- Email: [email protected]
Effect of naphtha diluent on greenhouse gases and reduced sulfur compounds emissions from oil sands tailings
Science of the Total Environment 598 (2017) 916–924
Contents lists available at ScienceDirect
Science of the Total Environment journal homepage: www.elsevier.com/locate/scitotenv
Effect of naphtha diluent on greenhouse gases and reduced sulfur compounds emissions from oil sands tailings Kathleen F. Gee, Ho Yin Poon, Zaher Hashisho, Ania C. Ulrich ⁎ Department of Civil and Environmental Engineering, University of Alberta, Canada
H I G H L I G H T S
G R A P H I C A L
A B S T R A C T
• Indigenous microbes degraded residual diluent in tailings thus producing emissions. • Reduced sulfur compounds were produced first at a max. of 0.12 μmol RSCs/mL MFT. • H2S and 2-methylthiophene contributed 81% of the RSCs gas produced. • CH4 and CO2 production occurred after week 5. • A max. of 40.7 μmol CH4/mL MFT and 5.9 μmol CO2/mL MFT was produced.
a r t i c l e
i n f o
Article history: Received 12 January 2017 Received in revised form 14 April 2017 Accepted 14 April 2017 Available online xxxx Editor: D. Barcelo Keywords: Greenhouse gases Reduced sulfur compounds Methanogens Sulfate reducing bacteria Oil sands tailings Diluent
a b s t r a c t The long-term storage of oil sands tailings has resulted in the evolution of greenhouse gases (CH4 and CO2) as a result of residual organics biodegradation. Recent studies have identified black, sulfidic zones below the tailingswater interface, which may be producing toxic sulfur-containing gases. An anaerobic mesocosm study was conducted over an 11-week period to characterize the evolution of CH4, CO2 and reduced sulfur compounds (RSCs) (including H2S) in tailings as it relates to naphtha-containing diluent concentrations (0.2, 0.8, and 1.5% w/v) and microbial activity. Our results showed that RSCs were produced first at 0.12 μmol°RSCs/mL MFT (1.5% w/v diluent treatment). RSCs contribution (from highest to lowest) was H2S and 2-methylthiophene N 2.5dimethylthiophene N 3-methylthiophene N thiofuran N butyl mercaptan N carbonyl sulfide, where H2S and 2methylthiophene contributed 81% of the gas produced. CH4 and CO2 production occurred after week 5 at 40.7 μmol CH4/mL MFT and 5.9 μmol CO2/mL MFT (1.5% w/v diluent treatment). The amount of H2S and CH4 generated is correlated to the amount of diluent present and to microbial activity as shown by corresponding increases in sulfate-reducers' Dissimilatory sulfite reductase (DsrAB) gene and methanogens' methyl-coenzyme M reductase (MCR) gene. © 2017 Elsevier B.V. All rights reserved.
1. Introduction
⁎ Corresponding author at: Department of Civil and Environmental Engineering, University of Alberta, Edmonton, Alberta T6G2W2, Canada. E-mail address: [email protected] (A.C. Ulrich).
http://dx.doi.org/10.1016/j.scitotenv.2017.04.107 0048-9697/© 2017 Elsevier B.V. All rights reserved.
The oil sands deposits in Alberta are the third largest in the world after Saudi Arabia and Venezuela, with an estimated oil reserve of 166 billion barrels of oil (Government of Alberta DoE, 2014). Bitumen is commonly extracted from surface-mined oil sands ore using the Clark
K.F. Gee et al. / Science of the Total Environment 598 (2017) 916–924
Hot Water Extraction Process, which, in turn, has generated enormous quantities of fluid tailings waste (Arkell et al., 2015). These fluid tailings, an alkaline slurry mixture of process water, sand, silt, clays, organics, inorganics, and unrecovered bitumen and diluent (Small et al., 2015) are deposited in large tailings ponds under a zero discharge policy (Government of Alberta DoE, 2013). Following the initial settling of coarse solids (N 44 μm) (Chalaturnyk et al., 2002), the suspended fines in the fluid fine tailings (FFT, ~ 10 wt% solids) gradually settle over 2–4 years to become mature fine tailings (MFT, ~ 30 wt% solids) (Arkell et al., 2015). Due to their poor consolidation properties, it may require several decades or more in situ before tailings sufficiently consolidate for reclamation (BGC Engineering Inc., 2010). Currently, the surface area of these ponds is approximately 182 km2 and contains an estimated 976 million m3 of accumulated tailings (Government of Alberta DoE, 2013; Government of Alberta DoE, 2014). One effect of long-term storage of tailings in the ponds is the evolution of gases. Greenhouse gases (GHG), methane (CH4) and carbon dioxide (CO2), are now emitted in most tailings ponds tested to date (Siddique et al., 2012; Small et al., 2015). Tailings ponds that receive froth treatment tailings (FTT), tailings that contain hydrocarbons and diluent, were reported to have higher GHG emissions and were more prone to intensive bubbling compared to using composite tailings and thickened tailings (Burkus et al., 2014). Diluents are added to improve recovery rates of bitumen (Small et al., 2015). The n-alkanes and BTEX (benzene, toluene, ethylbenzene, and xylenes) compounds found in naphtha diluent have been observed to stimulate the biological production of GHG (Siddique et al., 2006; Siddique et al., 2007). Recent research has also revealed the presence of black, sulfidic zones beneath the tailings-water interface (Chen et al., 2013; Chi Fru et al., 2013; Ramos-Padron et al., 2011; Stasik and Wendt-Potthoff, 2014) that are abundant in sulfate reducing bacteria (SRB) (Stasik and WendtPotthoff, 2014). Potentially, the SRB in these zones are consuming sulfate and producing considerable amounts of HS− and toxic, hydrogen sulfide (H2S) gas (Stasik and Wendt-Potthoff, 2014). The addition of gypsum to increase MFT densification increases the sulfate concentration in tailings, which in the presence of organics such as naphtha diluent, provides SRB with substrates for H2S production. Beside precipitation, H2S emissions are most likely prevented by the chemical (Ramos-Padron et al., 2011) and microbial re-oxidation (Boudens et al., 2016; Stasik et al., 2014). However, there is a lack of peerreviewed literature regarding H2S emissions and other reduced sulfur compounds (RSCs) in tailings. Indeed, H2S is a well-known toxic gas that impacts human health, and other RSCs (including H2S) compounds can be transformed into sulfur dioxide and/or sulfuric acid in the atmosphere (Bates et al., 1992). These transformed products can contribute to acidic precipitation and aerosol formation, which further affect human and environmental health (Bates et al., 1992; Small et al., 2015). Additionally, there is little peer-reviewed literature on the temporal relationship between CH4, CO2 and RSCs (including H2S) productions. As such, the overall objective of this study is to understand the evolution of CH4, CO2 and RSCs (including H2S) in tailings as it relates to: i) diluent concentration and ii) microbial activity. Studying the production of these compounds in tailings will aid in further understanding the emissions currently being released from tailings, and yield further insight on the effect of adding sulfate-containing substrates on gas production that require consideration in current or future tailings remediation plans. 2. Materials and methods
917
until use. Due to limited availability of pond water, surrogate pond water (SPW) was also prepared and used in this experiment (see Table S1 for recipe). Naphtha diluent was also provided by COSIA and was determined in our laboratory to have a specific gravity of 0.77 (T = 20 °C and P = 1 atm). 2.2. Mesocosm set-up Mesocosms were constructed using 1 L Pyrex® glass bottles and modified caps fitted with butyl rubber septa. MFT and pond water samples were oxygen-purged using. Argon gas (Praxair) prior to use. MFT were amended with typical diluent concentration in the tailings ponds – 0.8% w/v (b 1% mass) (Penner and Foght, 2010), whereas a range of diluent concentrations (0.2% w/v, 0.8% w/v, and 1.5% w/v) were studied using the SPW. All experimental mesocosms contained 400 mL MFT and 400 mL pond water (PW or SPW) with diluent. Two types of controls were used in both the PW and SPW groups: no-diluent control (400 mL MFT and 400 mL pond water) as a 0% w/v baseline, and a no-MFT control (800 mL pond water and 0.8% w/v diluent) to account for the presence of MFT in this study. All mesocosms were assembled in an anaerobic chamber (5% CO2, 5% H2, N2 balance) and incubated in the dark at 24 °C. Mesocosms were monitored for 14 weeks (for RSCs production) or 11 weeks (for all other parameters). 2.3. Gas analysis CH4 and CO2 were measured with a 7890A gas chromatograph with a thermal conductivity detector (GC-TCD), and RSCs (hydrogen sulfide, carbonyl sulfide, thiofuran, butyl mercapatan, 2-methylthiophene, 3methylthiophene, and 2,5-dimethylthiophene) were analyzed using a 7890A GC and an Agilent Technologies 355 sulfur chemiluminescence detector (GC-SCD) (See Supplementary information for detail analysis conditions). 2.4. Chemical analyses Prior to the experiment the solids, bitumen, and water content of the MFT were characterized using the Dean Stark procedure (Cao et al., 2014). The chemical analyses in this study were conducted on a mixture of released water and pore water but for the purposes of this study will be referred to as oil sands process water (OSPW). Prior to taking a liquid sample, the mesocosms were inverted to ensure complete mixing thus reducing sampling bias. After inversion, the mesocosms were placed upright and allowed to sit for approximately 1 h to minimize solids from clogging the sampling syringe. At each analytical time point, 20 mL of liquid sample was removed from the OSPW layer and measured for redox using an Accumet redox probe (4 M KCl internal solution) in the anaerobic chamber. The OSPW samples were then removed from the chamber, centrifuged at 8000 ×g for 5 min at 20 °C, and the supernatant was removed and filtered with 0.45 μm filters. Conductivity and pH were measured using an ExStik®II pH/Conductivity/ TDS Meter. Alkalinity was measured using a Mettler Toledo DL53 with 0.02 N H2SO4 as a titrant. Dissolved organic carbon samples were analyzed with a Shimadzu TOC-L CPH using the Non-Purgeable Organic Carbon (NPOC) method. OSPW samples for sulfate and nitrate analysis were filtered (0.22 μm) and analyzed using a Dionex ICS-2100 ion chromatography, or by the Department of Biological Sciences at the University of Alberta using a Dionex DX600 ion chromatography and EPA method 300.1 (USEPA 1997). Detailed explanation of the sulfate reduction rate calculation is included in the Supplementary information.
2.1. Mature fine tailings, pond water, and naphtha diluent 2.5. Microbial analysis MFT and pond water (PW) samples were provided by Canada's Oil Sands Innovation Alliance (COSIA). The samples were taken in June 2012 at a depth of 12 m below the surface and were stored at 4 °C
A 1.5 mL MFT and OSPW mixture was withdrawn from each mesocosm under anaerobic conditions in week 5 and 11. In addition,
918
K.F. Gee et al. / Science of the Total Environment 598 (2017) 916–924
MFT samples collected prior to commencing the experiment was used as a week 0 control. The MFT and OSPW mixture (1.5 mL) were then centrifuged for 30 s at 10000 × g, and supernatant was discarded. A 0.25 g MFT sample was used for the metagenomic DNA extraction with Power Soil DNA Extraction Kit (MoBio Laboratories, USA) according to the manufacturer recommended procedure. The Dissimilatory sulfite reductase (DsrAB) and methyl-coenzyme M reductase (MCR) genes were amplified from 0.5 μL of template DNA. PCR was performed with Phusion High-fidelity DNA polymerase (Thermo scientific, USA) with the initial denaturation 98 °C for 2 min and final extension 72 °C for 2 min. The amplification conditions, Dsr (Wagner et al., 1998) and mcrA (Springer et al., 1995) primers are listed in Table S2. The resulting PCR products were analyzed on 2% agarose gels. Microbial communities were characterized by 16S rRNA gene with Illumina MiSeq sequencing platform (detailed information can be found in the Supplementary information). 3. Results and discussion 3.1. Sample properties prior to diluent amendment Physical and chemical properties of the no-diluent controls at t = 0, which represent the initial sample properties in the MFT-containing mesocosms prior to diluent amendment, are summarized in Table 1. The physical and chemical parameters of MFT were characterized prior to the experiment (Table 1), and the 9.5% discrepancy observed was due to water evaporation during the Dean Stark extraction process. Regardless of pond water group, pH of the mesocosms were slightly alkaline and conductivity values ranged from 3.29 to 3.36 mS/cm. High, initial alkalinity concentrations were found in the PW and SPW groups, which provided buffering capacity in the tailings (Table 1). Positive redox potential values were observed for PW and SPW groups (Table 1). These values suggest that a small amount of oxygen was introduced into the tailings sample as the standard redox potential (pH 7 and T = 25 °C) for an oxygen to water coupling is significantly more positive at +820 mV (Liebensteiner et al., 2014). The initial DOC concentrations in the PW no-diluent controls (92 ± 0.3 mg/L) were nearly three times that in the SPW samples. As the SPW does not contain DOC, the 33 ± 3.8 mg/L found in the SPW samples was contributed by the MFT. The OSPW in both pond water groups were also analyzed for initial nitrate and sulfate concentrations. Nitrate concentrations in both PW and SPW mesocosms at t = 0 were b2 μmol, indicating that nitrate reduction by microorganisms was unlikely to be significant at this point in the study (Penner and Foght, 2010). The amount of sulfate in the tailings samples was found to be similar to those previously reported (ranged between 907.8 and 1207.6 μmol) (Allen, 2008). The SPW group had 633 μmol more sulfate than the PW samples at time 0, however, this discrepancy was likely attributed to the SPW being based on historical OSPW data received from the oil sands industry. Regardless, Table 1 Initial physical and chemical properties for the MFT and pond water samples prior to amendment. Parameter
PW with MFT
SPW with MFT
MFT
Bitumen (%) Solids (%) Water (%) Redox, EH (mV) pH Conductivity (mS/cm) Alkalinity (mg CaCO3/L) DOC (mg/L) NO− 3 (μmol) (μmol) SO2− 4
– – – 369 ± 12 7.84 ± 0.01 3.29 ± 0.01 779 ± 7 92 ± 0.3 1.58 ± 0.31 932.3 ± 20.2
– – – 190 ± 22 7.91 ± 0.02 3.36 ± 0.04 895 ± 6 33 ± 3.8 1.23 ± 0.09 1565.8 ± 9.2
2.5 51 37 – – – – – – –
Note: Measurements for PW and SPW with MFT are presented as averaged values from duplicate mesocosms. MFT, mature fine tailings; DOC, dissolved organic carbon; NO− 3 , nitrate; SO2− 4 , sulfate.
as a result of the high sulfate levels, sulfate reduction is likely to be a significant microbial process in both the PW and SPW mesocosms. 3.2. Changes in chemical parameters The chemical parameters outlined in Table 1 were monitored over an 11-week period to determine whether there were any observed trends during gas production.1 Over the course of 11 weeks, pH and conductivity values did not alter significantly: pH ranged from 7.59 (±na) to 7.95 (± 0.09), and conductivity ranged from 2.78 (± na) to 3.59 (±0.28) mS/cm (Fig. 1a–d). This indicates that any microbial processes occurring within the tailings was buffered by the high alkalinity of the samples, and that no significant ion release was detected during our study. A decrease in nitrate concentrations was observed in the PW, noMFT controls. Initially containing 94.6 μmol of nitrate, concentrations gradually declined to 2.7 μmol by week 11 (Fig. 2a & b). As the redox potentials in these mesocosms were largely positive throughout the study (254 ± 20 mg/L to 395 ± 0.8 mg/L) it is likely that microbial nitrate reduction was occurring during this time (Fig. 2e & f). It is possible that there was similar initial nitrate concentrations within all of the PW mesocosms prior to t = 0 but the nitrate may have been utilized in the days between mesocosm assembly and withdrawing the t = 0 liquid samples. Therefore these results indicate that microbial nitrate reduction likely occurred in the MFT-containing mesocosms prior to the t = 0 liquid sampling and was followed by microbial sulfate reduction until sulfate had become depleted around week 2. In contrast, there was a significant shift in redox potentials and sulfate concentrations within the first few weeks of the study (Fig. 2c–f). While the redox potentials in the no-MFT controls remained largely positive throughout the 11 weeks, the redox potentials in all of the MFT-containing mesocosms markedly decreased and remained negative for the remainder of the study (Fig. 2e & f). This decrease in redox potentials coincided with a decrease in sulfate concentrations and increase in alkalinity (Fig. 2c, d, g & h), suggesting that there was an increase in microbial sulfate reduction. A similar drop in redox values were also observed after one week in unamended fluid fine tailings (FFT) samples from Syncrude Canada Ltd.'s WIP pond (Chen et al., 2013). The authors suspected this was due to the production of highly reducing sulfide compounds by SRB (Chen et al., 2013). The standard reduction potential (E0) for SO24 −/HSO− 3 occurs at E° of approximately − 516 mV (Liebensteiner et al., 2014). Although our redox values for the MFT mesocosms did not appear to decrease below − 145 (± na) in the first two weeks during sulfate depletion (Fig. 2e & f), our redox measurements were conducted on OSPW and not a whole MFT sample. Salloum et al. (2002) reported that redox values in pore water samples could be up to 215 mV higher than those in a whole MFT sample. Therefore the redox values in the MFT of our mesocosms, where microbial activity is occurring, is likely lower than what was measured in OSPW in our study. Given the shifts in redox, sulfate, and alkalinity concentrations, and the high initial sulfate concentrations and low nitrate concentrations in the MFT-mesocosms at t = 0, the results of our study indicate that microbial sulfate reduction occurred during the first two weeks of this study. With respect to the DOC content of the samples, the no-MFT mesocosms had relatively stable DOC concentrations between 42 (± 0.9) mg/L and 79 (±7.6) mg/L throughout the 11 weeks (Fig. S1a & b). However, there is no clear trend concerning the fluctuating DOC concentrations between the different treatment levels (Fig. S1a & b). In some cases, there were significantly large differences in DOC content within duplicate mesocosms. For example, at week 11 in the SPW group, half of the no-diluent controls, 0.8% w/v and 1.5% w/v bottles 1 Due to sampling difficulties between week 1 and week 4 of this experiment, these measurements represent a single mesocosm and are denoted by a standard deviation of ±na [not available].
K.F. Gee et al. / Science of the Total Environment 598 (2017) 916–924
919
Fig. 1. pH (a and b) and conductivity (c and d) measurements for the PW and SPW groups over the 11-week study period. The measurements for the MFT containing mesocosms at weeks 1, 2, and 4 represent one measurement. All other values are averages from duplicate measurements and standard deviation bars, where visible, are plus and minus one standard deviation.
had a DOC concentration b 45 mg/L whereas the duplicate mesocosms contained DOC concentrations N 920 mg/L (Fig. S1a & b). In addition to this large range of fluctuating DOC, the DOC concentrations decreased very slightly unless a significant spike in DOC concentrations preceded it (Fig. S1a & b). One possibility is that the DOC content in the samples at t = 0 are largely recalcitrant (Penner and Foght, 2010) and the increases in DOC are due to microbial activity within the tailings. Some microorganisms have the ability to excrete extracellular polymeric substances (EPS) (Flemming and Wingender, 2001) that can be used by the microorganisms to emulsify hydrocarbons, thereby increasing hydrocarbon bioavailability (Vasconcellos et al., 2011). Bordenave et al. (2010) reported that tailings aggregates formed under various conditions (methanogenesis, nitrate reduction, and sulfate reduction) contained fine clays, microbial cells, and EPS. It is postulated that the EPS emulsifies hydrocarbons in the tailings to form readily available DOC, and this represents the subsequent spike of DOC in the OSPW before being taken up by microorganisms.
3.3. Sulfate reduction rates Sulfate reduction rates (SRR) were determined for mesocosms that contained MFT. In general, SRRs were found to be higher in mesocosms amended with greater amounts of diluent. In the SPW group, the maximum SRR observed in the no-diluent controls was 0.08 μmol/mL MFT/day (Fig. 3b). However, when amended with diluent, maximum SRRs increased by a factor of 2.6 to 3.4 times that of the no-diluent controls, ranging from 0.21 μmol/mL MFT/day (0.2% w/v) to 0.27 μmol/mL MFT/day (0.8% w/v) (Fig. 3b). Similar maximum SRRs were observed in the PW no-diluent controls (0.07 μmol/mL MFT/day) and 0.8% w/v mesocosms (0.29 μmol/mL MFT/day) but sulfate reduction in the 0.8% w/v mesocosms did not peak until week 2 (Fig. 3a). One possibility for the one-week lag between the PW and SPW pond water groups is due to the presence of toxic compounds in the PW water that temporarily inhibited the SRB. Regardless of this, after week 2, when
sulfate had become depleted (b 29 μmol), SRRs in all MFT-diluent amended mesocosms promptly declined. 3.4. Reduced sulfur compound production Six of the most abundant biogenic, reduced sulfur compounds (RSCs) in the environment are reported to be hydrogen sulfide (H2S), carbonyl sulfide (COS), methane thiol (or methyl mercaptan, MeSH), dimethyl sulfide (DMS), carbon disulfide (CS2), and dimethyl disulfide (DMDS) (Pandey and Kim, 2009; Wardencki, 1998). Originally, there were twelve RSCs to be analyzed in this study, including H2S, COS, MeSH, DMS, and CS2. RSC concentrations were anticipated to be low as those in the environment are typically bppb levels (Pandey and Kim, 2009; Wardencki, 1998). However by week 4, we switched our focus on these seven RSCs, H2S, COS, thiofuran, butyl mercaptan, 2methylthiophene, 3-methylthiophene, and 2,5-dimethylthiophene, as their concentrations greatly exceeded the 1 ppmv concentrations in the calibration standard. The RSC gas productions are plotted in Fig. 4 and Figs. S2 and S3. Of the seven RSCs analyzed, H2S was distinct as there was no H2S detected in the no-MFT controls; only mesocosms containing MFT produced H2S. This indicates that H2S production in tailings is associated with the microbial activity in the MFT (Fig. 4a–d). Indeed, PCR results of the dissimilatory sulfite reductase (DsrAB) gene showed that the SRB community was detected in the MFT samples at week 5 (MFT-PW-0.8%, MFT-SPW0.8%, and MFT-SPW-1.5%), and higher band intensity was observed in the samples with higher concentration of diluent (MFT-SPW-1.5%) (Fig. 4c). DsrAB gene was also detected in the week 11 MFT samples, but a lower band intensity was observed for the MFT-SPW with 1.5% diluent, which correspond to the decline observed in the H2S gas production data (Fig. 4c). A faint DsrAB band was observed on the week 0 MFT sample, which suggests that endogenous SRB community exists in the MFT prior the experiment and that the effect observed is likely due to the amendment stimulation of the endogenous community. Several SRB families were identified through the 16S rRNA gene
920
K.F. Gee et al. / Science of the Total Environment 598 (2017) 916–924
Fig. 2. Nitrate (a and b), sulfate (c and d), redox (e and f), and alkalinity (g and h) measurements for the PW and SPW groups over the 11 weeks study period. The measurements for the MFT containing mesocosms at weeks 1, 2, and 4 represent one measurement. All other values are averages from duplicate measurements and standard deviation bars, where visible, are plus and minus one standard deviation. Filled circles: PW or SPW + 0.8% diluent; open diamonds: +MFT; open triangle: +MFT / +0.2% diluent, open square: +MFT / +0.8% diluent; x: +MFT / +1.5% diluent.
amplicon sequencing, including Desulfobulbaceae, Desulfobacteraceae, Desulfuromonadaceae, and Desulfomicrobiaceae (Fig. 6). The relative abundance of the Desulfobulbaceae family mirrored the pattern of the DsrAB gene PCR results (Figs. 4c & 6 and Table S3). The majority of H2S production occurred within the first six weeks, before CH4 gas production began to increase (Figs. 4 and 5). Therefore, week 0 to week 6 were considered the RSC gas production timeframe and were the focus for RSC analysis. Following elevated SRRs in the first several weeks (Fig. 3), H2S production in the MFT-diluent amended mesocosms
also increased with increasing diluent concentrations. In the SPW group, mesocosms amended with 0.2%, 0.8%, and 1.5% w/v diluent resulted in 9.1 μmol, 16.3 μmol, and 19.7 μmol H2S, respectively, an increase of 1.3 to 2.9 times the H2S concentrations found in the nodiluent control (6.8 μmol). H2S generated in the PW-0.8% w/v diluent mesocosms resulted in 5.5 μmol H2S after six weeks. Although this amount is approximately one-third of the H2S found in the SPW-0.8% w/v diluent treatment, the increase in H2S generation between the PW-0.8% w/v and no-diluent control (2.1 μmol) yielded a similar factor
K.F. Gee et al. / Science of the Total Environment 598 (2017) 916–924
921
Fig. 3. Sulfate reduction rate for the (a) PW and (b) SPW groups over the 11 weeks study period. The amount of change in sulfate concentration (μmol SO4/mL of MFT/d) between 0 and 1, 1–2, 2–4, 4–6, and 6–11 weeks were plotted. Note that negative values indicating reduction in sulfate concentration. Open diamonds: PW or SPW + MFT; open triangle: +MFT / +0.2% diluent, open square: +MFT / +0.8% diluent; x: +MFT / +1.5% diluent.
increase of 2.7. These results further support that diluent stimulates sulfate reduction by the SRB and this in turn results in increased H2S generation from tailings. The higher than anticipated H2S concentrations in the mesocosm headspace (bppb levels) (Wardencki, 1998) may be due to several reasons. Salloum et al. (2002) reported that the release of H2S from tailings may become an issue if the tailings pH is low or if there is an insufficient amount of metals available to form metal sulfides. As the pH of our samples was slightly alkaline throughout the study, the generation of H2S may be due to the latter. Additionally, our RSC gas generation estimates are those yielded under anaerobic conditions; when exposed to oxygen, as found in the water cap of tailings ponds, H2S may be removed by conversion into sulfate (Pisz, 2008). In contrast to H2S, the other five RSCs measured in this study (excluding COS, b0.1 μmol throughout the study) 2-methylthiophene, 2methylthiophene, 2,5-dimethylthiophene, thiofuran, and butyl mercaptan (Fig. S2 and S3) were not only detected in the no-MFT controls at t = 0 but these concentrations were noticeably higher than the t = 0 production found in the MFT mesocosms. It is likely that these RSCs
originated from the diluent, and is further supported by two other observations in this study. Firstly, at the end of 14-weeks, the RSC concentrations in the MFT mesocosms were similar to the t = 0 concentrations found in the no-MFT controls. Secondly, there was a significant difference in the gases generated between the MFT-diluent amended mesocosms and the no-diluent controls. In the case of H2S, concentrations increased by a factor of 2.9 times in a worst-case diluent scenario of 1.5% w/v compared to that of no-diluent control. For 2methylthiophene, 3-methylthiophene, 2,5-dimethylthiophene, and thiofuran, the total amount of RSC production increased by a minimal factor of 4 at 0.2% w/v and reached up to 45 times that of the nodiluent control emissions at 1.5% w/v. Therefore, the marked difference in emissions produced between the treatments may be due to these RSCs originating from the diluent and that there is a greater amount of diluent in some of the treatments. Regarding the process behind the release of these RSCs from tailings, one possibility is that there is a sorption and release mechanism occurring between the RSCs and the MFT. However, as the partitioning coefficients for the RSCs analyzed in this
Fig. 4. Hydrogen sulfide measurement (upper panel) for the (a) PW and (b) SPW groups over the 11-week study period. Filled circles: PW or SPW + 0.8% diluent; open diamonds: +MFT; open triangle: +MFT / +0.2% diluent, open square: +MFT / +0.8% diluent; x: +MFT / +1.5% diluent. (c) Dissimilatory sulfite reductase (Dsr) (lower panel) gene was amplified from each of the duplicate MFT samples (indicated in parenthesis). MFT obtained prior to experiment (week 0) was used as control for both week 5 and 11 PCR. (d) The total reduced sulfur gas production between week 0 and 6.
922
K.F. Gee et al. / Science of the Total Environment 598 (2017) 916–924
Fig. 5. Methane measurement (upper panel) for the (a) PW and (b) SPW groups over the 11-week study period. Filled circles: PW or SPW + 0.8% diluent; open diamonds: +MFT; open triangle: +MFT / +0.2% diluent, open square: +MFT /+0.8% diluent; x: +MFT / +1.5% diluent. (c) Methyl-coenzyme M reductase (MCR) (lower panel) gene was amplified from each of the duplicate MFT samples (indicated in parenthesis). MFT obtained prior to experiment (week 0) was used as control.
study are largely unavailable in literature, further research would be required to verify whether sorption of these compounds is an important environmental fate process. Several studies have also found that microbial activity coincides with changes in tailings properties and increased MFT consolidation rates (Arkell et al., 2015; Bordenave et al., 2010). As RSC gases generation were the greatest in diluent amended mesocosms where microbial activity is stimulated, perhaps these changes in tailings properties also influence the release of RSCs over time. Further research would be required to determine the role microbial processes may play in these particular RSCs emissions release. Overall, the total amount of RSCs produced was higher in MFT mesocosms with greater concentrations of diluent (Fig. 4). The total combined RSC production rates between weeks 0 and 6 for the nodiluent controls, regardless of pond water group, ranged from 0.01– 0.02 μmol RSCs/mL MFT (Fig. 4d). Considering the worst-case diluent scenario of 1.5% w/v diluent with 400 mL of MFT, approximately 49 μmol total RSCs was produced in the first six weeks. The RSCs contribution to this amount from highest to lowest was H2S and 2methylthiophene N 2.5-dimethylthiophene N 3-methylthiophene N thiofuran N butyl mercaptan N COS, where H2S and 2-methylthiophene combined contributed to 81% of the gas produced. Based on our results,
further reducing the concentration of residual diluent in the tailings ponds is anticipated to reduce the amount of RSCs produced in the tailings. 3.5. Methane and carbon dioxide production Based on the results of Fedorak et al. (2002), it was hypothesized that methanogenesis would begin when there was approximately 17– 20 mg/L of sulfate remaining within the tailings samples. Although initial sulfate concentrations in our samples were N 200 mg/L (between 932 and 1566 μmol), there was small amounts of CH4 (b 205 μmol) detected in all of the MFT mesocosms at t = 0. Given that CH4 was undetected in the no-MFT controls throughout the 11 weeks, the initial CH4 concentrations in the MFT mesocosms were likely due to suppressed microbial methanogenic activity in the MFT. Following t = 0, sulfate concentrations immediately declined and became depleted in the MFT-diluent amended mesocosms around week 2. CH4 production rapidly increased after week 5, indicating there was a lag period of approximately 3 weeks between sulfate depletion and the start of methanogenesis (Fig. 5a & b). The PCR results of the methylcoenzyme M reductase (MCR) gene showed that the methanogenic
K.F. Gee et al. / Science of the Total Environment 598 (2017) 916–924
community was not detected in week 5 except for the MFT-SPW-1.5% samples (Fig. 5c). The MCR gene was detected in week 11 diluentcontaining samples (MFT-PW-0.8%, MFT-SPW-0.8%, and MFT-SPW1.5%), increasing band intensity was observed in the SPW samples with increased diluent concentration (MFT-SPW-0.8% and − 1.5%) (Fig. 5c). Two methanogenic families were identified from 16S rRNA sequencing, Methanoregulaceae and Methanotrichaceae (Fig. 6). The relative abundance observed for Methanoregulaceae and Methanotrichaceae was similar to the MCR gene PCR results (Figs. 5 & 6 and Table S3). Further evidence of this system transition from sulfur gas production to methanogenesis was denoted by the stabilization or decline of H2 S as CH4 spiked after week 5 (Figs. 4 and 5). Of the CH4 generated over the 11-week study period, N75% of CH4 in all MFT mesocosms were generated between week 5 and week 11. Therefore, this period was considered the CH4 production timeframe and was the focus of CH4 analysis. The molecular biology data and gas analysis data showed that the quantity of CH4 produced was found to increase with increasing diluent concentrations (Fig. 5a & b). The estimated amount of CH4 produced between weeks 5 and 11 in the PW no-diluent controls and 0.8% w/v diluent amended mesocosms was 751 μmol and 9928 μmol, respectively. In the SPW no-diluent controls, 0.2% w/v, 0.8% w/v, and 1.5%w/v diluent amended mesocosms, there was approximately 455 μmol, 766 μmol, 11,280 μmol, and 16,264 μmol CH4, respectively. In comparison to the no-diluent controls, the presence of diluent appears to be a greater influence on the generation of CH4 than that of H2S. CH4 production doubled at 0.2% w/v diluent amended and was up to 36 times the no-diluent controls at the 1.5% w/v worst-case diluent scenario. CH4 production
923
rates for the treatments were also estimated for the week 5 to week 11 timeframe. There was little difference between CH4 production rates in the no-diluent controls and the SPW-0.2% w/v diluent treatment, with rates ranging from 1.1 to 1.9 μmol CH4/mL MFT. Rates were markedly higher in the 0.8% w/v diluent treatments at 24.8– 28.2 μmol CH4/mL MFT (PW-SPW) and 1.5% w/v diluent treatments at 40.7 μmol CH4/mL MFT. Regarding CO2 generation, the majority of CO2 (N 55%) in the MFT mesocosms was also produced after week 5 (Fig. S1c & d). Between weeks 5 and week 11, the amount of CO2 produced in the PW nodiluent controls and 0.8% w/v diluent treatments was approximately 682 μmol and 1259 μmol CO2, respectively (Fig. S1c). In the SPW nodiluent controls, 0.2% w/v, 0.8% w/v, and 1.5% w/v diluent treatments, approximately 549 μmol, 609 μmol, 1760 μmol, and 2361 μmol CO2 respectively, was produced (Fig. S1d). Small amounts of CO2 were present in the mesocosms at t = 0 (up to 249 ± 122 μmol) due to the anaerobic chamber gas mixture where the mesocosms were assembled. However this amount is smaller than the maximum expected amount of CO2 in the mesocosm headspace at t = 0 (687 μmol), indicating that the increase in CO2 production in the MFT mesocosms were likely due to microbial activity. Similarly with CH4, CO2 production appeared to increase with increasing diluent concentrations but to a lesser extent. CO2 production increased by a minimum factor of 2 times the no-diluent controls at 0.8% w/v diluent amended and increased by a factor of 4 at the worst case 1.5% w/v diluent level (Fig. S1c & d). CO2 production rates followed similarly at 1.4 to 1.7 μmol CO2/mL MFT in the no-diluent controls and 0.2% w/v diluent treatments, 3.1 to 4.4 μmol CO2/mL MFT (PW-SPW)
Fig. 6. Microbial communities were characterized by 16S rRNA gene with Illumina MiSeq sequencing platform. The relative abundance (%) of microbial communities within the mesocosms were plotted at family levels.
924
K.F. Gee et al. / Science of the Total Environment 598 (2017) 916–924
at the 0.8% w/v treatments, and a maximum CO2 production rate of 5.9 μmol CO2/mL MFT in a worse-case scenario diluent level of 1.5% w/v. Interestingly, the quantities of CH4 found in this study relative to CO2 was opposite to that reported (Small et al., 2015). Small et al. (2015) compiled CH4 and CO2 emissions data obtained from flux chamber measurements by oil sands companies between 2010 and 2011, and found that the quantities of CO2 emissions from tailings ponds typically exceed CH4 emissions. Several potential factors that led to this difference in results between studies may include difference in solubility between CO2 and CH4 (Arkell et al., 2015), the types of methanogenic microorganisms active in the tailings samples (Siddique et al., 2011), and our gas generation estimates were produced under strictly anaerobic conditions. Additionally, within the water cap of tailings ponds there is evidence that CH4 may consumed by methanotrophs, therefore lowering CH4 emissions relative to CO2 before release to the atmosphere (Saidi-Mehrabad et al., 2013). 4. Conclusion Taken together, our data suggested there were three distinct stages that occurred in the MFT mesocosms over the 11 weeks: 1) Nitrate reduction prior to t = 0; 2) High SRRs, followed by RSCs production between weeks 0 to week 6; and 3) Methanogenesis from weeks 5 to week 11. In general, gases production collectively increased at higher diluent concentrations, and the genetic data presented here implicate two of the anaerobic microbial communities' involvement (methanogen and SRB). Furthermore, the microbial activities, influenced by the presence of diluent, could lead to the potential release of RSC under anoxic conditions. Finally, our results demonstrate that there is potential to reduce gas productions from tailings by further reducing the amount of diluent being lost to the tailings ponds. However, further research is required to determine to what extent these gas generation estimates, which are produced under anaerobic conditions, are released to the atmosphere after exposure to aerobic conditions. Acknowledgements This investigation was supported by funds from NSERC-CRD (CRDPJ386934-09), OSTRF for partnership support, and COSIA for providing materials. We would also like to thank Dena Cologgi, Emily Cao, Bin Ma, and Christine Hereygers for their technical assistance. Appendix A. Supplementary data Supplementary data to this article can be found online at http://dx. doi.org/10.1016/j.scitotenv.2017.04.107. References Allen, E.W., 2008. Process water treatment in Canada's oil sands industry: I. Target pollutants and treatment objectives. J. Environ. Eng. Sci. 7, 123–138. Arkell, N., Kuznetsov, P., Kuznetsova, A., Foght, J.M., Siddique, T., 2015. Microbial metabolism alters pore water chemistry and increases consolidation of oil sands tailings. J. Environ. Qual. 44, 145–153. Bates, T.S., Lamb, B.K., Guenther, A., Dignon, J., Stoiber, R.E., 1992. Sulfur emissions to the atmosphere from natural sources. J. Atmos. Chem. 14, 315–337. BGC Engineering Inc., 2010. Review of Reclamation Options for Oil Sands Tailings Substrates. Bordenave, S., Kostenko, V., Dutkoski, M., Grigoryan, A., Martinuzzi, R.J., Voordouw, G., 2010. Relation between the activity of anaerobic microbial populations in oil sands tailings ponds and the sedimentation of tailings. Chemosphere 81, 663–668.
Boudens, R., Reid, T., VanMensel, D., Prakasan, M.R.S., Ciborowski, J.J., Weisener, C.G., 2016. Bio-physicochemical effects of gamma irradiation treatment for naphthenic acids in oil sands fluid fine tailings. Sci. Total Environ. 539, 114–124. Burkus, Z., Wheler, J., Pletcher, S., 2014. GHG Emissions from Oil Sands Tailings Ponds: Overview and Modelling Based on Fermentable Substrates. Cao, E.S.R., Olubodun, A., Burns, T., Ulrich, A., 2014. Bioamendment of oil sands mature fine tailings accelerated dewatering and improved release water quality. IOSTC (Lake Louise, AB). Chalaturnyk, R.J., Scott, J.D., Ozum, B., 2002. Management of oil sands tailings. Pet. Sci. Technol. 20, 1025–1046. Chen, M., Walshe, G., Fru, E.C., Ciborowski, J.J.H., Weisener, C.G., 2013. Microcosm assessment of the biogeochemical development of sulfur and oxygen in oil sands fluid fine tailings. Appl. Geochem. 37, 1–11. Chi Fru, E., Chen, M., Walshe, G., Penner, T., Weisener, C., 2013. Bioreactor studies predict whole microbial population dynamics in oil sands tailings ponds. Appl. Microbiol. Biotechnol. 97, 3215–3224. Fedorak, P.M., Coy, D.L., Salloum, M.J., Dudas, M.J., 2002. Methanogenic potential of tailings samples from oil sands extraction plants. J Microbiol]–>Can. J. Microbiol. 48, 21–33. Flemming, H.C., Wingender, J., 2001. Relevance of microbial extracellular polymeric substances (EPSs)—part I: structural and ecological aspects. Water Sci. Technol. 43, 1–8. Government of Alberta DoE, 2013. Oil Sands Tailings. Government of Alberta DoE, 2014. Oil Sands: Facts and Statistics. 2016. Liebensteiner, M.G., Tsesmetzis, N., Stams, A.J., Lomans, B.P., 2014. Microbial redox processes in deep subsurface environments and the potential application of (per) chlorate in oil reservoirs. Front. Microbiol. 5, 428. Pandey, S.K., Kim, K.H., 2009. A Review of methods for the determination of reduced sulfur compounds (RSCs) in air. Environ. Sci. Technol. 43, 3020–3029. Penner, T.J., Foght, J.M., 2010. Mature fine tailings from oil sands processing harbour diverse methanogenic communities. J Microbiol]–>Can. J. Microbiol. 56, 459–470. Pisz, J., 2008. Characterization of Extremophilic Sulfur Oxidizing Microbial Communities Inhabiting the Sulfur Blocks Of Alberta's Oil Sands. University of Saskatchewan Saskatoon. Ramos-Padron, E., Bordenave, S., Lin, S., Bhaskar, I.M., Dong, X., Sensen, C.W., et al., 2011. Carbon and sulfur cycling by microbial communities in a gypsum-treated oil sands tailings pond. Environ. Sci. Technol. 45, 439–446. Saidi-Mehrabad, A., He, Z., Tamas, I., Sharp, C.E., Brady, A.L., Rochman, F.F., et al., 2013. Methanotrophic bacteria in oilsands tailings ponds of northern Alberta. ISME J. 7, 908–921. Salloum, M.J., Dudas, M.J., Fedorak, P.M., 2002. Microbial reduction of amended sulfate in anaerobic mature fine tailings from oil sand. Waste Manag. Res. 20, 162–171. Siddique, T., Fedorak, P.M., Foght, J.M., 2006. Biodegradation of short-chain n-alkanes in oil sands tailings under methanogenic conditions. Environ. Sci. Technol. 40, 5459–5464. Siddique, T., Fedorak, P.M., MacKinnon, M.D., Foght, J.M., 2007. Metabolism of BTEX and naphtha compounds to methane in oil sands tailings. Environ. Sci. Technol. 41, 2350–2356. Siddique, T., Penner, T., Semple, K., Foght, J.M., 2011. Anaerobic biodegradation of longerchain n-alkanes coupled to methane production in oil sands tailings. Environ. Sci. Technol. 45, 5892–5899. Siddique, T., Penner, T., Klassen, J., Nesbo, C., Foght, J.M., 2012. Microbial communities involved in methane production from hydrocarbons in oil sands tailings. Environ. Sci. Technol. 46, 9802–9810. Small, C.C., Cho, S., Hashisho, Z., Ulrich, A.C., 2015. Emissions from oil sands tailings ponds: Review of tailings pond parameters and emission estimates. J. Pet. Sci. Eng. 127, 490–501. Springer, E., Sachs, M.S., Woese, C.R., Boone, D.R., 1995. Partial gene sequences for the a subunit of methyl-coenzyme M reductase (mcrI) as a phylogenetic tool for the family Methanosarcinaceae. J Syst Bacteriol]–>Int. J. Syst. Bacteriol. 45, 554–559. Stasik, S., Wendt-Potthoff, K., 2014. Interaction of microbial sulphate reduction and methanogenesis in oil sands tailings ponds. Chemosphere 103, 59–66. Stasik, S., Loick, N., Knoller, K., Weisener, C., Wendt-Potthoff, K., 2014. Understanding biogeochemical gradients of sulfur, iron and carbon in an oil sands tailings pond. Chem. Geol. 382, 44–53. Vasconcellos, S.P., Dellagnezze, B.M., Wieland, A., Klock, J.H., Santos Neto, E.V., Marsaioli, A.J., et al., 2011. The potential for hydrocarbon biodegradation and production of extracellular polymeric substances by aerobic bacteria isolated from a Brazilian petroleum reservoir. J Microbiol Biotechnol]–>World J. Microbiol. Biotechnol. 27, 1513–1518. Wagner, M., Roger, A.J., Flax, J.L., Brusseau, G.A., Stahl, D.A., 1998. Phylogeny of dissimilatory sulfite reductases supports an early origin of sulfate respiration. J. Bacteriol. 180, 2975–2982. Wardencki, W., 1998. Problems with the determination of environmental sulphur compounds by gas chromatography. J. Chromatogr. A 793, 1–19.
Contents lists available at ScienceDirect
Science of the Total Environment journal homepage: www.elsevier.com/locate/scitotenv
Effect of naphtha diluent on greenhouse gases and reduced sulfur compounds emissions from oil sands tailings Kathleen F. Gee, Ho Yin Poon, Zaher Hashisho, Ania C. Ulrich ⁎ Department of Civil and Environmental Engineering, University of Alberta, Canada
H I G H L I G H T S
G R A P H I C A L
A B S T R A C T
• Indigenous microbes degraded residual diluent in tailings thus producing emissions. • Reduced sulfur compounds were produced first at a max. of 0.12 μmol RSCs/mL MFT. • H2S and 2-methylthiophene contributed 81% of the RSCs gas produced. • CH4 and CO2 production occurred after week 5. • A max. of 40.7 μmol CH4/mL MFT and 5.9 μmol CO2/mL MFT was produced.
a r t i c l e
i n f o
Article history: Received 12 January 2017 Received in revised form 14 April 2017 Accepted 14 April 2017 Available online xxxx Editor: D. Barcelo Keywords: Greenhouse gases Reduced sulfur compounds Methanogens Sulfate reducing bacteria Oil sands tailings Diluent
a b s t r a c t The long-term storage of oil sands tailings has resulted in the evolution of greenhouse gases (CH4 and CO2) as a result of residual organics biodegradation. Recent studies have identified black, sulfidic zones below the tailingswater interface, which may be producing toxic sulfur-containing gases. An anaerobic mesocosm study was conducted over an 11-week period to characterize the evolution of CH4, CO2 and reduced sulfur compounds (RSCs) (including H2S) in tailings as it relates to naphtha-containing diluent concentrations (0.2, 0.8, and 1.5% w/v) and microbial activity. Our results showed that RSCs were produced first at 0.12 μmol°RSCs/mL MFT (1.5% w/v diluent treatment). RSCs contribution (from highest to lowest) was H2S and 2-methylthiophene N 2.5dimethylthiophene N 3-methylthiophene N thiofuran N butyl mercaptan N carbonyl sulfide, where H2S and 2methylthiophene contributed 81% of the gas produced. CH4 and CO2 production occurred after week 5 at 40.7 μmol CH4/mL MFT and 5.9 μmol CO2/mL MFT (1.5% w/v diluent treatment). The amount of H2S and CH4 generated is correlated to the amount of diluent present and to microbial activity as shown by corresponding increases in sulfate-reducers' Dissimilatory sulfite reductase (DsrAB) gene and methanogens' methyl-coenzyme M reductase (MCR) gene. © 2017 Elsevier B.V. All rights reserved.
1. Introduction
⁎ Corresponding author at: Department of Civil and Environmental Engineering, University of Alberta, Edmonton, Alberta T6G2W2, Canada. E-mail address: [email protected] (A.C. Ulrich).
http://dx.doi.org/10.1016/j.scitotenv.2017.04.107 0048-9697/© 2017 Elsevier B.V. All rights reserved.
The oil sands deposits in Alberta are the third largest in the world after Saudi Arabia and Venezuela, with an estimated oil reserve of 166 billion barrels of oil (Government of Alberta DoE, 2014). Bitumen is commonly extracted from surface-mined oil sands ore using the Clark
K.F. Gee et al. / Science of the Total Environment 598 (2017) 916–924
Hot Water Extraction Process, which, in turn, has generated enormous quantities of fluid tailings waste (Arkell et al., 2015). These fluid tailings, an alkaline slurry mixture of process water, sand, silt, clays, organics, inorganics, and unrecovered bitumen and diluent (Small et al., 2015) are deposited in large tailings ponds under a zero discharge policy (Government of Alberta DoE, 2013). Following the initial settling of coarse solids (N 44 μm) (Chalaturnyk et al., 2002), the suspended fines in the fluid fine tailings (FFT, ~ 10 wt% solids) gradually settle over 2–4 years to become mature fine tailings (MFT, ~ 30 wt% solids) (Arkell et al., 2015). Due to their poor consolidation properties, it may require several decades or more in situ before tailings sufficiently consolidate for reclamation (BGC Engineering Inc., 2010). Currently, the surface area of these ponds is approximately 182 km2 and contains an estimated 976 million m3 of accumulated tailings (Government of Alberta DoE, 2013; Government of Alberta DoE, 2014). One effect of long-term storage of tailings in the ponds is the evolution of gases. Greenhouse gases (GHG), methane (CH4) and carbon dioxide (CO2), are now emitted in most tailings ponds tested to date (Siddique et al., 2012; Small et al., 2015). Tailings ponds that receive froth treatment tailings (FTT), tailings that contain hydrocarbons and diluent, were reported to have higher GHG emissions and were more prone to intensive bubbling compared to using composite tailings and thickened tailings (Burkus et al., 2014). Diluents are added to improve recovery rates of bitumen (Small et al., 2015). The n-alkanes and BTEX (benzene, toluene, ethylbenzene, and xylenes) compounds found in naphtha diluent have been observed to stimulate the biological production of GHG (Siddique et al., 2006; Siddique et al., 2007). Recent research has also revealed the presence of black, sulfidic zones beneath the tailings-water interface (Chen et al., 2013; Chi Fru et al., 2013; Ramos-Padron et al., 2011; Stasik and Wendt-Potthoff, 2014) that are abundant in sulfate reducing bacteria (SRB) (Stasik and WendtPotthoff, 2014). Potentially, the SRB in these zones are consuming sulfate and producing considerable amounts of HS− and toxic, hydrogen sulfide (H2S) gas (Stasik and Wendt-Potthoff, 2014). The addition of gypsum to increase MFT densification increases the sulfate concentration in tailings, which in the presence of organics such as naphtha diluent, provides SRB with substrates for H2S production. Beside precipitation, H2S emissions are most likely prevented by the chemical (Ramos-Padron et al., 2011) and microbial re-oxidation (Boudens et al., 2016; Stasik et al., 2014). However, there is a lack of peerreviewed literature regarding H2S emissions and other reduced sulfur compounds (RSCs) in tailings. Indeed, H2S is a well-known toxic gas that impacts human health, and other RSCs (including H2S) compounds can be transformed into sulfur dioxide and/or sulfuric acid in the atmosphere (Bates et al., 1992). These transformed products can contribute to acidic precipitation and aerosol formation, which further affect human and environmental health (Bates et al., 1992; Small et al., 2015). Additionally, there is little peer-reviewed literature on the temporal relationship between CH4, CO2 and RSCs (including H2S) productions. As such, the overall objective of this study is to understand the evolution of CH4, CO2 and RSCs (including H2S) in tailings as it relates to: i) diluent concentration and ii) microbial activity. Studying the production of these compounds in tailings will aid in further understanding the emissions currently being released from tailings, and yield further insight on the effect of adding sulfate-containing substrates on gas production that require consideration in current or future tailings remediation plans. 2. Materials and methods
917
until use. Due to limited availability of pond water, surrogate pond water (SPW) was also prepared and used in this experiment (see Table S1 for recipe). Naphtha diluent was also provided by COSIA and was determined in our laboratory to have a specific gravity of 0.77 (T = 20 °C and P = 1 atm). 2.2. Mesocosm set-up Mesocosms were constructed using 1 L Pyrex® glass bottles and modified caps fitted with butyl rubber septa. MFT and pond water samples were oxygen-purged using. Argon gas (Praxair) prior to use. MFT were amended with typical diluent concentration in the tailings ponds – 0.8% w/v (b 1% mass) (Penner and Foght, 2010), whereas a range of diluent concentrations (0.2% w/v, 0.8% w/v, and 1.5% w/v) were studied using the SPW. All experimental mesocosms contained 400 mL MFT and 400 mL pond water (PW or SPW) with diluent. Two types of controls were used in both the PW and SPW groups: no-diluent control (400 mL MFT and 400 mL pond water) as a 0% w/v baseline, and a no-MFT control (800 mL pond water and 0.8% w/v diluent) to account for the presence of MFT in this study. All mesocosms were assembled in an anaerobic chamber (5% CO2, 5% H2, N2 balance) and incubated in the dark at 24 °C. Mesocosms were monitored for 14 weeks (for RSCs production) or 11 weeks (for all other parameters). 2.3. Gas analysis CH4 and CO2 were measured with a 7890A gas chromatograph with a thermal conductivity detector (GC-TCD), and RSCs (hydrogen sulfide, carbonyl sulfide, thiofuran, butyl mercapatan, 2-methylthiophene, 3methylthiophene, and 2,5-dimethylthiophene) were analyzed using a 7890A GC and an Agilent Technologies 355 sulfur chemiluminescence detector (GC-SCD) (See Supplementary information for detail analysis conditions). 2.4. Chemical analyses Prior to the experiment the solids, bitumen, and water content of the MFT were characterized using the Dean Stark procedure (Cao et al., 2014). The chemical analyses in this study were conducted on a mixture of released water and pore water but for the purposes of this study will be referred to as oil sands process water (OSPW). Prior to taking a liquid sample, the mesocosms were inverted to ensure complete mixing thus reducing sampling bias. After inversion, the mesocosms were placed upright and allowed to sit for approximately 1 h to minimize solids from clogging the sampling syringe. At each analytical time point, 20 mL of liquid sample was removed from the OSPW layer and measured for redox using an Accumet redox probe (4 M KCl internal solution) in the anaerobic chamber. The OSPW samples were then removed from the chamber, centrifuged at 8000 ×g for 5 min at 20 °C, and the supernatant was removed and filtered with 0.45 μm filters. Conductivity and pH were measured using an ExStik®II pH/Conductivity/ TDS Meter. Alkalinity was measured using a Mettler Toledo DL53 with 0.02 N H2SO4 as a titrant. Dissolved organic carbon samples were analyzed with a Shimadzu TOC-L CPH using the Non-Purgeable Organic Carbon (NPOC) method. OSPW samples for sulfate and nitrate analysis were filtered (0.22 μm) and analyzed using a Dionex ICS-2100 ion chromatography, or by the Department of Biological Sciences at the University of Alberta using a Dionex DX600 ion chromatography and EPA method 300.1 (USEPA 1997). Detailed explanation of the sulfate reduction rate calculation is included in the Supplementary information.
2.1. Mature fine tailings, pond water, and naphtha diluent 2.5. Microbial analysis MFT and pond water (PW) samples were provided by Canada's Oil Sands Innovation Alliance (COSIA). The samples were taken in June 2012 at a depth of 12 m below the surface and were stored at 4 °C
A 1.5 mL MFT and OSPW mixture was withdrawn from each mesocosm under anaerobic conditions in week 5 and 11. In addition,
918
K.F. Gee et al. / Science of the Total Environment 598 (2017) 916–924
MFT samples collected prior to commencing the experiment was used as a week 0 control. The MFT and OSPW mixture (1.5 mL) were then centrifuged for 30 s at 10000 × g, and supernatant was discarded. A 0.25 g MFT sample was used for the metagenomic DNA extraction with Power Soil DNA Extraction Kit (MoBio Laboratories, USA) according to the manufacturer recommended procedure. The Dissimilatory sulfite reductase (DsrAB) and methyl-coenzyme M reductase (MCR) genes were amplified from 0.5 μL of template DNA. PCR was performed with Phusion High-fidelity DNA polymerase (Thermo scientific, USA) with the initial denaturation 98 °C for 2 min and final extension 72 °C for 2 min. The amplification conditions, Dsr (Wagner et al., 1998) and mcrA (Springer et al., 1995) primers are listed in Table S2. The resulting PCR products were analyzed on 2% agarose gels. Microbial communities were characterized by 16S rRNA gene with Illumina MiSeq sequencing platform (detailed information can be found in the Supplementary information). 3. Results and discussion 3.1. Sample properties prior to diluent amendment Physical and chemical properties of the no-diluent controls at t = 0, which represent the initial sample properties in the MFT-containing mesocosms prior to diluent amendment, are summarized in Table 1. The physical and chemical parameters of MFT were characterized prior to the experiment (Table 1), and the 9.5% discrepancy observed was due to water evaporation during the Dean Stark extraction process. Regardless of pond water group, pH of the mesocosms were slightly alkaline and conductivity values ranged from 3.29 to 3.36 mS/cm. High, initial alkalinity concentrations were found in the PW and SPW groups, which provided buffering capacity in the tailings (Table 1). Positive redox potential values were observed for PW and SPW groups (Table 1). These values suggest that a small amount of oxygen was introduced into the tailings sample as the standard redox potential (pH 7 and T = 25 °C) for an oxygen to water coupling is significantly more positive at +820 mV (Liebensteiner et al., 2014). The initial DOC concentrations in the PW no-diluent controls (92 ± 0.3 mg/L) were nearly three times that in the SPW samples. As the SPW does not contain DOC, the 33 ± 3.8 mg/L found in the SPW samples was contributed by the MFT. The OSPW in both pond water groups were also analyzed for initial nitrate and sulfate concentrations. Nitrate concentrations in both PW and SPW mesocosms at t = 0 were b2 μmol, indicating that nitrate reduction by microorganisms was unlikely to be significant at this point in the study (Penner and Foght, 2010). The amount of sulfate in the tailings samples was found to be similar to those previously reported (ranged between 907.8 and 1207.6 μmol) (Allen, 2008). The SPW group had 633 μmol more sulfate than the PW samples at time 0, however, this discrepancy was likely attributed to the SPW being based on historical OSPW data received from the oil sands industry. Regardless, Table 1 Initial physical and chemical properties for the MFT and pond water samples prior to amendment. Parameter
PW with MFT
SPW with MFT
MFT
Bitumen (%) Solids (%) Water (%) Redox, EH (mV) pH Conductivity (mS/cm) Alkalinity (mg CaCO3/L) DOC (mg/L) NO− 3 (μmol) (μmol) SO2− 4
– – – 369 ± 12 7.84 ± 0.01 3.29 ± 0.01 779 ± 7 92 ± 0.3 1.58 ± 0.31 932.3 ± 20.2
– – – 190 ± 22 7.91 ± 0.02 3.36 ± 0.04 895 ± 6 33 ± 3.8 1.23 ± 0.09 1565.8 ± 9.2
2.5 51 37 – – – – – – –
Note: Measurements for PW and SPW with MFT are presented as averaged values from duplicate mesocosms. MFT, mature fine tailings; DOC, dissolved organic carbon; NO− 3 , nitrate; SO2− 4 , sulfate.
as a result of the high sulfate levels, sulfate reduction is likely to be a significant microbial process in both the PW and SPW mesocosms. 3.2. Changes in chemical parameters The chemical parameters outlined in Table 1 were monitored over an 11-week period to determine whether there were any observed trends during gas production.1 Over the course of 11 weeks, pH and conductivity values did not alter significantly: pH ranged from 7.59 (±na) to 7.95 (± 0.09), and conductivity ranged from 2.78 (± na) to 3.59 (±0.28) mS/cm (Fig. 1a–d). This indicates that any microbial processes occurring within the tailings was buffered by the high alkalinity of the samples, and that no significant ion release was detected during our study. A decrease in nitrate concentrations was observed in the PW, noMFT controls. Initially containing 94.6 μmol of nitrate, concentrations gradually declined to 2.7 μmol by week 11 (Fig. 2a & b). As the redox potentials in these mesocosms were largely positive throughout the study (254 ± 20 mg/L to 395 ± 0.8 mg/L) it is likely that microbial nitrate reduction was occurring during this time (Fig. 2e & f). It is possible that there was similar initial nitrate concentrations within all of the PW mesocosms prior to t = 0 but the nitrate may have been utilized in the days between mesocosm assembly and withdrawing the t = 0 liquid samples. Therefore these results indicate that microbial nitrate reduction likely occurred in the MFT-containing mesocosms prior to the t = 0 liquid sampling and was followed by microbial sulfate reduction until sulfate had become depleted around week 2. In contrast, there was a significant shift in redox potentials and sulfate concentrations within the first few weeks of the study (Fig. 2c–f). While the redox potentials in the no-MFT controls remained largely positive throughout the 11 weeks, the redox potentials in all of the MFT-containing mesocosms markedly decreased and remained negative for the remainder of the study (Fig. 2e & f). This decrease in redox potentials coincided with a decrease in sulfate concentrations and increase in alkalinity (Fig. 2c, d, g & h), suggesting that there was an increase in microbial sulfate reduction. A similar drop in redox values were also observed after one week in unamended fluid fine tailings (FFT) samples from Syncrude Canada Ltd.'s WIP pond (Chen et al., 2013). The authors suspected this was due to the production of highly reducing sulfide compounds by SRB (Chen et al., 2013). The standard reduction potential (E0) for SO24 −/HSO− 3 occurs at E° of approximately − 516 mV (Liebensteiner et al., 2014). Although our redox values for the MFT mesocosms did not appear to decrease below − 145 (± na) in the first two weeks during sulfate depletion (Fig. 2e & f), our redox measurements were conducted on OSPW and not a whole MFT sample. Salloum et al. (2002) reported that redox values in pore water samples could be up to 215 mV higher than those in a whole MFT sample. Therefore the redox values in the MFT of our mesocosms, where microbial activity is occurring, is likely lower than what was measured in OSPW in our study. Given the shifts in redox, sulfate, and alkalinity concentrations, and the high initial sulfate concentrations and low nitrate concentrations in the MFT-mesocosms at t = 0, the results of our study indicate that microbial sulfate reduction occurred during the first two weeks of this study. With respect to the DOC content of the samples, the no-MFT mesocosms had relatively stable DOC concentrations between 42 (± 0.9) mg/L and 79 (±7.6) mg/L throughout the 11 weeks (Fig. S1a & b). However, there is no clear trend concerning the fluctuating DOC concentrations between the different treatment levels (Fig. S1a & b). In some cases, there were significantly large differences in DOC content within duplicate mesocosms. For example, at week 11 in the SPW group, half of the no-diluent controls, 0.8% w/v and 1.5% w/v bottles 1 Due to sampling difficulties between week 1 and week 4 of this experiment, these measurements represent a single mesocosm and are denoted by a standard deviation of ±na [not available].
K.F. Gee et al. / Science of the Total Environment 598 (2017) 916–924
919
Fig. 1. pH (a and b) and conductivity (c and d) measurements for the PW and SPW groups over the 11-week study period. The measurements for the MFT containing mesocosms at weeks 1, 2, and 4 represent one measurement. All other values are averages from duplicate measurements and standard deviation bars, where visible, are plus and minus one standard deviation.
had a DOC concentration b 45 mg/L whereas the duplicate mesocosms contained DOC concentrations N 920 mg/L (Fig. S1a & b). In addition to this large range of fluctuating DOC, the DOC concentrations decreased very slightly unless a significant spike in DOC concentrations preceded it (Fig. S1a & b). One possibility is that the DOC content in the samples at t = 0 are largely recalcitrant (Penner and Foght, 2010) and the increases in DOC are due to microbial activity within the tailings. Some microorganisms have the ability to excrete extracellular polymeric substances (EPS) (Flemming and Wingender, 2001) that can be used by the microorganisms to emulsify hydrocarbons, thereby increasing hydrocarbon bioavailability (Vasconcellos et al., 2011). Bordenave et al. (2010) reported that tailings aggregates formed under various conditions (methanogenesis, nitrate reduction, and sulfate reduction) contained fine clays, microbial cells, and EPS. It is postulated that the EPS emulsifies hydrocarbons in the tailings to form readily available DOC, and this represents the subsequent spike of DOC in the OSPW before being taken up by microorganisms.
3.3. Sulfate reduction rates Sulfate reduction rates (SRR) were determined for mesocosms that contained MFT. In general, SRRs were found to be higher in mesocosms amended with greater amounts of diluent. In the SPW group, the maximum SRR observed in the no-diluent controls was 0.08 μmol/mL MFT/day (Fig. 3b). However, when amended with diluent, maximum SRRs increased by a factor of 2.6 to 3.4 times that of the no-diluent controls, ranging from 0.21 μmol/mL MFT/day (0.2% w/v) to 0.27 μmol/mL MFT/day (0.8% w/v) (Fig. 3b). Similar maximum SRRs were observed in the PW no-diluent controls (0.07 μmol/mL MFT/day) and 0.8% w/v mesocosms (0.29 μmol/mL MFT/day) but sulfate reduction in the 0.8% w/v mesocosms did not peak until week 2 (Fig. 3a). One possibility for the one-week lag between the PW and SPW pond water groups is due to the presence of toxic compounds in the PW water that temporarily inhibited the SRB. Regardless of this, after week 2, when
sulfate had become depleted (b 29 μmol), SRRs in all MFT-diluent amended mesocosms promptly declined. 3.4. Reduced sulfur compound production Six of the most abundant biogenic, reduced sulfur compounds (RSCs) in the environment are reported to be hydrogen sulfide (H2S), carbonyl sulfide (COS), methane thiol (or methyl mercaptan, MeSH), dimethyl sulfide (DMS), carbon disulfide (CS2), and dimethyl disulfide (DMDS) (Pandey and Kim, 2009; Wardencki, 1998). Originally, there were twelve RSCs to be analyzed in this study, including H2S, COS, MeSH, DMS, and CS2. RSC concentrations were anticipated to be low as those in the environment are typically bppb levels (Pandey and Kim, 2009; Wardencki, 1998). However by week 4, we switched our focus on these seven RSCs, H2S, COS, thiofuran, butyl mercaptan, 2methylthiophene, 3-methylthiophene, and 2,5-dimethylthiophene, as their concentrations greatly exceeded the 1 ppmv concentrations in the calibration standard. The RSC gas productions are plotted in Fig. 4 and Figs. S2 and S3. Of the seven RSCs analyzed, H2S was distinct as there was no H2S detected in the no-MFT controls; only mesocosms containing MFT produced H2S. This indicates that H2S production in tailings is associated with the microbial activity in the MFT (Fig. 4a–d). Indeed, PCR results of the dissimilatory sulfite reductase (DsrAB) gene showed that the SRB community was detected in the MFT samples at week 5 (MFT-PW-0.8%, MFT-SPW0.8%, and MFT-SPW-1.5%), and higher band intensity was observed in the samples with higher concentration of diluent (MFT-SPW-1.5%) (Fig. 4c). DsrAB gene was also detected in the week 11 MFT samples, but a lower band intensity was observed for the MFT-SPW with 1.5% diluent, which correspond to the decline observed in the H2S gas production data (Fig. 4c). A faint DsrAB band was observed on the week 0 MFT sample, which suggests that endogenous SRB community exists in the MFT prior the experiment and that the effect observed is likely due to the amendment stimulation of the endogenous community. Several SRB families were identified through the 16S rRNA gene
920
K.F. Gee et al. / Science of the Total Environment 598 (2017) 916–924
Fig. 2. Nitrate (a and b), sulfate (c and d), redox (e and f), and alkalinity (g and h) measurements for the PW and SPW groups over the 11 weeks study period. The measurements for the MFT containing mesocosms at weeks 1, 2, and 4 represent one measurement. All other values are averages from duplicate measurements and standard deviation bars, where visible, are plus and minus one standard deviation. Filled circles: PW or SPW + 0.8% diluent; open diamonds: +MFT; open triangle: +MFT / +0.2% diluent, open square: +MFT / +0.8% diluent; x: +MFT / +1.5% diluent.
amplicon sequencing, including Desulfobulbaceae, Desulfobacteraceae, Desulfuromonadaceae, and Desulfomicrobiaceae (Fig. 6). The relative abundance of the Desulfobulbaceae family mirrored the pattern of the DsrAB gene PCR results (Figs. 4c & 6 and Table S3). The majority of H2S production occurred within the first six weeks, before CH4 gas production began to increase (Figs. 4 and 5). Therefore, week 0 to week 6 were considered the RSC gas production timeframe and were the focus for RSC analysis. Following elevated SRRs in the first several weeks (Fig. 3), H2S production in the MFT-diluent amended mesocosms
also increased with increasing diluent concentrations. In the SPW group, mesocosms amended with 0.2%, 0.8%, and 1.5% w/v diluent resulted in 9.1 μmol, 16.3 μmol, and 19.7 μmol H2S, respectively, an increase of 1.3 to 2.9 times the H2S concentrations found in the nodiluent control (6.8 μmol). H2S generated in the PW-0.8% w/v diluent mesocosms resulted in 5.5 μmol H2S after six weeks. Although this amount is approximately one-third of the H2S found in the SPW-0.8% w/v diluent treatment, the increase in H2S generation between the PW-0.8% w/v and no-diluent control (2.1 μmol) yielded a similar factor
K.F. Gee et al. / Science of the Total Environment 598 (2017) 916–924
921
Fig. 3. Sulfate reduction rate for the (a) PW and (b) SPW groups over the 11 weeks study period. The amount of change in sulfate concentration (μmol SO4/mL of MFT/d) between 0 and 1, 1–2, 2–4, 4–6, and 6–11 weeks were plotted. Note that negative values indicating reduction in sulfate concentration. Open diamonds: PW or SPW + MFT; open triangle: +MFT / +0.2% diluent, open square: +MFT / +0.8% diluent; x: +MFT / +1.5% diluent.
increase of 2.7. These results further support that diluent stimulates sulfate reduction by the SRB and this in turn results in increased H2S generation from tailings. The higher than anticipated H2S concentrations in the mesocosm headspace (bppb levels) (Wardencki, 1998) may be due to several reasons. Salloum et al. (2002) reported that the release of H2S from tailings may become an issue if the tailings pH is low or if there is an insufficient amount of metals available to form metal sulfides. As the pH of our samples was slightly alkaline throughout the study, the generation of H2S may be due to the latter. Additionally, our RSC gas generation estimates are those yielded under anaerobic conditions; when exposed to oxygen, as found in the water cap of tailings ponds, H2S may be removed by conversion into sulfate (Pisz, 2008). In contrast to H2S, the other five RSCs measured in this study (excluding COS, b0.1 μmol throughout the study) 2-methylthiophene, 2methylthiophene, 2,5-dimethylthiophene, thiofuran, and butyl mercaptan (Fig. S2 and S3) were not only detected in the no-MFT controls at t = 0 but these concentrations were noticeably higher than the t = 0 production found in the MFT mesocosms. It is likely that these RSCs
originated from the diluent, and is further supported by two other observations in this study. Firstly, at the end of 14-weeks, the RSC concentrations in the MFT mesocosms were similar to the t = 0 concentrations found in the no-MFT controls. Secondly, there was a significant difference in the gases generated between the MFT-diluent amended mesocosms and the no-diluent controls. In the case of H2S, concentrations increased by a factor of 2.9 times in a worst-case diluent scenario of 1.5% w/v compared to that of no-diluent control. For 2methylthiophene, 3-methylthiophene, 2,5-dimethylthiophene, and thiofuran, the total amount of RSC production increased by a minimal factor of 4 at 0.2% w/v and reached up to 45 times that of the nodiluent control emissions at 1.5% w/v. Therefore, the marked difference in emissions produced between the treatments may be due to these RSCs originating from the diluent and that there is a greater amount of diluent in some of the treatments. Regarding the process behind the release of these RSCs from tailings, one possibility is that there is a sorption and release mechanism occurring between the RSCs and the MFT. However, as the partitioning coefficients for the RSCs analyzed in this
Fig. 4. Hydrogen sulfide measurement (upper panel) for the (a) PW and (b) SPW groups over the 11-week study period. Filled circles: PW or SPW + 0.8% diluent; open diamonds: +MFT; open triangle: +MFT / +0.2% diluent, open square: +MFT / +0.8% diluent; x: +MFT / +1.5% diluent. (c) Dissimilatory sulfite reductase (Dsr) (lower panel) gene was amplified from each of the duplicate MFT samples (indicated in parenthesis). MFT obtained prior to experiment (week 0) was used as control for both week 5 and 11 PCR. (d) The total reduced sulfur gas production between week 0 and 6.
922
K.F. Gee et al. / Science of the Total Environment 598 (2017) 916–924
Fig. 5. Methane measurement (upper panel) for the (a) PW and (b) SPW groups over the 11-week study period. Filled circles: PW or SPW + 0.8% diluent; open diamonds: +MFT; open triangle: +MFT / +0.2% diluent, open square: +MFT /+0.8% diluent; x: +MFT / +1.5% diluent. (c) Methyl-coenzyme M reductase (MCR) (lower panel) gene was amplified from each of the duplicate MFT samples (indicated in parenthesis). MFT obtained prior to experiment (week 0) was used as control.
study are largely unavailable in literature, further research would be required to verify whether sorption of these compounds is an important environmental fate process. Several studies have also found that microbial activity coincides with changes in tailings properties and increased MFT consolidation rates (Arkell et al., 2015; Bordenave et al., 2010). As RSC gases generation were the greatest in diluent amended mesocosms where microbial activity is stimulated, perhaps these changes in tailings properties also influence the release of RSCs over time. Further research would be required to determine the role microbial processes may play in these particular RSCs emissions release. Overall, the total amount of RSCs produced was higher in MFT mesocosms with greater concentrations of diluent (Fig. 4). The total combined RSC production rates between weeks 0 and 6 for the nodiluent controls, regardless of pond water group, ranged from 0.01– 0.02 μmol RSCs/mL MFT (Fig. 4d). Considering the worst-case diluent scenario of 1.5% w/v diluent with 400 mL of MFT, approximately 49 μmol total RSCs was produced in the first six weeks. The RSCs contribution to this amount from highest to lowest was H2S and 2methylthiophene N 2.5-dimethylthiophene N 3-methylthiophene N thiofuran N butyl mercaptan N COS, where H2S and 2-methylthiophene combined contributed to 81% of the gas produced. Based on our results,
further reducing the concentration of residual diluent in the tailings ponds is anticipated to reduce the amount of RSCs produced in the tailings. 3.5. Methane and carbon dioxide production Based on the results of Fedorak et al. (2002), it was hypothesized that methanogenesis would begin when there was approximately 17– 20 mg/L of sulfate remaining within the tailings samples. Although initial sulfate concentrations in our samples were N 200 mg/L (between 932 and 1566 μmol), there was small amounts of CH4 (b 205 μmol) detected in all of the MFT mesocosms at t = 0. Given that CH4 was undetected in the no-MFT controls throughout the 11 weeks, the initial CH4 concentrations in the MFT mesocosms were likely due to suppressed microbial methanogenic activity in the MFT. Following t = 0, sulfate concentrations immediately declined and became depleted in the MFT-diluent amended mesocosms around week 2. CH4 production rapidly increased after week 5, indicating there was a lag period of approximately 3 weeks between sulfate depletion and the start of methanogenesis (Fig. 5a & b). The PCR results of the methylcoenzyme M reductase (MCR) gene showed that the methanogenic
K.F. Gee et al. / Science of the Total Environment 598 (2017) 916–924
community was not detected in week 5 except for the MFT-SPW-1.5% samples (Fig. 5c). The MCR gene was detected in week 11 diluentcontaining samples (MFT-PW-0.8%, MFT-SPW-0.8%, and MFT-SPW1.5%), increasing band intensity was observed in the SPW samples with increased diluent concentration (MFT-SPW-0.8% and − 1.5%) (Fig. 5c). Two methanogenic families were identified from 16S rRNA sequencing, Methanoregulaceae and Methanotrichaceae (Fig. 6). The relative abundance observed for Methanoregulaceae and Methanotrichaceae was similar to the MCR gene PCR results (Figs. 5 & 6 and Table S3). Further evidence of this system transition from sulfur gas production to methanogenesis was denoted by the stabilization or decline of H2 S as CH4 spiked after week 5 (Figs. 4 and 5). Of the CH4 generated over the 11-week study period, N75% of CH4 in all MFT mesocosms were generated between week 5 and week 11. Therefore, this period was considered the CH4 production timeframe and was the focus of CH4 analysis. The molecular biology data and gas analysis data showed that the quantity of CH4 produced was found to increase with increasing diluent concentrations (Fig. 5a & b). The estimated amount of CH4 produced between weeks 5 and 11 in the PW no-diluent controls and 0.8% w/v diluent amended mesocosms was 751 μmol and 9928 μmol, respectively. In the SPW no-diluent controls, 0.2% w/v, 0.8% w/v, and 1.5%w/v diluent amended mesocosms, there was approximately 455 μmol, 766 μmol, 11,280 μmol, and 16,264 μmol CH4, respectively. In comparison to the no-diluent controls, the presence of diluent appears to be a greater influence on the generation of CH4 than that of H2S. CH4 production doubled at 0.2% w/v diluent amended and was up to 36 times the no-diluent controls at the 1.5% w/v worst-case diluent scenario. CH4 production
923
rates for the treatments were also estimated for the week 5 to week 11 timeframe. There was little difference between CH4 production rates in the no-diluent controls and the SPW-0.2% w/v diluent treatment, with rates ranging from 1.1 to 1.9 μmol CH4/mL MFT. Rates were markedly higher in the 0.8% w/v diluent treatments at 24.8– 28.2 μmol CH4/mL MFT (PW-SPW) and 1.5% w/v diluent treatments at 40.7 μmol CH4/mL MFT. Regarding CO2 generation, the majority of CO2 (N 55%) in the MFT mesocosms was also produced after week 5 (Fig. S1c & d). Between weeks 5 and week 11, the amount of CO2 produced in the PW nodiluent controls and 0.8% w/v diluent treatments was approximately 682 μmol and 1259 μmol CO2, respectively (Fig. S1c). In the SPW nodiluent controls, 0.2% w/v, 0.8% w/v, and 1.5% w/v diluent treatments, approximately 549 μmol, 609 μmol, 1760 μmol, and 2361 μmol CO2 respectively, was produced (Fig. S1d). Small amounts of CO2 were present in the mesocosms at t = 0 (up to 249 ± 122 μmol) due to the anaerobic chamber gas mixture where the mesocosms were assembled. However this amount is smaller than the maximum expected amount of CO2 in the mesocosm headspace at t = 0 (687 μmol), indicating that the increase in CO2 production in the MFT mesocosms were likely due to microbial activity. Similarly with CH4, CO2 production appeared to increase with increasing diluent concentrations but to a lesser extent. CO2 production increased by a minimum factor of 2 times the no-diluent controls at 0.8% w/v diluent amended and increased by a factor of 4 at the worst case 1.5% w/v diluent level (Fig. S1c & d). CO2 production rates followed similarly at 1.4 to 1.7 μmol CO2/mL MFT in the no-diluent controls and 0.2% w/v diluent treatments, 3.1 to 4.4 μmol CO2/mL MFT (PW-SPW)
Fig. 6. Microbial communities were characterized by 16S rRNA gene with Illumina MiSeq sequencing platform. The relative abundance (%) of microbial communities within the mesocosms were plotted at family levels.
924
K.F. Gee et al. / Science of the Total Environment 598 (2017) 916–924
at the 0.8% w/v treatments, and a maximum CO2 production rate of 5.9 μmol CO2/mL MFT in a worse-case scenario diluent level of 1.5% w/v. Interestingly, the quantities of CH4 found in this study relative to CO2 was opposite to that reported (Small et al., 2015). Small et al. (2015) compiled CH4 and CO2 emissions data obtained from flux chamber measurements by oil sands companies between 2010 and 2011, and found that the quantities of CO2 emissions from tailings ponds typically exceed CH4 emissions. Several potential factors that led to this difference in results between studies may include difference in solubility between CO2 and CH4 (Arkell et al., 2015), the types of methanogenic microorganisms active in the tailings samples (Siddique et al., 2011), and our gas generation estimates were produced under strictly anaerobic conditions. Additionally, within the water cap of tailings ponds there is evidence that CH4 may consumed by methanotrophs, therefore lowering CH4 emissions relative to CO2 before release to the atmosphere (Saidi-Mehrabad et al., 2013). 4. Conclusion Taken together, our data suggested there were three distinct stages that occurred in the MFT mesocosms over the 11 weeks: 1) Nitrate reduction prior to t = 0; 2) High SRRs, followed by RSCs production between weeks 0 to week 6; and 3) Methanogenesis from weeks 5 to week 11. In general, gases production collectively increased at higher diluent concentrations, and the genetic data presented here implicate two of the anaerobic microbial communities' involvement (methanogen and SRB). Furthermore, the microbial activities, influenced by the presence of diluent, could lead to the potential release of RSC under anoxic conditions. Finally, our results demonstrate that there is potential to reduce gas productions from tailings by further reducing the amount of diluent being lost to the tailings ponds. However, further research is required to determine to what extent these gas generation estimates, which are produced under anaerobic conditions, are released to the atmosphere after exposure to aerobic conditions. Acknowledgements This investigation was supported by funds from NSERC-CRD (CRDPJ386934-09), OSTRF for partnership support, and COSIA for providing materials. We would also like to thank Dena Cologgi, Emily Cao, Bin Ma, and Christine Hereygers for their technical assistance. Appendix A. Supplementary data Supplementary data to this article can be found online at http://dx. doi.org/10.1016/j.scitotenv.2017.04.107. References Allen, E.W., 2008. Process water treatment in Canada's oil sands industry: I. Target pollutants and treatment objectives. J. Environ. Eng. Sci. 7, 123–138. Arkell, N., Kuznetsov, P., Kuznetsova, A., Foght, J.M., Siddique, T., 2015. Microbial metabolism alters pore water chemistry and increases consolidation of oil sands tailings. J. Environ. Qual. 44, 145–153. Bates, T.S., Lamb, B.K., Guenther, A., Dignon, J., Stoiber, R.E., 1992. Sulfur emissions to the atmosphere from natural sources. J. Atmos. Chem. 14, 315–337. BGC Engineering Inc., 2010. Review of Reclamation Options for Oil Sands Tailings Substrates. Bordenave, S., Kostenko, V., Dutkoski, M., Grigoryan, A., Martinuzzi, R.J., Voordouw, G., 2010. Relation between the activity of anaerobic microbial populations in oil sands tailings ponds and the sedimentation of tailings. Chemosphere 81, 663–668.
Boudens, R., Reid, T., VanMensel, D., Prakasan, M.R.S., Ciborowski, J.J., Weisener, C.G., 2016. Bio-physicochemical effects of gamma irradiation treatment for naphthenic acids in oil sands fluid fine tailings. Sci. Total Environ. 539, 114–124. Burkus, Z., Wheler, J., Pletcher, S., 2014. GHG Emissions from Oil Sands Tailings Ponds: Overview and Modelling Based on Fermentable Substrates. Cao, E.S.R., Olubodun, A., Burns, T., Ulrich, A., 2014. Bioamendment of oil sands mature fine tailings accelerated dewatering and improved release water quality. IOSTC (Lake Louise, AB). Chalaturnyk, R.J., Scott, J.D., Ozum, B., 2002. Management of oil sands tailings. Pet. Sci. Technol. 20, 1025–1046. Chen, M., Walshe, G., Fru, E.C., Ciborowski, J.J.H., Weisener, C.G., 2013. Microcosm assessment of the biogeochemical development of sulfur and oxygen in oil sands fluid fine tailings. Appl. Geochem. 37, 1–11. Chi Fru, E., Chen, M., Walshe, G., Penner, T., Weisener, C., 2013. Bioreactor studies predict whole microbial population dynamics in oil sands tailings ponds. Appl. Microbiol. Biotechnol. 97, 3215–3224. Fedorak, P.M., Coy, D.L., Salloum, M.J., Dudas, M.J., 2002. Methanogenic potential of tailings samples from oil sands extraction plants. J Microbiol]–>Can. J. Microbiol. 48, 21–33. Flemming, H.C., Wingender, J., 2001. Relevance of microbial extracellular polymeric substances (EPSs)—part I: structural and ecological aspects. Water Sci. Technol. 43, 1–8. Government of Alberta DoE, 2013. Oil Sands Tailings. Government of Alberta DoE, 2014. Oil Sands: Facts and Statistics. 2016. Liebensteiner, M.G., Tsesmetzis, N., Stams, A.J., Lomans, B.P., 2014. Microbial redox processes in deep subsurface environments and the potential application of (per) chlorate in oil reservoirs. Front. Microbiol. 5, 428. Pandey, S.K., Kim, K.H., 2009. A Review of methods for the determination of reduced sulfur compounds (RSCs) in air. Environ. Sci. Technol. 43, 3020–3029. Penner, T.J., Foght, J.M., 2010. Mature fine tailings from oil sands processing harbour diverse methanogenic communities. J Microbiol]–>Can. J. Microbiol. 56, 459–470. Pisz, J., 2008. Characterization of Extremophilic Sulfur Oxidizing Microbial Communities Inhabiting the Sulfur Blocks Of Alberta's Oil Sands. University of Saskatchewan Saskatoon. Ramos-Padron, E., Bordenave, S., Lin, S., Bhaskar, I.M., Dong, X., Sensen, C.W., et al., 2011. Carbon and sulfur cycling by microbial communities in a gypsum-treated oil sands tailings pond. Environ. Sci. Technol. 45, 439–446. Saidi-Mehrabad, A., He, Z., Tamas, I., Sharp, C.E., Brady, A.L., Rochman, F.F., et al., 2013. Methanotrophic bacteria in oilsands tailings ponds of northern Alberta. ISME J. 7, 908–921. Salloum, M.J., Dudas, M.J., Fedorak, P.M., 2002. Microbial reduction of amended sulfate in anaerobic mature fine tailings from oil sand. Waste Manag. Res. 20, 162–171. Siddique, T., Fedorak, P.M., Foght, J.M., 2006. Biodegradation of short-chain n-alkanes in oil sands tailings under methanogenic conditions. Environ. Sci. Technol. 40, 5459–5464. Siddique, T., Fedorak, P.M., MacKinnon, M.D., Foght, J.M., 2007. Metabolism of BTEX and naphtha compounds to methane in oil sands tailings. Environ. Sci. Technol. 41, 2350–2356. Siddique, T., Penner, T., Semple, K., Foght, J.M., 2011. Anaerobic biodegradation of longerchain n-alkanes coupled to methane production in oil sands tailings. Environ. Sci. Technol. 45, 5892–5899. Siddique, T., Penner, T., Klassen, J., Nesbo, C., Foght, J.M., 2012. Microbial communities involved in methane production from hydrocarbons in oil sands tailings. Environ. Sci. Technol. 46, 9802–9810. Small, C.C., Cho, S., Hashisho, Z., Ulrich, A.C., 2015. Emissions from oil sands tailings ponds: Review of tailings pond parameters and emission estimates. J. Pet. Sci. Eng. 127, 490–501. Springer, E., Sachs, M.S., Woese, C.R., Boone, D.R., 1995. Partial gene sequences for the a subunit of methyl-coenzyme M reductase (mcrI) as a phylogenetic tool for the family Methanosarcinaceae. J Syst Bacteriol]–>Int. J. Syst. Bacteriol. 45, 554–559. Stasik, S., Wendt-Potthoff, K., 2014. Interaction of microbial sulphate reduction and methanogenesis in oil sands tailings ponds. Chemosphere 103, 59–66. Stasik, S., Loick, N., Knoller, K., Weisener, C., Wendt-Potthoff, K., 2014. Understanding biogeochemical gradients of sulfur, iron and carbon in an oil sands tailings pond. Chem. Geol. 382, 44–53. Vasconcellos, S.P., Dellagnezze, B.M., Wieland, A., Klock, J.H., Santos Neto, E.V., Marsaioli, A.J., et al., 2011. The potential for hydrocarbon biodegradation and production of extracellular polymeric substances by aerobic bacteria isolated from a Brazilian petroleum reservoir. J Microbiol Biotechnol]–>World J. Microbiol. Biotechnol. 27, 1513–1518. Wagner, M., Roger, A.J., Flax, J.L., Brusseau, G.A., Stahl, D.A., 1998. Phylogeny of dissimilatory sulfite reductases supports an early origin of sulfate respiration. J. Bacteriol. 180, 2975–2982. Wardencki, W., 1998. Problems with the determination of environmental sulphur compounds by gas chromatography. J. Chromatogr. A 793, 1–19.