levels using multiple analytical methods. In a 3-year outdoor mesocosm experiment, the analysis of naphthenic acid (NA) species was conducted using ultra-performance liquid chromatography time-of-flight mass spectrometry (UPLC-TOFMS). The results revealed the removal of both total NAs (38% and 35%) and classical NAs (O2-NAs, 58% and 49%) in undiluted and half-diluted OSPW, respectively. The increased ratios of oxidized NAs (O3-O6 NAs) to classical NAs suggested a transformation trend. The results also indicated that O2-NAs with higher carbon number and lower double bond equivalent (DBE) were more easily degraded in the mesocosm systems. Biomimetic extraction using solid-phase microextraction (BE-SPME) measurement displayed 26% (undiluted OSPW) and 30% (half-diluted OSPW) decrease in total bioavailable organics over 3 years. Naphthenic acids fraction compounds (NAFCs) obtained by liquid-liquid extraction (LLE) were also determined using Fourier transform infrared spectroscopy (FTIR). Reduction in acute toxicity for undiluted (43%) and half-diluted (26%) OSPW was observed over 3 years, which are well correlated with the decreases of NAs and BE-SPME concentrations. Moreover, BE-SPME values were found to be linearly correlated with total NAs concentrations (r = 0.96) and NAFCs (r = 0.96). Additionally, the linear relationships of individual O2-O6 NA species and BE-SPME concentrations unveiled the changes in the relative abundances of O2-O6 NA species in total bioavailable organics over time in the mesocosms. The present study has provided comprehensive insights by integrating various analytical methods, contributing valuable information for assessing the effectiveness of aquatic mesocosm systems in studying the temporal changes of organics in OSPW.

Low adsorption affinity of athabasca oil sands naphthenic acid fraction compounds to a peat-mineral mixture

Much of the toxicity in oil sands process-affected water in Athabasca oil sands tailings has been attributed to naphthenic acids (NAs) and associated naphthenic acid fraction compounds (NAFCs). Previous work has characterized the environmental behaviour and fate of these compounds, particularly in the context of constructed treatment wetlands. There is evidence that wetlands can attenuate NAFCs in natural and engineered contexts, but relative contributions of chemical, biotic, and physical adsorption with sequestration require deconvolution. In this work, the objective was to evaluate the extent to which prospective wetland substrate material may adsorb NAFCs using a peat-mineral mix (PMM) sourced from the Athabasca Oil Sands Region (AOSR). The PMM and NAFCs were first mixed and then equilibrated across a range of NAFC concentrations (5-500 mg/L) with moderate ionic strength and hardness (∼200 ppm combined Ca2+ and Mg2+) that approximate wetland water chemistry. Under these experimental conditions, low sorption of NAFCs to PMM was observed, where sorbed concentrations of NAFCs were approximately zero mg/kg at equilibrium. When NAFCs and PMM were mixed and equilibrated together at environmentally relevant concentrations, formula diversity increased more than could be explained by combining constituent spectra. The TOC present in this PMM was largely cellulose-derived, with low levels of thermally recalcitrant carbon (e.g., lignin, black carbon). The apparent enhancement of the concentration and diversity of components in PMM/NAFCs mixtures are likely related to aqueous solubility of some PMM-derived organic materials, as post-hoc combination of dissolved components from PMM and NAFCs cannot replicate enhanced complexity observed when the two components are agitated and equilibrated together.

Evaluations of Weathering of Polar and Nonpolar Petroleum Components in a Simulated Freshwater-Oil Spill by Orbitrap and Fourier Transform Ion Cyclotron Resonance Mass Spectrometry

The comprehensive chemical characterization of crude oil is important for the evaluation of the transformation and fate of components in the environment. Molecular-level speciation of naphthenic acid fraction compounds (NAFCs) was investigated in a mesoscale spill tank using both negative-ion electrospray ionization (ESI) Orbitrap mass spectrometry (MS) and positive-ion atmospheric pressure photoionization Fourier transform ion cyclotron resonance mass spectrometry (APPI-FT-ICR-MS). Both ionization techniques are coupled to high-resolution mass spectrometric detectors (ESI: Orbitrap MS; APPI: FT-ICR-MS at 9.4 T), enabling insight into the behavior and fate of petrogenic compounds during a simulated freshwater crude oil spill. Negative-ion ESI Orbitrap-MS reveals that oxygen-containing (Ox) classes are detected early in the spill, whereby species with more oxygen per molecule evolve later in the simulated spill. The O2-containing species gradually decreased in relative abundance, while O3 and O4 species increased in relative abundance throughout the simulated spill, which could correspond to a relative degree of oxygen incorporation. Nonpolar speciation by positive-ion APPI 9.4 T FT-ICR-MS allowed for the identification of water-soluble nonpolar and less polar acidic species. Molecular-level graphical representation of elemental compositions derived from simulated spill water-soluble and oil-soluble species suggest that biological activity is the primary degradation mechanism and that biodegradation was the dominant mechanism based on the negative-ion ESI Orbitrap-MS results.

Microbial degradation of naphthenic acids using constructed wetland treatment systems: metabolic and genomic insights for improved bioremediation of process-affected water

Naphthenic acids (NAs) are a complex mixture of organic compounds released during bitumen extraction from mined oil sands that are important contaminants of oil sands process-affected water (OSPW). NAs can be toxic to aquatic organisms and, therefore, are a main target compound for OSPW. The ability of microorganisms to degrade NAs can be exploited for bioremediation of OSPW using constructed wetland treatment systems (CWTS), which represent a possible low energy and low-cost option for scalable in situ NA removal. Recent advances in genomics and analytical chemistry have provided insights into a better understanding of the metabolic pathways and genes involved in NA degradation. Here, we discuss the ecology of microbial NA degradation with a focus on CWTS and summarize the current knowledge related to the metabolic pathways and genes used by microorganisms to degrade NAs. Evidence to date suggests that NAs are mostly degraded aerobically through ring cleavage via the beta-oxidation pathway, which can be combined with other steps such as aromatization, alpha-oxidation, omega-oxidation, or activation as coenzyme A (CoA) thioesters. Anaerobic NA degradation has also been reported via the production of benzoyl-CoA as an intermediate and/or through the involvement of methanogens or nitrate, sulfate, and iron reducers. Furthermore, we discuss how genomic, statistical, and modeling tools can assist in the development of improved bioremediation practices.

Evaluating the attenuation of naphthenic acids in constructed wetland mesocosms planted with Carex aquatilis

Surface oil sands mining and extraction in northern Alberta's Athabasca oil sands region produce large volumes of oil sands process-affected water (OSPW). OSPW is a complex mixture containing major contaminant classes including trace metals, polycyclic aromatic hydrocarbons, and naphthenic acid fraction compounds (NAFCs). Naphthenic acids (NAs) are the primary organic toxicants in OSPW, and reducing their concentrations is a priority for oil sands companies. Previous evidence has shown that constructed wetland treatment systems (CWTSs) are capable of reducing the concentration of NAs and the toxicity of OSPW through bioremediation. In this study, we constructed greenhouse mesocosms with OSPW or lab process water (LPW) (i.e., water designed to mimic OSPW minus the NAFC content) with three treatments: (1) OSPW planted with Carex aquatilis; (2) OSPW, no plants; and (3) LPW, no plants. The OSPW-C. aquatilis treatment saw a significant reduction in NAFC concentrations in comparison to OSPW, no plant treatments, but both changed the distribution of the NAFCs in similar ways. Upon completion of the study, treatments with OSPW saw fewer high-molecular-weight NAs and an increase in the abundance of O3- and O4-containing formulae. Results from this study provide invaluable information on how constructed wetlands can be used in future remediation of OSPW in a way that previous studies were unable to achieve due to uncontrollable environmental factors in field experiments and the active, high-energy processes used in CWTSs pilot studies.

Site-specific spatiotemporal occurrence and molecular congener distributions of naphthenic acids in Athabasca oil sands wetlands of Alberta, Canada

The Athabasca oil sands region (AOSR) of Alberta, Canada is notable for its considerable unconventional petroleum extraction projects, where bitumen is extracted from naturally-occurring oil sands ore. The large scale of these heavy crude oil developments raises concerns because of their potential to distribute and/or otherwise influence the occurrence, behaviour, and fate of environmental contaminants. Naphthenic acids (NAs) are one such contaminant class of concern in the AOSR, so studies have examined the occurrence and molecular profiles of NAs in the region. We catalogued the spatiotemporal occurrence and characteristics of NAs in boreal wetlands in the AOSR over a 7-year period, using derivatized liquid chromatography-tandem mass spectrometry (LC-MS/MS). Comparing median concentrations of NAs across these wetlands revealed a pattern of NAs suggesting NAs in surface waters derived from oil sands deposits. Opportunistic wetlands that formed adjacent to reclaimed overburden and other reclamation activities had the highest concentrations of NAs and consistent patterns suggestive of bitumen-derived inputs. However, similar patterns in the occurrence of NAs were also observed in undeveloped natural wetlands located above the known surface-mineable oil sands deposit that underlies the region. Intra-annual sampling results along with inter-annual comparisons across wetlands demonstrated that differences in the spatial and temporal NA concentrations were dependent on local factors, particularly when naturally occurring oil sands ores were observed in the wetland or drainage catchment.