A critical review of analytical methods for comprehensive characterization of produced water

Produced water is the largest waste stream associated with oil and gas production. It has a complex matrix composed of native constituents from geologic formation, chemical additives from fracturing fluids, and ubiquitous bacteria. Characterization of produced water is critical to monitor field operation, control processes, evaluate appropriate management practices and treatment effectiveness, and assess potential risks to public health and environment during the use of treated water. There is a limited understanding of produced water composition due to the inherent complexity and lack of reliable and standardized analytical methods. A comprehensive description of current analytical techniques for produced water characterization, including both standard and research methods, is discussed in this review. Multi-tiered analytical procedures are proposed, including field sampling; sample preservation; pretreatment techniques; basic water quality measurements; organic, inorganic, and radioactive materials analysis; and biological characterization. The challenges, knowledge gaps, and research needs for developing advanced analytical methods for produced water characterization, including target and nontarget analyses of unknown chemicals, are discussed.


Produced Water Quality and Temporal Variability in Different Basins
The United States produces large volumes of produced water (PW) from unconventional oil and gas development (UD).The production increase of the UD in the U.S. is mainly from seven key oil and gas basins: Appalachia including Marcellus and Utica (Pennsylvania, Ohio, and West Virginia), Bakken (North Dakota and Montana), Eagle Ford (South Texas), Haynesville (Louisiana and East Texas), Niobrara (Colorado and Wyoming), and the Permian Basin (West Texas and Southeast New Mexico) [1].Table S1 summarizes the general physicochemical parameters of PW quality from primary UD plays in the U.S. Fig. S1 shows the temporal change of PW quality in Marcellus formation in Pennsylvania and Niobrara formation in Colorado.Because of the higher proportion of formation brine, PW typically has considerably higher total dissolved solids (TDS) concentrations than flowback water (FW).However, FW can have higher organics due to organic additives in fracturing fluid [2][3][4].Fig. S1.Temporal variation of PW qualities in Marcellus shale, PA, two well sites [5]; and Niobrara formation, CO, two well sites [3,6].Note: data compiled from 2020 Scanlon et al. [7], 2020 Rodriguez et al. [8] , 2019 Chang et al. [9], 2018 Lipus et al. [10].n/a: data not available.TSS: total suspended solids.

Field sampling and preservation
Environment sampling is a crucial process to ensure the certainty of analytical results.Several important aspects need to be considered for the field sampling process, including sampling points and locations, analyte selection and the number of samples, field measurements and sampling log, containers and preservation, quality assessment samples, and other related information.Based on the methods generally used by commercial labs under the EPA guidance, Table S2 summarizes the analytical methods, containers, preservations, and holding times for PW analysis of wet chemistry, anions, total metals, organics, radioactive, and WET testing.
Sampling equipment and preservation techniques are important for environmental samples.The EPA approved analytical methods for drinking water or wastewater have detailed instructions on sample collection, preservation, and storage.For example, the EPA Method 625 (for organic analysis) states that "samples must be collected in glass containers", "All samples must be iced or refrigerated at 4 C from the time of collection until extraction", "If residual chlorine is present, add 80 mg sodium thiosulfate per liter of sample".Thus, the sampling equipment and preservation techniques used for PW samples are even more crucial due to their extremely complex matrices and high oxidation potential of certain constituents (e.g., iron).The target analytes can be easily adsorbed to the container or degraded during conveyance or storage.For waste and wastewater matrices, the EPA's SW-846 compendium is a more suitable candidate for PW sample preparation and analysis.A major concern for PW samples is the biodegradation of numerous organic compounds.Acids or sodium azide are often used to preserve PW samples [11].Immediate transfer of hydrophobic analytes out of the aqueous phase in the field using a water-immiscible organic solvent is another method to minimize biodegradation.Another concern is some of the highly reactive HF additives and metal ions [12].To detect those highly reactive targets in PW, their transformation/degradation has to be minimized at the time of sampling by adding chemical preservatives, extracting into an organic solvent, or onsite filtration.Targeting the more stable intermediates from transformation during analysis is also a good approach if the parent compounds are known.However, for those preservation methods, their compatibility with the fundamental analysis must be considered before application because they may suit one target analyte but not the others [13].
Lipus et al. [14] used 16S rRNA sequencing to monitor the changes in microbial community composition in PW at different sample storage conditions.Their results suggested an ideal handling method for microbiology analysis would be filtering the sample on-site and preserving the filters with chemical preservatives (such as TRIZOL).If on-site sample processing is not possible, the best way to maintain the original microbial communities would include collecting samples in sterile bottles in the field and placing them on ice as soon as possible, but at least within 24 h, followed by shipping the samples on ice over 2-3 days.Furthermore, samples should be stored at the -20 C or -80 C for long-term storage.
The widely used containers for PW samples include sterile or precleaned photo resistant glass bottle (e.g., amber glass bottle) and clean plastic bottles (e.g., high-density polyethylene (HDPE) and polytetrafluoroethylene (PTFE)).They are suitable for almost all samples with several particular cases.The biological sample requires sterile HDPE or polypropylene bottles, while the boron and silica sample requires plastic, PTFE, or quartz containers.Minimal headspace is necessary during sampling to minimize portioning into the gaseous phase.The collected sample should be stored with ice and sent to the lab for analysis as soon as possible.If immediate analysis cannot be performed, samples should be stored in a refrigerator and analyzed as soon as possible.The following preservation methods are recommended to enhance the accuracy of the results.To determine the dissolved inorganic elements, samples must be filtered through a 0.45 µ m membrane filter at the time of collection or as soon as practicable.To determine the total recoverable inorganic elements, samples are not filtered but acidified with trace metal grade nitric acid to pH < 2. Acid is often used to preserve some biocides, and chloroform is added for anions.Organics should be preserved in a fridge (2 -6 C) or -20 C, and should not be stored longer than two weeks before analysis [15].
For quality assessment, duplicate samples are collected to evaluate precision, including variability in sample collection, handling, preparation, and analysis.Sample blanks are also required to increase analysis confidence by eliminating possible contamination during sample collection, conveyance, or preparation [16], such as analytical blanks [17,18], field blanks [18,19], trip blanks [6], equipment blanks, laboratory blanks [18], and storage blanks [17].1. Alkalinity: 14 days holding time for treated samples and should be analyzed as soon as possible for untreated samples.

Sample preparation
Sample preparation is essential for PW analysis.It has several goals: 1) to concentrate or dilute target analytes to meet the capability of analytical instrumentation; 2) to remove materials in the matrix that might interfere with the chromatographic separation, ionization, or detection of target analytes.For inorganic analysis, these goals are usually met by removing particles and diluting the sample to meet instrument performance.For organic compound analyses, removing inorganic ions in PW while retaining specific organics in the final solution is often required.The EPA's SW-846 compendium consists of over 200 analytical methods for sampling and analyzing waste and other matrices.It includes the 3000 series for inorganic sample preparation, 3500 series for organic sample extraction, and 3600 series for organic extract cleanup.A variety of sample preparation methods suitable for PW samples are discussed in the following sections.

Dilution, filtration, and centrifugation
Dilution is a useful way to address the sample matrix, making it more suitable for the analytical instrument and adjusting the concentration of analytes into the calibration range.Filtration and centrifugation are two simple sample preparation methods.They both remove particulate materials in PW to make samples compatible with analytical methods and protect instruments, such as to prevent clogging and high backpressure for ion chromatography (IC) and liquid chromatography (LC) columns [20].However, filtration and centrifugation do not concentrate the sample or change the dissolved fraction of the sample matrix, which may be required when analyzing PW, especially when targeting trace amounts of organic analytes.Thus, these methods usually can only be applied to bulk and inorganic measurements and need to be coupled with other pretreatment methods for organic sample analysis [21].Another important consideration for these methods is their bias toward chemical constituents adsorbed to the suspended solids in the matrix, which are often removed during the filtration process [13].Thus, the filtered solids are sometimes collected and treated (e.g., acid digested) to analyze the PW sample comprehensively [17].

Solid-phase extraction
Solid-phase extraction (SPE) is a powerful and widely used extraction technique that offers high selectivity, flexibility, and automation.The EPA Method 3535A is a procedure for isolating target organic analytes from aqueous samples using SPE media.SPE has been widely applied to concentrate and purify analytes from different water matrices, including wastewater and PW [6,22,23].Table S3 summarizes the SPE cartridges used in literature for organic analysis.Table S3.SPE cartridges used for organic extraction Ref.
SPE, analytes, and analytical method

Bulk Measurements and Basic Water Quality Parameters
Bulk measurements are essential for any water analysis because they are quick and cost-effective and provide overall information about the water matrix.The informative bulk measurements include pH, conductivity, temperature, alkalinity, salinity, total suspended and dissolved solids (TSS and TDS), total organic carbon (TOC)/dissolved organic carbon (DOC), total nitrogen (TN), total petroleum hydrocarbons (TPH), oxidation-reduction potential (ORP), and others [16].These basic parameters are valuable for monitoring well operation and guiding subsequent detailed analysis.Some industries use these parameters as process control, only performing a more detailed analysis when fluctuation is observed [29].These measurements can be performed on-site with probes/sensors or in the lab with a relatively simple instrument.Currently, there are commercial probes available for on-site measurements.For example, YSI Professional Plus multi-parametric probe can be used to measure temperature, dissolved oxygen, conductivity, TDS, salinity, pH, turbidity, and ORP [30].
Alkalinity in PW is caused by carbonate and bicarbonate ions, which affect the pH of the solution and have the potential to induce scaling with cations (e.g., Ca 2+ ) present in the solution [31].Alkalinity can be measured by titration using the EPA Method 310.1 and the colorimetric testing EPA Method 310.2.The EPA Method 310.1 is more suitable for PW measurement because PWs usually present in yellow color, which can affect the accuracy of colorimetric testing, and using a pH meter to titrate the sample to endpoint pH 4.5 would be more accurate [32,33].There are many different alkalinity measurement test kits available in the market that are suitable for onsite testing.
Solids refer to the substances suspended or dissolved in PW.Total solids (TS) includes TSS and TDS.TSS are particles mainly comprised of formation sands and clays, proppants, and corrosion byproducts.TDS are primarily charged particles (major cations and anions).TDS levels can vary considerably in a given region.For example, PW in Bakken shale varies from 1,800 to 350,000 mg/L TDS [9].There are two principal methods for measuring TDS: gravimetric analysis and conductivity.The gravimetric method is more accurate than the conductivity method, while the latter is more convenient.Dilution is often required for the conductivity method to yield accurate results within the instrument measurement range.Currently, TS, TSS, and TDS are often measured by the Standard Methods 2540 A-F (gravimetric methods, range up to 20,000 mg/L) approved by the EPA to analyze solids residue from domestic and industrial wastewater [34].TS is measured by evaporating a well-mixed sample in a weighed dish and dried to constant weight in an oven at 103 to 105 ºC.The increase in dish weight represents the TS (method 2540B).TSS and TDS can be measured at the same time.A well-mixed sample is first filtered through a weighed standard glass-fiber filter.The residue retained on the filter is dried to a constant weight at 103 to 105 ºC; the filter weight increase represents the TSS (method 2540D).TDS is obtained by evaporating the filtrate in a weighed dish and dried to constant weight at 180 ºC; the dish weight increase represents the TDS (method 2540C) [32,35].
TOC provides the concentration of organic carbon in water.It is a more convenient and accurate measurement to perform in the lab than biochemical oxygen demand (BOD) or chemical oxygen demand (COD) methods.The EPA Method 415.3 or the Standard Method 5310C is usually used to measure TOC.Samples are first acidified by HCl, H3PO4, or H2SO4 to pH < 2, to remove the inorganic carbon (carbonate and bicarbonate).The organic carbon is then oxidized to carbon dioxide by combustion or chemical oxidation, which is then detected by a conductivity detector or a nondispersive infrared (NDIR) detector [36].DOC is another commonly measured parameter representing the dissolved (filtered) organic compounds in water.The procedure requires the sample to be filtered by a 0.45 m filter before analysis by a TOC analyzer (e.g., Shimadzu TOC analyzer TOC-L or TOC-V series) [37].Dilution sometimes is needed when the concentration of DOC exceeds the optimum range of the instrument [38].
TN is the sum of the inorganic nitrogen, organic nitrogen, and ammonia.Inorganic nitrite and nitrate are analyzed using the EPA Method 353.2: nitrate in a filtered sample is reduced to nitrite, then all the nitrite is measured colorimetrically.The sum of organic nitrogen and ammonia can be analyzed using EPA Methods 351.2 and EPA-NERL 351.4.The sample is digested to convert total Kjeldahl nitrogen (total nitrogen in organic substances and inorganic ammonia/ammonium) into ammonia.Then the concentration of ammonia is measured using an ion-selective electrode [32,33].Another method (ASTM D8083) to determine TN is to convert all nitrogen compounds to NO, followed by photoelectric measurement of radiation emitted when NO2 relaxes [39].
These methods are easy to perform if the samples are correctly prepared.Dilution is usually a convenient way to avoid interferences because these bulk parameters do not measure constituents at trace levels.Table S1 includes some measurement results of the typical water quality parameters from different PW sources.

Organic Analysis
Table S4 summarizes 25 peer-reviewed publications analyzing organic compounds in shale gas PW from 2016 to date.In summary, 14 publications used LC-MS, while 13 used GC-based techniques (the overlap is because some publications used both techniques).This trend may be a result of advances in HRMS and ultra-HRMS, in addition to the concerns surrounding undisclosed proprietary chemicals used during HF and their transformation products during well production.Orbitrap (7 publications) and Q-ToF (7 publications) have become the dominant HRMS/MS analyzers because of their high resolution and relatively low price.In comparison, only 2 publications from the same group used FT-ICR-MS, likely due to its high cost despite the high resolution.

Table S1 .
Comparison of general physicochemical parameters of PW in primary UD plays in minimummaximum/mean values