River water influenced by shale gas wastewater discharge for paddy irrigation has limited effects on soil properties and microbial communities.

The processes of hydraulic fracturing to extract shale gas generate a large amount of wastewater, and the potential impacts of wastewater discharge after treatment are concerning. In this field study, we investigated the effects of the irrigation of paddy fields for 2 consecutive years by river water that has been influenced by shale gas wastewater discharge on soil physicochemical properties, microbial community structure and function, and rice grain quality. The results showed that conductivity, chloride and sulfate ions in paddy soils downstream of the outfall showed an accumulative trend after two years of irrigation, but these changes occurred on a small scale (<500 m). Two-year irrigation did not cause the accumulation of trace metals (barium, cadmium, chromium, copper, lead, strontium, zinc, nickel, and uranium) in soil and rice grains. Among all soil parameters, the accumulation of chloride ions was the most pronounced, with concentrations in the paddy soil at the discharge site 13.3 times higher than at the upstream control site. The use of influenced river water for paddy irrigation positively increased the soil microbial diversity, but these changes occurred after two years of irrigation and did not occur after one year of irrigation. Overall, the use of river water affected by shale gas wastewater discharge for agricultural irrigation has limited effects on agroecosystems over a short period. Nevertheless, the possible negative effects of contaminant accumulation in soil and rice caused by longer-term irrigation should be seriously considered.

A review of membrane development in membrane distillation for emulsified industrial or shale gas wastewater treatments with feed containing hybrid impurities.

Investigations on membrane materials for membrane distillation (MD) and its applications have been ongoing since the 1990s. However, a lack of materials that produce robustly stable and up-to-the-mark membranes for MD for different industrial applications remains an ongoing problem. This paper provides an overview of materials developed for MD applications. Although key aspects of published articles reviewed in this paper pertain to MD membranes synthesized for desalination, future MD can also be applied to organic wastewater containing surfactants with inorganic compounds, either with the help of hybrid treatment processes or with customized membrane materials. Many industrial discharges produce effluents at a very high temperature, which is an available driving force for MD. However, there remains a lack of cost-effective membrane materials. Amphiphobic and omniphobic membranes have recently been developed for treating emulsified and shale gas produced water, but the problem of organic fouling and pore wetting remains a major challenge, especially when NaCl and other inorganic impurities are present, which further deteriorate separation performance. Therefore, further advancements in materials are required for the treatment of emulsified industrial wastewater containing surfactants, salts, and for oil or shale gas wastewater for its commercialized reuse. Integrated MD systems, however, may represent a major change in shale gas wastewater and emulsified wastewater that are difficult to treat.

Characteristics and potential cytotoxicity of halogenated organic compounds in shale gas wastewater-impacted surface waters in Chongqing area， China.

Recent screening surveys have shown the presence of unknown source halogenated organic compounds (HOCs) in shale gas wastewater. However, their occurrence, profile, transport in surrounding surface water and environmental risk potentials remain unclear. Here, a method for the extraction and quantitative determination of 13 HOCs in water by solid phase extraction combined with gas chromatography-mass spectrometry (GC-MS) was established. All of the targeted HOCs were detected and peaked at the outfall, while these contaminants were generally not detected in samples upstream of the outfall, suggesting that these contaminants originated from the discharge of shale gas wastewater; this was further supported by the fact that these pollutants were generally detected in downstream samples, with a tendency for pollutant concentrations to decrease progressively with increasing distance from the outfall. However，different HOCs had different transport potential in water. In addition, the toxicological effects of typical HOCs were evaluated using HepG2 as a model cell. The results indicated that diiodoalkanes suppressed HepG2 cell proliferation and induced ROS generation in a concentration-dependent manner. Mechanistic studies showed that diiodoalkanes induced apoptosis in HepG2 cells via the ROS-mediated mitochondrial pathway, decreasing mitochondrial membrane potential and increasing intercellular ATP and Ca2+ levels. On the other hand, RT-qPCR and Western blot assays revealed that the SLC7A11/GPX4 signaling pathway and HO-1 regulation of ferritin autophagy-dependent degradation (HO-1/FTL) pathway were involved in the ferroptosis pathway induced by diiodoalkane in HepG2 cells. Our study not only elucidates the contamination profiles and transport of HOCs in surface water of typical shale gas extraction areas in China, but also reveals the toxicity mechanism of typical diiodoalkane.

Removal of 6-methylquinoline from shale gas wastewater using electrochemical carbon nanotubes filter.

6-Methylquinoline (6-MQ) is identified as a high-concentration organic compound pervasive in shale gas wastewater (SGW) and poses a significant risk of environmental pollution. In response, this study aimed to address these challenges by introducing an innovative electrochemical membrane constructed with multi-walled carbon nanotubes (CNTs) for the removal of 6-MQ. The investigation systematically explored the impact of voltage, initial pollutant concentration, and salinity on the performance of the electrochemical CNTs filter. It was found a positive correlation between removal efficiency and increasing voltage and salinity levels. Conversely, as the initial concentration of pollutants increased, the efficiency showed a diminishing trend. The electrochemical CNTs filter exhibited remarkable efficacy in both adsorption removal and electrochemical oxidation of 6-MQ. Notably, the CNTs membrane exhibited robust adsorption capabilities, evidenced by the sustained adsorption of 6-MQ for over 33 h. Furthermore, applying an electrochemical oxidation voltage of 3 V consistently maintained a removal rate exceeding 34.0% due to both direct and indirect oxidation, underscoring the sustained efficacy of the electrochemical membranes. Besides, real wastewater experiments, while displaying a reduction in removal efficiency compared to synthetic wastewater experiments, emphasized the substantial potential of the electrochemical CNTs filter for practical applications. This study underscores the significant promise of electrochemical membranes in addressing low molecular weight contaminants in SGW, contributing valuable insights for advancing SGW treatment strategies.

Effective and mechanistic insights into shale gas wastewater reverse osmosis concentrate treatment using ozonation-biological activated carbon process.

Reverse osmosis (RO) plays a pivotal role in shale gas wastewater resource utilization. However, managing the reverse osmosis concentrate (ROC) characterized by high salinity and increased concentrations of organic matter is challenging. In this study, we aimed to elucidate the enhancement effects and mechanisms of pre-ozonation on organic matter removal efficacy in ROC using a biological activated carbon (BAC) system. Our findings revealed that during the stable operation phase, the ozonation (O3 and O3/granular activated carbon)-BAC system removes 43.6-72.2 % of dissolved organic carbon, achieving a 4-7 fold increase in efficiency compared with that in the BAC system alone. Through dynamic analysis of influent and effluent water quality, biofilm performance, and microbial community structure, succession, and function prediction, we elucidated the following primary enhancement mechanisms: 1) pre-ozonation significantly enhances the biodegradability of ROC by 4.5-6 times and diminishes the organic load on the BAC system; 2) pre-ozonation facilitates the selective enrichment of microbes capable of degrading organic compounds in the BAC system, thereby enhancing the biodegradation capacity and stability of the microbial community; and 3) pre-ozonation accelerates the regeneration rate of the granular activated carbon adsorption sites. Collectively, our findings provide valuable insights into treating ROC through pre-oxidation combined with biotreatment.

Characterizing hazardous substances of shale gas wastewater from the upper Yangtze River: A focus on heavy metals and organic compounds.

In the last decade, rapid shale gas exploration in upper Yangtze River ecological zone in China has led to increasing concerns about the environmental impact of shale gas wastewater (SGW). However, our understanding of the types of potential hazardous substances of SGW remains limited. In this study, eight SGW samples from three shale gas regions in upper Yangtze River: the Sichuan Basin, the Guizhou Plateau, and the Three Gorges Area were collected, and their general water quality, trace metals, and organic compounds were comprehensively analyzed. Our in-depth analysis detected 55 kinds of trace heavy metals, with 24 exceeding detection limits. Most of them were of the concentration below 100 µg/L. Concentrations of primary pollutants, including Cd, Cr, As, Pb, and Ni, remained below Integrated Wastewater Discharge Standard (GB 8978-1996), indicating minimal environmental risk. The organic analysis identified 45 to 104 kinds of volatile and semi-volatile organic compounds in SGW samples from different regions. SGW samples from the Sichuan Basin exhibited a balanced proportion of aliphatic and aromatic compounds, with oxygen and nitrogen-substituted heteroatomic compounds prevailing, while SGW samples from the Guizhou Plateau and the Three Gorges Area were dominated by aromatic compounds, particularly hydrocarbons. Several organic substances exhibited high response strengths across multiple SGW samples, including isoquinoline, dibenzylamine, 2,4-di-tert-butylphenol, 1,2,3,4-tetrahydro-naphthalene, and 1,2,3,4-tetrahydro-6-methyl-naphthalene. The Globally Harmonized System (GHS) of Classification and Labelling of Chemicals classified most high-response organics as high acute and chronic aquatic hazards. Our findings indicate that high salinity and a variety of high-risk organic pollutants, rather than heavy metals, are the primary pollutants in SGW, underscoring the urgency of safety management of SGW.

Impact of shale gas wastewater discharge on the trace elements of the receiving river in the Sichuan Basin, China.

The potential contamination of shale gas wastewater generated from hydraulic fracturing to water resources is of growing concern, yet minimum attention has been paid to the impact of shale gas wastewater on the trace elements of the receiving waters. In this study, we analyzed the levels of 50 trace elements of a river that receives effluent from a shale gas wastewater treatment facility in the Sichuan Basin, China. Sixteen trace elements were detected in the surface water sample from the effluent discharge site, all of which were of higher concentrations than the upstream background level. Among the 16 shale gas wastewater-related elements, Sr, Ba, and Li were of elevated levels in the downstream water samples (24.9-44.2%, 5.0-8.0 times, and 17.8-22.8 times higher than the upstream background level, respectively). Shale gas wastewater effluent may be related to the accumulation of Sr, Ba, Li, and Cs in riverbed sediments near and/or downstream of the effluent discharge site and may lead to elevated pollution level of Sr and Li in downstream sediments. The ecological risk of the riverbed sediments was of medium to high level, with Cd contributing to the most risk, while shale gas wastewater-related elements are of low potential risk throughout the river. Our results suggested that shale gas wastewater effluent discharge had limited impacts on the trace elements of the receiving river within two years.

Dissolved organic matter in complex shale gas wastewater analyzed with ESI FT-ICR MS: Typical characteristics and potential of biological treatment.

Knowledge on the composition and characteristics of dissolved organic matter (DOM) in complex shale gas wastewater (SGW) is critical to evaluate environmental risks and to determine effective management strategies. Herein, five SGW samples from four key shale gas blocks in the Sichuan Basin, China, were comprehensively characterized. Specifically, FT-ICR MS was employed to provide insights into the sources, composition, and characteristics of SGW DOM. Organic matter was characterized by low average molecular weight, high saturation degree, and low aromaticity. Notably, the absence of correlations between molecular-level parameters and spectral indexes might be attributed to the high complexity and variability of SGW. The unique distribution depicted in van Krevelen diagrams suggested various sources of DOM in SGW, such as microbially derived organics in shales and biochemical transformations. Moreover, linear alkyl benzene sulfonates, as well as associated biodegraded metabolites and coproducts, were identified in SGW, implying the distinct anthropogenic imprints and abundant microbial activities. Furthermore, high DOC removal rates (31.42-79.23 %) were achieved by biological treatment, fully supporting the inherently labile nature of SGW and the feasibility of biodegradation for SGW management. Therefore, we conclude that DOM in SGW is a complex but mostly labile mixture reflecting both autochthonous and anthropogenic sources.

Combining eDNA and morphological approaches to reveal the impacts of long-term discharges of shale gas wastewaters on receiving waters.

The potential threats of shale gas wastewater discharges to receiving waters is of great concern. In this study, chemical analyses and biomonitoring were performed three times in a small river that received treated wastewater over a two-year period. The results of chemical analyses showed that the concentrations of chloride, conductivity, barium, and strontium increased at the discharge site, but their concentrations decreased considerably farther downstream (≥500 m). The concentrations of toxic organic compounds (16 US EPA priority polycyclic aromatic hydrocarbons and 6 priority phthalates), trace metals (strontium, arsenic, zinc, copper, chromium, lead, cadmium, nickel, and neodymium), and natural radionuclides (40K, 238U, 226Ra, and 232Th) were comparable to the corresponding background values or did not exhibit obvious accumulation in sediments with continued discharge. Morphological and environmental DNA approaches were used to reveal the potential effects of wastewater discharges on aquatic ecosystems. The results showed that the community structure of benthic invertebrates was not altered by the long-term discharges of shale gas wastewaters. However, the biodiversity indices (richness and Shannon) from the two approaches showed inconsistencies, which were caused by multiple reasons, and that substrates had a strong influence on the morphological biodiversity indices. A multimetric index was proposed to further analyze morphological and environmental DNA data, and the results showed no significant difference between the upstream and downstream sites. Generally, the chemical and biological results both demonstrated that the discharges of shale gas wastewaters had limited impacts on river ecosystems within two years.

Detection and treatment of organic matters in hydraulic fracturing wastewater from shale gas extraction: A critical review.

Although shale gas has shown promising potential to alleviate energy crisis as a clean energy resource, more attention has been paid to the harmful environmental impacts during exploitation. It is a critical issue for the management of shale gas wastewater (SGW), especially the organic compounds. This review focuses on analytical methods and corresponding treatment technologies targeting organic matters in SGW. Firstly, detailed information about specific shale-derived organics and related organic compounds in SGW were overviewed. Secondly, the state-of-the art analytical methods for detecting organics in SGW were summarized. The gas chromatography paired with mass spectrometry was the most commonly used technique. Thirdly, relevant treatment technologies for SGW organic matters were systematically explored. Forward osmosis and membrane distillation ranked the top two most frequently used treatment processes. Moreover, quantitative analyses on the removal of general and single organic compounds by treatment technologies were conducted. Finally, challenges for the analytical methods and treatment technologies of organic matters in SGW were addressed. The lack of effective trace organic detection techniques and high cost of treatment technologies are the urgent problems to be solved. Advances in the extraction, detection, identification and disposal of trace organic matters are critical to address the issues.

Risk assessment of pollutants in flowback and produced waters and sludge in impoundments.

Due to the growing hydraulic fracturing (HF) practices in China, the environmental risks of pollutants in flowback and produced waters (FPW) and sludge in impoundments for FPW reserves have drawn increasing attention. In this context, we first characterized the comparative geochemical characteristics of the FPW and the sludge in impoundments that collected FPW from 75 shale gas wells, and then the risks associated with the pollutants were assessed. The results demonstrated that four organic compounds detected in the FPW, naphthalene, acenaphthene, dibutyl phthalate, and bis(2-ethylhexyl)phthalate, were potential threats to surface waters. The concentrations of trace metals (copper, cadmium, manganese, chromium, nickel, zinc, arsenic, and lead) in the FPW and sludge were low; however, those of iron, barium, and strontium were high. The accumulation of chromium, nickel, zinc, and lead in the sludge became more evident as the depth increased. The environmental risks from heavy metals in the one-year precipitated sludge were comparable to those reported in the environment. However, the radium equivalent activities were 10-41 times higher than the recommended value for human health safety, indicating potential radiation risks. Although hydrophobic organic compounds, such as high-molecular-weight polycyclic aromatic hydrocarbons (PAHs), phthalate esters (PAEs), benzene, ethylbenzene, toluene, and xylene (BTEX), tended to accumulate in the sludge, their environmental risks were within tolerable ranges after proper treatment. Multiple antibiotic resistance genes (ARGs), such as those for macrolide, lincosamide, streptogramin (MLS), tetracycline, and multidrug resistances, were detected in the shale gas wastewaters and sludge. Therefore, the environmental risks of these emerging pollutants upon being discharged or leaked into surface waters require further attention.

Shale gas wastewater characterization: Comprehensive detection, evaluation of valuable metals, and environmental risks of heavy metals and radionuclides.

Shale gas wastewater (SGW) has great potential for the recovery of valuable elements, but it also poses risks in terms of environmental pollution, with heavy metals and naturally occurring radioactive materials (NORM) being of major concerns. However, many of these species have not been fully determined. For the first time, we identify the elements present in SGW from the Sichuan Basin and consequently draw a comprehensive periodic table, including 71 elements in 15 IUPAC groups. Based on it, we analyze the elements possessing recycling opportunities or with risk potentials. Most of the metal elements in SGW exist at very low concentrations (< 0.2 mg/L), including rare earth elements, revealing poor economic feasibility for recovery. However, salts, strontium (Sr), lithium (Li), and gallium (Ga) are in higher concentrations and have impressive market demands, hence great potential to be recovered. As for environmental burdens related to raw SGW management, salinity, F, Cl, Br, NO3-, Ba, B, and Fe, Cu, As, Mn, V, and Mo pose relatively higher threats in view of the concentrations and toxicity. The radioactivity is also much higher than the safety range, with the gross α activity and gross ß activity in SGW ranging from 3.71-83.4 Bq/L, and 1.62-18.7 Bq/L, respectively and radium-226 as the main component. The advanced combined process "pretreatment-disk tube reverse osmosis (DTRO)" with pilot-scale is evaluated for the safe reuse of SGW. This process has high efficiency in the removal of metals and total radioactivity. However, the gross α activity of the effluent (1.3 Bq/L) is slightly higher than the standard for discharge (1 Bq/L), which is thus associated with potential long-term environmental hazards.

Efficient removal of organic compounds from shale gas wastewater by coupled ozonation and moving-bed-biofilm submerged membrane bioreactor.

Shale gas wastewater (SGW) with complex composition and high salinity needs an economical and efficient method of treatment with the main goal to remove organics. In this study, a coupled system consisting of ozonation and moving-bed-biofilm submerged membrane bioreactor (MBBF-SMBR) was comprehensively evaluated for SGW treatment and compared with a similar train comprising ozonation and submerged membrane bioreactor (SMBR) without addition of carriers attaching biofilm. The average removal rates of MBBF-SMBR were 77.8% for dissolved organic carbon (DOC) and 37.0% for total nitrogen (TN), higher than those observed in SMBR, namely, 73.9% for DOC and 18.6% for TN. The final total membrane resistance in SMBR was 40.1% higher than that in MBBF-SMBR. Some genera that specifically contribute to organic removal were identified. Enhanced gene allocation for membrane transport and nitrogen metabolism was found in MBBF-SMBR biofilm, implying that this system has significant industrial application potential for organics removal from SGW.

Integration of coagulation and ozonation with flat-sheet ceramic membrane filtration for shale gas hydraulic fracturing wastewater treatment: A laboratory study.

The performance of the integrated process of coagulation and ozonation with ceramic membrane filtration was evaluated for the treatment of shale gas hydraulic fracturing flowback wastewater (HFFW). The removal efficiencies of carbon oxygen demand (CODCr ), dissolved organic carbon (DOC), petroleum oils, and turbidity in effluent by the combined process were 87.1%, 72.2%, 94.3%, and 99.6%, respectively. Compared with sole membrane filtration, the transmembrane pressure (TMP) of ceramic membrane filtration was reduced by >99% with the integrated process. The coagulation and ozonation can effectively remove the organics with high molecular weights in the cake layer of ceramic membrane. To the best of our knowledge, this work proposed the combined process of coagulation, ozonation, and flat-sheet ceramic membrane filtration for the treatment of HFFW for the first time. The water quality of the effluent met the discharge standard (Comprehensive Wastewater Discharge Standard GB8978-1996). The findings can provide an important technical foundation for the innovation of integrated equipment for HFFW treatment. PRACTITIONER POINTS: An integrated process combining coagulation and ozonation with flat-sheet ceramic membrane ultrafiltration for the treatment of shale gas wastewater. The water quality of this integrated process met the discharge standard. Coagulation and ozonation effectively alleviated the membrane fouling related to organics with high molecular weights. A new avenue for on-site treatment of shale gas wastewater and an alternative of the current centralized wastewater management.

Organics removal from shale gas wastewater by pre-oxidation combined with biologically active filtration.

Biological treatment technology is increasingly explored in shale gas wastewater (SGW) treatment owing to its cost effectiveness and requires efforts to improve its efficacy. In this work, ozone and ferrate(VI) oxidation pre-treatment were evaluated to enhance the performance of the subsequent biologically active filtration (BAF) in the removal of organic contaminants. The oxidation improved the SGW biodegradability and organic composition under relative high salinity (~20 g/L). Due to the degradation activity of microorganisms, the organics removal efficiency in the BAF system was observed to gradually improve and then reaching stability in long-term continuous-mode operation. The removal rate of dissolved organic carbon (DOC) of the ozone-BAF (O3-BAF) and the ferrate(VI)-BAF (Fe(VI)-BAF) systems was 83.2% and 82.8% , respectively, higher than that of BAF alone (80.9%). This increase was attributed to higher activity and content of microorganisms in O3-BAF and Fe(VI)-BAF systems. Two uncultured bacterial species with high abundance of 7.2-21.0% and 2.24-22.31% in genus Rehaibacterium and genus Methyloversatilis were significantly correlated with DOC removal and fluorescent organics removal, respectively. More research is needed to understand whether the species were new and their specific function. This study provides valuable suggestions for extracting safe water from SGW with an efficient treatment train.

Can pre-ozonation be combined with gravity-driven membrane filtration to treat shale gas wastewater?

Low-cost gravity-driven membrane (GDM) filtration has the potential to efficiently manage highly decentralized shale gas wastewater (SGW). In this work, the feasibility of combining low dosage pre-ozonation with the GDM process was evaluated in the treatment of SGW. The results showed that pre-ozonation significantly increased the stable flux (372%) of GDM filtration, while slightly deteriorating the quality of the effluent water in terms of organic content (-14%). These results were mainly attributed to the conversion of macromolecular organics to low-molecular weight fractions by pre-ozonation. Interestingly, pre-ozonation markedly increased the flux (198%) in the first month of operation also for a GDM process added with granular activated carbon (GGDM). Nevertheless, the flux of O3-GGDM systems dropped sharply around the 25th day of operation, which might be due to the rapid accumulation of pollutants in the high flux stage and the formation of a dense fouling layer. Pre-ozonation remarkably influenced the microbial community structure. And O3-GDM systems were characterized by distinct core microorganisms, which might degrade specific organics in SGW. Furthermore, O3-GDM outperformed simple GDM as a pretreatment for RO. These findings can provide valuable references for combining oxidation technologies with the GDM process in treating refractory wastewater.