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Jan 10, 2017 - Fourier Transform Ion Cyclotron Resonance Mass Spectrometry. Characterization of Athabasca Oil Sand Process-Affected Waters. Incubated ...
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Fourier Transform Ion Cyclotron Resonance Mass Spectrometry Characterization of Athabasca Oil Sand Process-Affected Waters Incubated in the Presence of Wetland Plants Chukwuemeka Ajaero,*,†,‡ Dena W. McMartin,*,† Kerry M. Peru,‡ Jon Bailey,‡ Monique Haakensen,§ Vanessa Friesen,§ Rachel Martz,§ Sarah A. Hughes,∥ Christine Brown,⊥ Huan Chen,# Amy M. McKenna,# Yuri E. Corilo,# and John V. Headley‡ †

Environmental Systems Engineering, University of Regina, 3737 Wascana Parkway, Regina, Saskatchewan S4S 0A2, Canada Watershed Hydrology and Ecology Research Division, Water Science and Technology Directorate, Environment and Climate Change Canada, 11 Innovation Boulevard, Saskatoon, Saskatchewan S7N 3H5, Canada § Contango Strategies, Limited, 15-410 Downey Road, Saskatoon, Saskatchewan S7N 4N1, Canada ∥ Shell Health−Americas, One Shell Plaza, 910 Louisiana, Houston, Texas 77002, United States ⊥ Shell Canada, Ltd. 400 − Fourth Ave S.W., P.O. Box 100, Station M, Calgary, Alberta T2P 2H5, Canada # National High Magnetic Field Laboratory, Florida State University, 1800 E. Paul Dirac Dr., Tallahassee, Florida United States ‡

S Supporting Information *

ABSTRACT: Naphthenic acid fraction compounds (NAFCs) are naturally present in the oil sand. These compounds become integrated into the oil sands process-affected water (OSPW) during the bitumen extraction process. NAFCs have been identified as causing toxicity in the OSPW to aquatic organisms. Water treatment technologies that are largely passive, such as constructed treatment wetlands, are a sought-after technology for the degradation of NAFCs in aquatic environments, partly because of their low energy intensity. However, it can be challenging to accurately assess the performance regarding decreased NAFC concentration and biodegradation characteristics in water samples that have been exposed to such systems. This is due to interferences of biological products such as fatty acids and humic-like materials, which may give false-positive information on NAFCs estimation with conventional analytical sample cleanup methods such as liquid−liquid extraction (LLE). It is recognized that this same issue exists when attempting to characterize NAFCs in natural wetlands for environmental monitoring purposes and, therefore, an analytical method that can remove background interferences in water samples is desirable on several fronts. Studies were thus conducted to develop and compare methods for NAFC isolation in an experimental wetland setting. A controlled greenhouse experiment was conducted with sedge (Carex aquatilis), bulrush (Schoenoplectus acutus), and cattail (Typha latifolia) grown in OSPW. Two methodsthe Isolute Biotage ENV+ SPE method and a new weak anion exchange (WAX SPE)were assessed for their ability to isolate, clean up, and concentrate NAFCs in OSPW and municipal tap water (control) that were exposed to samples of plants and associated microbes. Negative-ion-electrospray ionization Fourier transform ion cyclotron resonance mass spectrometry (ESI-FT-ICR-MS) data revealed that WAX SPE method has better relative enhancement (5%−50%) of O2 classes in OSPW exposed to wetland plants, compared to ENV+ SPE method. The WAX SPE method is a good candidate for the isolation of organic compounds in complex environmental matrices and supports the development of analytical protocols for isolation and characterization of NAFCs. Compound classes from negative-ion ESI-FTICR-MS data were further probed using principal component analysis (PCA) to evaluate the NAFCs that are potential indicators of efficiency of engineered wetlands for monitoring in future wetland studies. Given the PCA results, future wetland NAFC degradation investigations should target O2 classes for detailed evaluation of the performance of treatment systems, or measurement of the fate and distributions of NAFCs in natural wetlands exposed to OSPW.

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INTRODUCTION The extraction of bitumen from oil sands in the Athabasca region of northeastern Alberta, Canada generates considerable volumes of oil sands process-affected water (OSPW), which is stored on site in tailings ponds. For every cubic meter of oil produced from bitumen extraction process, ∼1.5 m3 of fresh water is used (including water from the Athabasca River, groundwater aquifers, precipitation, and contained surface water runoff) which generates the OSPW and ∼85%1 of the OSPW is recycled for reuse in the extraction process. Although currently no oil sands mining operations are approved to © XXXX American Chemical Society

discharge their treated OSPW to the environment prior to mine closure, many operators are evaluating technologies to treat OSPW for the safe return to the natural environment during operations. This will allow operators to minimize their footprint by reducing the growth of tailings ponds as mining advances, and to better manage water quantity and quality issues on site. Received: October 11, 2016 Revised: December 20, 2016 Published: January 10, 2017 A

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Energy & Fuels Process-related problems in the refinery such as corrosion2,3 and toxicity4−6 are previously associated with naphthenic acids (NAs). Characterization of OSPW is generally based on the salt content, metals, and organic compounds (primarily naphthenic acid fraction compounds (NAFCs)).7,8 NAFCs are diverse heteroatom compounds in OSPW with the general formula CnH2n+zOxNβSγ, where n is the carbon number, z is a negative, even-numbered integer that indicates hydrogen displacement due to formation of rings or double bonds, and x, β, and γ are the oxygen, nitrogen, and sulfur numbers, respectively.9,10 The sum of classical NAs (CnH2n+zO2), oxy-NAs, and S- and Ncontaining NAs constitutes NAFCs. The name NAs has been widely used to designate these compounds in the literature to date. The application of high-resolution instruments such as Fourier transform ion cyclotron resonance mass spectrometry (FT-ICR-MS) in OSPW analysis have revealed that 2). This depth in composition was well-articulated by Grewer et al.11 There have been recommendations for common designation of NAs because of their compositional heterogeneity. Nomenclatures such as acid-extractable organics (AEOs), oil sands tailings water acid-extractable organics (OSTWAEO),11 and NAFCs have been suggested.12 In this paper, the term NAFC is used to designate this diverse group of compounds in OSPW. Several mass spectrometry methods have been applied for the analysis of NAFCs in environmental samples, such as gas chromatography−mass spectrometry (GC-MS),13 two-dimensional gas chromatography−mass spectrometry (2D-GC-MS or GC×GC-MS)14,15 and liquid chromatography−mass spectrometry (LC-MS).16,17 More recently, FT-ICR-MS10,18 and Orbitrap-MS19,20 are gaining popularity as analytical tools in the characterization of OSPW constituents. FT-ICR-MS is a powerful tool for molecular-level characterization of complex samples such as crude oils and OSPW. The unique capability of FT-ICR-MS for wider compositional coverage in the analysis of OSPW polar compounds employing high mass accuracy and ultrahigh-resolution power has been reported.9,12,21−23 Principal Component Analysis (PCA) has been used extensively in environmental studies for data analysis. PCA is well-suited for assessment of the compositional differences between samples. For instance, Headley et al. employed PCA to determine the compositional variations in oil sand environmental samples using data from negative-ion ESI Orbitrap MS.12 The correlation of NA sources into natural waters close to the oil sand industry using PCA and negative-ion electrospray ionization−liquid chromatography−time-of-flight mass spectrometry (ESI-LC-TOF-MS) data has been previously reported.24 For the latter, PCA plots showed that 57.6% of the variance correlated to different NA sources. PC1 revealed that NAs in the background samples were mainly fatty acids from biological sources, while PC2 showed that the second group of NAs was primarily contributed from oil sand industry. PCA and data from negative-ion ESI-FT-ICR-MS were successfully evaluated for the compositional differences in environmental samples from the Athabasca oil sand area.25 The polar organic compounds found in the Athabasca Rivers, lakes, and tributaries were likely from similar sources and showed markedly different composition from snow samples. PCA was successfully applied in distinguishing NA sources from OSPW and river waters. The variable significantly contributing to the differences between the samples were based on the molecular structure of their dominant NA classes. The NA from river

water were mainly acyclic NAs while those of OSPW are primarily two-ring and three-ring compounds.11 Similarly, PCA was used as diagnostic tool to unravel the compositional differences in tailing pond OSPW from two industrial sources and surface water samples using datasets from Fourier transform ion cyclotron resonance mass spectrometry (FTICR-MS). It was observed that the ratio of the sulfur-containing species were characteristic of OSPW from tailings.22 The NAFCs in OSPW are also of concern, because of their potential persistence and toxicity.26 The development and application of OSPW treatment technologies are primarily driven by the aquatic toxicity associated with NAFCs. Approaches that have been evaluated for the treatment of NAFCs include ozonation27,28 and ultraviolet/visible light (UV/vis) radiation.29 Moreover, passive water treatment such as constructed treatment wetlands are sought-after water treatment technologies for NAFCs, since it has been demonstrated that wetland plants are capable of mitigating the toxicity of NAs (likely through the activity of microbes associated with root zones).30 However, it can be challenging to accurately assess the performance of biological systems, in terms of decreased NAFC concentration, and biodegradation characteristics of the system, because of interferences with conventional NAFC detection methods. The challenges arise from interferences of biological products such as fatty acids and humic-like materials which may give false positive information on NAFC estimation with conventional extraction methods such as liquid−liquid extraction (LLE). Furthermore, the conventional methods of isolation may be biased toward NAFC compounds with higher carbon chains and molecular weight.31 In recognition that this same issue exists for characterization of NAFCs in natural wetlands, an analytical method that can remove background interferences is desirable on several fronts. To this extent, a study was conducted with nursery wetland plants in a controlled greenhouse setting. Sedge (Carex aquatilis), bulrush (Schoenoplectus acutus), and cattail (Typha latifolia) were planted in pails with OSPW or tap water (control), and water was characterized for NAFCs using different extraction methods after ∼100 days of plant growth in the static system. Microbes were acclimated in OSPW prior to use in the study. This study evaluated the ability of two solid-phase extraction (SPE) methodsIsolute Biotage ENV+ SPE method and a new WAX SPEto isolate, clean up, and concentrate NAFCs associated with water samples of plants grown in OSPW and municipal tap water. NAFCs may likely be preferentially isolated in the WAX SPE method due to the high ionic character of the column and analytes at the pH of extraction (pH ∼7.0). While information is available on the use of ENV+ for the isolation, quantification, and characterization of NAFCs in environmental samples, to our knowledge, information is lacking on the application of WAX SPE analytical methods for the isolation and characterization of NAFCs in wetlands samples. We will demonstrate that the WAX SPE is useful for the isolation of organic compounds in complex environmental and biological samples and support the advancement of analytical techniques for the isolation and characterization of organic compounds such as NAFCs found in complex matrices. Furthermore, it will be demonstrated that PCA of the negativeion ESI-FT-ICR-MS data can establish the identity of principal classes of compounds in OSPW associated with plants, for monitoring the performance of future treatment wetlands. B

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Figure 1. Measurements total plant growth over time for cattail (Typha latifolia), hardstem bulrush (Schoenoplectus acutus), and aquatic sedge (Carex aquatilis) in OSPW and tap water.

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(Phenomenex, Torrance, CA, USA). The Phenomenex Strata-XAW (200 mg, 33 μm polymeric weak anion solid phase adsorbent) cartridge was conditioned with 5 mL of methanol and equilibrated with 5 mL of Milli-Q H2O. The 10 mL samples (adjusted to pH ∼7.0 with 1% formic acid) were passed through the cartridge at 1 drop/s. The cartridges were washed with 2 mL of 25 mM ammonium acetate and 1 mL methanol. The cartridges were then dried for 5 min under house vacuum. The analytes were subsequently eluted with 5 mL methanol/5% ammonium hydroxide. The eluents were collected in 20 mL test tubes and brought to complete dryness under a stream of nitrogen and water bath temperature of ∼40 °C. The samples were redissolved with 1 mL of 50%/50% acetonitrile/Milli-Q H2O + 0.1% ammonium hydroxide and vortexed for ∼30 s. The samples were transferred into 2 mL amber vials for analysis. The ENV+ SPE method used was described previously.33 ISOLUTE ENV+ is a tradename for sorbent used in SPE of polar organic compounds from environmental samples. Sample Preparation. All solvents were high-performance liquid chromatography (HPLC) grade (Sigma−Aldrich Chemical Co., St. Louis, MO) and OSPW extracts were run with negative-ion electrospray ionization. Prior to FT-ICR mass spectral analysis, dried extracts were reconstituted in 50:50 acetonitrile/methanol with a final concentration range of 100−500 μg/L. For negative-ion electrospray, 1% ammonium hydroxide (by volume) was added to increase deprotonation. Instrumentation: ESI Source. The sample solution was infused via a microelectrospray source34 (50 μm i.d. fused silica emitter) at 400 nL/min by a syringe pump. Typical conditions for negative ion formation were as follows: emitter voltage, −2.5 kV; tube lens, −250 V; and heated metal capillary current, 7 A. OSPW extracts were analyzed with a custom-built FT-ICR mass spectrometer35 equipped with a 9.4 T horizontal 220-mm-bore-diameter superconducting solenoid magnet operated at room temperature, and a modular ICR data station (Predator) facilitated instrument control, data acquisition, and data analysis.36 Ions generated at atmospheric pressure were accumulated in an external linear quadrupole ion trap for 300−700 ms and transferred by radio-frequency (rf)-only quadrupoles (2.0 MHz and 255 Vp‑p amplitude) to the ICR cell.37 ICR time-domain transients were collected from a seven-segment open cylindrical cell with capacitively coupled excitation electrodes,38 based on the Tolmachev configuration.39,40 One hundred individual transients of 5.8−6.1 s duration collected for OSPW extracts were averaged, apodized with a

MATERIALS AND METHODS

The OSPW used in this study were obtained from Shell’s Muskeg River Mine external tailings facility and brought to Contango Strategies, Ltd. (Saskatoon, SK, Canada) from the Athabasca oil sand region of Alberta, Canada on July 15, 2015 in a 1000 L plastic tote. The wetland plants were from a boreal ecozone, which is similar to that of Fort McMurray. The emergent macrophytes investigated are commonly known as cattail (Typha latifolia), hardstem bulrush (Schoenoplectus acutus), and aquatic sedge (Carex aquatilis). Aquatic mosses (bryophytes) were also used in some of the trials. Pea gravel was chosen as the substrate to avoid sorption of the OSPW to organic material that might be present in other types of soils. Plant Growth Conditions. Six 19-L pails were each filled with ∼9 L of washed pea gravel, for a total depth of 18 cm. Bulrush, cattails, and sedges were separately planted four plants per pail, in duplicate to allow for one pail of OSPW and a control pail of dechlorinated tap water for each plant type. Aquatic moss, in a clump ∼5 cm in diameter, was added to the cattail and bulrush pails. One pail of each plant type was filled with 3.5 L of 50% OSPW, 50% tap water mixture sourced from Saskatoon, while the duplicate pails for each plant type were filled with 3.5 L of tap water. This volume of liquid resulted in a water depth of 1.9−2.5 cm above the gravel and was selected to accommodate plant growth. Plants were trimmed to the height of the water after planting, to enable monitoring of growth through the trial. The trial was setup in the greenhouse facility at Contango, at an indoor temperature of 20 °C. Pails were monitored weekly over a period of 105 days to evaluate plant growth and water or water/OSPW mixture was added as needed to maintain a water depth similar to the original water depth of 1.9−2.5 cm (based on volumes lost through evapotranspiration). Weekly monitoring included measuring plant heights (from the surface of the water) and recording observations. A volume of 120 mL each of the OSPW and tap water plant derived aqueous samples sets were collected for analysis 105 days after setup. Therefore, the samples of OSPW analyzed in this study were not a single batch of OSPW incubated for 105 days with the vegetation, but rather, these samples represent water where OSPW was added multiple times over this period to pails that contain plants to replenish as per evapotranspiration losses. Extraction and Analysis. For the WAX SPE method, the samples were extracted based on a modified previously described method,32 using the Phenomenex Strata-XAW solid-phase extraction cartridge C

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Figure 2. Class distribution for selected classes from OSPW (OS) incubated with cattail plants (C1) generated by negative-ion ESI-FT-ICR-MS data with WAX and ENV+ SPE methods. The relative standard error of the relative abundances for the NAs, oxy-NAs, and heteroatom NAs distributions is 15%.

Figure 3. Class distribution for selected classes from OSPW (OS) incubated with bulrush plants (B1) generated by negative-ion ESI-FT-ICR-MS data with WAX and ENV+ SPE methods. The relative standard error of the relative abundances for the NAs, oxy-NAs, and heteroatom NAs distributions is 15%. appropriate linear combination of real and imaginary FT spectra to recover the absorption-mode display. The absorption-mode spectral resolving power is higher than the magnitude-mode resolving power, by a factor of up to 2. Frequency-to-m/z calibration and data analysis are subsequently performed, as for magnitude-mode display. Mass Calibration and Data Analysis. ICR frequencies were converted to ion masses based on the quadrupolar trapping potential approximation.44,45 Internal calibration of mass spectra was performed with abundant homologous alkylation series whose members differ in mass by integer multiples of 14.01565 Da (mass of a CH2 unit).46−49 Data were analyzed and peak lists generated with PetroOrg software50 for all mass spectral signals of >6σ root mean square (RMS) noise. Peak lists were generated for all mass spectra by use of a single set of constraints (C = 1−100, H = 4−200, N = 0−2, O = 0−30, S = 0−0 or 2, DBE = 0−30) or multiple sets of constraints applied in series. Principal Component Analysis. PCA was performed with PetroOrg software using the percent relative abundances for the heteroatomic classes obtained from FT-ICR-MS of selective OSPW samples. The deconvolution method used was the nonlinear iterative

Hanning weight function, and zero-filled once prior to fast Fourier transformation (FFT). For all mass spectra, the achieved spectral resolving power approached the theoretical limit over the entire mass range of 150−800 (e.g., average resolving power, m/Δm50%, where Δm50% is the mass spectral peak full width at half-maximum peak height (∼1 000 000−1 300 000 for absorption mode at m/z 500 for all mass spectra)). Broadband Phase Correction. Because of increased complexity at higher m/z, broadband phase correction41,42 was applied to the entire mass spectrum for the SPE extracts to increase the resolution of isobaric species as previously described.43 Briefly, Fourier transformation of a discrete time-domain dataset yields real and imaginary frequency-domain spectraRe(ω) and Im(ω), respectivelythat are linear combinations of the absorption- and dispersion-mode components, A(ω) and D(ω), respectively. Therefore, FT-ICR spectra are conventionally displayed in magnitude (absolute-value) mode, M(ω) = {[Re(ω)]2 + [Im(ω)]2}1/2. As described in detail elsewhere,41,42 it is possible to determine the phase angle, φ = tan−1[Im(ω)/Re(ω)], for each peak, and then construct the D

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Figure 4. Class distribution for selected classes from OSPW (OS) incubated with sedge plants (S1) generated by negative-ion ESI-FT-ICR-MS data with WAX and ENV + SPE methods. The relative standard error of the relative abundances for the NAs, oxy-NAs, and heteroatom NAs distributions is 15%.

Figure 5. Class distribution for selected classes from initial OSPW (i-OSPW) prior to incubating with wetland plants generated by negative-ion ESIFT-ICR-MS data with WAX and ENV+ SPE methods. The relative standard error of the relative abundances for the NAs, oxy-NAs, and heteroatom NAs distributions is 15%. partial least-squares (NIPALS) method. Data were visualized using a principal component (PC) score plot of the first two principal components (PC1 and PC2 with 98.88% summed explained variance). Each point in the score plot represents an OSPW sample (extracted using the WAX SPE method). The loading plot represented major heteroatom classes in the samples and was used to identify the heteroatom classes that contributed most to the variations among samples in the score plot.

beginning of November when new shoots began to sprout (Figure 1). The total water and OSPW mixture added to the pails throughout the trials mirrored the growth of plants, with sedges experiencing the most growth and having the most OSPW mixture or tap water added to replenish amounts that had been removed from evapotranspiration. These results suggest that all three of these plant species are potential candidates for wetland treatment system designs for OSPW (pending water chemistry and composition), since they were capable of growing in the OSPW to a similar extent as in the tap water controls, with sedges being the best candidate, based on growth in this study. The class analysis from negative-ion ESI-FT-ICR-MS for OSPW associated with wetland plant samples are displayed in Figures 2−4. The data showed that Ox (x = 2−4) classes are generally the most abundant classes found in all samples. The profiles of the heteroatom class distribution for the WAX SPE and ENV+ SPE isolation methods are identical. However, the Ox (x = 2−4) compounds have increased relative abundance with the WAX SPE method, compared to the ENV+ SPE method. For example, the ESI-FT-ICR-MS data revealed that

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RESULTS AND DISCUSSION During the plant acclimation test, no deleterious growth effects were observed in OSPW, in comparison to tap water controls. The trial revealed that sedges had the most growth, compared to other plant species, and also grew better in the OSPW mixture than in tap water (Figure 1). Sedges had the most growth, growing the longest and the fastest, with growth leveling off after ∼60 days. The bulrush grew more in the OSPW mixture than in tap water pails, with maximum growth achieved in both by 46 days. The bulrush and cattail planted in tap water had similar total growth to those grown exposed to OSPW. The cattail pails had growth at the beginning of the trial in the first 15 days, and then growth leveled off until the E

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Figure 6. Isoabundance contour plots of DBE versus carbon number for O2 classes derived from negative-ion ESI-FT-ICR-MS data with WAX and ENV+ SPE methods for (a) cattails in tap water and OSPW, (b) sedges in tap water and OSPW, and (c) bulrushes in tap water and OSPW with WAX and ENV+ SPE methods. [Abbreviations legend: TW, tap water; OS, OSPW; B1, bulrush; C1, cattail; and S1, sedge.]

likely candidates for the microbial oxidation of the classical NAs (x = 2). The isoabundance contour plots of relative-abundance weighted average double bond equivalents (DBE) and carbon number for the O2 class of compounds in tap water and OSPW associated with wetland plants obtained from the two extraction methods are shown in Figure 6. The isoabundance color contour plot of DBE versus carbon number provides information on the degree of alkylation and aromaticity (chemical structure) in a particular heteroatom class. The most abundant O2 species in the tap water-plant samples (control sample) was observed at DBE = 1−2, which suggest nonaromatic long-chain background fatty acids with maximum intensities of carbon number from 14 to 18. These carboxylic acids are likely exudates from biodegradation.55 However, DBE values >1 were observed for the O2 species in some tap water plant samples. The higher DBE observed in tap water samples may likely be due to enhanced response of negative-ion ESI for these compounds. The maximum relative abundance for all OSPW-plant samples was at DBE = 3−4, which suggests a low number of rings and or double bonds are present in these samples. DBE values of 3−4 indicate that classical naphthenic acid structures, as opposed to condensed aromatic class compounds, are more abundant in the samples investigated. The carbon number range for the O2 species is 12−22 for the OSPW and tap water samples with the most abundant species having carbon numbers at 14−16. DBE values of ≤4 are typical for compound structures with two cyclic alkane rings and single carboxylic acid with carbon numbers of ∼20.8 In addition, a prevalence of components with DBE ≈ 8 was observed, showing the presence of multirings and double bonds species. These components are likely aromatic structures with carbon numbers of 12−20. This DBE trend is in agreement to that observed for NAFCs in OSPW sample reported earlier.21 SOx compounds are the second-most-abundant compound class in the OSPW and tap water samples. The maximum relative intensities of their DBE and carbon numbers were almost the same to those of the O2 classes. For example, the DBE of one of the most dominant species (SO4) in the OSPW plant matrix showed the greatest relative intensity (∼2−5) at carbon numbers from 10 to 16 (Figure 7). Although we did not determine the nature of these SOx compound classes in the

WAX SPE method has better relative enhancement (5%−50%) of O2 classes in OSPW exposed to wetland plants, compared to the ENV+ SPE method. At lower pH, there is protonation of the NAs and consequently decreased low levels of Ox species.51 The ENV+ SPE extraction was performed at pH ∼2.0 and could account for the relatively lower abundances of NAs from the method in comparison to WAX SPE. Furthermore, the WAX SPE method may have enhanced solubility of the NAs at the higher pH of extraction. Some investigations have reported the influence of pH on the distribution of NAFCs in Athabasca OSPW.10,16,33,52 The difference in the relative abundances obtained from the WAX and ENV+ extraction methods may likely be due to the mechanism of interaction of the Ox species in the columns. The pH of extraction can induce charges on the amine group of the WAX SPE column to enhance retention and selectivity. At the pH regime of WAX SPE extraction, the carboxylic acids groups in the OSPW are negatively charged and amine groups of the SPE column are positively charged, causing higher retention of the ionized Ox species. Therefore, the WAX SPE is more efficient for the isolation of Ox species specifically the O2, in comparison to the ENV+ SPE method. Overall, ESI data for the SPE methods showed a decrease in the relative abundance of O2 species in the OSPW containing wetland plants, compared to samples of the initial OSPW without wetland plants (Figure 5). This decrease in the relative abundance of O2 species was accompanied by a discernible increase in the relative abundances of O3 species. These changes in the relative abundance of the Ox classes in the OSPW exposed to wetland plants may be due to sorption or oxidation of NAs to oxy-NAs (x ≥ 3). Low-molecular-weight carboxylic acids have the lowest ionization potential in negativeion ESI and therefore are the most easily ionized compounds in these samples. If there are higher-order oxygen-containing classes, they will be subjected to ion suppression that occurs due to easily ionized O2 compounds. Standard mixtures should be used to further verify this method in the future. Oxidative degradation of NAs converts the O2 species to O3 species,53,54 and this may be catalyzed by a combination of wetland plants and microbes. Wetland systems provide a platform for diverse microbial communities, and plant association with microbes can enhance contaminant metabolism.9 These wetland plant species, combined with associated microbial populations, are F

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and the O3 and O4 species are most likely their oxidation products, which could be created from biological oxidation in the trials (i.e., by microbes and/or plants). This result indicates that the O2 species are distinctly the primary compound class for future monitoring of OSPW associated with wetland plants (e.g., natural or treatment wetlands). The PCA plots of samples from WAX SPE and Isolute ENV+ extraction methods yielded similar patterns. The reader is directed to the Supporting Information for more details on comparison of the WAX PCA data with the ENV+ PCA data.

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CONCLUSIONS Treatment technologies that are largely passive are gaining recognition for naphthenic acid fraction compound (NAFC) abatements; however, the assessment of their efficiency to attenuate these compounds is a major analytical challenge, because of the complexity of oil sands process-affected waters (OSPWs), in addition to the interference of plant or microbially derived compounds that are present in largely passive treatment systems, such as constructed wetlands. This same issue exists for the characterization of NAFCs in natural wetlands. Therefore, there is a need for analytical techniques for the selective isolation and characterization of NAs. WAX and Isolute ENV+ SPE methods were employed for the isolation and characterization of NAs in OSPW associated with wetland plants to try to improve analytical sensitivity and reduce sample matrix effects via selective isolation of NAs. Negative-ionelectrospray ionization Fourier transform ion cyclotron resonance mass spectrometry (ESI-FT-ICR-MS) techniques provided insight on the performance of the extraction methods. WAX SPE isolation provided enhancement of the O2 species, compared to its ENV+ SPE counterpart and is a reliable method for the isolation and characterization of NAs in complex matrices, such as when OSPW is associated with plants. The O2 classes from the tap-water-plant-derived samples are probably carboxylic acids from plant exudates and those of the OSPW plant systems are primarily bitumen-derived NAs, based on isoabundance data. The nurseries were not optimized for OSPW treatment and thus a more pronounced effect on degradation of NAFCs may occur for real outdoor conditions. Principal Component Analysis (PCA) showed that OSPW with wetland plants consisted predominantly of Ox classes (x = 2−4) while tap water with plant samples are mainly Ox classes (x ≥ 5). PCA analysis indicated that future measurement of OSPW

Figure 7. Isoabundance contour plots of DBE versus carbon number for SO4 classes derived from negative-ion ESI-FT-ICR-MS data for cattails (C1) in OSPW (OS).

samples, they may be surfactants present in the original OSPW sample.9 Low relative abundances for NOx classes were observed in the negative-ion ESI-FT-ICR-MS analysis (Figures 1−4). PCA was used to analyze the Ox classes observed in the negative-ion ESI-FT-ICR-MS of OSPW and tap water (control) containing cattail, sedges, and bulrushes. PCA of the score and loading plots are shown in (Figure 8). The score plot displays the distributions of OSPW and tap water associated with plants while the loading plot shows the distinct species in each set of samples. PCA analysis of these samples showed compositional differences between the OSPW and tap water samples. The loading plot shows the distribution of the Ox species. Overall, the OSPW samples are dominated by O2, O3, and O4 species. The tap water with plants is dominated by Ox (x ≥ 5) species with high positive correlation on the PC1 and PC2 axes. We speculate that these classes (x ≥ 5) are likely biological materials or oxidized biological materials. On the other hand, O3 and O4 species are close together and have high positive correlation on the PC2 axis, while the O2 species appear distinctively of the negative axis of the PC plot. Individual species observed close to each other in the PC spaces may likely have similar characteristics. Data suggest that the O2 class is the dominant species in the initial OSPW sample

Figure 8. PCA analysis of Ox Classes in the samples of OSPW and tap water growing plants, derived from negative-ion ESI-FT-ICR-MS data of WAX SPE. G

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NA degradation in treatment wetland systems should focus primarily on O2 classes (along with O3 and O4 classes) in the assessment of treatment efficiency.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.energyfuels.6b02643. PCA analysis of Ox classes in the samples of OSPW and tap-water-growing plants, derived from negative-ion ESIFT-ICR-MS data of WAX SPE and ENV+ SPE (PDF)

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AUTHOR INFORMATION

Corresponding Authors

*Tel.: 306-975-5914. E-mail: [email protected] (C. Ajaero). *Tel.: 306-585-4703. E-mail: [email protected] (D. McMartin). ORCID

Dena W. McMartin: 0000-0002-0824-0565 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We are grateful to the Program of Energy Research and Development (PERD) for their financial support and the National Science Foundation (NSF), and to Florida State University for their contribution in the FT-ICR-MS analysis and data processing. Shell Canada, Ltd. is acknowledged for providing the tailings samples used for the study.

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REFERENCES

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