Letter Cite This: Environ. Sci. Technol. Lett. XXXX, XXX, XXX−XXX
pubs.acs.org/journal/estlcu
Aerated Electrolysis for Reducing Impacts of Shale Gas Production Wastewater on Water Sources regarding Disinfection Byproduct Formation Hao L. Tang,† Linlin Tang,‡ and Yuefeng F. Xie*,‡ †
Department of Chemistry, Indiana University of Pennsylvania, Indiana, Pennsylvania 15705, United States Environmental Programs, The Pennsylvania State University, Middletown, Pennsylvania 17057, United States
‡
Environ. Sci. Technol. Lett. Downloaded from pubs.acs.org by UNIV OF SUNDERLAND on 10/05/18. For personal use only.
S Supporting Information *
ABSTRACT: Advances in treatment technologies of shale gas production wastewater are needed to minimize its toxic potency in polluted water sources. An aerated electrolysis (AE) process was employed for treatment of both synthetic and field production wastewaters. Results showed AE led to a 64% reduction in the formation of total disinfection byproducts (DBPs) analyzed in this study and 79% reduction in the formation of more toxic brominated DBPs (Br-DBPs) in polluted natural water in an 8-h treatment, suggesting a potential of scaling up as an in situ pretreatment strategy to reduce negative impacts caused by accidental spills or surface discharges during transportation. The mechanisms for reduced formation of DBPs were associated with bromide oxidation followed by bromine stripping from production wastewater, as evidenced by first-order kinetics on bromide removal. Along with bromide removal that led to the reduction of Br-DBP formation and decreased bromine substitution factors (BSFs), the results also revealed alteration of organic DBP precursors during the AE process by exploring the profile of formed DBP species. Formation of bromate in production wastewater was minimal, as bromate was consistently found below a reporting limit of 500 μg/L during the 8-h AE treatment. The lowest energy consumption at 5.9 kWh/m3 warrants further investigations on process optimization. Application of this technology on site of shale gas exploration is beneficial for water utilities that are facing challenges due to contamination of water sources by production wastewater and inability of conventional water treatment processes in attenuation of DBP precursors.
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INTRODUCTION Shale gas exploration is reshaping the energy landscape of the United States and many other countries nowadays. The gas extraction process, however, unavoidably produces large volumes of wastewater that causes serious environmental concerns.1,2 In 2015, 10,000 unconventional Marcellus Shale gas wells in Pennsylvania produced 6.4 billion liters of wastewater3 and the volume is expected to rise given the accelerated rate of shale gas drilling in recent years. Underground injection to Class II injection wells appears to be the best management practice to date for production wastewater to minimize its impact on the environment.4 However, there are growing restrictions on underground injection,2 and in some regions where deep injection is not available or restricted locally, transportation of the production wastewater to longer distances is required. Nonetheless, the transportation process has triggered nearly 3900 spills between 2007−2015 in North Dakota alone,5 and the intensity of the shale gas development is believed to cause a high frequency of the spills and an overall increase of salinity in watersheds.6 In addition, the accidental spills or surface discharges of © XXXX American Chemical Society
production wastewater to water sources pose further risks to human health and the environment as a result of pollutions from heavy metals, anthropogenic (chemicals injected during drilling or fracturing) and naturally occurring organic compounds, and naturally occurring radioactive materials.7,8 Therefore, the receiving water bodies under the influence of the production wastewater can be quite different in chemical composition compared to pristine water sources. Recently, such chemical composition differences have been reported to result in increased formation of disinfection byproducts (DBPs) in receiving streams, attributable to a significant bromide enrichment from the production wastewater.9−11 What makes things even worse is the enhanced formation of brominated DBPs (Br-DBPs),12,13 which tend to present significantly higher developmental toxicity and growth inhibition than their chlorinated analogues.14,15 Water utilities Received: September 13, 2018 Revised: September 26, 2018 Accepted: September 28, 2018
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DOI: 10.1021/acs.estlett.8b00482 Environ. Sci. Technol. Lett. XXXX, XXX, XXX−XXX
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Environmental Science & Technology Letters
synthetic and field production wastewaters, respectively, throughout an 8-h experimental period. Air flow was maintained at 20 mL/min. After each experiment, the cathode was cleaned in 5% (v/v) sulfuric acid to remove debris of precipitates. Chlorination and DBP Analyses. At time zero and every 2 h of the AE experiments, a 250 μL aliquot of production wastewater was withdrawn from the reactor and added to a natural water sample in a 250 mL amber borosilicate bottle, leading to a blend ratio of 0.1%. The natural water was a 0.45μm-membrane-filtered grab sample from Swatara Creek (Middletown, Pennsylvania) (water quality parameters shown in Table S2), which was assumed free of production wastewater impact. A chlorine dose of 12 mg/L was applied to the mixture to initiate a DBP formation potential test, and no headspace was allowed in the bottle to minimize volatilization.23,24 The chlorination took 24 h in absence of light at room temperature (21 °C) before chlorine residuals were quenched, and the samples were analyzed for DBPs. The chlorine quenching agents were sodium sulfite for THM analyses and ammonium chloride for HAA analyses, respectively, per EPA Methods 551.1 and 552.3. 1,2Dibromopropane at a concentration of 300 μg/L in MtBE was used as an internal standard during sample extraction. Details on the sample extraction procedures and program settings of gas chromatographs can be found elsewhere.25 DBP concentrations were validated against a comprehensive set of procedural blanks, matrix spikes, and replicate samples. The DBP results presented in the study were averages of duplicate analyses. Bromide and bromate in AE-treated production wastewater was analyzed by ion chromatography following USEPA Method 300.1. The reporting limit of bromate was 500 μg/L as a result of a 200× dilution.
drawing water from such water sources, therefore, are at risk of violating the Stage 2 Disinfectants/Disinfection Byproducts Rule (DBPR).16 Methods for reducing their negative impacts regarding DBP formation are now needed. Fresh production wastewater is typically stored in engineered impoundments on site. If it is sent for treatment instead of underground injection due to limited capacity of disposal wells, commercial wastewater treatment (CWT) facilities can be its destination, where it goes through a series of chemical precipitation, flocculation, and solids separation processes before surface discharge or reuse. However, none of these conventional processes is designed nor successful in removing the monovalent halide (i.e., bromide),17 and the BrDBP formation is our concern. Moreover, since CWT is not in situ treatment, it does not reduce risks arising from accidental spills during transportation. Advances in unconventional processes for production wastewater treatment are in urgent need. In this study, we developed an aerated electrolysis (AE) process as an in situ treatment strategy for shale gas production wastewater, with the purpose of reducing the formation of DBPs in polluted water sources. The standard equilibrium potential (E0) (V vs standard hydrogen electrode) for the bromide oxidation half reaction is 1.09 V. Electrolysis for bromide oxidation in production wastewater followed by bromine volatilization has been found possible by Sun et al.18 The impact of the process on DBP formation remains unclear. Kimbrough et al.19−21 applied electrolysis and volatilization for bromide removal in drinking water. Still, the regulated trihalomethanes (THMs) and haloacetic acids (HAAs) were not addressed, and drinking water can be quite different from production wastewater due to their significant differences in chemical composition. Since electrolysis is capable of altering bromide and organics on the anode and the formation of DBPs largely depend on these two as inorganic and organic DBP precursors during chlorination of natural waters, the applicability of an AE process for DBP control is appealing. This is the first study employing the AE process for control of DBPs in production wastewater-impacted water sources.
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RESULTS AND DISCUSSION AE for Synthetic Production Wastewater Treatment. Figure S2a shows AE led to reduced formation of total DBPs (i.e., the mass sum of all analyzed DBPs including four THMs and nine HAAs) and Br-DBPs (i.e., the mass sum of all brominated DBPs in the analyzed THMs and HAAs) in the production wastewater-impacted natural water. The AE was found to reduce the formation of total DBPs from 509 to 374 μg/L (27% removal) after an 8-h period. It was especially effective in reducing the formation of Br-DBPs, as the Br-DBPs decreased from 270 to 56 μg/L, a 79% removal. The impacts of the synthetic production wastewater blending on a baseline reference of DBP formation potential in river water have been quantified by the authors,9,23,24 and the alteration of DBP formation was attributed to the introduction of bromide from production wastewater. The results of the present study implied the occurrence of bromide removal during the AE treatment of synthetic wastewater. The reduced bromide level would protect receiving water bodies from forming high levels of Br-DBPs, and thus protect consumers from potential negative health impacts. Direct evidence of bromide oxidation followed by bromine volatilization from production wastewater included bromide reduction (up to 98.3% removal) during an 8-h AE treatment (Figure S3a) and the first-order kinetics for bromide removal (Figure S3b). Formation of bromate in production wastewater was minimal, as bromate was consistently found below the reporting limit of 25 μg/L. Along with bromide removal and formation of less Br-DBPs, the relatively increased ratio of Cl and Br suppressed bromine
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MATERIALS AND METHODS Shale Gas Production Wastewater. Both synthetic and field shale gas production wastewaters were employed in this study. Synthetic production wastewater was made in the laboratory based on representative shale gas production water characteristics summarized by Warner et al.22 Chemicals used in the synthesis included sodium chloride (1210 mM), sodium bromide (9.3 mM), sodium sulfate (0.22 mM), calcium chloride (241 mM), magnesium chloride hexahydrate (44.0 mM), barium chloride (12.4 mM), and strontium chloride hexahydrate (18.2 mM). The resulting parameters on the basis of both mass and molar concentrations are detailed in Table S1. Field production wastewater was obtained from an active shale gas well in production, courtesy of JKLM Energy (Sewickley, Pennsylvania), and its conductivity and total dissolved solids (TDS) were 247 mS/cm and 247 g/L, respectively. AE Experiments. The AE reactor consisted of a 600 mL beaker on a magnetic stirring plate, two graphite rod electrodes (0.6 cm diameter and 10 cm length) connected to a potentiostat, and air supply tubes with a diffusion stone at the anode side (Figure S1). Each AE experiment started with 500 mL of production wastewater. The initial current was set at 600 mA, while the voltage was maintained at 4.5 or 3.4 V for B
DOI: 10.1021/acs.estlett.8b00482 Environ. Sci. Technol. Lett. XXXX, XXX, XXX−XXX
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Figure 1. AE treatment of field production wastewater: (a) formation of DBPs after blending with Swatara Creek water at a ratio of 0.1% and chlorination, (b) bromine substitution factors (BSFs) of three DBP classes, (c) current change, and (d) energy consumption.
production wastewater. If a 2-h AE was adopted, the energy consumption was 0.10 Wh for reducing formation of 1 μg total DBPs and 0.11 Wh for reducing formation of 1 μg more toxic Br-DBPs (Figure S2d). On a volume basis, the used energy was 5.9 kWh/m.3 As the electrolysis time increased, the energy consumption increased linearly on the basis of total DBPs and volume. The energy consumption on the basis of Br-DBPs increased exponentially after 4 h, suggesting extra efforts required for Br-DBPs removal. Note that the energy needed for outgassing the produced bromine from production wastewater was not included, which could cause higher energy consumption than the reported values here. AE for Field Production Wastewater Treatment. Although the field and synthetic wastewaters differed in TDS by a factor of 2.7, their bromide percentages (grams of bromide in grams of TDS) were at similar levels (0.83% and 0.80%, respectively). AE of field wastewater demonstrated much better capability on reducing the formation of total DBPs compared to that of synthetic wastewater. The starting total DBPs was 700 μg/L, and an 8-h AE brought it down to 255 μg/L (a 64% reduction) (Figure 1a), while for the synthetic wastewater, the percentage was only 27%. For BrDBPs, the percentage (77%) was comparable to that of the synthetic wastewater (79%). The enhanced total DBP removal was likely due to the beneficial removal of organic DBP precursors along with bromide in field production wastewater by AE. Those organic DBP precursors were associated with the organic chemicals in the field production wastewater, which
substitution during chlorination and formed more non-BrDBPs (i.e., chlorinated DBPs with no bromine substitutions including trichloromethane, monochloroacetic acid, dichloroacetic acid (DCAA), and trichloroacetic acids (TCAA)). We detected the non-Br-DBPs to have increased from 239 to 318 μg/L as a result of the suppression in bromine substitution. Bromine substitution factor (BSF), a parameter evaluating the degree of bromination in different classes of DBPs, was calculated following Hua and Reckhow’s method,26 and the values for THMs, dihaloacetic acids (DHAAs), and trihaloacetic acids (THAAs) are presented in Figure S2b. The BSF for THMs demonstrated a monotonic decreasing pattern from 0.28 to 0.04, while the BSF for DHAAs starting at 0.34 showed a slight increase and then decreased consistently to 0.11. The BSF for THAAs, although presented in relative low values compared to the other two, also demonstrated the same pattern like DHAAs. The BSF plot confirmed the beneficial aspects of AE on reducing bromine substitution during chlorination. Along with bromide oxidation at the anode, the cathode promoted chemical precipitations as a result of the formation of hydroxide and carbonate.27 As the hardness metals were precipitated, the conductivity of the solution decreased, causing a decrease in the current from 600 to 250 mA as shown in Figure S2c. Some precipitates coated on the cathode, and acid cleaning was needed prior to the next run. The amount of consumed energy was determined by multiplying current, voltage, and time during the AE treatment of synthetic C
DOI: 10.1021/acs.estlett.8b00482 Environ. Sci. Technol. Lett. XXXX, XXX, XXX−XXX
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shown in Figure S4 could be ascribed to faster degradation of hydrophilic compounds that are more associated with DCAA precursors34 due to stronger charge-dipole electrostatic interaction between the anode and hydrophilic compounds.35,36 Nevertheless, the toxic potency of the production wastewater decreased, as revealed in the decreased total DBPs and especially the Br-DBPs. Bromate was consistently found below a reporting limit of 500 μg/L during the 8-h treatment, which did not appear to be a problem in this AE application. Although Br-DBPs are major toxicity drivers, it is important to note that AE is helpful to reduce both THM/HAA and the brominated species to meet DBPR at water utilities. The AE application in production wastewater treatment is feasible, considering the potential concerns can be addressed: (1) cathode cleaning by acid rinsing is possible since the scaling is due to the formation of calcium carbonate and (2) proper ventilation can be done to cope with gas phase emission of bromine. Compared to alternative technologies37 such as membrane processes and distillation, AE excels in simple operation, low maintenance, and a potential to be used as an in situ treatment technology. There are certainly some factors that warrant further investigation and optimization. It would be appealing to investigate the iodinated DBPs due to its greater toxicity than Br-DBPs.38 Low cost and resistance to poisoning were considered when selecting the graphite electrodes,18 which could lead to partial oxidation of organics. Electrodes made with other materials might improve organic DBP precursor removal from production wastewater. Moreover, the cathode needs acid cleaning after 8 h to remove precipitates. Softening production wastewater by chemical precipitation would be helpful to alleviate the cathode issue. Last but not least, considering that 3−10 kWh/m3 of energy is required for seawater desalination by reverse osmosis the lowest energy consumption of 5.9 kWh/m3 at 2 h by AE indicates room for improvement by adjusting the applied voltage to allow selective oxidation of bromide15 and control of other undesirable DBP formation in production wastewater.
has been characterized with saturated, aliphatic compounds and a small fraction of aromatic, resin, and asphaltene compounds.28 The organic chemicals used in hydraulic fracturing are complex and usually remain proprietary and undisclosed to the public in many states.28 Improved understanding of these chemicals are helpful to better assess the proportion and speciation of DBPs formed in polluted natural waters. The BSF values for THMs, DHAAs, and THAAs behaved similarly to those of synthetic wastewater, and overall decreases of all three BSF values were observed (Figure 1b), confirming a shift to a direction of less bromine incorporation. The temporary increases in BSF values at 2 h for DHAAs and THAAs could be ascribed to the alteration of organic matter in the production wastewater by the short-lived electrochemically produced oxidants29−32 such as •OH, O3, H2O2, and S2O82− in the AE reactor. The oxidization of organic matter may form greater amounts of a methyl-ketone-like structure, which could cause a temporary shift to more bromine substitution.33 On the other hand, some organics could be preferentially oxidized at the anode due to lower E0 and thereby compete with bromide oxidation. Sun et al.18 summarized a list of species with lower E0 than bromide that may exist in production wastewater such as formic acid and iodide. In addition, the E0 for chloride is only 0.27 V higher than bromide. Therefore, it is an engineering challenge to selectively oxidize bromide. Considering chloride is not posing a health risk, the study did not initiate investigations toward chloride removal. The precipitation of hardness metals caused the current to decrease as shown in Figure 1c, and the majority of the decrease occurred in the first 4 h. Note that the AE of field wastewater demonstrated better energy consumption. If a 2-h AE was adopted, the energy consumption would be 0.035 Wh for reducing formation of 1 μg total DBPs, which was approximately 3 folds lower than that of synthetic wastewater. On Br-DBPs and volume bases, the used energy was 0.15 Wh/μg Br-DBPs and 6.1 kWh/m3, which were similar to that of synthetic wastewater. The reduced energy consumption on the basis of total DBPs was attributable to the beneficial removal of organic DBP precursors by the AE treatment. Similar to the pattern of synthetic wastewater, as the electrolysis time increased, the energy consumption increased linearly on bases of total DBPs and volume. The energy consumption on the basis of Br-DBPs increased exponentially after 4 h, suggesting greater efforts involved for a higher Br-DBP removal efficiency. Outlook. The use of AE for shale gas production wastewater treatment achieved up to 64% reduction in the formation of total DBPs and up to 79% reduction in the formation of more toxic Br-DBPs during an 8-h treatment, suggesting a potential of scaling-up for in situ treatment to minimize the impacts of spills or surface discharge on water sources and to help the affected water utilities in compliance with the Stage 2 DBPR. Along with the removal of bromide by oxidation and volatilization, the organics in the production wastewater could be altered, changing the speciation of DBPs in the affected receiving water bodies. Two replications of the AE experiments reported in this study produced consistent trends on mitigation of DBP formation, and the DBP species and energy consumption of the two replicate experiments are included in Tables S3 and S4. As the Cl/Br ratio decreased, a shift to the direction of more chlorine substitution during chlorination might lead to formation of more non-Br-DBPs. Moreover, the overall decreased ratio of DCAA and TCAA
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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.estlett.8b00482.
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Description of chemicals used for quantification of DBPs. Schematic diagram of the AE reactor (Figure S1). System performance during AE treatment of synthetic production wastewater (Figure S2). Effect of AE treatment on bromide removal (Figure S3). Ratios of DCAA and TCAA during AE treatment of both synthetic and field production wastewaters (Figure S4). Water quality parameters of the synthetic production wastewater (Table S1) and the Swatara Creek water (Table S2). Concentrations of DBP species and energy consumption in replicate experiment #1 (Table S3). Concentrations of DBP species and energy consumption in replicate experiment #2 (Table S4). (PDF)
AUTHOR INFORMATION
Corresponding Author
*Yuefeng F. Xie. E-mail:
[email protected]. Tel: +1 717-948-6415. Fax: +1 717-948-6580. D
DOI: 10.1021/acs.estlett.8b00482 Environ. Sci. Technol. Lett. XXXX, XXX, XXX−XXX
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Environmental Science & Technology Letters ORCID
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Yuefeng F. Xie: 0000-0002-1005-1114 Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS This research was supported by the Pennsylvania Water Resources Research Center USGS 104B small grants program and the start-up fund provided by the College of Natural Sciences and Mathematics at Indiana University of Pennsylvania. The authors gratefully acknowledge Joseph Harrick from JKLM Energy (Sewickley, Pennsylvania) for providing shale gas production wastewater used in this research.
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