Emergence and fate of volatile iodinated organic compounds during biological treatment of oil and gas produced water

Science of the Total Environment 699 (2020) 134202

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Science of the Total Environment journal homepage: www.elsevier.com/locate/scitotenv

Emergence and fate of volatile iodinated organic compounds during biological treatment of oil and gas produced water Nohemi Almaraz a, Julia Regnery a,b,⇑, Gary F. Vanzin a, Stephanie M. Riley a,c, Danika C. Ahoor a, Tzahi Y. Cath a,** a b c

Department of Civil and Environmental Engineering, Colorado School of Mines, Golden, CO, USA Department of Biochemistry and Ecotoxicology, Federal Institute of Hydrology, Koblenz, Germany Water Quality Research and Development Division, Southern Nevada Water Authority, Henderson, NV, USA

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


Iodinated organic compounds (IOC)

emerge in biological active filter (BAF) effluent.
IOC formation correlated with decreasing iodide concentrations after treatment.
Seemingly, BAF treatment performance was not affected by the presence of IOCs.
Iodide oxidizing bacteria were found at high abundance in BAF effluent.

a r t i c l e

i n f o

Article history: Received 25 June 2019 Received in revised form 29 August 2019 Accepted 29 August 2019 Available online 02 September 2019 Editor: Frederic Coulon Keywords: Iodinated disinfection byproducts Biologically active filters

a b s t r a c t Oil and gas (O&G) production in the United States is expected to grow at a substantial rate over the coming decades. Environmental sustainability related to water consumption during O&G extraction can be addressed through treatment and reuse of water returning to the surface after well completion. Water quality is an important factor in reuse applications, and specific treatment technologies must be utilized to remove different contaminants. Among others, biological active filtration can remove dissolved organic matter as a pre-treatment for surface discharge or to facilitate reuse in such applications as hydraulic fracturing, dust suppression, road stabilization, and crop irrigation. Yet, the formation of byproducts during treatment of O&G wastewater remains a concern when evaluating reuse applications. In this study, we investigated the previously unnoticed biotic formation of iodinated organic compounds (IOCs) such as triiodomethane during biological treatment of O&G wastewater for beneficial reuse. Iodide and several

Abbreviations: ASVs, Amplicon sequence variants; BAF, Biological active filtration; Br, Bromide; BrCHI2, Bromodiiodomethane; BQL, Below quantification limit; O&G, Oil and gas; Ca2+, Calcium; CH2ClI, Chloroiodomethane; CH2I2, Diiodomethane; CHI3, Triiodomethane; Cl2, Chlorine; Cl, Chloride; ClCHI2, Chlorodiiodomethane; DOC, Dissolved organic carbon; DJ, Denver Julesburg; DNA, Deoxyribonucleic acid; EC, Electrical conductivity; Fe2+, Iron (II); Fe3+, Iron (III); FB, Flowback; GAC, Granular activated carbon; HSSPME-GC–MS, Headspace solid-phase micro extraction gas chromatography mass spectrometry; I2, Iodine; I, Iodide; IC, Ion chromatography; ICP-AES, Inductive coupled plasma-atomic emission spectroscopy; I-DBPs, Iodinated disinfection byproducts; IOB, Iodide oxidizing bacteria; IOCs, Iodinated organic compounds; KMnO4, Potassium permanganate; MDL, Method detection limits; Mg2+, Magnesium; Mn2+, Manganese; MQL, Method quantification limits; MS, Mass Spectrometer; NIST, National Institute of Standards and Technology; Na+, Sodium; NaBr, Sodium bromide; NaCl, Sodium chloride; NaI, Sodium iodide; NH3-N, Ammonia; NO2, Nitrite; NO3, Nitrate; NSF, National Science Foundation; NPDES, National Pollution Discharge Elimination system; PCR, Polymerase chain reaction; PDMS-DVB, Polydimethylsiloxane; PTV, Programmable temperature vaporizing inlet; PVC, Polyvinyl chloride; PW, Produced water; S, Sulfur; SM, Supplementary material; TN, Total nitrogen; U.S. EPA, United States Environmental Protection Agency; 16S rRNA, 16S ribosomal RNA. ⇑ Corresponding author. ⇑⇑ Corresponding author. E-mail addresses: [email protected] (J. Regnery), [email protected] (T.Y. Cath). https://doi.org/10.1016/j.scitotenv.2019.134202 0048-9697/Ó 2019 Elsevier B.V. All rights reserved.

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Hydraulic fracturing Water reuse Iodide oxidation Produced water

IOCs were quantified in O&G produced water before and after treatment in biological active filters filled with different media types over 13 weeks of operation. While iodide and total IOCs were measured at concentrations <53 mg/L and 147 lg/L, respectively, before biological treatment, total IOCs were measured at concentrations close to 4 mg/L after biological treatment. Triiodomethane was the IOC that was predominantly present. IOC formation had a negative strong correlation (r = 0.7 to 0.8, p < 0.05, n = 9) with iodide concentration in the treated O&G wastewater, indicating that iodide introduced to the biological active filter system was utilized in various reactions, including biologically mediated halogenation of organic matter. Additionally, iodide-oxidizing bacteria augmented in the treated produced water pointed towards potential negative environmental implications when releasing biologically treated halide-rich wastewater effluents to the aquatic environment. Ó 2019 Elsevier B.V. All rights reserved.

1. Introduction

shown the presence of halogenated organic compounds in O&G wastewater, including 20 different biogenic and xenobiotic iodinated organic compounds (IOCs) (Luek et al., 2017) and the abiotic formation of a subgroup of IOCs; iodinated disinfection byproducts (I-DBPs) such as triiodomethane (also referred to as iodoform) during disinfection of water impacted by iodide-rich O&G wastewater (Hladik et al., 2014; Liberatore et al., 2017). IOCs are organic compounds that contain one or more carbon–iodine bonds, the majority thereof featuring iodide connected to one carbon center. I-DBPs are shown to be more cytotoxic and genotoxic than their brominated and chlorinated analogs, with detrimental environmental impacts and adverse health effects at sub lg/L levels (Dickenson et al., 2008; Dong et al., 2017; Gallard and von Gunten, 2002; Liberatore et al., 2017; Liu et al., 2017; Luek et al., 2017; Richardson et al., 2007). Iodine can be found in multiple oxidation states and its speciation, bioavailability, and mobility are dictated by biological interactions, physical, and chemical environmental conditions (Yeager et al., 2017). During disinfection of saline waters (e.g., O&G wastewater disposed of at wastewater treatment facilities (Hladik et al., 2014)), iodide can be oxidized by chlorine or chloramines to form hypoiodous acid, which may react with organic matter to form I-DBPs (Allard et al., 2013; Gong and Zhang, 2015; Kim et al., 2015). Further oxidation of hypoiodous acid can form iodate, a non-toxic sink of iodine; thus, mitigating the formation of IOCs. However, during disinfection concurrent formation of known and regulated human carcinogens such as bromate is highly undesirable (EPA, 1998; WHO, 2011). Formation of I-DBPs during oxidation of iodide-containing waters by alternative disinfectants such as potassium permanganate (KMnO4) and chlorine dioxide has also been reported (Kim et al., 2015; Ye et al., 2012). Moreover, iodine is bioavailable in the form of iodide (Agarwal et al., 2017), and several IOCs (e.g., chloroiodomethane, diiodomethane) are known to form through enzymatic and iodide methylation reactions of iodide oxidizing bacteria (IOB) (Amachi et al., 2005) and other marine microorganisms (Schall et al., 2004; van Pée and Unversucht, 2003). Therefore, attention should be paid not only to the abiotic formation of halogenated byproducts during disinfection of haliderich waters, but also to biotic formation of IOCs and other halogenated compounds during biological treatment; thus, better understanding the environmental implications of reusing and releasing treated O&G waste streams into the aquatic environment. Biological treatment of O&G PW for example has shown substantial removal of organic matter, increasing O&G wastewater quality, and preparing it for downstream applications (Frank et al., 2017; Freedman et al., 2017; Freire et al., 2001; Lu et al., 2009; Riley et al., 2016). Studies by Freedman et al. and Riley et al. (Freedman et al., 2017; Riley et al., 2018a; Riley et al., 2018b; Riley et al., 2016) have demonstrated 75–92% removal of dissolved organic carbon (DOC) from PW and FB using biological active filtration (BAF). In these studies, biofilm acclimation and high performance of BAFs with O&G wastewater of different chemistries showed the robustness and flexibility of this promising technology

Development of new oil and gas (O&G) extraction technologies such as hydraulic fracturing, in conjunction with increased energy demands, have enhanced production of O&G. With global energy demands expected to grow 28% by 2040 (U.S. Energy Information Administration, 2017), hydraulic fracturing provides the technology and economic means to exploit unconventional O&G reserves to meet this demand. Hydraulic fracturing as a well completion/ stimulation technique is a water-intensive process and requires several million liters of water to fracture a single well (Clark and Veil, 2009; Kargbo et al., 2010; Soeder and Kappel, 2009). Over the lifetime of a well, 10–70% of the water pumped into the well to fracture the rock formation returns to the surface as flowback (FB) together with large volumes of produced water (PW) (Lester et al., 2015). Currently, O&G wastewater management strategies are mostly inclined towards injection into Class II disposal wells for economic reasons (Vengosh et al., 2014). Yet, increasing seismic activity near injection well sites has increased the necessity to reduce this management practice and shift towards potential treatment and reuse of O&G wastewater (Kondash et al., 2017; Vengosh et al., 2014). However, water reuse for crop irrigation, streamflow augmentation, well drilling, and dust suppression can be achieved only following partial or full treatment to remove contaminants of concern (Guerra et al., 2011; Vengosh et al., 2014). FB and PW contain a variety of drilling fluid chemicals (e.g., proppants, friction reducers, surfactants, biocides, etc.), dissolved organic matter, metals, naturally occurring radioactive material, volatile and aromatic hydrocarbons, dissolved gases, and high concentrations of salts that can exceed that of seawater (Harkness et al., 2015; Kargbo et al., 2010; Lester et al., 2015). Notably, the upstream O&G sector has experienced an increase in wastewater generation containing high concentrations of halides. Halides naturally present in O&G wastewater include chloride (Cl), bromide (Br), and iodide (I), likely of geogenic origin from contact with ancient marine shale formations (Vengosh et al., 2017). In seawater, chloride, bromide, and iodide can be found at concentrations up to 23,000 mg/L, 80 mg/L, and 60 mg/L, respectively (Kim et al., 2015; Mullaney et al., 2007). In the Denver-Julesburg (DJ) basin and the Barnett shale in the U.S., chloride has been measured at concentrations ranging from 8900 to 31,300 mg/L, bromide from 125 to 170 mg/L, while iodide concentrations were up to 40 and 55 mg/L, respectively (Freedman et al., 2017; Liberatore et al., 2017; Oetjen et al., 2017). In comparison, typical concentrations of chloride (0.3–230 mg/L) (Mullaney et al., 2007), bromide (5–150 lg/L) (Hem, 1985; Neal et al., 2007), and iodide (<10 lg/L) (Yeager et al., 2017) in freshwater are orders of magnitude lower than those found in PW and seawater. Thus, there is a growing concern of diminishing the quality of freshwater resources by introducing persistent, mobile, and potentially toxic substances derived from O&G activities into natural aquatic environments during potential wastewater reuse, surface water discharge, or accidental release. Emerging research has

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for O&G water reclamation. The potential formation of halogenated byproducts during biological treatment of O&G wastewater has not been investigated so far; their main research objectives were directed towards improving the removal of organic matter during BAF treatment, particularly beyond 90% DOC removal in 24 h. However, in this study the emergence and fate of five volatile IOCs—chloroiodomethane, diiodomethane, triiodomethane, chlorodiiodomethane, and bromodiiodomethane—were investigated during BAF treatment of O&G PW at different time periods (I–III) using headspace solid-phase microextraction gas chromatography mass spectrometry (HS-SPME-GC–MS). Selection of targeted IOCs was made prior to this study based on an initial GC–MS screening of BAF-treated PW and mass spectral identification of prominent peaks. A qualitative and quantitative analytical approach was employed to monitor and establish relationships between the occurrence of volatile IOCs in BAF effluent and three different granular activated carbon (GAC) media types that had been used in a previous investigation (Riley et al., 2018a). Results from Riley et al. (2018a) suggested that GAC media selection can affect biodegradability of organic matter during BAF treatment (and consequently formation of transformation products) as GAC properties (e.g., pore size distribution) have an impact on biofilm formation and stability in the GAC pores. In our study the presence of IOB in the aqueous phase, as suggested by earlier research on microbial communities in O&G impoundments (Mohan et al., 2013), was also evaluated. Assessment of IOC formation mechanisms (abiotic vs. biotic) was evaluated through microbial community analysis and basic oxidation jar testing experiments under controlled conditions. Furthermore, understanding the fate of IOCs after treatment is critical in understanding handling and storage of BAF treated effluent. Therefore, samples analyzed at time period III were re-examined 137 days after collection to understand the fate of IOCs after prolonged storage and potential implications for O&G wastewater management.

2. Materials and methods 2.1. Biological active filter (BAF) system Water samples for this study were obtained from a laboratoryscale BAF system consisting of nine biologically active filter columns. In brief, columns were constructed with clear polyvinyl chloride (PVC) pipe (5 cm inner diameter, 147 cm length, 76 cm media depth) and were connected to individual storage tanks using clear polyethylene tubing. Columns were connected to a peristaltic pump and operated in up-flow, batch configuration as shown in Fig. 1. Three sets of three columns were filled with three distinct GAC media as specified in Table 1. Details regarding column optimization and specifications can be found elsewhere (Riley et al., 2018a).

Table 1 Media specifications for the BAF columns investigated in this study. All GAC media were steam activated with different surface area characteristics and pore size distribution. Adapted from (Riley et al., 2018a).

BAF column designation Origin Physical form Mesh size Activation method Surface area (m2/g) Micropores (%) Mesopores (%) Macropores (%) Biologically active?

NoritÒ G816a

NoritÒ G400b

DarcoÒ D1240b

1, 2, 3 Bituminous Granular 8  16 Steam 511 32% 68% 0.40% Yes

4, 5, 6 Bituminous Granular 12  40 Steam 862 42% 57% 0.65 No

7, 8, 9 Lignite Granular 12  40 Steam 1240 60% 40% 0.47% No

a Spent media from Peter D. Binney Water Purification Facility, Aurora Water, Aurora, Colorado. b Cabot Corporation, Alpharetta, GA.

Fig. 1. Schematic of individual biological active filter (BAF) column setup. Three sets of three BAF columns were operated independently. Arrows indicate flow direction. Refer to Table 1 for GAC specifications of each column.

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2.1.1. Experiment set up Raw PW (i.e., a blend of O&G wastewater from several production wells of the same well pad) had been obtained from the DJ Basin (Colorado, U.S.) and was stored in opaque 1 m3 totes (with headspace) at ambient temperature (~21 °C) until experiments started. Each tote was replenished with PW from the same well pad every 3 to 4 weeks. The raw PW was oxidized with 10 mg/L KMnO4 at least 24 h prior to each experiment. Oxidant dosage was optimized and primarily used as an additional pre-treatment step to remove dissolved iron and manganese and prevent clogging in the filters as a result of the formation of iron and manganese oxides. Each column was continuously aerated (10 mL/min) at the inlet of the columns and operated in batch mode at 2.44 m/h (1 gpm/ft2; 83 mL/min) hydraulic loading rate with approximately 10 L of PW. Columns were operated in parallel over a 13-week period with a new batch of PW treated at 72-h intervals (referred to as BAF influent). After 72 h of treatment, BAF effluent was collected immediately after passing through the filter, columns were backwashed with chlorinated tap water (50% bed expansion, 1.1 mg/L chlorine residue typically present in tap water from City of Golden, CO) for 10 min, and each column was replenished with fresh PW. To allow drawing conclusions from previous research, the partially open design and operation of the bench-scale BAF system had not been altered for the purpose of this study. Accordingly, potential losses of volatile organic compounds to the ambient air were to be expected and were omitted in this initial investigation. 2.1.2. Sample collection and analyses One BAF influent sample was collected before treatment (after KMnO4 pre-treatment) while BAF effluent samples were collected after 72 h of treatment at three arbitrary time points over a 13 week period treatment time periods: at week 4 (time period I), week 8 (time period II), and week 13 (time period III) after starting the system (i.e., week 1). Duplicate samples were collected without headspace in 40 mL amber vials and stored at 4 °C until analysis (less than three days). pH, electrical conductivity (EC), DOC, IOC, and iodide concentrations were measured at every sampling time period whereas major anion and cation constituents were only measured during time periods I and II. Additionally, samples collected during time period III were stored after their initial analysis with approximately 5 mL of headspace at 4 °C in the dark (presence of headspace is representative of typical storage conditions of treated PW for further downstream applications), with no preservatives or pH adjustments, and were re-analyzed after 137 days of storage to evaluate the fate of IOCs. 2.1.3. Produced water quality analyses DOC was analyzed using a Shimadzu TOC-L analyzer (Columbia, MD). Samples were filtered using 0.45 lm polypropylene filters and acidified (pH  2) with hydrochloric acid. Samples with DOC exceeding 100 mg/L DOC were diluted (1:10) with ultrapure water. Samples were analyzed for iodide concentration, EC (Cole Parmer, Vernon Hills, IL), and pH (VWR, Radnor, PA) using calibrated handheld probes at room temperature (20 °C). For iodide concentrations, samples were diluted 1:20 with ultrapure water. Samples were analyzed using an iodide double-junction ion-selective electrode (Cole Parmer) and concentrations calculated using a fivepoint calibration (0.1, 1, 5, 10, 50 mg/L I) made by dilution of a 1000 mg/L I stock solution (Cole Parmer). Analysis of major cations and anions was conducted using an inductively coupled plasma-atomic emission spectroscopy (ICP-AES) system (5300DV, Perkin Elmer, Waltham, MA) and Dionex ion chromatography (IC) system (ICS-900, Thermo Scientific, Waltham, MA), respectively. Samples were diluted 1:50 (Cl < 300 mg/L) and 1:60 (Cl < 100 mg/L) for ICP-AES and IC, respectively, to prevent chloride instrument saturation.

2.2. Oxidation jar testing experiments Basic oxidation experiments to assess the influence of abiotic formation of I-DBPs during BAF treatment were conducted in duplicates in 250 mL amber bottles without headspace. Aliquots of PW and GAC were sterilized in a MarketForge Sterilmatic autoclave for 30 min at 121 °C and 100 kPa according to manufacturer specifications to serve as a biologically inactive control. KMnO4 was added to the respective autoclaved and non-autoclaved raw PW to a final concentration of 10 mg/L KMnO4. After 24 h water samples were analyzed for IOCs and other major constituents. The effect of chlorine on IOC formation after 10 min of GAC backwashing with chlorinated tap water was also tested. 40 g of autoclaved dry GAC was submerged for 10 min in 500 mL of tap water containing 1.1 mg/L Cl2 (representative of the water that was in contact with GAC during backwashing). Water was decanted and 8 g of wet GAC was added to each of the two 250 mL glass bottles containing non-autoclaved PW and each of the two 250 mL glass bottles containing autoclaved PW. Water samples were analyzed for IOCs and other major constituents after 24 h of contact with the GAC. Chlorine residual in the 250 mL bottles was estimated by calculating the mass of chlorine added from excess water in the wet GAC using Eq. (1):

C Cl2 ;PW ¼


M wet  Mdry 1
C Cl2 ;tap
0:250 L densityH2O

ð1Þ

where Mwet and Mdry are the wet and dry mass of GAC, respectively; CCl2,tap and CCl2,PW are the concentration of chlorine in tap water and 250 mL PW, respectively; assuming a water density of 1000 g/L and homogenous GAC/water distribution. For simplification, we also assumed that chlorine concentration in the GAC pores was the same as the initial chlorine in the tap water and that there was no chlorine decay. Cl2 concentrations were measured using Hach Method 8167. 2.3. Analysis of iodinated organic compounds (IOCs) 2.3.1. Chemical reagents Reagents used for chemical preparations were HPLC grade and were obtained from Fisher Scientific (Hampton, NH). Analytical grade diiodomethane (99%), triiodomethane (99%), chloroiodomethane (97%), triiodomethane-d (99%), and diiodomethane-d2 (99%) were obtained from Sigma-Aldrich (St. Louis, MO) and chloroiodomethane-d2 (98%) was obtained from Cambridge Isotope Laboratories, Inc. (Tewskbury, MA). 2.3.2. Chemical reagent preparation Standard stock solutions of chloroiodomethane, diiodomethane, and triiodomethane in pure and deuterated form were prepared separately. Standard stock solutions (1 lg/lL) were made by adding 0.01 g of each compound into a 10 mL glass flask and diluted with methanol. Stock solutions were pipetted into 2 mL amber glass vials with no headspace, closed with screw caps sealed with parafilm, and stored in dark conditions at 4 °C to avoid photo degradation and evaporation. 2.3.3. HS-SPME-GC–MS IOCs were analyzed by HS-SPME-GC–MS using a polydimethylsiloxane/divinylbenzene (PDMS/DVB) fiber (Supelco, Bellafonte, PA). Evaluation of four important extraction parameters (i.e., fiber material, extraction temperature and time, and desorption time) during method development can be found in the supplementary material (SM) (Figs. S1–S4). SPME sample preparation (n = 1) consisted of 5 mL of sample pipetted into a 20 mL GCSPME amber vial covered with a 1.3 mm thick PTFE/silicone septa

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and magnetic screw cap, followed by the addition of deuterated internal standard stock solution. Recoveries for this method were within 70–110% and standard deviations ranging from 1% to 14% (Fig. S5). Analysis was performed with a Trace 1310 gas chromatograph (Thermo Scientific, Waltham, MA) equipped with a capillary column and a split/splitless baffle liner (2 mm  2.75  120) (Thermo Scientific). The instrument was equipped with a Thermo Scientific TriPlusTM RSH autosampler and a TSQTM 8000 Evo Triple Quadrupole mass spectrometer (MS). The MS was operated in full scan mode (mass range 45–500 m/z) using electron-ionization (70 eV ionization energy). Extraction, GC, and MS method details are summarized in Table 2. 2.3.4. Identification and quantitation of IOCs Full scan mode was employed with the purpose of tracking other emerging IOCs and transformation products in the water at Table 2 HS-SPME-GC–MS method parameters. GC parameters Carrier gas Flow Sample volume Headspace volume Internal Standard Inlet mode Injector temperature (PTV) Capillary column Column dimensions

Helium 1.0 mL/min 5 mL 15 mL 2 lL of 1 lg/lL in MeOH (C = 400 lg/L) 1:4 Split 230 °C Rxi-5 Sil MS column phenylene dimethyla polysiloxane 30 m  0.32 mm  0.25 lm film thickness

Extraction parameters 65 lm (PDMS/DVB)b Sequential 250 RPM 40 mm 60 min 45 °C 3 min 250 °C 1 min (2 min total)

Fiber material Agitator speed Sampling depth Extraction time Extraction temperature Desorption time Pre- and post-desorption temp Pre- and post-desorption time

Oven program Initial temperature: hold time Initial temperature ramp: hold time Final temperature ramp:

35 °C; 3 min 10 °C/min to 80 °C held 2.5 min 7 °C/min to 180 °C held 0 min 15 °C/min to 280 °C

any given time period of sampling. Total ion chromatograms (Fig. S6) and extracted ion chromatograms were analyzed using Chromeleon 7.2 software (Thermo Scientific). Compound identification was achieved using mass spectra and retention times of analytical standards as well as reference database spectra (NIST standard reference database, version 2.2). Quantification of chloroiodomethane, diiodomethane, and triiodomethane was achieved based on isotope dilution using selected target ions and appropriate isotope-labeled analogs (Table 3). At least five calibration solutions were prepared in both ultrapure water and BAF matrix (calibration standards concentrations were 1, 5, 25, 50, 100, 500, 1000, 2500, and 5000 lg/L) to establish matrix correction factors and adjust calculated concentrations accordingly. Calibrations achieved acceptable linearity for each analyte (R2 > 0.989). Correction factor details can be found in Table S1 in the SM. Other volatile organic compounds that exhibited prominent peaks in the total ion chromatograms were identified based on their mass spectra (Table 3, additional information provided in the SM, Table S2) and semi-quantified using internal standard method with diiodomethane-d2 as the internal standard. Semi-quantification was achieved as described elsewhere (Regnery et al., 2016) assuming a response factor of 1 with the internal standard. Method detection and quantification limits (MDL and MQL, respectively) for target analytes were calculated based on the linear regression method (Shrivastava and Gupta, 2011) using the following expressions:

MDL ¼ 3
Sa =b

ð2Þ

MQL ¼ 10
Sa =b

ð3Þ

where Sa represents the standard deviation of the response, and b represents slope of the low range calibration curve. It should be noted that due to difficulties with the extraction of triiodomethane-d in BAF influent samples, triiodomethane-d was not used as an internal standard for quantification of triiodomethane. Thus, triiodomethane was quantified using diiodomethane-d2 as the reference internal standard in all samples and correction factors applied accordingly for concentration corrections. MDL and MQL were in the range of 0.8–6.7 lg/L and 2.6– 22.5 lg/L, respectively, and were deemed acceptable for this study (Table 3). Analyte specific details including retention time, selected target ions, and detection and quantification limits are summarized in Table 3.

MS parameters Ion source Source temperature Electron energy Transfer line temperature Emission energy Scan range a b

Electron ionization 220 °C 70 eV 250 °C 50 lA 45–500 Da

Perkin Elmer, Waltham, MA. Supelco/Sigma Aldrich, St. Louis, MO.

2.3.5. Quality assurance and quality control All analyses consisted of control blanks, method blanks, fiber blanks, ultrapure water blanks, and check standards. In addition, thermal degradation products of triiodomethane were evaluated at established method parameters by analyzing spiked samples (400 lg/L triiodomethane). Signal responses of all blanks were established to be below MDLs and all check standards and recoveries were 100 ± 30% of the spiked concentration.

Table 3 Analytical parameters of five iodinated organic compounds analyzed in this study. Compound

Labeled analog/internal standard

Primary m1/z

Secondary m1/z

Formula

MW [g/mol]

RT [min]

MDL [lg/L]

MQL [lg/L]

Chloroiodomethane Diiodomethane Triiodomethane Chlorodiiodomethane Bromodiiodomethane

Chloroiodomethane-d2 Diiodomethane-d2 Triiodomethane-d Diiodomethane-d2 Diiodomethane-d2

141/143 268/270 268/269 175 219

178/180 141/143 394/395 302 346

CH2ClI CH2I2 CHI3 ClCHI2 BrCHI2

176.4 267.8 393.7 302.2 346.7

2.44 5.89 13.68 9.45 11.55

1.2 0.8 6.7 n.a. n.a.

3.9 2.6 22.5 n.a. n.a.

Abbreviations: MW- molecular weight. RT – retention time. m/z – quantifier ions. MDL - method detection limit. MQL - method quantification limit. n.a. – not applicable.

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2.4. Microbial community analysis Effluent samples (n = 4) were collected from the four best performing (in terms of DOC removal) BAF columns (BAF 1–4) in sterile 1 L amber bottles after 72 h of BAF treatment. 300–500 mL of each sample was filtered through 0.2 lm SterivexTM filters (Millipore Sigma, Burlington, MA), and the filters were used to obtain microbial DNA. As a control, a single BAF influent sample of PW was collected and processed in a similar manner. Genomic DNA was extracted using a DNeasy PowerLyzer PowerSoil DNA extraction Kit (Qiagen, Inc., Germantown, MD) as specified by the manufacturer. DNA preparation included a 45 s bead-beating step using a BioSpec MiniBeadbeater-16 (BioSpec Products Inc. Bartlesville, OK). DNA was quantified using a Qubit Fluorometer and a Qubit dsDNA High Sensitivity Assay Kit (Thermo-Fisher, Inc.). Microbial cell concentrations were approximated from DNA yield assuming an average genome size similar to E. coli, where one cell contains ~51015 g of DNA (Torsvik and Goksøyr, 1978). DNA samples (1 ng) were amplified using 515F (50 GTGCCAGCMGCCGCGGTAA 30 ) and 806R (50 GGACTACHVGGGTWTCTAAT 30 ) primers following the two-step amplification and barcoding strategy described elsewhere (Stamps et al., 2016). Illumina MiSeq sequencing was performed by the Duke University Center for Genomic and Computational Biology using Illumina 2  250 chemistry. Post sequencing samples were demultiplexed and barcodes were removed using Sabre (https:// github.com/najoshi/sabre), allowing for zero barcode mismatches. The rRNA gene sequences (called ‘amplicon sequence variants’ or ASVs) (Callahan et al., 2016) were initially analyzed using DADA2 (Callahan et al., 2017) for the following: removal of PCR primer sequences and low-quality bases, merging paired end reads, chimera removal, taxonomy assignment using Silva Version 128 (Pruesse et al., 2007), and ASV table construction. The ASV table, taxonomy table, and metadata were imported into Phyloseq (McMurdie and Holmes, 2013). Prior to data visualization the ASV table was converted to compositional (i.e., relative percent) and filtered to retain ASVs representing >0.1% of a samples’ composition. 3. Results and discussion 3.1. Produced water quality The BAF influent and effluent water quality is summarized in Table 4. Removal of DOC and total nitrogen (TN) were variable throughout the

different time periods of treatment with an average removal of 54 ± 22% DOC (min. 7%/max. 79%) and 43 ± 25% TN (min. 17%/max. 94%), respectively. TN removal was likely attributed to the biodegradation of organic nitrogen (not measured) and ammonia, which accounted for most of the nitrogen (25 mg/L NH3-N) present in the PW. Concentrations of nitrate and phosphate were below their respective MDLs in all samples. Dissolved iron concentrations in the BAF influent were relatively low compared to raw PW, as expected from the oxidation pretreatment with KMnO4. Slightly elevated dissolved manganese levels in the BAF influent compared to raw PW (Table 5) resulted from KMnO4 addition. Under the prevalent operational BAF conditions, remaining aqueous Fe(II) was oxidized to insoluble Fe(III) by dissolved oxygen within 72 h of treatment. As the reaction of aqueous Mn(II) with dissolved oxygen is considerably slower (i.e., at least 106 times) at pH 7 compared to Fe(II) (Martin, 2005), no further decrease in dissolved manganese concentration was observed (Table 4). 3.2. Halide concentrations Iodide, chloride, and bromide were measured in the BAF influent at concentrations substantially above those typical of freshwater environments (Table 4). A study by Oetjen et al. (2017) showed a linear increase of iodide (~0.2 to 20 mg/L), bromide (~2 to 90 mg/ L), and chloride (~65 to 7000 mg/L) concentrations as an O&G production well transitioned from FB to PW (~6 days), at which concentrations remained relatively constant during the PW period. This suggests that a substantial portion of halides in PW originates from the rock formation (geogenic) rather than the fracking fluid injected (anthropogenic) to stimulate well production. Nevertheless, NaBr, NaI, and NaCl are additives occasionally used in some fracking fluids as corrosion inhibitors, biocides, and to adjust fluid density (Ground Water Protection Commission, 2019) and might have contributed to observed halide levels in the utilized O&G wastewater. Iodide concentrations in the BAF influent and effluent ranged from 40 to 53 mg/L I and 5 to 62 mg/L I, respectively (Table 4). Iodide concentrations in the BAF effluent seemed to decrease over the runtime of the experiment. Generally higher iodide concentrations in BAF effluent were observed for time period I compared to time period III (Table 5). Bromide was measured in the BAF influent at a concentration ranging from 104 to 148 mg/ L Br. Its concentration in BAF effluent samples ranged from 104 to 132 mg/L Br (Table 4). Notably, a reduction of Br concentration by 30% after treatment was observed in BAF 9 (lignite coal acid

Table 4 General water quality parameters of DJ Basin PW before and after KMnO4 oxidation. Values represent average raw PW, BAF influent, and BAF effluent values and standard deviations. Values below the respective detection limit are denoted ‘BDL’, ‘n.a.’ indicates not applicable/measured. Parameter

Unit

Raw PW

Influent

Effluent

Sample size (n) pH EC DOC DOC removal TN TN removal Iodide (I) I transformation

S.U. mS/cm mg/L % mg/L % mg/L %

4 7.5 ± 0.3 30.5 ± 1.3 369 ± 99 n.a. 32 ± 2 n.a. 47 ± 1 n.a.

3 6.7 ± 0.0 31.4 ± 0.8 420 ± 141 n.a. 31 ± 2 n.a. 48 ± 6 n.a.

27 7.2 ± 0.2 32.0 ± 0.7 192 ± 95 54 ± 22 22 ± 5 43 ± 25 33 ± 16 32 ± 30

Sample size (n) Bromide (Br) Chloride (Cl) Nitrite (NO 2) Nitrate (NO 3) Calcium (Ca2+) Magnesium (Mg2+) Iron (Fe2+) Manganese (Mn2+) Sulfur (S) Sodium (Na+)

mg/L mg/L mg/L mg/L mg/L mg/L mg/L mg/L mg/L mg/L

4 130 ± 4 11,306 ± 97 8.7 ± 0.5 BDL 340 ± 19 41 ± 3 22.4 ± 18.2 0.5 ± 0.9 16.1 ± 5.8 5788 ± 544

3 125 ± 18 10,306 ± 729 15.0 ± 3.6 BDL 330 ± 20 38 ± 2 1.0 ± 0.2 3.2 ± 1.0 7.7 ± 2.3 5606 ± 704

17 114 ± 9 10,214 ± 503 16.0 ± 3.3 BDL 324 ± 28 38 ± 2 0.8 ± 1.3 3.4 ± 1.9 9.2 ± 1.9 5330 ± 375

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Table 5 Concentration of three IOCs and several water quality parameters - iodide, TN, DOC, pH, and EC measured in BAF influent and after 72 h of biological treatment at three treatment time periods (I: week 4; II: week 8; and III: week 13 after starting the system (week 1)). ‘BQL’ indicates values below the respective method quantification limit. BAF column 1S CH2ClI CH2I2 CHI3 DOC TN Iodide pH EC

Unit lg/L lg/L lg/L mg/L mg/L mg/L S.U. mS/cm

CH2ClI CH2I2 CHI3 DOC TN Iodide pH EC

lg/L lg/L lg/L

CH2ClI CH2I2 CHI3 DOC TN Iodide pH EC

lg/L lg/L lg/L

CH2ClI CH2I2 CHI3 CHClI2* CHBrI2* Iodide DOC TN

lg/L lg/L lg/L lg/L lg/L

mg/L mg/L mg/L S.U. mS/cm

mg/L mg/L mg/L S.U. mS/cm Unit

mg/L mg/L mg/L

2S

3S

4N

5N

6N

7D

8D

9D

Influent

Time period I BQL BQL 7.6 2.9 92 175 212 182 18 13 47 62 7.3 6.9 32.8 32.5

BQL 15.1 3979 101 7 5 7.0 32.7

BQL 10.6 164 209 20 46 7.3 33.2

BQL 7.1 180 259 18 46 7.0 33.0

BQL BQL 191 349 19 33 7.6 32.6

BQL 10.7 118 300 19 52 6.9 32.4

BQL 37.4 118 283 19 56.6 7.1 32.6

BQL 5.1 212 385 22 43 7.4 33.3

BQL 21.8 125 616 33 53 6.6 32.1

Time period II BQL BQL 13.7 7.6 BQL 570 34 18 18 16 24 12 7.3 7.4 31.1 31.8

BQL 6.0 124 19 19 40 6.9 31.7

BQL 8.7 1156 116 28 20 7.5 31.2

BQL 4.9 226 165 29 35 7.5 31.8

BQL 41.7 765 183 29 20 7.4 31.8

BQL 14.5 173 123 27 39 7.2 31.4

14.6 426 75 177 26 10 7.4 31.9

BQL BQL 319 225 28 34 7.3 31.1

BQL BQL 68.9 292 31 40 6.7 31.1

Time period III BQL BQL 9.0 8.7 334 174 89 111 19 19 40 50 7.2 6.7 30.7 31.3

BQL 9.1 197 107 20 50 7.1 31.7

BQL 129 1220 211 24 25 7.1 31.6

BQL 28.0 662 262 25 16 6.9 30.9

BQL 14.4 802 239 25 20 6.9 31.8

BQL 18.9 805 263 25 17 7.4 32.3

BQL 18.8 372 268 26 6 7.4 32.3

BQL 8.2 574 293 24 43 7.4 32.0

BQL BQL 95 351 28 52 6.7 30.6

84.2 >2500 >2500 2021 100 0.4 246 28.1

33.0 1918 >2500 >2500 337 1.0 237 27.9

124 >2500 >2500 1157 46.6 0.4 244 28.1

306 >2500 BDL 30.2 BDL n.m 234 27.9

131 >2500 >2500 >2500 364 4.8 283 29.7

BDL BDL BDL BDL BDL n.m 340 35.1

Samples from time period III after 137 days of storage 21.4 17.0 42.7 132 895 594 >2500 >2500 >2500 >2500 >2500 >2500 >2500 >2500 >2500 >2500 284 541 563 153 3.9 2.5 4.1 0.6 90.5 112 105 207 20.7 20.3 22.2 28.0

S: Spent media, N: Norit media, D: Darco media, CH2ClI: chloroiodomethane, CH2I2: diiodomethane, CHI3: triiodomethane, CHClI2: chlorodiiodomethane, n.m.: not measured. * Note: These compounds were only semi-quantified due to their unexpected detection after 137 days of sample storage with headspace; Gas-phase concentrations were not considered/measured during aqueous concentration calculations.

washed GAC media) at time period II. Chloride concentration in the BAF influent exceeded 11,000 mg/L Cl, and the concentration in the BAF effluents ranged from 9626 to 10,811 mg/L Cl (Table 4). Overall, halide concentrations, especially iodide, decreased during BAF treatment, indicating utilization of halides within the BAF in halogenation reactions and the potential formation of iodinated, brominated, and/or chlorinated compounds. However, bromide and chloride concentrations did not correlate with iodide concentrations or DOC removal except for BAF 3 (spent GAC media) during time period I. Of particular interest was the observed reduction in iodide (90% decrease from 53 to 5 mg/L I), bromide (13% decrease from 123 to 106 mg/L Br), and chloride (15% decrease from 11,337 to 9627 mg/L Cl) concentrations in BAF 3 at time period I, which corresponded with the highest DOC removal and measured IOC formation in this time period (83% DOC removal; ~4 mg/ L IOCs) as discussed in Section 3.3. Additionally, iodide concentrations were reduced between 90% and 99% in BAF effluent samples from time period III sampling after storage with headspace for 137 days (Table 5). This corresponded with the detection of high concentrations of IOCs. 3.3. Emergence of IOCs during BAF treatment Representative total ion chromatograms of BAF influent and effluent for quantification of IOCs are shown in Fig. S6. Five IOCs, namely chloroiodomethane, diiodomethane, triiodomethane,

chlorodiiodomethane, and bromodiiodomethane were analyzed in O&G PW treated in the BAFs (Table 5). While all five compounds contained iodide moieties, three of them also contained bromide and chloride moieties; they can also be classified as chlorinated and/or brominated organic compounds. Despite the observed slight reduction of bromide concentration in several BAFs (max. 30%), brominated compounds were not detected during the initial sampling time periods I–III. Levels of iodide and detected IOCs in the BAF effluent varied substantially throughout the three sampling time periods (Table 5) for each individual column (BAF 1–9). However, chloroiodomethane, diiodomethane, and triiodomethane were frequently measured at substantially higher concentrations in BAF effluent samples compared to the BAF influent, highlighting their formation during biological treatment (Table 5). As mentioned earlier, losses of IOCs to the ambient atmosphere during BAF operation due to volatilization were not accounted for in this study as the partially open BAF system had not been specifically designed to focus on the fate of volatile organic compounds during treatment. Triiodomethane was detected in BAF influent samples during each time period (69–125 lg/L), whereas diiodomethane was only present during time period I (Table 5). Although both IOCs have relevance as industrial chemicals (Coday et al., 2015; Sheen et al., 2008; Shibata et al., 1986), they are not used for hydraulic fracturing according to FracFocus (Ground Water Protection Commission, 2019). As none of the targeted IOCs were present in samples (n = 2)

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of raw PW that had been collected directly from the tote prior to KMnO4 dosing, their abiotic formation during KMnO4 oxidation prior to the start of the biological treatment was assumed. Previous studies observed the formation of triiodomethane and other I-DBPs during wastewater oxidation with KMnO4 (Ye et al., 2012), as the use of strong oxidants can act as a catalyst to oxidize iodide to reactive iodine species. In our study KMnO4 was mainly added with the purpose of removing dissolved iron to prevent clogging of the filters despite the likelihood of IOC formation during oxidation. Evaluation of the effect of 10 mg/L KMnO4 added during pretreatment of BAF influent as well as residual backwash chlorine (~1.1 mg/L Cl2) was conducted in jar test experiments containing autoclaved raw PW (as a biologically inactive control) and raw PW (Table 6). Only triiodomethane was measured above its MDL in this series of experiments in autoclaved PW after 10 mg/L KMnO4 dosage with an average triiodomethane concentration of 80 lg/L (Table 6). This was in the range of triiodomethane concentrations measured in BAF influent during time periods I to III (Table 5). The iodide concentration in samples spiked with KMnO4 showed no change compared to iodide concentrations in raw PW (Tables 4 and 6), indicating rather insignificant contribution of KMnO4 pre-treatment to iodide oxidation and IOC formation in O&G wastewater in our study. Additionally, the effect of residual chlorine after backwashing with tap water did not seem to play a critical role in formation of the targeted volatile IOCs. Although jars with chlorine and GAC had lower iodide concentrations compared to solely PW, none of the analyzed IOCs were detected above their respective MDLs (Table 3). Unfortunately, iodate was not measured in these samples to confirm iodide oxidation to iodate by chlorine. Moreover, potential formation of other non-volatile IOCs was not assessed in this initial study. Ye et al. (2012) had demonstrated that triiodomethane can form at concentrations up to 390 lg/L at pH 7 during oxidation of water with 15.8 mg/L KMnO4 (DOC = 2.3 mg/L; I = 0.1 mM (12.7 mg/L)). In our study DOC and I concentrations were substantially higher; thus, it remains to be tested if higher oxidant concentrations are required to catalyze the oxidation of iodide to reactive iodine species in relevant quantities in complex matrices such as PW. Nevertheless, abiotic formation of volatile IOCs due to KMnO4 pre-treatment and chlorine residues played a minor role in this treatment scheme as shown in the jar test experiments. An overview of the distribution (evaluated by the location of the minimum and maximum) and variability (evaluated by the interquartile range) of total quantified IOC formation, iodide removal, and DOC removal in columns with the same GAC and sampling time period is shown in Fig. 2. Overall there were no clear trends between the presence of specific GAC media types and the

formation of IOCs. Measured IOC concentrations had variable fluctuations among different columns and over the three sampling periods. In accordance with findings by Riley et al. (2018a), BAF columns filled with spent GAC media had substantially higher DOC removal performance throughout the study. While spent GAC media provided enhanced DOC removal performance, GAC media type revealed no statistically significant differences with regard to analyzed IOC. Across three sampling time periods IOC concentrations increased irrespective of media type (Fig. 2(a–ii)), with similar variability during time periods II and III. Iodide concentration decreased over the three time periods (depicted as an increase in iodide removal/transformation in Fig. 2(a–iii)), which coincided with an increase of IOC concentration overtime (Fig. 2 (a–ii)). The established negative correlation (r = 0.7 to 0.8, p < 0.05, n = 9) between iodide concentration and IOC occurrence suggests that iodide is oxidized and used in halogenation reactions, resulting in the formation of IOCs. The observed increase in IOC concentrations over the three sampling time periods, in conjunction with increasing iodide transformation, supports our research hypothesis of biologically mediated iodide oxidation during BAF treatment as discussed in Section 3.5. Looking at individual columns in more detail, the highest concentration of triiodomethane measured was 3979 lg/L in BAF 3 during time period I. Analysis of iodide concentrations in BAF 3 showed that approximately 48 mg/L (90%) of iodide transformed into other species. Although a representative iodide mass balance cannot be fully established for the partially open bench-scale BAF system due to the volatility of IOCs and the lack of analytical methods to measure all iodide species (i.e., iodine, iodate, other IOCs that were not HS-SPME-GC–MS amenable) in complex water matrices such as O&G wastewater, Table S6 in the SM provides a general estimation for the utilization of iodide, chloride, and bromide in mg/L basis to form the measured concentrations of IOCs (1.03 CHI3 – I = CHI3; 1.05 CH2I2 – I = CH2I2; 1.4 CH2ClI – I = CH2ClI, 1.36 CHBrI2 – I = CHBrI2; 1.2 CHClI2 – I = CHClI2). According to this estimation, the highest observed concentration of iodide utilized in volatile IOC formation corresponds to 3.9 mg/L I which was transformed to 3979 lg/L triiodomethane (30 mM I) (Table S3). For BAF 3, the overall IOC concentration declined over the next two sampling time periods, while system performance remained at 93% and 70% DOC removal in time period II and III, respectively. Previous reports indicate that bactericidal properties of triiodomethane and other reactive iodine species might impair microbial communities degrading organic matter in the GAC biofilm at high IOC levels (Gottardi, 2001). The highest concentration of diiodomethane was 426 lg/L in BAF 8 (lignite coal acid washed GAC media) during time period II (Table 5). Chloroiodomethane

Table 6 PW quality parameters after addition of oxidant and 24 h incubation at ambient temperature conditions (n = 2; CCl2,PW = 0.017 mg/L). Parameter

Unit

PW

PWa

PW + Cl2

PWa + Cl2

PW + KMnO4

PWa + KMnO4

IOCs* pH EC DOC TN Iodide (I) Bromide (Br) Chloride (Cl) Calcium (Ca2+) Iron (Fe2+) Manganese (Mn2+) Sulfur (S) Sodium (Na+)

lg/L

BDL 7.2 ± 0.0 31.2 ± 0.6 173 ± 0.2 33 ± 0.9 45 ± 0.3 127 ± 0.5 11.3 ± 0.03 356 ± 1.7 0.8 ± 0.08 0.6 ± 0.0 18.9 ± 1.7 5.8 ± 0.2

BDL 7.7 ± 0.0 29.8 ± 0.2 172 ± 1.6 32 ± 0.6 48 ± 0.3 133 ± 1.7 11.3 ± 0.05 324 ± 3.6 0.4 ± 0.0 0.4 ± 0.0 10.7 ± 0.0 5.8 ± 0.2

BDL 7.1 ± 0.0 31.1 ± 0.7 76 ± 2.0 27 ± 0.3 39 ± 0.2 131 ± 1.4 11.3 ± 0.01 332 ± 3.5 0.3 ± 0.03 0.4 ± 0.01 6.5 ± 2.6 5.7 ± 0.3

BDL 7.5 ± 0.0 30.1 ± 0.0 64 ± 1.0 25 ± 0.0 36 ± 0.8 129 ± 0.4 11.4 ± 0.03 288 ± 2.5 BDL 0.21 ± 0.0 10.4 ± 0.0 5.7 ± 0.3

BDL 7.3 ± 0.0 31.8 ± 0.5 171 ± 1.1 32 ± 0.1 48 ± 0.1 134 ± 0.3 11.6 ± 0.03 359 ± 4.6 0.2 ± 0.1 3.89 ± 0.02 10.8 ± 0.0 5.7 ± 0.2

80 ± 6.8 7.6 ± 0.0 31.9 ± 0.3 168 ± 0.7 31 ± 0.8 48 ± 0.9 144 ± 3.8 11.7 ± 0.05 331 ± 6.0 BDL 0.71 ± 0.01 10.4 ± 0.0 5.8 ± 0.2

S.U. mS/cm mg/L mg/L mg/L mg/L g/L mg/L mg/L mg/L mg/L g/L

Abbreviations: PWa: autoclaved produced water; BDL: below detection limit. *only triiodomethane was detected.

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Fig. 2. Evaluation of distribution (evaluated by minimum and maximum) and variability (evaluated by interquartile range) of BAFs from a given (a) time period and (b) GAC media for (i) system performance (i.e., DOC removal), (ii) concentration of selected IOCs, and (iii) iodide transformation. Note that each data set is composed of nine (n = 9) samples analyzed for a given time period and GAC media. DOC and iodide removal/transformation were evaluated based on the ratio of effluent/final concentration (C) to influent/initial concentration (Co) ([Co-C]/Co).

was not detected above its MQL of 3.9 lg/L in any of the BAF column effluents, except for BAF 8 at treatment time period II, where its concentration was 14.6 lg/L. The concentration of chloroiodomethane is likely underestimated due to the high volatility of this compound in a partially open system. Chlorodiiodomethane and bromodiiodomethane were not detected in BAF samples during the initial sampling and analysis but emerged at substantial levels when samples collected at time period III were re-analyzed after prolonged storage (137 days) with headspace. For all BAF column effluents, there was a substantial increase in IOC concentrations from the initial sampling campaign at time period III compared to the samples evaluated after 137 days of storage. IOC distribution was mostly dominated by triiodomethane, diiodomethane, and chlorodiiodomethane (Fig. S7 (d)). Diiodomethane now exceeded concentrations of 2500 lg/L in 6 samples after storage. Chloroiodomethane was also measured above its MQL in all samples with a maximum concentration of 306 lg/L (Table 5). In this set of samples, BAF 8 was the only effluent sample where triiodomethane was below MDL, concentrations in all other effluent samples exceeded 2500 lg/L. The two semiquantified compounds chlorodiiodomethane and bromodiiodomethane were measured at concentrations above 2500 lg/L and 563 lg/L, respectively. In good agreement, iodide concentrations decreased from 6 to 52 mg/L at time period III sampling to 0.4–4.8 mg/L after storage (Table 5), indicating that BAF treated PW has oxidizing capabilities after treatment. Evaluation of the relative contribution of each compound to the total IOC concentration at each treatment time period revealed that triiodomethane is predominantly found in most effluent samples (at least 90% contribution of the quantified IOCs among 21 samples analyzed in time periods I, II, and III) (Fig. S7). Notably, BAF 8 showed a distinct pattern of IOC distribution compared to other BAF samples analyzed during the same sampling campaign. During time period II, BAF 8 was predominantly composed of diiodomethane followed by chloroiodomethane, and after prolonged storage (137 days) of time period III samples, it was primarily composed of diiodomethane while triiodomethane was below detection limits (Fig. S7 and Table 5).

3.4. Microbial community analysis Studies of BAFs used in O&G PW treatment have conducted microbial community analysis of GAC in BAFs and observed IOBs forming in GAC biofilm after extended operation (Fig. S8) (Chan, 2017; Riley, 2018), but these studies did not reported microbial community analysis of the treated wastewater. Because GAC and the treated water provide different micro-niches for microbial development and the presence of IOB in O&G PW was previously reported in literature (Mohan et al., 2013), microbial communities of PW and biologically treated water were analyzed in efforts to identify IOB and other microbial community members that develop in the water during treatment and that might be involved in the formation of IOCs during treatment and storage. 16S rRNA gene analysis of the four best performing BAF columns (BAF 1–4) contain ASVs classified within the Roseovarius and Iodidimonas genera (Fig. 3), both representing IOB (Amachi et al., 2005; Iino et al., 2016; Mohan et al., 2013). Interestingly, organisms of the Roseovarius and Iodidimonas genera were found at <0.2% relative abundance in BAF influent pre-treated with KMnO4. In comparison, in PW treated through BAFs (BAF effluent) the relative abundance was substantially higher (51.5% and 5.9%, respectively). Additionally, Roseovarius and Iodidimonas were not detected in the GAC of a pilot scale BAF system at the beginning of a different study in our research group (Fig. S8) but increased after prolonged operation (69–78 days) with PW. Because the aim of the BAF is to retain biomass within the system, it is likely that our assessments of IOB underestimate the total IOB population. 3.5. Evaluation of biotransformation pathways The formation of halogenated organic compounds mediated by biological iodide oxidation has been observed in previous studies (Amachi et al., 2005; Laturnus, 1995; Laturnus, 1996; Schall et al., 1994). Two compounds analyzed in the current study, diiodomethane and chloroiodomethane, are known to form biotically (Laturnus, 1995; Laturnus, 1996; Luek et al., 2017; Schall et al., 2004) by microorganisms such as Bjerkandera adusta (Fungi)

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Fig. 3. (a) Absolute and (b) relative abundance of microorganisms in PW and the effluent of the best performing BAF columns. DNA sequences are shown at the Genus level. Absolute cell abundance was calculated for each sample using the approximate microbial cell concentration (as described in the Materials and methods section) times the fractional abundance of each taxon shown.

(Manley, 2002) and multiple a-proteobacteria (Amachi et al., 2005). Amachi et al. (2005) presented and isolated several IOB responsible for extracellular enzymatic iodide oxidation to iodine. Organic iodine species (diiodomethane and chloroiodomethane) in cultivations with IOB strains were also found at concentrations higher than iodine. Given the nature of IOB, these organisms are more tolerant to high levels of halogens, iodide, and iodine and can readily produce I2. The formation of organic iodine was determined to originate from reactions of biologically mediated reactive iodine species with organic matter. Our results are consistent with other studies where IOB are found in halogen-rich PW samples. IOB have been previously found in O&G wastewater impoundments (Mohan et al., 2013), natural gas brines (Iino et al., 2016), and other engineered and environmental settings (Table S4), but seem to be augmented when specific growth conditions are developed in iodiderich wastewater. The presence of microorganisms of the iodide-oxidizing genus reinforces the formation of IOCs and I-DBPs during treatment as a biogenic byproduct of treatment. Observations supporting IOB as the iodide selective oxidizing mechanism that contributes to the formation of I-DBPs and IOCs in the BAFs include the presence of two halogenated organic compounds (diiodomethane and triiodomethane) with only iodide moieties formed at high concentrations and the significant change of iodide, for example during the initial sampling of BAF 3, when iodide was transformed by 90% and triiodomethane was measured up to 3979 lg/L. In addition, the presence of two compounds of biogenic origin (diiodomethane and chloroiodomethane) highlights the biogenic influence on IOC formation in BAFs treating PW. IOB likely oxidize iodide using an extracellular oxidase transforming iodide to reactive iodine species such as I2 (Amachi et al., 2005). Organic matter utilization by bacterial consortia developed within the GAC and treated water transform complex organic matter into organic matter that can readily interact with reactive iodine species, initiating halogenation of organic matter into IOC and I-DBPs. I-DBPs in this study likely formed through haloform reaction of methyl ketones and other organic matter. Studies have suggested that the nature of organic matter can affect the formation of different DBPs (Dickenson et al., 2008; Hua et al., 2006; Yang et al., 2015). The aforementioned I-DBPs are part of a sub-category of disinfection byproducts that have been previously

studied to identify organic precursors. These precursors include metadihydroxybenzene phenolic structures, b-diketones, and bketoacids (Dickenson et al., 2008; Gallard and von Gunten, 2002). Furthermore, dissolved methane present in O&G wastewater could have an impact on IOC formation and should be further investigated. Due to its highly volatile nature and prevalent operating conditions (e.g., age of raw PW, storage conditions, partially open treatment system), dissolved methane was not detected in multiple PW samples analyzed (data not shown). 3.6. Broader implications for O&G wastewater management strategies Wastewater management strategies such as streamflow augmentation can increase water supply in surface streams in arid regions in the U.S. while reducing the need to dispose of O&G derived waste streams in injection wells. This management strategy has the potential to not only increase water availability in oil-rich, water scarce regions but also to reduce the risk of spills, transportation, and maintenance of injection wells. Yet, the presence of known and unknown waste-derived organic contaminants can impose negative ecological risks that must be understood before such practice is put in place. Although streamflow augmentation discharge of waste-derived streams in the U.S. requires permits, monitoring, and reporting to the National Pollutant Discharge Elimination System (NPDES) as authorized by the Clean Water Act (EPA, 1972; Plumlee et al., 2012), risks can derive from the lack of regulations of unknown contaminants in treated wastewater. Although the I-DBPs (triiodomethane, bromodiiodomethane, and chlorodiiodomethane) investigated in this study, or any other I-DBPs, are not regulated by the United States Environmental Protection Agency (U.S. EPA), four other similar DBPs (chloroform, bromodichloromethane, dibromochloromethane, and bromoform) are regulated to a total concentration of 80 lg/L in drinking water by the U.S. EPA (EPA, 1998; Richardson et al., 2007). In addition, DBP standards for discharge into the environment have been established. In 1980, the U.S. EPA published ambient water quality criteria for halomethanes, which included two of the trihalomethanes: bromoform and bromodichloromethane (EPA, 1980). In 2004, the updated National Recommended Water Quality Criteria included the trihalomethanes (EPA, 2004). Besides ambient standards, some states specifically limit trihalomethane discharges

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in NPDES permits. For example, the Florida Department of Environmental Protection limits are designed to protect in-stream water uses including recreation, shellfish propagation, and maintenance of a healthy, well-balanced population of fish and wildlife (deSilva and Milligan, 2006; Friedman and Heaney, 2009). Hence, the potential formation of regulated and non-regulated DBPs at concentrations that exceed the established regulatory concentrations could hinder beneficial reuse of reclaimed O&G wastewater or streamflow augmentation due to potential risks to water quality of downstream potable water supplies. Moreover, naturally occurring microbial communities in surface environments could be vulnerable to high concentrations of IOCs and reactive iodine species mediated by IOB (Gottardi, 2001). Additional formation and re-occurrence of IOCs during storage of biologically treated O&G wastewater can have implications in management strategies that would require storage and addition of chemicals to quench reactive iodine species. Due to the complexity of PW and the large volumes of water, addition of chemicals during storage might not be a feasible alternative and requires evaluation of treatment trains to reduce cost of treatment and environmental impacts. Downstream membrane treatment processes can provide the mechanisms necessary to achieve higher water quality for reuse. Yet, due to the physical and chemical properties of IOCs, they are not readily rejected during membrane filtration. Previous studies have shown that low rejection of I-DBPs by high pressure membrane treatment such as reverse osmosis and nanofiltration is influenced by molecular size and the hydrophobic nature of these compounds (Doederer et al., 2014). Additionally, management of the waste generated (concentrate) during membrane processes needs to be evaluated because IOB, IOCs, and iodine species can be present in waste brines. Based on these observations, management of halides in PW is suggested to begin before any treatment processes. Iodide is an important element for many pharmaceutical and commercial applications. Recovery of iodide from PW can provide additional economic advantages to reclaim and treat PW to minimize its impacts on the environment. Ion exchange resins can be a feasible pre-treatment alternative to recover iodide from PW. Yet, the presence of other extractable constituents like chloride can potentially hinder recovery strategies of halogens of interest. 3.7. Conclusions In this study we were able to demonstrate a predominantly biologically mediated increase in concentration of IOCs particularly triiodomethane in nine individually operated BAF columns treating halide-rich O&G PW. Until now, biotic formation of IOCs during biological treatment of O&G wastewater for downstream reuse applications had not been reported. Iodide concentrations ranging from 40 to 52 mg/L were measured in the untreated PW and were measured at concentrations as low as 5 mg/L immediately after BAF treatment. As the decrease in iodide concentrations in PW correlated with increasing volatile IOC concentrations, iodide was utilized in the formation of IOCs. In addition, the presence of IOB in BAF columns reinforces the role of iodide-oxidizing organisms in the oxidation of iodide and formation of IOCs. Through this study we were able to establish that BAF can lead to the oxidation of iodide and formation of IOCs. Consequentially, formation of IOCs and I-DBPs that are mediated by biological reactions, require re-thinking of management strategies of O&G derived waste streams that could amplify IOB and their proliferation in other surface environments during reuse or discharge. Naturally occurring microbial communities in surface environments could be vulnerable to high concentrations of IOCs and reactive iodine species mediated by IOB. In order to enhance our knowledge with regard to the role of microorganisms during

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I-DBP formation, further research is necessary to better quantify the biotic and abiotic contributions to IOC formation. Future studies of BAF treating PW in a closed system under controlled conditions and on-line monitoring of critical parameters such as redox potential, dissolved oxygen, and iodine species are suggested to be able to establish a comprehensive overview of halogenation reactions and byproduct formation within the filters. Acknowledgements The authors thank the National Science Foundation (NSF) through the AirWaterGas SRN under Cooperative Agreement CBET-1240584 for supporting this study, as well as to the ConocoPhillips Center for a Sustainable WE2ST at the Colorado School of Mines. The authors would like to thank Chamandika Warusavitharana for the operation of the BAF columns during sampling and Estefani Bustos, Kate Spangler, Mike Veres, Kevin Chan, and Karl Oetjen for technical and scientific support. Appendix A. Supplementary data Supplementary data to this article can be found online at https://doi.org/10.1016/j.scitotenv.2019.134202. References Agarwal, V., Miles, Z.D., Winter, J.M., Eustaquio, A.S., El Gamal, A.A., Moore, B.S., 2017. Enzymatic halogenation and dehalogenation reactions: pervasive and mechanistically diverse. Chem. Rev. 117, 5619–5674. Allard, S., Nottle, C.E., Chan, A., Joll, C., von Gunten, U., 2013. Ozonation of iodidecontaining waters: selective oxidation of iodide to iodate with simultaneous minimization of bromate and I-THMs. Water Res. 47, 1953–1960. Amachi, S., Muramatsu, Y., Akiyama, Y., Miyazaki, K., Yoshiki, S., Hanada, S., et al., 2005. Isolation of iodide-oxidizing bacteria from iodide-rich natural gas brines and seawaters. Microb. Ecol. 49, 547–557. Callahan, B.J., McMurdie, P.J., Rosen, M.J., Han, A.W., Johnson, A.J., Holmes, S.P., 2016. DADA2: high-resolution sample inference from Illumina amplicon data. Nat. Methods 13, 581–583. Callahan, B.J., McMurdie, P.J., Holmes, S.P., 2017. Exact sequence variants should replace operational taxonomic units in marker-gene data analysis. The ISME Journal 11, 2639–2643. Chan, K.E., 2017. Microbial Ecology of Activated Sludge and Granular Activated Carbon Communities Treating Oil and Gas Produced Water Master of Science thesis. Colorado School of Mines, p. 71. Clark, C.E., Veil, J.A., 2009. Produced Water Volumes and Management Practices in the United States. Argone National Laboratory, p. 60. Coday, B.D., Almaraz, N., Cath, T.Y., 2015. Forward osmosis desalination of oil and gas wastewater: impacts of membrane selection and operating conditions on process performance. J. Membr. Sci. 488, 40–55. deSilva, V., Milligan, J., 2006. Operational challenges to meeting new copper & THM effluent limits at clearwater APCFS. Florida Water Resources Journal 4, 49–57. Dickenson, E.R.V., Summers, R.S., Croué, J.-P., Gallard, H., 2008. Haloacetic acid and trihalomethane formation from the chlorination and bromination of aliphatic bdicarbonyl acid model compounds. Environmental Science & Technology 42, 3226–3233. Doederer, K., Farré, M.J., Pidou, M., Weinberg, H.S., Gernjak, W., 2014. Rejection of disinfection by-products by RO and NF membranes: influence of solute properties and operational parameters. J. Membr. Sci. 467, 195–205. Dong, S., Masalha, N., Plewa, M.J., Nguyen, T.H., 2017. Toxicity of wastewater with elevated bromide and iodide after chlorination, chloramination, or ozonation disinfection. Environ Sci Technol 51, 9297–9304. EPA, 1972. In: Clean Water Act National Pollutant Discharge Elimination System. https://www.epa.gov/npdes, last accessed on April 30, 2019. EPA, 1980. In: Ambient water quality criteria for halomethanes. U.S. Environmental Protection Agency, Washington DC, USA, p. 135. EPA, 1998. U.S. Environmental Protection Agency. National Primary Drinking Water Regulations: Disinfectants and Disinfection Byproducts. Federal Register 63 (241), 69390–69476. EPA, 2004. National Recommended Water Quality Criteria. Office of Water OoSaT, U. S. Environmental Protection Agency, Washington DC, USA, p. 23. Frank, V.B., Regnery, J., Chan, K.E., Ramey, D.F., Spear, J.R., Cath, T.Y., 2017. Cotreatment of residential and oil and gas production wastewater with a hybrid sequencing batch reactor-membrane bioreactor process. Journal of Water Process Engineering 17, 82–94. Freedman, D.E., Riley, S.M., Jones, Z.L., Rosenblum, J.S., Sharp, J.O., Spear, J.R., et al., 2017. Biologically active filtration for fracturing flowback and produced water treatment. Journal of Water Process Engineering 18, 29–40.

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