Polyamide Membranes with Tunable Surface Charge Induced by Dipole-Dipole Interaction for Selective Ion Separation.

Nanofiltration (NF) has the potential to achieve precise ion-ion separation at the subnanometer scale, which is necessary for resource recovery and a circular water economy. Fabricating NF membranes for selective ion separation is highly desirable but represents a substantial technical challenge. Dipole-dipole interaction is a mechanism of intermolecular attractions between polar molecules with a dipole moment due to uneven charge distribution, but such an interaction has not been leveraged to tune membrane structure and selectivity. Herein, we propose a novel strategy to achieve tunable surface charge of polyamide membrane by introducing polar solvent with a large dipole moment during interfacial polymerization, in which the dipole-dipole interaction with acyl chloride groups of trimesoyl chloride (TMC) can successfully intervene in the amidation reaction to alter the density of surface carboxyl groups in the polyamide selective layer. As a result, the prepared positively charged (PEI-TMC)-NH2 and negatively charged (PEI-TMC)-COOH composite membranes, which show similarly high water permeance, demonstrate highly selective separations of cations and anions in engineering applications, respectively. Our findings, for the first time, confirm that solvent-induced dipole-dipole interactions are able to alter the charge type and density of polyamide membranes and achieve tunable surface charge for selective and efficient ion separation.

Antiscalants for mitigating silica scaling in membrane desalination: Effects of molecular structure and membrane process.

Silica scaling is a major type of mineral scaling that significantly constrains the performance and efficiency of membrane desalination. While antiscalants have been commonly used to control mineral scaling formed via crystallization, there is a lack of antiscalants for silica scaling due to its unique formation mechanism of polymerization. In this study, we performed a systematic study that investigated and compared antiscalants with different functional groups and molecular weights for mitigating silica scaling in membrane distillation (MD) and reverse osmosis (RO). The efficiencies of these antiscalants were tested in both static experiments (for hindering silicic acid polymerization) as well as crossflow, dynamic MD and RO experiments (for reducing water flux decline). Our results show that antiscalants enriched with strong H-accepters and H-donors were both able to hinder silicic acid polymerization efficiently in static experiments, with their antiscaling performance being a function of both molecular functionality and weight. Although poly(ethylene glycol) (PEG) with abundant H-accepters exhibited high antiscaling efficiencies during static experiments, it displayed limited performance of mitigating silica scaling during MD and RO. Poly (ethylene glycol) diamine (PEGD), which has a PEG backbone but is terminated by two amino groups, was efficient to both hinder silicic acid polymerization and reduce water flux decline in MD and RO. Antiscalants enriched with H-donors, such as poly(ethylenimine) (PEI) and poly(amidoamine) (PAMAM), were effective of extending the water recovery of MD but conversely facilitated water flux decline of RO in the presence of supersaturated silica. Further analyses of silica scales formed on the membrane surfaces confirmed that the antiscalants interacted with silica via hydrogen bonding and showed that the presence of antiscalants governed the silica morphology. Our work indicates that discrepancy in antiscalant efficiency exists between static experiments and dynamic membrane filtration as well as between different membrane processes associated with silica scaling, providing valuable insights on the design principle and mechanisms of antiscalants tailored to silica scaling.

A Tale of Two Foulants: The Coupling of Organic Fouling and Mineral Scaling in Membrane Desalination.

Membrane desalination that enables the harvesting of purified water from unconventional sources such as seawater, brackish groundwater, and wastewater has become indispensable to ensure sustainable freshwater supply in the context of a changing climate. However, the efficiency of membrane desalination is greatly constrained by organic fouling and mineral scaling. Although extensive studies have focused on understanding membrane fouling or scaling separately, organic foulants commonly coexist with inorganic scalants in the feedwaters of membrane desalination. Compared to individual fouling or scaling, combined fouling and scaling often exhibits different behaviors and is governed by foulant-scalant interactions, resembling more complex but practical scenarios than using feedwaters containing only organic foulants or inorganic scalants. In this critical review, we first summarize the performance of membrane desalination under combined fouling and scaling, involving mineral scales formed via both crystallization and polymerization. We then provide the state-of-the-art knowledge and characterization techniques pertaining to the molecular interactions between organic foulants and inorganic scalants, which alter the kinetics and thermodynamics of mineral nucleation as well as the deposition of mineral scales onto membrane surfaces. We further review the current efforts of mitigating combined fouling and scaling via membrane materials development and pretreatment. Finally, we provide prospects for future research needs that guide the design of more effective control strategies for combined fouling and scaling to improve the efficiency and resilience of membrane desalination for the treatment of feedwaters with complex compositions.

Exploring the Knowledge Attained by Machine Learning on Ion Transport across Polyamide Membranes Using Explainable Artificial Intelligence.

Recent studies have increasingly applied machine learning (ML) to aid in performance and material design associated with membrane separation. However, whether the knowledge attained by ML with a limited number of available data is enough to capture and validate the fundamental principles of membrane science remains elusive. Herein, we applied explainable artificial intelligence (XAI) to thoroughly investigate the knowledge learned by ML on the mechanisms of ion transport across polyamide reverse osmosis (RO) and nanofiltration (NF) membranes by leveraging 1,585 data from 26 membrane types. The Shapley additive explanation method based on cooperative game theory was used to unveil the influences of various ion and membrane properties on the model predictions. XAI shows that the ML can capture the important roles of size exclusion and electrostatic interaction in regulating membrane separation properly. XAI also identifies that the mechanisms governing ion transport possess different relative importance to cation and anion rejections during RO and NF filtration. Overall, we provide a framework to evaluate the knowledge underlying the ML model prediction and demonstrate that ML is able to learn fundamental mechanisms of ion transport across polyamide membranes, highlighting the importance of elucidating model interpretability for more reliable and explainable ML applications to membrane selection and design.

Which Surface Is More Scaling Resistant? A Closer Look at Nucleation Theories for Heterogeneous Gypsum Nucleation in Aqueous Solutions.

Developing engineered surfaces with scaling resistance is an effective means to inhibit surface-mediated mineral scaling in various industries including desalination. However, contrasting results have been reported on the relationship between scaling potential and surface hydrophilicity. In this study, we combine a theoretical analysis with experimental investigation to clarify the effect of surface wetting property on heterogeneous gypsum (CaSO4·2H2O) formation on surfaces immersed in aqueous solutions. Theoretical prediction derived from classical nucleation theory (CNT) indicates that an increase of surface hydrophobicity reduces scaling potential, which contrasts our experimental results that more hydrophilic surfaces are less prone to gypsum scaling. We further consider the possibility of nonclassical pathway of gypsum nucleation, which proceeds by the aggregation of precursor clusters of CaSO4. Accordingly, we investigate the affinity of CaSO4 to substrate surfaces of varied wetting properties via calculating the total free energy of interaction, with the results perfectly predicting experimental observations of surface scaling propensity. This indicates that the interactions between precursor clusters of CaSO4 and substrate surfaces might play an important role in regulating heterogeneous gypsum formation. Our findings provide evidence that CNT might not be applicable to describing gypsum scaling in aqueous solutions. The fundamental insights we reveal on gypsum scaling mechanisms have the potential to guide rational design of scaling-resistant engineered surfaces.

The water footprint of hydraulic fracturing under different hydroclimate conditions in the Central and Western United States.

The oil and gas (O&G) exploitation via hydraulic fracturing (HF) has augmented both energy production and water demand in the United States. Despite the geographical coincidence of U.S. shale plays with water-scarce areas, the water footprint of HF under drought conditions, as well as its impacts on local water allocation, have not been well understood. In this study, we investigated the water consumption by HF activities under different hydroclimate conditions in eleven O&G-producing states in the central and western U.S. from 2011 to 2020. Our results show that the water consumption under abnormally dry or drought climates accounted for 49.7 % (475.3 billion gallons or 1.8 billion m3) of total water usage of HF, with 9 % (86.1 billion gallons or 325.9 million m3) of water usage occurring under extreme or exceptional drought conditions. The water usage of HF under arid conditions can translate to high densities of water footprint at the local scale, equivalent to >10 % and 50 % of the annual water usage by the irrigation and domestic sectors in 6-29 irrigation-active counties and 11-51 counties (depending on the specific year), respectively. Such water stress imposed by O&G production, however, can be effectively mitigated by the reuse of flowback and produced water. Our findings, for the first time, quantify the water footprint of HF as a function of hydroclimate condition, providing evidence that the water consumption by HF intensifies local water competition and alters water supply threatened by climate variability. This renders wastewater reuse necessary to maintain water sustainability of O&G-producing regions in the context of both a rising O&G industry and a changing climate.

Long-Chain PFASs-Free Omniphobic Membranes for Sustained Membrane Distillation.

Omniphobic membranes possessing high wetting resistance have been created for the treatment of challenging hypersaline feedwaters with low surface tension through membrane distillation (MD). However, virtually all such membranes are fabricated with long-chain per- and polyfluoroalkyl substances (PFASs, ≥8 fluorinated carbons). The environmental risks and high bioaccumulation potential of long-chain PFASs have raised increasing concerns. Developing highly wetting-resistant MD membranes while avoiding the use of long-chain PFASs is essential to improve the viability of MD for resilient and sustainable water purification. We demonstrate that MD membranes with exceptional wetting resistance can be designed through the combination of hierarchically structured membranes consisting of re-entrant texture at different length scales and (ultra)short-chain fluorocarbons, which have lower acute toxicity and bioaccumulation potentials than long-chain PFASs. Our hierarchically structured membrane with three-tier micro/nanostructure fabricated with short-chain fluorocarbon possesses superior wetting resistance, which is comparable to or higher than the long-chain PFASs-based omniphobic membranes reported in the literature. Furthermore, the hierarchically structured membranes fabricated with ultrashort-chain fluorocarbons display improved wetting resistance against feedwaters with low surface tension. Our findings indicate that long-chain PFASs are not required when designing wetting-resistant membranes and that the balance between sustainability and wetting resistance should be tailored to the wetting potential of the feedwater.

Superhydrophobic Carbon Nanotube Network Membranes for Membrane Distillation: High-Throughput Performance and Transport Mechanism.

Despite increasing sustainable water purification, current desalination membranes still suffer from insufficient permeability and treatment efficiency, greatly hindering extensive practical applications. In this work, we provide a new membrane design protocol and molecule-level mechanistic understanding of vapor transport for the treatment of hypersaline waters via a membrane distillation process by rationally fabricating more robust metal-based carbon nanotube (CNT) network membranes, featuring a superhydrophobic superporous surface (80.0 ± 2.3% surface porosity). With highly permeable ductile metal hollow fibers as substrates, the construction of a superhydrophobic (water contact angle ∼170°) CNT network layer endows the membranes with not only almost perfect salt rejection (over 99.9%) but a promising water flux (43.6 L·m-2·h-1), which outperforms most existing inorganic distillation membranes. Both experimental and molecular dynamics simulation results indicate that such an enhanced water flux can be ascribed to an ultra-low liquid-solid contact interface (∼3.23%), allowing water vapor to rapidly transport across the membrane structure via a combined mechanism of Knudsen diffusion (more dominant) and viscous flow while efficiently repelling high-salinity feed via forming a Cassie-Baxter state. A more hydrophobic surface is more in favor of not only water desorption from the CNT outer surface but superfast and frictionless water vapor transport. By constructing a new superhydrophobic triple-phase interface, the conceptional design strategy proposed in this work can be expected to be extended to other membrane material systems as well as more water treatment applications.

Capillary-Assisted Fabrication of Thin-Film Nanocomposite Membranes for Improved Solute-Solute Separation.

Efficient separation of harmful contaminants (e.g., per- and polyfluoroalkyl substances, PFASs) from valuable components (water and nutrients) is essential to the resource recovery from domestic wastewater for agricultural purposes. Such selective recovery requires precise separation at the angstrom scale. Although nanofiltration (NF) has the potential to achieve solute-solute separation, the state-of-the-art polyamide (PA) membranes are typically constrained by limited precision of solute-solute selectivity and their well-documented permeability-selectivity trade-off. Herein, we present a novel capillary-assisted interfacial polymerization (CAIP) approach to generate metal-organic framework (MOF)-PA nanocomposite membranes with reduced surface charges and more uniform pore sizes that favor solute selectivity by enhanced size exclusion. By uniquely regulating the PA-MOF interactions using the capillary force, CAIP results in effective exposure of MOF nanochannels on the membrane surface and a PA matrix with a high cross-linking gradient in the vertical direction, both of which contribute to an exceptional water permeance of ∼18.7 LMH/bar and an unprecedentedly high selectivity between nutrient ions and PFASs. Our CAIP approach breaks new ground for utilizing nanoparticles with nanochannels in fabricating the next-generation, fit-for-purpose NF membranes for improved solute-solute separations.

Robust ultrathin nanoporous MOF membrane with intra-crystalline defects for fast water transport.

Rational design of high-performance stable metal-organic framework (MOF) membranes is challenging, especially for the sustainable treatment of hypersaline waters to address critical global environmental issues. Herein, a molecular-level intra-crystalline defect strategy combined with a selective layer thinning protocol is proposed to fabricate robust ultrathin missing-linker UiO-66 (ML-UiO-66) membrane to enable fast water permeation. Besides almost complete salt rejection, high and stable water flux is achieved even under long-term pervaporation operation in hash environments, which effectively addresses challenging stability issues. Then, detailed structural characterizations are employed to identify the type, chemical functionality, and density of intra-crystalline missing-linker defects. Moreover, molecular dynamics simulations shed light on the positive atomistic role of these defects, which are responsible for substantially enhancing structural hydrophilicity and enlarging pore window, consequently allowing ultra-fast water transport via a lower-energy-barrier pathway across three-dimensional sub-nanochannels during pervaporation. Unlike common unfavorable defect effects, the present positive intra-crystalline defect engineering concept at the molecular level is expected to pave a promising way toward not only rational design of next-generation MOF membranes with enhanced permeation performance, but additional water treatment applications.

Predicting Micropollutant Removal by Reverse Osmosis and Nanofiltration Membranes: Is Machine Learning Viable?

Predictive models for micropollutant removal by membrane separation are highly desirable for the design and selection of appropriate membranes. While machine learning (ML) models have been applied for such purposes, their reliability might be compromised by data leakage due to inappropriate data splitting. More importantly, whether ML models can truly understand the mechanisms of membrane separation has not been revealed. In this study, we evaluate the capability of the XGBoost model to predict micropollutant removal efficiencies of reverse osmosis and nanofiltration membranes. Our results demonstrate that data leakage leads to falsely high prediction accuracy. By utilizing a model interpretation method based on the cooperative game theory, we test the knowledge of XGBoost on the mechanisms of membrane separation via quantifying the contributions of input variables to the model predictions. We reveal that XGBoost possesses an adequate understanding of size exclusion, but its knowledge of electrostatic interactions and adsorption is limited. Our findings suggest that future work should focus more on avoiding data leakage and evaluating the mechanistic knowledge of ML models. In addition, high-quality data from more diverse experimental conditions, as well as more informative variables, are needed to improve the accuracy of ML models for predicting membrane performance.

Contrasting Behaviors between Gypsum and Silica Scaling in the Presence of Antiscalants during Membrane Distillation.

Mineral scaling is a major constraint that limits the performance of membrane distillation (MD) for hypersaline wastewater treatment. Although the use of antiscalants is a common industrial practice to mitigate mineral scaling, the effectiveness and underlying mechanisms of antiscalants in inhibiting different mineral scaling types have not been systematically investigated. Herein, we perform a comparative investigation to elucidate the efficiencies of antiscalant candidates with varied functional groups for mitigating gypsum scaling and silica scaling in MD desalination. We show that antiscalants with Ca(II)-complexing moieties (e.g., carboxyl group) are the most effective to inhibit gypsum scaling formed via crystallization, whereas amino-enriched antiscalants possess the best performance to mitigate silica scaling created by polymerization. A set of microscopic and spectroscopic analyses reveal distinct mechanisms of antiscalants required for those two common types of scaling. The mitigating effect of antiscalants on gypsum scaling is attributed to the stabilization of scale precursors and nascent CaSO4 nuclei, which hinders phase transformation of amorphous CaSO4 toward crystalline gypsum. In contrast, antiscalants facilitate the polymerization of silicic acid, immobilizing active silica precursors and retarding the gelation of silica scale layer on the membrane surface. Our study, for the first time, demonstrates that antiscalants with different functionalities are required for the mitigation of gypsum scaling and silica scaling, providing mechanistic insights on the molecular design of antiscalants tailored to MD applications for the treatment of wastewaters containing different scaling types.

Differentiating Solutes with Precise Nanofiltration for Next Generation Environmental Separations: A Review.

Selective removal or enrichment of targeted solutes including micropollutants, valuable elements, and mineral scalants from complex aqueous matrices is both challenging and pivotal to the success of water purification and resource recovery from unconventional water resources. Membrane separation with precision at the subnanometer or even subangstrom scale is of paramount importance to address those challenges via enabling "fit-for-purpose" water and wastewater treatment. So far, researchers have attempted to develop novel membrane materials with precise and tailored selectivity by tuning membrane structure and chemistry. In this critical review, we first present the environmental challenges and opportunities that necessitate improved solute-solute selectivity in membrane separation. We then discuss the mechanisms and desired membrane properties required for better membrane selectivity. On the basis of the most recent progress reported in the literature, we examine the key principles of material design and fabrication, which create membranes with enhanced and more targeted selectivity. We highlight the important roles of surface engineering, nanotechnology, and molecular-level design in improving membrane selectivity. Finally, we discuss the challenges and prospects of highly selective NF membranes for practical environmental applications, identifying knowledge gaps that will guide future research to promote environmental sustainability through more precise and tunable membrane separation.

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.

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.

Achieving low-cost, highly selective nitrate removal with standard anion exchange resin by tuning recycled brine composition.

This study demonstrated the presence of a critical equivalent ratio of the competing anion (i.e., sulfate and bicarbonate) to chloride ion in recycled brine to achieve highly-selective nitrate removal from nitrate-rich groundwater in the standard-anion exchange resin (AER) (i.e., with trimethylamine functional groups) column process. With increasing bicarbonate (or sulfate):chloride equivalent ratio in brine used to circularly activate/regenerate the standard-AER column, considerable bicarbonate (sulfate) removal and dumping were observed. The critical bicarbonate (sulfate):chloride equivalent ratio of 2:5 (8:1) in brine effectively achieved zero net bicarbonate (sulfate) removal (<5%) from feedwater during long-term exhaustion-regeneration cyclic operation. The feed rate (6-18 BV/h) played a key role in determining the critical sulfate:chloride equivalent ratio in brine, while the feed sulfate concentration (145-345 mg/L) slightly changed the critical sulfate:chloride equivalent ratio. The use of optimized ternary brine (with a sulfate:chloride:bicarbonate equivalent ratio of 42:5:2) stably achieved long-term highly-selective nitrate removal from groundwater in the standard-AER column process with brine electrochemical treatment. The possible mechanism for nitrate selectivity included the modification of the sulfate: and bicarbonate:chloride equivalent ratios in the standard-AER column by the optimized brine in circular activation/regeneration mode; this changed the column elution and breakthrough curves, inhibited the competition of sulfate and bicarbonate for ion exchange sites during exhaustion according to the separation factor, and finally achieved selective nitrate removal from feedwater.

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.

Electrochemical Oxidation of Hexafluoropropylene Oxide Dimer Acid (GenX): Mechanistic Insights and Efficient Treatment Train with Nanofiltration.

Hexafluoropropylene oxide dimer acid (HFPO-DA, trade name GenX) is a perfluoroalkyl ether carboxylic acid (PFECA) that has been detected in watersheds around the world. Similar to other per- and polyfluoroalkyl substances (PFASs), few processes are able to break HFPO-DA's persistent carbon-fluorine bonds. This study provides both experimental and computational lines of evidence for HFPO-DA mineralization during electrochemical oxidation at a boron-doped diamond anode with a low potential for the generation of stable organofluorine intermediates. Our density functional theory calculations consider the major operative mechanism, direct electron transfer, throughout the entire pathway. Initial oxidative attack does not break the ether bond, but leads to stepwise mineralization of the acidic side chain. Our mechanistic investigations reveal that hydroxyl radicals are unreactive toward HFPO-DA, while electrochemically activated sulfate facilitates its oxidation. Furthermore, we demonstrate that an NF90 membrane is capable of removing 99.5% of HFPO-DA from contaminated water. Electrochemical treatment of the nanofiltration rejectate is shown to reduce both energy and electrode costs by more than 1 order of magnitude compared to direct electrochemical treatment of the raw water. Overall, a nanofiltration-electrochemical oxidation treatment train is a sustainable destructive approach for the cost-effective elimination of HFPO-DA and other PFASs from contaminated water.

Trade-off in membrane distillation with monolithic omniphobic membranes.

Omniphobic membranes are attractive for membrane distillation (MD) because of their superior wetting resistance. However, a design framework for MD membrane remains incomplete, due to the complexity of omniphobic membrane fabrication and the lack of fundamental relationship between wetting resistance and water vapor permeability. Here we present a particle-free approach that enables rapid fabrication of monolithic omniphobic membranes for MD desalination. Our monolithic omniphobic membranes display excellent wetting resistance and water purification performance in MD desalination of hypersaline feedwater containing surfactants. We identify that a trade-off exists between wetting resistance and water vapor permeability of our monolithic MD membranes. Utilizing membranes with tunable wetting resistance and permeability, we elucidate the underlying mechanism of such trade-off. We envision that our fabrication method as well as the mechanistic insight into the wetting resistance-vapor permeability trade-off will pave the way for smart design of MD membranes in diverse water purification applications.