Desalination and reuse of high-salinity shale gas produced water: drivers, technologies, and future directions

Thermo-osmosis-Coupled Thermally Regenerative Electrochemical Cycle for Efficient Lithium Extraction

Lithium (Li) production based on the soda evaporation process is time-consuming and unsustainable. The emerging electrochemical Li extraction is time-efficient but requires high-concentration Li sources and significant electrical energy input. Here, we demonstrate a fast, energy-saving, and environment-friendly Li production process by coupling a thermally regenerative electrochemical cycle (TREC) using lithium manganese oxide (LMO) and nickel hexacyanoferrate (NiHCF) electrodes with poly(vinylidene fluoride) membrane-based thermo-osmosis (denoted as TO-TREC). The characterization of LMO and NiHCF electrodes confirmed that the relatively high temperature of TO-TREC has negligible adverse effects on the ion intercalation in LMO and NiHCF electrodes. The LMO/NiHCF pair has a positive temperature coefficient of 0.843 mV K^-1. In the TO-TREC process, Li ions are selectively extracted from a Li-containing brine warmed by low-grade heat and then released into a room-temperature recovery solution such as LiCl with a production rate of 50-60 mmol Li⁺ m^-2 h^-1. Li source solutions are concentrated by thermo-osmosis simultaneously, making it possible to utilize previously unusable Li-containing sources, such as concentrated brines from desalination plants and industrial effluents. Besides, the TREC harvests thermal energy from the heated brine, saving >20% of electrical energy compared to conventional electrochemical methods. The new process shows the potential to meet the growing global Li demands for many applications.

A Critical Review of the Physicochemical Impacts of Water Chemistry on Shale in Hydraulic Fracturing Systems

Hydraulic fracturing of unconventional hydrocarbon resources involves the sequential injection of a high-pressure, particle-laden fluid with varying pH's to make commercial production viable in low permeability rocks. This process both requires and produces extraordinary volumes of water. The water used for hydraulic fracturing is typically fresh, whereas "flowback" water is typically saline with a variety of additives which complicate safe disposal. As production operations continue to expand, there is an increasing interest in treating and reusing this high-salinity produced water for further fracturing. Here we review the relevant transport and geochemical properties of shales, and critically analyze the impact of water chemistry (including produced water) on these properties. We discuss five major geochemical mechanisms that are prominently involved in the temporal and spatial evolution of fractures during the stimulation and production phase: shale softening, mineral dissolution, mineral precipitation, fines migration, and wettability alteration. A higher salinity fluid creates both benefits and complications in controlling these mechanisms. For example, higher salinity fluid inhibits clay dispersion, but simultaneously requires more additives to achieve appropriate viscosity for proppant emplacement. In total this review highlights the nuances of enhanced hydrogeochemical shale stimulation in relation to the choice of fracturing fluid chemistry.

Hole-Type Spacers for More Stable Shale Gas-Produced Water Treatment by Forward Osmosis

An appropriate spacer design helps in minimizing membrane fouling which remains the major obstacle in forward osmosis (FO) systems. In the present study, the performance of a hole-type spacer (having holes at the filament intersections) was evaluated in a FO system and compared to a standard spacer design (without holes). The hole-type spacer exhibited slightly higher water flux and reverse solute flux (RSF) when Milli-Q water was used as feed solution and varied sodium chloride concentrations as draw solution. During shale gas produced water treatment, a severe flux decline was observed for both spacer designs due to the formation of barium sulfate scaling. SEM imaging revealed that the high shear force induced by the creation of holes led to the formation of scales on the entire membrane surface, causing a slightly higher flux decline than the standard spacer. Simultaneously, the presence of holes aided to mitigate the accumulation of foulants on spacer surface, resulting in no increase in pressure drop. Furthermore, a full cleaning efficiency was achieved by hole-type spacer attributed to the micro-jets effect induced by the holes, which aided to destroy the foulants and then sweep them away from the membrane surface.

Investigating the Potential Toxicity of Hydraulic Fracturing Flowback and Produced Water Spills to Aquatic Animals in Freshwater Environments: A North American Perspective

Unconventional methods of oil and natural gas extraction have been a growing part of North America's energy sector for the past 20-30 years. Technologies such as horizontal hydraulic fracturing have facilitated the exploitation of geologic reserves that were previously resistant to standard drilling approaches. However, the environmental risks associated with hydraulic fracturing are relatively understudied. One such hazard is the wastewater by-product of hydraulic fracturing processes: flowback and produced water (FPW). During FPW production, transport, and storage, there are many potential pathways for environmental exposure. In the current review, toxicological hazards associated with FPW surface water contamination events and potential effects on freshwater biota are assessed. This review contains an extensive survey of chemicals commonly associated with FPW samples from shale formations across North America and median 50% lethal concentration values (LC₅₀) of corresponding chemicals for many freshwater organisms. We identify the characteristics of FPW which may have the greatest potential to be drivers of toxicity to freshwater organisms. Notably, components associated with salinity, the organic fraction, and metal species are reviewed. Additionally, we examine the current state of FPW production in North America and identify the most significant obstacles impeding proper risk assessment development when environmental contamination events of this wastewater occur. Findings within this study will serve to catalyze further work on areas currently lacking in FPW research, including expanded whole effluent testing, repeated and chronic FPW exposure studies, and toxicity identification evaluations.

Effects of salinity on the treatment of synthetic petroleum-industry wastewater in pilot vertical flow constructed wetlands under simulated hot arid climatic conditions

Petroleum-industry wastewater (PI-WW) is a potential source of water that can be reused in areas suffering from water stress. This water contains various fractions that need to be removed before reuse, such as light hydrocarbons, heavy metals and conditioning chemicals. Constructed wetlands (CWs) can remove these fractions, but the range of PI-WW salinities that can be treated in CWs and the influence of an increasing salinity on the CW removal efficiency for abovementioned fractions is unknown. Therefore, the impact of an increasing salinity on the removal of conditioning chemicals benzotriazole, aromatic hydrocarbon benzoic acid, and heavy metal zinc in lab-scale unplanted and Phragmites australis and Typha latifolia planted vertical-flow CWs was tested in the present study. P. australis was less sensitive than T. latifolia to increasing salinities and survived with a NaCl concentration of 12 g/L. The decay of T. latifolia was accompanied by a decrease in the removal efficiency for benzotriazole and benzoic acid, indicating that living vegetation enhanced the removal of these chemicals. Increased salinities resulted in the leaching of zinc from the planted CWs, probably as a result of active plant defence mechanisms against salt shocks that solubilized zinc. Plant growth also resulted in substantial evapotranspiration, leading to an increased salinity of the CW treated effluent. A too high salinity limits the reuse of the CW treated water. Therefore, CW treatment should be followed by desalination technologies to obtain salinities suitable for reuse. In this technology train, CWs enhance the efficiency of physicochemical desalination technologies by removing organics that induce membrane fouling. Hence, P. australis planted CWs are a suitable option for the treatment of water with a salinity below 12 g/L before further treatment or direct reuse in water scarce areas worldwide, where CWs may also boost the local biodiversity. Graphical abstract.

Tuning the Pore Structures of Organosilica Membranes for Enhanced Desalination Performance via the Control of Calcination Temperatures

Microporous organosilica membranes based on 1,2-bis(triethoxylsilyl)ethane (BTESE) were fabricated via an acid-catalyzed sol-gel technique. In the preparation process, the calcination temperature plays a significant role in structural and surface properties of the organosilica networks. With an increase in calcination temperature, the surface hydrophilicity decreased due to the enhanced condensation of Si-OH groups in the networks. N₂ adsorption results suggest that the pore structures of BTESE membranes was clearly dependent on the calcination temperature. The pore sizes of the membranes were quantitatively determined by using the Normalized Knudsen-based permeance (NKP) model. In pervaporation tests, the membranes with higher calcination temperatures showed higher salt rejections and lower water permeances, which was attributed to the changes in pore size and surface chemistry of pore walls. The BTESE membranes calcined at 200 °C exhibited superior hydrothermal stability in temperature cycles up to 70 °C and high reproducibility in concentration cycles with NaCl concentrations of 0.2-13 wt%, showing great promise for desalination applications of high-salinity water.

Multifunctional nanocoated membranes for high-rate electrothermal desalination of hypersaline waters

Surface heating membrane distillation overcomes several limitations inherent in conventional membrane distillation technology. Here we report a successful effort to grow in situ a hexagonal boron nitride (hBN) nanocoating on a stainless-steel wire cloth (hBN-SSWC), and its application as a scalable electrothermal heating material in surface heating membrane distillation. The novel hBN-SSWC provides superior vapour permeability, thermal conductivity, electrical insulation and anticorrosion properties, all of which are critical for the long-term surface heating membrane distillation performance, particularly with hypersaline solutions. By simply attaching hBN-SSWC to a commercial membrane and providing power with an a.c. supply at household frequency, we demonstrate that hBN-SSWC is able to support an ultrahigh power intensity (50 kW m^-2) to desalinate hypersaline solutions with exceptionally high water flux (and throughput), single-pass water recovery and heat utilization efficiency while maintaining excellent material stability. We also demonstrate the exceptional performance of hBN-SSWC in a scalable and compact spiral-wound electrothermal membrane distillation module.

On-site treatment capacity of membrane distillation powered by waste heat or natural gas for unconventional oil and gas wastewater in the Denver-Julesburg Basin

Leveraging waste heat has been considered to have significant potential for promoting the economic feasibility of wastewater treatment in unconventional oil and gas (UOG) production. However, its availability near well sites has not been fully understood and other energy sources may be also feasible. In this work, we quantitatively investigate the viability of using waste heat and well-pad natural gas to power on-site wastewater treatment by membrane distillation (MD) for twenty randomly selected wells located in the Denver-Julesburg (DJ) Basin, U.S. Results show that waste heat produced from on-site electrical loads is insufficient for MD treatment of all the wastewater generated during UOG production (2.2-24.3% of thermal energy required for MD treatment). Waste heat from hydraulic fracturing, which persists only for a short timeframe, is able to meet the full or partial energy requirement during the peak period of wastewater production (17-1005% of thermal energy required for MD treatment within the first two months of production), but this scenario varies among wells and is dependent on the energy efficiency of MD. Compared to waste heat, natural gas is a more consistent energy source. The treatment capacity of MD powered by natural gas at the well pad exceeds full wastewater treatment demands for all the twenty wells, with only two wells requiring short-term wastewater storage. Our work indicates that although waste heat has the potential to reduce the electricity consumption and cost of UOG wastewater treatment, it is unlikely to supply sufficient thermal energy required by MD for long-term treatment. Natural gas can serve as an alternative or complementary energy resource. Further investigations, in particular techno-economic analyses, are needed to identify the best suitable energy source or combination for on-site UOG wastewater treatment.

Review on the Evaluation of the Impacts of Wastewater Disposal in Hydraulic Fracturing Industry in the United States

This paper scrutinized hydraulic fracturing applications mainly in the United States with regard to both groundwater and surface water contamination with the purpose of bringing forth objective analysis of research findings. Results from previous studies are often unconvincing due to the incomplete database of chemical additives; after and before well-founded water samples to define the change in parameters; and specific sources of water pollution in a particular region. Nonetheless, there is a superior chance of both surface and groundwater contamination induced by improper and less monitored wastewater disposal and management practices. This report has documented systematic evidence for total dissolved solids, salinity, and methane contamination regarding drinking water correlated with hydraulic fracturing. Methane concentrations were found on an average rate of 19.2 mg/L, which is 17 times higher than the acceptance rate and the maximum value was recorded as 64.2 mg/L near the active hydraulic fracturing drilling and extraction zones than that of the nonactive sites (1.1 mg/L). The concentration of total dissolved solids (350 g/L) was characterized as a voluminous amount of saline wastewater, which was quite unexpectedly high. The paper concludes with plausible solutions that should be implemented to avoid further contamination.

Utica Shale Play Oil and Gas Brines: Geochemistry and Factors Influencing Wastewater Management

The Utica and Marcellus Shale Plays in the Appalachian Basin are the fourth and first largest natural gas producing plays in the United States, respectively. Hydrocarbon production generates large volumes of brine ("produced water") that must be disposed of, treated, or reused. Though Marcellus brines have been studied extensively, there are few studies from the Utica Shale Play. This study presents new brine chemical analyses from 16 Utica Shale Play wells in Ohio and Pennsylvania. Results from Na-Cl-Br systematics and stable and radiogenic isotopes suggest that the Utica Shale Play brines are likely residual pore water concentrated beyond halite saturation during the formation of the Ordovician Beekmantown evaporative sequence. The narrow range of chemistry for the Utica Shale Play produced waters (e.g., total dissolved solids = 214-283 g/L) over both time and space implies a consistent composition for disposal and reuse planning. The amount of salt produced annually from the Utica Shale Play is equivalent to 3.4% of the annual U.S. halite production. Utica Shale Play brines have radium activities 580 times the EPA maximum contaminant level and are supersaturated with respect to barite, indicating the potential for surface and aqueous radium hazards if not properly disposed of.

Large-pore-size membranes tuned by chemically vapor deposited nanocoatings for rapid and controlled desalination

Though membranes with pore size larger than 1 μm are much desired to increase the permeate flux of membrane distillation (MD), the vulnerability of large-pore-size membranes to pore wetting results in the penetration of saline water and consequent failure of MD operation. We report modification of large-pore-size membranes by chemically vapor deposited nanocoatings to achieve both high salt rejection and high permeate flux. The chemical vapor modification not only led to enhanced surface hydrophobicity and increased liquid entry pressure in membranes, but also significantly improved membrane wetting resistance at high temperature. Membranes with 1.0 and 2.0 μm pore size were successfully used for MD desalination with salt rejection higher than 99.99% achieved. Enlarging the pore size from 0.2 μm to 2.0 μm contributed to 48-73% enhancement in the permeate flux of the modified membranes. The modified large-pore-size membranes maintained the high permeate flux at elevated saline concentration and extended the operation time.

Thermally Sensitized Membranes for Crude Oil-Water Remediation under Visible Light

Effective remediation of produced water requires separating crude oil-water mixture and removing the dissolved organic pollutants. Membranes with selective wettability for water over oil enable the gravity-driven separation of an oil-water mixture by allowing water to permeate through while repelling oil. However, these membranes are often limited by their inability to remove the dissolved organic pollutants. In this work, a membrane with in-air superhydrophilic and underwater superoleophobic wettability is fabricated by thermal annealing of a stainless steel mesh. The resulting membrane possesses a hierarchical surface texture covered with a photocatalytic oxide layer composed of iron oxide and chromium oxide. The membrane exhibits chemical and mechanical robustness, which makes it suitable for remediation of crude oil and water mixture. Further, after being fouled by crude oil, the membrane can recover its inherent water-rich permeate flux upon visible light irradiation. Finally, the membrane demonstrates that it can separate surfactant-stabilized crude oil-in-water emulsion under gravity and decontaminate water-rich permeate by photocatalytic degradation of dissolved organic pollutants upon continuous irradiation of visible light.

Effect of produced water treatment technologies on irrigation-induced metal and salt accumulation in wheat (Triticum aestivum) and sunflower (Helianthus annuus)

Produced water (PW), a wastewater resulting from hydraulic fracturing and oil and gas production, has been utilized in arid regions for irrigation purposes and potentially presents a new water source for crop irrigation in areas of increasing water scarcity. However, there is a potential for both synthetic and geogenic contaminants in these waters to accumulate in irrigated food crops. This study assessed how water treatment technologies targeted at removal of salinity (i.e., total dissolved solids) and organic chemical content (i.e., dissolved organic carbon) from PW to achieve agricultural irrigation standards altered the impact of inorganic contaminants and nutrient uptake on two salt-tolerant food crops, sunflower (Helianthus annuus) and wheat (Triticum aestivum). The impacts of the treatment technologies on inorganic contaminant loadings in the irrigated soils were also assessed. Treatment technologies to improve PW quality decreased the adverse impacts on plant health; however, plant health was more affected by dilutions of PW than by the treatment technologies employed. Phenotypically, plants irrigated with 90% dilution (low) treatment groups, regardless of treatment technology, were comparable to controls; however, plants watered with high proportions (50%) of raw or treated PW displayed stunted growth, with reduced height and leaf area, and sunflower seed saw 100% yield loss. Although phenotypically similar, plants of the low treatment groups exhibited changes in the ionome, illustrating the influence of PW on plant uptake, translocation, and accumulation of metals, salts, and micronutrients. In addition, bioavailability of metals and nutrients was impacted by the unique and complex PW matrix: bioconcentration factors traditionally used to evaluate risk may therefore over or underestimate accumulation.

Transport of produced water through reactive porous media

During hydraulic fracturing (or fracking) large volumes of wastewater (flow-back and produced water) are generated, which are naturally rich in heavy metals and radionuclides, such as radium. Spills may occur during operations and contaminate the groundwater. Therefore, there is an urgent need to identify a practice that can mitigate the negative impact of accidental leaks on water resources. Here, we present an experimental and modeling work on the transport of alkaline earth elements in produced water, which are congeners of radium, namely, barium (Ba²⁺), strontium (Sr²⁺), calcium (Ca²⁺), and magnesium (Mg²⁺) in addition to sodium (Na⁺). Column-flood tests were conducted using produced water from a shale-gas site and reactive porous media made of ubiquitous minerals such as sand, hydrous ferric oxide, activated alumina, and manganese oxide. In all cases, no retardation of the ions was observed at the salinity conditions of the produced water, but strong retardation in the pH front was measured, indicating that adsorption indeed occurred. When using manganese oxide and upon dilution of produced water, the concentration fronts of all major divalent cations were retarded. However, a fast wave of solute, traveling at the average flow velocity, emerged. This phenomenon confirmed that significant adsorption occurred under those conditions. But, pH-dependent adsorption and hydrodynamic dispersion favored fast solute transport. Overall, these results suggest that manganese oxide could be used as a reactive material in the lining of temporary storage tanks and in the well cases in order to retard the migration of the major toxic elements in produced water. However, mixing must be controlled to avoid the emergence of an instability at the concentration fronts favoring the formation of fast waves.

Increasing net water recovery of reverse osmosis with membrane distillation using natural thermal differentials between brine and co-located water sources: Impacts at large reclamation facilities

Maximizing water recovery and minimizing the volume of RO concentrate (i.e., brine) produced is a growing challenge, especially for inland communities that lack ocean disposal options. In such regions, transitioning towards zero liquid discharge (ZLD) can avoid detrimental impacts associated with salt disposal via regional sewer discharge or deep-well injection. On-site ZLD energy requirements are proportional to the RO brine flowrate. Thus, system-level strategies that reduce RO brine flows will lower ZLD costs while simultaneously increasing the overall water recovery for beneficial reuse in reclamation facilities. We investigated a membrane distillation (MD) system operating using co-located, cooler source water to treat warmer wastewater RO brine. Using experimentally-quantified MD fluxes based on observed monthly water temperatures of co-located water and RO brine at a facility in central Arizona, and based on the previously reported performance of large-scale MD systems, energy consumption and operating cost were estimated to evaluate the potential capabilities of MD to treat RO brine at full scale facilities. When the RO unit was combined with MD brine treatment, net water recovery at the full-scale facility can increase from 85% to up to 91% while brine flow can be reduced by 26%. A 25% lower thermal energy was required to achieve RO net water recovery of 95% when using co-located water, compared against conventional MD without using co-located water. Overall, this work demonstrates the potential to use local thermal gradients to reduce RO brine volumes, thereby reducing ZLD costs.

On-Site Treatment of Shale Gas Flowback and Produced Water in Sichuan Basin by Fertilizer Drawn Forward Osmosis for Irrigation

Fertilizer drawn forward osmosis (FDFO) was proposed to extract fresh water from flowback and produced water (FPW) from shale gas extraction for irrigation, with fertilizer types and membrane orientations assessed. The draw solution (DS) with NH₄H₂PO₄ displayed the best performance, while the DS with (NH₄)₂HPO₄ resulted in the most severe membrane fouling. The DS with KCl and KNO₃ led to substantial reverse solute fluxes. The FDFO operation where the active layer of the membrane was facing the feed solution outperformed that when the active layer was facing the DS. The diluted DS and diluted FPW samples were used for irrigation of Cherry radish and Chinese cabbage. Compared to deionized water, irrigation with the diluted DS (total dissolved solid (TDS) = 350 mg·L^-1) promoted plant growth. In contrast, inhibited plant growth was observed when FPW with high salinity (TDS = 5000 mg·L^-1) and low salinity (TDS = 1000 mg·L^-1) was used for irrigation of long-term (8-week) plant cultures. Finally, upregulated genes were identified to illustrate the difference in plant growth. The results of this study provide a guide for efficient and safe use of FPW after FDFO treatment for agricultural application.

Elucidating the Trade-off between Membrane Wetting Resistance and Water Vapor Flux in Membrane Distillation

Membrane distillation (MD) has been receiving considerable attention as a promising technology for desalinating industrial wastewaters. While hydrophobic membranes are essential for the process, increasing membrane surface hydrophobicity generally leads to the reduction of water vapor flux. In this study, we investigate the mechanisms responsible for this trade-off relation in MD. We prepared hydrophobic membranes with different degrees of wetting resistance through coating quartz fiber membranes with a series of alkylsilane molecules while preserving the fiber structures. A trade-off between wetting resistance and water vapor flux was observed in direct-contact MD experiments, with the least-wetting-resistant membrane exhibiting twice as high vapor flux as the most wetting-resistant membrane. Electrochemical impedance analysis, combined with fluorescence microscopy, elucidated that a lower wetting resistance (still water-repelling) allows deeper penetration of the liquid-air interfaces into the membrane, resulting in an increased interfacial area and therefore a larger evaporative vapor flux. Finally, we performed osmotic distillation experiments employing anodized alumina membranes that possess straight nanopores with different degrees of wetting resistance, observed no trade-off, and substantiated this proposed mechanism. Our study provides a guideline to tailor the membrane surface wettability to ensure stable MD operations while maximizing the water recovery rate.

Zero Liquid Discharge of Ultrahigh-Salinity Brines with Temperature Swing Solvent Extraction

Zero liquid discharge (ZLD) of hypersaline brines is technically and energetically challenging. This study demonstrates ZLD of ultrahigh-salinity brines using temperature swing solvent extraction (TSSE), a membrane-less and nonevaporative desalination technology. TSSE utilizes a low-polarity solvent to extract water from brine and then releases the water as a product with the application of low-temperature heat. Complete extraction of water from a hypersaline feed, simulated by 5.0 M NaCl solution (≈292 g/L TDS), was achieved using diisopropylamine solvent. Practically all of the salt is precipitated as mineral solid waste and the product water contains <5% of NaCl relative to the hypersaline feed brine. Consistent ZLD performance of high salt removals and product water quality was maintained in three repeated semibatch TSSE cycles, highlighting recyclability of the solvent. The practical applicability of the technique for actual field samples was demonstrated by ZLD of an irrigation drainage water concentrate. This study establishes the potential of TSSE as a more sustainable alternative to current thermal evaporation methods for zero liquid discharge of ultrahigh-salinity brines.

Chemical characterization in hydraulic fracturing flowback and produced water (HF-FPW) of shale gas in Sichuan of China

Large amounts of fracturing flowback and wastewater with complex compositions are produced during hydraulic fracturing. Characterization of hydraulic fracturing flowback and produced water (HF-FPW) is an important initial step in efforts to determine a suitable treatment method for this type of wastewater. In the present study, fracturing flowback and produced water samples were obtained from well CN-F and well CN-E in the prophase and anaphase stages of the Changning shale gas mining area. Chemical characterization of inorganic and organic substances was then conducted. Metal contents were determined by inductively coupled plasma optical emission spectroscopy (ICP-OES), and all inorganic anions involved were determined by ion chromatography. The organic pollutant components were analyzed in detail by combining Fourier transform infrared spectrometer (FTIR) and gas chromatography-mass spectrometer (GC-MS). Results showed that samples contained salt (TDS = 30,000-50,000 mg/L), metals (e.g., 650 ± 50 mg/L calcium), and total organic carbon (TOC = 32-178 mg/L). The organic substances detected in all samples could be divided into six categories, alkanes, aromatics, halogenated hydrocarbons, alcohols, esters, and ketones. C₆-C₂₁ straight-chain alkanes and C₇-C₁₃ naphthenes had the highest amount of organic matter, reaching more than 48%. The organic matter contained fracturing fluid additives, such as surfactants (e.g., ethylene glycol), and nitrotrichloromethane, which is a chlorinated product of some additives. These results provide information on the chemical composition of HF-FPW in Sichuan, China, as well as a basis for subsequent processing.

Potential water recovery during lithium mining from high salinity brines

Lithium extraction from continental brines involves the evaporation of large amounts of water in open air ponds, in order to concentrate the brine. The evaporitic technology implies the evaporation of large water volumes, raising environmental concerns. If we envision the use of desalination processes for the concentration of lithium-rich brines, then fresh water production/recovery becomes a process well integrated with lithium extraction. Here we apply the Pitzer thermodynamic model with effective molality to estimate activity coefficients for 8 different native brines, and for the resulting concentrated solutions produced by a hypothetical advanced desalinization technique. In all cases, rational activity coefficients deviate considerably from unity. We calculate next the least work of separation for a hypothetical desalination process for the 8 different brines. Because of the large total salinity, the calculation shows that the least work of separation ranges from 18 until 42 kJ kg^-1 at nil recovery ratio, and escalating from those numbers as more water is recovered. We can also predict the boiling point elevation, the vapour pressure lowering, and the osmotic pressure. Our calculations show that results are not strictly proportional to the total dissolved solids. Results are strongly dependent with the specific chemical composition of each brine, with the amount of divalent ions (Mg-Ca-SO₄^2-) in particular strongly influencing calculations. Fresh water and lithium minerals production could be part of a single integrated production system.

Water Treatment: Are Membranes the Panacea?

Alongside the rising global water demand, continued stress on current water supplies has sparked interest in using nontraditional source waters for energy, agriculture, industry, and domestic needs. Membrane technologies have emerged as one of the most promising approaches to achieve water security, but implementation of membrane processes for increasingly complex waters remains a challenge. The technical feasibility of membrane processes replacing conventional treatment of alternative water supplies (e.g., wastewater, seawater, and produced water) is considered in the context of typical and emerging water quality goals. This review considers the effectiveness of current technologies (both conventional and membrane based), as well as the potential for recent advancements in membrane research to achieve these water quality goals. We envision the future of water treatment to integrate advanced membranes (e.g., mixed-matrix membranes, block copolymers) into smart treatment trains that achieve several goals, including fit-for-purpose water generation, resource recovery, and energy conservation.

Volume reduction and water reclamation of reverse osmosis concentrate from coal chemical industry by forward osmosis with an osmotic backwash strategy

Coal chemical industry (CCI) generally utilizes reverse osmosis (RO) for water reclamation, which generates a highly concentrated stream containing refractory organic substances and high-concentration total dissolved solids (TDS). To address this issue, the present work focuses on volume reduction of RO concentrate (ROC) produced from CCI by forward osmosis (FO). We investigated the effects of membrane orientation and draw solution (DS) concentration on FO performance. Foulant removal was tested by using chemical cleaning, physical cleaning and osmotic backwash (OB). AL-FS (active layer facing feed solution) mode outcompeted AL-DS (active layer facing draw solution) mode, achieving a flux of 26.4 LMH, 92.5% water reclamation and energy consumption of 0.050 kWh·m^-3 with 4 M NaCl as DS. The FO process was able to reject >98% SO₄ ^2-, Mg²⁺and Ca²⁺, 92-98% Si and 33-55% total organic carbon (TOC). Ten-cycle (10 × 20 h) accelerated fouling test demonstrated approximately 30% flux decline in association with Si-containing foulants, which could be removed almost completely through OB with 97.1% flux recovery. This study provides a proof-of-concept demonstration of FO for volume reduction and water reclamation of ROC produced from CCI, making the treatment of ROC more efficient and more energy effective.

Can we beneficially reuse produced water from oil and gas extraction in the U.S.?

There is increasing interest in beneficial uses of large volumes of wastewater co-produced with oil and gas extraction (produced water, PW) because of water scarcity, potential subsurface disposal limitations, and regional linkages to induced seismicity. Here we quantified PW volumes relative to water demand in different sectors and PW quality relative to treatment and reuse options for the major U.S. shale oil and gas plays. PW volumes from these plays totaled ~600 billion liters (BL, 160 billion gallons, Bgal) in 2017. One year of PW is equal to ~60% of one day of freshwater use in the U.S. For these plays, the total irrigation demand exceeded PW volumes by ~5× whereas municipal demand exceeded PW by ~2×. If PW is reused for hydraulic fracturing (HF) within the energy sector, there would be no excess PW in about half of the plays because HF water demand exceeds PW volumes in those plays. PW quality can be highly saline with median total dissolved solids up to 255 g/L in the Bakken play, ~7× seawater. Intensive water treatment required for PW from most unconventional plays would further reduce PW volumes by at least 2×. Desalination would also result in large volumes of salt concentrates, equivalent to ~3000 Olympic swimming pools in the Permian Delaware Basin in 2017. While water demands outside the energy sector could accommodate PW volumes, much lower PW volumes relative to water demand in most regions would not substantially alleviate water scarcity. However, large projected PW volumes relative to HF water demand over the life of the play in the Permian Delaware Basin may provide a substantial new water source for beneficial use in the future. Large knowledge gaps in PW quality, lack of appropriate regulations, and economic factors currently preclude beneficial uses outside the energy sector in most regions.

The Use of Noble Gas Isotopes in Detecting Methane Contamination of Groundwater in Shale Gas Development Areas: An Overview of Technology and Methods

Groundwater contamination by stray gas (mainly methane) in areas of shale-gas development has captured publics, political and scientific attention. However, the sources and potential mechanisms of groundwater contamination are still debated. Noble gases can provide useful information on fluid migration for discerning the scale, conditions, and physical mechanisms. In this study, details about analytical technology and theoretical approach of noble gases in tracing groundwater contaminations are presented. In addition, applications of noble-gases isotopes for determining contamination sources and potential pathways are explored and reviewed. Recent developments are discussed and highlighted with focusing on new utilities of noble-gas isotope parameters in evaluating groundwater contamination. Some usages of indicators (⁴He/²⁰Ne, CH₄/³⁶Ar, ⁴He/CH₄, etc.) are discussed through specific research articles. And it is a new trend to make comprehensive use of multiple geochemical parameters to determine the occurrence, source, and process of methane pollution in groundwater.

Energy efficiency of membrane distillation: Simplified analysis, heat recovery, and the use of waste-heat

Membrane distillation (MD) is a thermal desalination process that is advantageous due to its ability to harness low-grade waste heat to separate highly saline feedstock. However, like any thermal desalination process, the energy efficiency depends on the ability to recover latent heat from condensation in the distillate. In direct contact MD (DCMD), this can be achieved by integrating a heat exchanger (HX) to recover latent heat stored in the distillate stream to preheat the incoming feed stream. Based on the principle of equal heat capacity flows, we derive a simple and intuitive expression for the optimal flow rate ratio between the feed and distillate streams to best recover this latent. Following the principle of energy balance, we derive simple expressions for the specific thermal energy consumption (SEC_th) and gained output ratio (GOR) of DCMD with and without a coupled HX for latent heat recovery, revealing an intuitive critical condition that indicates whether DCMD should or should not be coupled with HX. As MD is attractive for its ability to use low-grade waste heat as a heat source, we also evaluate the energy efficiency of DCMD powered by a waste heat stream. A waste heat stream differs fundamentally from a conventional constant-temperature heat source in that the temperature of the waste heat stream decreases as heat is extracted from it. We discuss the implication of this fundamental difference on energy efficiency and how we should analyze the energy efficiency of DCMD powered by waste heat streams. A new metric, namely specific yield, is proposed to quantify the performance of DCMD powered by waste heat stream. Our analysis suggests that, for a single-stage DCMD powered by a waste heat stream, whether implementing latent heat recovery or not only affects conventional metrics for energy efficiency (e.g. SEC_th and GOR) but not the specific yield. Overall, this analysis presents an intuitive and important framework for evaluating and optimizing energy efficiency in DCMD.

Highly Efficient Bromide Removal from Shale Gas Produced Water by Unactivated Peroxymonosulfate for Controlling Disinfection Byproduct Formation in Impacted Water Supplies

Shale gas extraction processes generate a large amount of hypersaline wastewater, whose spills or discharges may significantly increase the bromide levels in downstream water supplies and result in the formation of brominated disinfection byproducts (DBPs) upon chlorination. Although a few studies have investigated selective bromide removal from produced water, the low removal efficiencies and complex system setups are not desirable. In this study, we examined a simple cost-effective approach for selective bromide removal from produced water relying on the oxidation by unactivated peroxymonosulfate. More than 95% of bromide was removed as Br₂(g) in less than 10 min under weakly acidic conditions without significant formation of Cl₂(g) even when the chloride concentration was more than 2 orders of magnitude higher. A kinetic model considering the involved reactions was then developed to describe the process well under various reaction conditions. The organic compounds in the produced water neither noticeably lowered the bromide removal efficiency nor reacted with the halogen species to form halogenated byproducts. The tests in batch and continuously stirred tank reactor systems suggested that it was feasible to achieve both high bromide removal and neutral effluent pH such that further pH adjustment was not necessary before discharge. After the treatment, the effect of the produced water on DBP formation was largely eliminated.

Fit-for-purpose treatment goals for produced waters in shale oil and gas fields

Hydraulic fracturing (HF), or "fracking," is the driving force behind the "shale gas revolution," completely transforming the United States energy industry over the last two decades. HF requires that 4-6 million gallons per well (15,000-23,000 m³/well) of water be pumped underground to stimulate the release of entrapped hydrocarbons from unconventional (i.e., shale or carbonate) formations. Estimated U.S. produced water volumes exceed 150 billion gallons/year across the industry from unconventional wells alone and are projected to grow for at least another two decades. Concerns over the environmental impact from accidental or incidental release of produced water from HF wells ("U-PW"), along with evolving regulatory and economic drivers, has spurred great interest in technological innovation to enhance U-PW recycling and reuse. In this review, we analyze U-PW quantity and composition based on the latest U.S. Geographical Survey data, identify key contamination metrics useful in tracking water quality improvement in the context of HF operations, and suggest "fit-for-purpose treatment" to enhance cost-effective regulatory compliance, water recovery/reuse, and resource valorization. Drawing on industrial practice and technoeconomic constraints, we further assess the challenges associated with U-PW treatment for onshore U.S. operations. Presented are opportunities for targeted end-uses of treated U-PW. We highlight emerging technologies that may enhance cost-effective U-PW management as HF activities grow and evolve in the coming decades.

Evaluation of soil treatment techniques on remediated brine spill sites in semi-arid rangelands

Unconventional oil and gas development (UOG) generates high volumes of flowback and produced water, byproducts of hydraulic fracturing operations, that are often released or spilled on the soil surface. Soil contamination with these wastewaters, commonly referred to as brine, has the potential to inhibit vegetation growth indefinitely. Natural attenuation of brine is not expedient in arid and semi-arid regions where most United States UOG developments are located, including the Bakken region of North Dakota. In situ (at-site) and ex situ (off-site) soil treatment techniques are commonly employed to remediate brine-contaminated soils in the Bakken. However, little is known regarding each technique's efficacy despite differences in application, cost, and efficiency. We selected 10 sites previously remediated with chemical amendments (in situ) and 11 sites with topsoil excavation (ex situ) in the United States Forest Service Little Missouri National Grasslands. We paired each remediated site with a reference to examine the ability of each strategy to return brine-contaminated sites to conditions reflective of the current state of the surrounding semi-arid rangeland ecosystem. At each site, we quantified soil electrical conductivity (EC_e) as an indicator of soil salinity and measured vegetation cover, biomass production, bare ground, and litter. The difference between paired reference and remediated sites was used for analysis. Brine contamination was still evident as soil EC_e was similarly increased on chemical amendment and topsoil excavation remediated sites over paired references at all soil depths tested. Due to the nature of the topsoil excavation treatment, elevated EC_e in the 0-15 cm depth suggested resalinization of the new topsoil. Remediation techniques also resulted in similar plant community composition marked by an increase in exotic forb biomass, largely due to the invasion of kochia (Bassia scoparia) which was absent from reference sites. However, remediation techniques differed substantially in vegetation establishment. We found 15% more bare ground on sites remediated with chemical amendment treatment than paired references and 55% more with topsoil excavation. Our results indicate that in situ strategies may be more suitable than ex situ strategies for brine-spill remediation in semi-arid rangelands like the Bakken in North Dakota as they cause less disturbance and likely require less post-remediation management to establish adequate vegetation cover to protect the soil from further erosion.

Hydrodynamic-Colloidal Interactions of an Oil Droplet and a Membrane Surface

Membranes have been shown to be exceptionally successful in the challenging separation of stable oil/water emulsions but suffer from severe fouling that limits their performance. Understanding the mechanisms leading to oil deposition on the membrane surface, as influenced by hydrodynamics and colloidal surface interactions, is imperative for informing better engineered membrane surfaces and process conditions. Here, we study the interactions between an oil droplet and a membrane surface. Hydrodynamics within the water film, confined between the droplet and the membrane, are captured within the framework of the lubrication approximation, coupled with the van der Waals (vdW) and electrostatic interactions through the droplet shape, which is governed by an augmented Young-Laplace equation. The model is used to calculate possible equilibrium positions, where the droplet is held at a finite distance from the membrane by a balance of the forces present. An equilibrium phase diagram is constructed as a function of various process parameters and is shown in terms of the scaled permeation rate through the membrane. The phase diagram identifies the range of conditions leading to deposition, characterized by a "critical" permeation rate, beyond which no equilibrium exists. When equilibrium positions are permitted, we find that these may be classified as stable/unstable, in the kinetic sense. Further, our results demonstrate the link between the deformation of the droplet and the stability of equilibria. An upward deflection of the droplet surface, owing to a dominant, long-range repulsion, has a stabilizing effect, as it maintains the separation between the droplet and membrane. Conversely, a downward deflection is destabilizing because of the self-amplifying effect of strongly increasing attractive forces with separation distance-as the surfaces are pulled together because of deformation, the attractive force increases, causing further deformation. This is also manifested by a dependence of the bistable region on the deformability of the droplet, which is represented by a capillary number, modified so as to account for the effect of the permeable boundary. As the droplet becomes more easy to deform, the transition from an unconditionally stable region of the phase diagram to a point beyond which there is no equilibrium (interpreted as deposition) becomes abrupt. These results provide valuable physical insights into the mechanisms that govern oil fouling of membrane surfaces.

Exploring the Operation Factors that Influence Performance of a Spiral-Wound Forward Osmosis Membrane Process for Scale-up Design

Forward osmosis (FO) technology has increasingly attracted attention owing to its low operational energy and low fouling propensity. Despite extensive investigations on FO, very few module-scale FO studies on the operation and design of the FO process have been conducted. In this paper, a simple and practical FO process design parameter called normalized membrane area is suggested based on a performance analysis of spiral-wound FO elements. The influence of operation factors on operating pressures and water recovery was investigated using 8-inch spiral wound elements in the continuous operation mode. The membrane area was adjusted by series connection of FO elements to a maximum value of 46 m² (three elements). The feed and draw flow rates were varied between 5 and 15 LPM under various feed (10, 20, and 30 g/L) and draw (58.4 and 233.8 g/L) concentration combinations. The analysis of flow rates (feed, draw, and permeate flow rates) indicated not only high flow channel resistance on the draw side but also high permeate flow rates can induce higher operating pressures owing to strong mutual interaction of the feed and the draw streams. Feed water recovery was focused on as a key performance index, and the experimental recovery (R_Exp) and theoretical maximum recovery (R_Th) values were compared. The results revealed the significance of the feed flow rate and the membrane area in terms of enhancing the water recovery performance. In addition, a clear relationship was observed between the membrane area normalized by the initial feed flow rates and the water recovery ratio (R_Exp/R_Th), even though the applied operation conditions were different. Finally, an empirical equation to estimate the required membrane area of spiral-wound FO was proposed for the FO process design. The equation can be used to predict water recovery of FO systems as well, for example, if an FO system is operated at 0.08 m²L⁻¹h of the normalized membrane area, the system is expected to offer 78% of the R_Th value.

Minimal and zero liquid discharge with reverse osmosis using low-salt-rejection membranes

Minimal and zero liquid discharge (MLD/ZLD) are wastewater management strategies that are attracting heightened attention worldwide. While conventional reverse osmosis (RO) has been proposed as a promising technology in desalination and MLD/ZLD processes, its application is limited by the maximum hydraulic pressures that current RO membranes and modules can withstand. In this study, we develop low-salt-rejection RO (LSRRO), a novel staged RO process, that employs low-salt-rejection membranes to desalinate or concentrate highly saline feed streams, requiring only moderate hydraulic pressures. Based on process modeling, we demonstrate that LSRRO can overcome the hydraulic pressure limitations of conventional RO, achieving hypersaline brine salinities (>4.0 M NaCl or 234 g L^-1 NaCl) that are required for MLD/ZLD applications, without using excessively high hydraulic pressures (≤70 bar). In addition, we show that the energy efficiency of LSSRO is substantially higher than traditional thermally-driven phase-change-based technologies, such as mechanical vapor compressor (MVC). For example, to concentrate a saline feed stream from 0.1 to 1.0 M NaCl, the specific energy consumption (SEC) of 4-stage LSRRO ranges from 2.4 to 8.0 kWh of electrical energy per m³ of feedwater treated, around four times less than that of MVC, which requires 20-25 kWh_e m^-3. Furthermore, compared to osmotically mediated RO technologies that require bilateral countercurrent stages to treat hypersaline brines, LSRRO is eminently more practical as it can be readily implemented by using 'loose' RO or nanofiltration membranes in conventional RO. Our study highlights LSRRO's potential for energy efficient brine concentration using moderate hydraulic pressures, which would drastically improve the energetic and economic performance of MLD/ZLD processes.

Environmental and Economic Water Management in Shale Gas Extraction

This paper introduces a comprehensive study of the Life Cycle Impact Assessment (LCIA) of water management in shale gas exploitation. First, we present a comprehensive study of wastewater treatment in the shale gas extraction, including the most common technologies for the pretreatment and three different desalination technologies of recent interest: Single and Multiple-Effect Evaporation with Mechanical Vapor Recompression and Membrane Distillation. The analysis has been carried out through a generic Life Cycle Assessment (LCA) and the ReCiPe metric (at midpoint and endpoint levels), considering a wide range of environmental impacts. The results show that among these technologies Multiple-Effect Evaporation with Mechanical Vapor Recompression (MEE-MVR) is the most suitable technology for the wastewater treatment in shale gas extraction, taking into account its reduced environmental impact, the high water recovery compared to other alternatives as well as the lower cost of this technology. We also use a comprehensive water management model that includes previous results that takes the form of a new Mixed-Integer Linear Programming (MILP) bi-criterion optimization model to address the profit maximization and the minimization Life Cycle Impact Assessment (LCIA), based on its results we discuss the main tradeoffs between optimal operation from the economic and environmental points of view.

Synergistic utilization of inherent halides and alcohols in hydraulic fracturing wastewater for radical-based treatment: A case study of di-(2-ethylhexyl) phthalate removal

The degradation of di-(2-ethylhexyl) phthalate (DEHP) was examined as an example to capitalize on the potential interactions of peroxydisulfate (PS) and ferrous iron (Fe²⁺) in the model Day-1/Day-90 and on-site hydraulic fracturing wastewater (FWW). The primary oxidative radicals in the Fe²⁺/PS system (i.e., SO₄^- and OH) were less effective for the degradation of DEHP (6.45%) in ultrapure water. Both chloride (Cl^-) and bromide (Br^-) at equivalent molar ratio with PS enhanced DEHP degradation (15.6% and 45.5%, respectively) via the generation of Cl and Br radicals, whereas the degradation rate was inhibited by the excessive amount of Cl^- or Br^- in the Day-1/Day-90 FWW. However, the co-presence of ethylene glycol (C₂H₄(OH)₂, 0.043% v/v in the FWW) and halide ions (Cl^- or Br^-, 0.05 mM) resulted in the highest removal efficiency of 82.6 - 88.5% within 10 min by Fe²⁺/PS. Further investigation revealed that the formation of reductive alcohol radicals (C₂H₃(OH)₂) slowed down or replenished the Fe²⁺ exhaustion. This study demonstrated that the Fe²⁺/PS-based advanced oxidation may show a synergistic interplay with Cl^-/Br^- and C₂H₄(OH)₂ in the FWW, which depends on their relative concentrations. Thus, the inherent constituents in the fracturing wastewater can be utilized for the catalytic degradation of co-existing organic contaminants.

Distinct Behaviors between Gypsum and Silica Scaling in Membrane Distillation

Mineral scaling constrains membrane distillation (MD) and limits its application in treating hypersaline wastewater. Addressing this challenge requires enhanced fundamental understanding of the scaling phenomenon. However, MD scaling with different types of scalants may have distinctive mechanisms and consequences which have not been systematically investigated in the literature. In this work, we compared gypsum and silica scaling in MD and demonstrated that gypsum scaling caused earlier water flux decline and induced membrane wetting that was not observed in silica scaling. Microscopic imaging and elemental mapping revealed contrasting scale morphology and distribution for gypsum and silica, respectively. Notably, while gypsum crystals grew both on the membrane surface and deep in the membrane matrix, silica only formed on the membrane surface in the form of a relatively thin film composed of connected submicrometer silica particles. We attribute the intrusion of gypsum into membrane pores to the crystallization pressure as a result of rapid, oriented crystal growth, which leads to pore deformation and the subsequent membrane wetting. In contrast, the silica scale layer was formed via polymerization of silicic acid and gelation of silica particles, which were less intrusive and had a milder effect on membrane pore structure. This hypothesis was supported by the result of tensile testing, which showed that the MD membrane was significantly weakened by gypsum scaling. The fact that different scaling mechanisms could yield different consequences on membrane performance provides valuable insights for the future development of cost-effective strategies for scaling control.

Energy Efficiency of Desalination: Fundamental Insights from Intuitive Interpretation

Desalination has become an essential toolset to combat the worsening water stress resulting from population and industrial growth and exacerbated by climate change. Various technologies have been developed to desalinate feedwater with a wide spectrum of salinity. While energy consumption is an important consideration in many desalination studies, it is challenging to make (intuitive) sense of energy efficiency due to the different mechanisms of various desalination processes and the very different separations achieved. This perspective aims to provide an intuitive, thermodynamics-based interpretation of energy efficiency by illustrating how energy consumption breaks down into minimum energy of separation and the irreversible energy dissipation. The energy efficiencies of different desalination processes are summarized and rationalized based on their working mechanisms. Notably, a new concept called the minimum mean voltage is proposed as a convenient tool to evaluate the energy efficiency of electrochemical desalination processes. Lastly, the intrinsic trade-off between energy efficiency and desalination rate and the relevance of energy efficiency in different desalination applications are discussed.