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Membrane filtration has been widely adopted in various water treatment applications, but its use in selective solute separation for resource extraction and recovery is an emerging research area. When a membrane process is applied for solute-solute separation to extract solutes as the product, the performance metrics and process optimization strategies should differ from a membrane process for water production because of separation goals are fundamentally different. In this analysis, we used lithium (Li) magnesium (Mg) separation as a representative solute-solute separation to illustrate the deficiency of existing performance evaluation framework developed for water-solute separation using nanofiltration (NF). We performed coupon and module scale analyses of mass transfer to elucidate how membrane properties and operating conditions affect the performance of Li/Mg separation in NF. Notably, we identified an important operational tradeoff between Li/Mg selectivity and Li recovery, which is critical for process optimization. We also established a new framework for evaluating membrane performance based on the success criteria of Li purity and recovery. This analysis lays the theoretical foundation for performance evaluation and process optimization for NF-based selective solute separation.

Securing decarbonized economies for energy and commodities will require abundant and widely available green H

Clarifying the reaction pathways at the solid–water interface and in bulk water solution is of great significance for the design of heterogeneous catalysts for selective oxidation of organic pollutants. However, achieving this goal is daunting because of the intricate interfacial reactions at the catalyst surface. Herein, we unravel the origin of the organic oxidation reactions with metal oxide catalysts, revealing that the radical-based advanced oxidation processes (AOPs) prevail in bulk water but not on the solid catalyst surfaces. We show that such differing reaction pathways widely exist in various chemical oxidation (e.g., high-valent Mn 3+ and MnO X ) and Fenton and Fenton-like catalytic oxidation (e.g., Fe 2+ and FeOCl catalyzing H 2 O 2 , Co 2+ and Co 3 O 4 catalyzing persulfate) systems. Compared with the radical-based degradation and polymerization pathways of one-electron indirect AOP in homogeneous reactions, the heterogeneous catalysts provide unique surface properties to trigger surface-dependent coupling and polymerization pathways of a two-electron direct oxidative transfer process. These findings provide a fundamental understanding of catalytic organic oxidation processes at the solid–water interface, which could guide the design of heterogeneous nanocatalysts.

The successful implementation of thin-film composite membranes (TFCM) for challenging solute-solute separations in the pharmaceutical industry requires a fine control over the microstructure (size, distribution, and connectivity of the free-volume elements) and thickness of the selective layer. For example, desalinating antibiotic streams requires highly interconnected free-volume elements of the right size to block antibiotics but allow the passage of salt ions and water. Here, we introduce stevioside, a plant-derived contorted glycoside, as a promising aqueous phase monomer for optimizing the microstructure of TFCM made via interfacial polymerization. The low diffusion rate and moderate reactivity of stevioside, together with its nonplanar and distorted conformation, produced thin selective layers with an ideal microporosity for antibiotic desalination. For example, an optimized 18-nm membrane exhibited an unprecedented combination of high water permeance (81.2 liter m −2 hour −1 bar −1 ), antibiotic desalination efficiency (NaCl/tetracycline separation factor of 11.4), antifouling performance, and chlorine resistance.

We performed nonequilibrium molecular dynamics (NEMD) simulations and solvent permeation experiments to unravel the mechanism of water transport in reverse osmosis (RO) membranes. The NEMD simulations reveal that water transport is driven by a pressure gradient within the membranes, not by a water concentration gradient, in marked contrast to the classic solution-diffusion model. We further show that water molecules travel as clusters through a network of pores that are transiently connected. Permeation experiments with water and organic solvents using polyamide and cellulose triacetate RO membranes showed that solvent permeance depends on the membrane pore size, kinetic diameter of solvent molecules, and solvent viscosity. This observation is not consistent with the solution-diffusion model, where permeance depends on the solvent solubility. Motivated by these observations, we demonstrate that the solution-friction model, in which transport is driven by a pressure gradient, can describe water and solvent transport in RO membranes.

The release of wastewaters containing relatively low levels of nitrate (NO 3 − ) results in sufficient contamination to induce harmful algal blooms and to elevate drinking water NO 3 − concentrations to potentially hazardous levels. In particular, the facile triggering of algal blooms by ultra-low concentrations of NO 3 − necessitates the development of efficient methods for NO 3 − destruction. However, promising electrochemical methods suffer from weak mass transport under low reactant concentrations, resulting in long treatment times (on the order of hours) for complete NO 3 − destruction. In this study, we present flow-through electrofiltration via an electrified membrane incorporating nonprecious metal single-atom catalysts for NO 3 − reduction activity enhancement and selectivity modification, achieving near-complete removal of ultra-low concentration NO 3 − (10 mg-N L −1 ) with a residence time of only a few seconds (10 s). By anchoring Cu single atoms supported on N-doped carbon in a carbon nanotube interwoven framework, we fabricate a free-standing carbonaceous membrane featuring high conductivity, permeability, and flexibility. The membrane achieves over 97% NO 3 − removal with high N 2 selectivity of 86% in a single-pass electrofiltration, which is a significant improvement over flow-by operation (30% NO 3 − removal with 7% N 2 selectivity). This high NO 3 − reduction performance is attributed to the greater adsorption and transport of nitric oxide under high molecular collision frequency coupled with a balanced supply of atomic hydrogen through H 2 dissociation during electrofiltration. Overall, our findings provide a paradigm of applying a flow-through electrified membrane incorporating single-atom catalysts to improve the rate and selectivity of NO 3 − reduction for efficient water purification.

Understanding the salt-water separation mechanisms of reverse osmosis (RO) membranes is critical for the further development and optimization of RO technology. The solution-diffusion (SD) model is widely used to describe water and salt transport in RO, but it does not describe the intricate transport mechanisms of water molecules and ions through the membrane. In this study, we develop an ion transport model for RO, referred to as the solution-friction model, by rigorously considering the mechanisms of partitioning and the interactions among water, salt ions, and the membrane. Ion transport through the membrane is described by the extended Nernst-Planck equation, with the consideration of frictions between the species (i.e., ion, water, and membrane matrix). Water flow through the membrane is governed by the hydraulic pressure gradient and the friction between the water and membrane matrix as well as the friction between water and ions. The model is validated using experimental measurements of salt rejection and permeate water flux in a lab-scale, cross-flow RO setup. We then investigate the effects of feed salt concentration and hydraulic pressure on salt permeability, demonstrating strong dependence of salt permeability on feed salt concentration and applied pressure, starkly disparate from the SD model. Lastly, we develop a framework to analyze the pressure drop distribution across the membrane, demonstrating that cross-membrane transport dominates the overall pressure drop in RO, in marked contrast to the SD model that assumes no pressure drop across the membrane.

Membrane separation has enjoyed tremendous advances in relevant material and engineering sciences, making it the fastest growing technology in water treatment. Although membranes as a broad-spectrum physical barrier have great advantages over conventional treatment processes in a myriad of applications, the need for higher selectivity and specificity in membrane separation is rising as we move to target contaminants at trace concentrations and to recover valuable chemicals from wastewater with low energy consumption. In this review, we discuss the drivers, fundamental science, and potential enabling materials for high selectivity membranes, as well as their applications in different water treatment processes. Membrane materials and processes that show promise to achieve high selectivity for water, ions, and small molecules—as well as the mechanisms involved—are highlighted. We further identify practical needs, knowledge gaps, and technological barriers in both material development and process design for high selectivity membrane processes. Finally, we discuss research priorities in the context of existing and future water supply paradigms.

The lack of electron donors in oxygen-rich aquatic environments limits the ability of natural denitrification to remove excess nitrate, leading to eutrophication of aquatic ecosystems. Herein, we demonstrate that electron rich substances in river or lake sediments could participate in long-distance electron rebalancing to reduce nitrate in overlying water. A microstructure containing Dechloromonas and consisting of an inner layer of green rust and an outer layer of lepidocrocite forms in the sediment-water system through synergetic evolution and self-assembly. The microstructure enables long-distance electron transfer from the sediment to dilute nitrate in the overlying water. Specifically, the inner green rust adsorbs nitrate and reduces the kinetic barrier for denitrification via an Fe(II)/Fe(III) redox mediator. Our study reveals the mechanism of spontaneous electron transfer between distant and dilute electron donors and acceptors to achieve denitrification in electron-deficient aquatic systems.

Engineered nanoporous materials have been extensively employed in the environmental field to take advantage of increased surface area and tunable size exclusion. Beyond those benefits, recent studi...

Electrified membranes (EMs) have the potential to address inherent limitations of conventional membrane technologies. Recent studies have demonstrated that EMs exhibit enhanced functions beyond sep...

Aggregation kinetics and surface charging properties of rod-like sulfated cellulose nanocrystals (CNCs) have been investigated in aqueous suspensions containing monovalent, divalent, or trivalent inorganic salts. Electrophoresis and time-resolved dynamic light scattering (DLS) were used to characterize the surface charge and colloidal stability of the CNCs, respectively. The surface charge and aggregation kinetics of the sulfated CNCs were found to be independent of solution pH (pH range 2-10). For the monovalent salts (CsCl, KCl, NaCl, and LiCl), the critical coagulation concentration (CCC) followed the order of Cs+ Ca2+ > Ba2+, which is in the reverse order of the counterion ionic size. For the trivalent salts (LaCl3, AlCl3, and FeCl3), the CNCs suspension was destabilized much more effectively. The observed complex stability curves with AlCl3 and FeCl3 are attributed to charge neutralization and charge reversal imparted by the adsorption of aluminum and ferric hydrolysis species on the CNC surface. The significant charge reversal induced by the ferric hydrolysis species led to the restabilization of suspensions. Our results on the colloidal stability of CNCs are of central importance to the nanotechnology and materials science communities working on various applications of CNCs.

Though electrodialysis (ED) and reverse osmosis (RO) are both mature, proven technologies for brackish water desalination, RO is currently utilized to desalinate over an order of magnitude more bra...

Within the last decade, in addition to water desalination, capacitive deionization (CDI) has been used for resource recovery and selective separation of target ions in multicomponent solutions. In this review, we summarize the mechanisms of selective ion removal utilizing different electrode materials, carbon and non-carbon together with or without membranes, from a mixture of salt solutions, by a detailed review of the literature from the beginning until the state-of-the-art. In this venture, we review the advances made in the preparation, theoretical understanding, and the role of electrodes and membranes. We also describe how ion selectivity has been defined and used in literature. Finally, we present a theory of selective ion removal for intercalation materials that, for the first time, considers mixtures of different cations, evidencing the time-dependent selectivity of these electrodes.

Membrane desalination is a promising technology for addressing the global challenge of water scarcity by augmenting fresh water supply. Continuous progress in this technology relies on development of membrane materials. The state-of-the-art membranes used in a wide range of desalination applications are polyamide thin-film composite (TFC) membranes which are formed by interfacial polymerization (IP). Despite the wide use of such membranes in desalination, their real-world application is still hampered by several technical obstacles. These challenges of the TFC membranes largely stem from the inherent limitations of the polyamide chemistry, as well as the IP reaction mechanisms. In the past decade, we have witnessed substantial progress in the understanding of polyamide formation mechanisms and the development of new IP strategies that can potentially lead to the redesign of TFC membranes. In this Tutorial, we first present a brief history of the development of desalination membranes and highlight the major challenges of the existing TFC membranes. We then proceed to discuss the pros and cons of emerging IP-based fabrication strategies aiming at improving the performance of TFC membranes. Next, we present technical obstacles and recent efforts in the characterization of TFC membranes to enable fundamental understanding of relevant mechanisms. We conclude with a discussion of the current gap between industrial needs and academic research in designing high-performance TFC membranes, and provide an outlook on future research directions for advancing IP-based fabrication processes.

Emerging organic contaminants (EOCs) discharged from wastewater effluents into drinking water resources are of growing concern for human health and the environment. In this study, we demonstrate th...

Minimizing the energy consumption of desalination processes is an important goal for augmenting freshwater production and mitigating water scarcity. Chemical, civil, mechanical, and environmental engineering students can derive and analyze the energy consumption of desalination processes by applying engineering fundamentals such as thermodynamics, transport phenomena, and process design. We explore the fundamental thermodynamic limits of the most prominent desalination technologies in a format designed for engineering students and instructors. Two thermodynamically reversible processes for reverse osmosis (RO) and electrodialysis (ED) are developed to demonstrate that reversible processes consume the theoretical minimum energy, which is the Gibbs free energy of separation. We then quantify the practical minimum energy consumption for RO and ED, showing that the energy consumption of these processes approaches the minimum thermodynamic limit with increased process staging.

Transmembrane protein channels, including ion channels and aquaporins that are responsible for fast and selective transport of water, have inspired membrane scientists to exploit and mimic their performance in membrane technologies. These biomimetic membranes comprise discrete nanochannels aligned within amphiphilic matrices on a robust support. While biological components have been used directly, extensive work has also been conducted to produce stable synthetic mimics of protein channels and lipid bilayers. However, the experimental performance of biomimetic membranes remains far below that of biological membranes. In this review, we critically assess the status and potential of biomimetic desalination membranes. We first review channel chemistries and their transport behavior, identifying key characteristics to optimize water permeability and salt rejection. We compare various channel types within an industrial context, considering transport performance, processability, and stability. Through a re-examination of previous vesicular stopped-flow studies, we demonstrate that incorrect permeability equations result in an overestimation of the water permeability of nanochannels. We find in particular that the most optimized aquaporin-bearing bilayer had a pure water permeability of 2.1 L m-2 h-1 bar-1, which is comparable to that of current state-of-the-art polymeric desalination membranes. Through a quantitative assessment of biomimetic membrane formats, we analytically show that formats incorporating intact vesicles offer minimal benefit, whereas planar biomimetic selective layers could allow for dramatically improved salt rejections. We then show that the persistence of nanoscale defects explains observed subpar performance. We conclude with a discussion on optimal strategies for minimizing these defects, which could enable breakthrough performance.

Synthetic membranes with pores at the subnanometre scale are at the core of processes for separating solutes from water, such as water purification and desalination. While these membrane processes have achieved substantial industrial success, the capability of state-of-the-art membranes to selectively separate a single solute from a mixture of solutes is limited. Such high-precision separation would enable fit-for-purpose treatment, improving the sustainability of current water-treatment processes and opening doors for new applications of membrane technologies. Herein, we introduce the challenges of state-of-the-art membranes with subnanometre pores to achieve high selectivity between solutes. We then analyse experimental and theoretical literature to discuss the molecular-level mechanisms that contribute to energy barriers for solute transport through subnanometre pores. We conclude by providing principles and guidelines for designing next-generation single-species selective membranes that are inspired by ion-selective biological channels.

Separating molecules or ions with sub-Angstrom scale precision is important but technically challenging. Achieving such a precise separation using membranes requires Angstrom scale pores with a high level of pore size uniformity. Herein, we demonstrate that precise solute-solute separation can be achieved using polyamide membranes formed via surfactant-assembly regulated interfacial polymerization (SARIP). The dynamic, self-assembled network of surfactants facilitates faster and more homogeneous diffusion of amine monomers across the water/hexane interface during interfacial polymerization, thereby forming a polyamide active layer with more uniform sub-nanometre pores compared to those formed via conventional interfacial polymerization. The polyamide membrane formed by SARIP exhibits highly size-dependent sieving of solutes, yielding a step-wise transition from low rejection to near-perfect rejection over a solute size range smaller than half Angstrom. SARIP represents an approach for the scalable fabrication of ultra-selective membranes with uniform nanopores for precise separation of ions and small solutes., Separating molecules or ions with sub-Angstrom scale precision is important but technically challenging. Here, the authors demonstrate that precise solute-solute separation can be achieved using polyamide membranes formed via surfactant-assembly regulated interfacial polymerization.

Electro-driven technologies are viewed as a potential alternative to the current state-of-the-art technology, reverse osmosis, for the desalination of brackish waters. Capacitive deionization (CDI), based on the principle of electrosorption, has been intensively researched under the premise of being energy efficient. However, electrodialysis (ED), despite being a more mature electro-driven technology, has yet to be extensively compared to CDI in terms of energetic performance. In this study, we utilize Nernst-Planck based models for continuous flow ED and constant-current membrane capacitive deionization (MCDI) to systematically evaluate the energy consumption of the two processes. By ensuring equivalently sized ED and MCDI systems-in addition to using the same feed salinity, salt removal, water recovery, and productivity across the two technologies-energy consumption is appropriately compared. We find that ED consumes less energy (has higher energy efficiency) than MCDI for all investigated conditions. Notably, our results indicate that the performance gap between ED and MCDI is substantial for typical brackish water desalination conditions (e.g., 3 g L-1 feed salinity, 0.5 g L-1 product water, 80% water recovery, and 15 L m-2 h-1 productivity), with the energy efficiency of ED often exceeding 30% and being nearly an order of magnitude greater than MCDI. We provide further insights into the inherent limitations of each technology by comparing their respective components of energy consumption, and explain why MCDI is unable to attain the performance of ED, even with ideal and optimized operation.

As the threat of global water scarcity continues to grow, a myriad of scientific effort is directed towards advancing water desalination technologies. Reverse osmosis (RO), solar thermal desalination (STD), and capacitive deionization (CDI), have dominated recent pressure-, thermal-, and electro-driven desalination research efforts, respectively. Despite being based on distinctive driving forces and separation mechanisms, research of these three processes has primarily shared the same fundamental goal and approach: the minimization of energy consumption for desalination through the development of novel materials. A variety of materials have been studied and proposed to enhance RO membrane permeability, STD solar absorptivity, and CDI electrode capacitance. Here, we critically discuss the advanced materials investigated and assess their efficacy in augmenting the energy efficiency of desalination. Through our systematic analysis, we show that materials have relatively insignificant impact on further increasing energy efficiency, regardless of the process applied. We provide insights into the inherent limitations of advanced materials for improving the energy efficiency of each of the evaluated technologies and propose more effective materials-based research directions. We conclude by highlighting the opportunity for considerable improvement in energy efficiency via system design, reinforcing the critical need for a paradigm shift in desalination research.

State-of-the-art polymeric membranes are unable to perform the high-precision ion separations needed for technologies essential to a circular economy and clean energy future. Coordinative interactions are a mechanism to increase sorption of a target species into a membrane, but the effects of these interactions on membrane permeability and selectivity are poorly understood. We use a multilayered polymer membrane to assess how ion-membrane binding energies affect membrane permeability of similarly sized cations: Cu 2+ , Ni 2+ , Zn 2+ , Co 2+ , and Mg 2+ . We report that metals with higher binding energy to iminodiacetate groups of the polymer more selectively permeate through the membrane in multisalt solutions than single-salt solutions. In contrast, weaker binding species are precluded from diffusing into the polymer membrane, which leads to passage proportional to binding energy and independent of membrane thickness. Our findings demonstrate that selectivity of polymeric membranes can markedly increase by tailoring ion-membrane binding energy and minimizing membrane thickness.

Description, Machine learning unveils molecular transport mechanisms obscured by entropy-enthalpy compensation in polymeric membranes., Designing single-species selective membranes for high-precision separations requires a fundamental understanding of the molecular interactions governing solute transport. Here, we comprehensively assess molecular-level features that influence the separation of 18 different anions by nanoporous cellulose acetate membranes. Our analysis identifies the limitations of bulk solvation characteristics to explain ion transport, highlighted by the poor correlation between hydration energy and the measured permselectivity (R2 = 0.37). Entropy-enthalpy compensation, spanning 40 kilojoules per mole, leads to a free-energy barrier (∆G‡) variation of only ~8 kilojoules per mole across all anions. We apply machine learning to elucidate descriptors for energetic barriers from a set of 126 collected features. Notably, electrostatic features account for 75% of the overall features used to describe ∆G‡, despite the relatively uncharged state of cellulose acetate. Our work presents an approach for studying ion transport across nanoporous membranes that could enable the design of ion-selective membranes.

The quality of municipal water supplies is of fundamental importance to public health and national security. Here, we assess the tap water quality in 31 provinces across China and examine the effects of natural and anthropogenic conditions on water quality and associated health risks. Precipitation is a crucial factor influencing the organic matter content and ionic conductivity in tap water, especially for arid and semi-arid regions. Although the concentration of disinfection byproducts (DBPs) is closely related to the organic matter content, the occurrence of highly toxic DBPs was more significantly affected by anthropogenic factors such as economic development and pollution emission. We confirmed nanofiltration as an effective point-of-use treatment to reduce the adverse effects of DBPs in public water supplies. Since DBPs in tap water is a long-term global problem, our results highlight the potential health hazards of drinking water brought about by social development and conclude that countries and regions with rapid development might face high DBP toxicity.

Since the advent of thin-film composite polyamide membranes brought forth a breakthrough in desalination and water purification membranes nearly half a century ago, recent years have only witnessed marginal improvements in the water-salt selectivity of these membranes. The slow progression is partly attributable to limited understanding of membrane synthesis–structure–performance relationships. A centralized archive of reverse osmosis membrane (RO) characterization data may lead to a shared understanding of features that maximize RO performance and unify research efforts. The Open Membrane Database (OMD), which can be found at www.openmembranedatabase.org , is a growing database of over 600 water purification and desalination membranes that are sourced from peer-reviewed journals, patents, and commercial product data. Here, we outline the detailed functionality of the database, the transport theory underlying the membrane performance calculations, and best practices for membrane performance testing and reporting. The user-sourced, open-access database may be used to benchmark novel RO membranes against the state of the art, conduct meta-analyses, and develop synthesis–structure–performance relationships, each of which will be critical to advancing membrane development.

ispartof: ENVIRONMENTAL SCIENCE & TECHNOLOGY LETTERS vol:8 issue:9 pages:818-824 status: published

Nanofiltration membranes have limited ion-ion selectivity in water treatment applications, especially when separating ions with similar size and charge. To achieve greater size-based selectivity in nanofiltration, more control of pore structure is required during membrane fabrication. We demonstrate how to tailor membrane pore size and thickness using polyelectrolyte layer-by-layer assembly by alternately applying two strong polyelectrolytes, PDADMAC and PSS, to a polysulfone substrate while systematically controlling the polyelectrolyte and salt concentrations in the deposition solution. Results suggest that increasing polyelectrolyte concentration or salt concentration in the deposition solution increases polyelectrolyte multilayer thickness, but the effects on pore size may be categorized into two distinct regimes. In the first growth regime, increasing polyelectrolyte concentration in the deposition solution led to larger polymer deposition rates and smaller pore sizes. In the second growth regime, increasing polyelectrolyte concentration produced larger pore sizes. We attribute the second regime to less adsorbed polyelectrolyte on the membrane and/or less coiled polymer chains as a result of changing polyelectrolyte-salt interactions. Overall, results show that pore size modification is achievable using layer-by-layer assembly by tuning polyelectrolyte-salt interactions and can be used to study and improve size-based selectivity in membrane separation processes.

We explored the mechanisms governing the selectivity of anion- and cation-exchange membranes for the transport of four monovalent anions (i.e., fluoride, chloride, bromide, and nitrate) and four monovalent cations (i.e., sodium, potassium, cesium, and ammonium), respectively. Our ion adsorption and transport tests with mixed ion solutions reveal that an ion with larger ionic radius and lower hydration energy is more favorably adsorbed onto the ion-exchange membrane but diffuses more slowly through the polymer matrix compared to an ion with smaller ionic radius and higher hydration energy. Individual anion (as sodium salt) or cation (as chloride salt) permeation tests at different temperatures were performed to evaluate the activation behavior of ion transport through the ion-exchange membranes by calculating the energy barrier and pre-exponential factor (i.e., the ion flux when the energy barrier is negligible) for ion transport from an Arrhenius-type equation. Our results show that an ion with smaller ionic radius and higher hydration energy experiences higher energy barrier (e.g., fluoride, 10.3 kcal mol−1) and possesses higher pre-exponential factor compared to an ion with larger ionic radius and lower hydration energy (e.g., bromide, 4.6 kcal mol−1). This correlation corroborates our main hypothesis that the activation behavior observed for ion transport is a result of ion dehydration at the water-membrane interface. Our proposed ion selectivity mechanism elucidates how ion dehydration governs the extent of ion permeation into the membrane and the subsequent transport through the charged polymer matrix. Future membrane design that promotes dehydration of target ions is challenging but can result in unprecedented ion selectivity.

Not all nanopores are created equal. By definition, nanopores have characteristic diameters or conduit widths between ∼1 and 100 nm. However, the narrowest of such pores, perhaps best called Single...