Pressure is the defining operational parameter in nanofiltration (NF) and reverse osmosis (RO), yet its influence on the intrinsic transport coefficients commonly used to describe water and salt transport is often implicitly assumed to be constant. Here, we systematically investigate the pressure dependence of water and salt permeability in two commercial polyamide thin-film composite membranes (NF270 and SW30). Experiments conducted over a pressure range of 5–40 bar reveal a non-monotonic dependence of water permeability on applied pressure in both membranes. At low pressures, water permeability increases with pressure, whereas at higher pressures it decreases, with a more pronounced effect observed for the NF270 membrane. Pressure-cycling experiments demonstrate that the pressure dependence of permeability is largely reversible, although each membrane exhibits a distinct degree of irreversible permeability loss (≈43% for NF270 and ≈20% for SW30). We further show that the pressure dependence of water permeability is influenced by solution conditions: lower pH and higher temperature lead to reduced permeability and enhanced sensitivity to pressure. Analysis of salt transport indicates that applied pressure reduces salt permeability, moderately amplifies concentration polarization effects, but does not significantly alter trends associated with ion dehydration. Together, these results demonstrate that applied pressure can substantially modify experimentally determined water and salt permeabilities, even in the absence of permanent membrane damage. Rather than attributing these observations to a specific transport model, this work provides a systematic experimental mapping of pressure-dependent permeability and highlights the importance of accounting for pressure effects when interpreting transport parameters and comparing membrane performance across operating conditions.

Monovalent–divalent ion selectivity in nanofiltration (NF) is central to many separation processes, such as water softening and lithium recovery, yet the role of feed ionic composition in regulating selectivity remains insufficiently understood. Here, we systematically show how ion concentration and composition can modulate monovalent–divalent ion selectivity in a polyamide NF membrane by coupling zeta potential measurements, single- and mixed-salt filtration, concentration polarization analysis, and temperature-dependent permeation experiments. Increasing salinity enhanced charge screening, reducing the rejection of monovalent salts (e.g., NaCl and LiCl) while increasing the rejection of divalent salts (e.g., MgCl₂ and CaCl₂). In mixed-salt systems, high flux of chloride associated with strongly rejected divalent cations promoted electromigration effects; that is, cotransport of monovalent cations to maintain electroneutrality, dramatically lowering monovalent salt rejection and, under highly screened conditions, inducing negative LiCl rejection. Transition-state theory analysis revealed that electromigration reduces the effective free-energy barrier for Na⁺ transport primarily through favorable entropic contributions. Case studies simulating groundwater softening and filtration of lithium-rich brines demonstrate that tuning ionic composition can strategically enhance monovalent–divalent selectivity without altering membrane material properties. These findings establish charge screening and electroneutrality-driven electromigration as complementary and controllable mechanisms for optimizing NF separations under realistic multicomponent conditions.

Large amounts of saline municipal wastewater are often produced in global coastal areas due to the application of seawater toilet flushing and/or occasional seawater intrusions into municipal wastewater treatment plants (WWTPs). Mainstream anammox has been rated as the best nitrogen removal technology that enables the upgrade of existing WWTPs from energy-intensive to energy-positive. Application of anammox for saline municipal wastewater treatment is yet to be explored. In this study, we investigated the stability and capability of a granular mainstream partial nitritation and anammox (PN/A) process experiencing the recurrent seawater intrusions. Despite fluctuant salinity and ammonium in the wastewater, PN/A achieved stable nitrogen removal of 150.0±8.0 and 78.5±12.1 mgN/L/d on freshwater and saline municipal wastewater, respectively, producing high-quality effluent of <4.5 mgNH₄⁺−N/L and <6.5 mgTN/L. Activity assays revealed that anammox bacteria, ammonium-oxidizing bacteria, and denitrifying bacteria reserved average activities of 402.8, 291.7, and 93.2 mgN/L/d, respectively, over the recurrent seawater intrusions. 16S rRNA gene amplicon sequencing analysis indicated that Nitrosomonas predominating in flocs responded to ammonium oxidation, and anammox bacteria mainly residing at granules shifted from Brocadia (11.3%) to Kuenenia (20.4%) while saline wastewater was supplied. Notably, an unclassified anammox lineage proliferated in flocs and retained with dominance of 8.2%−30.3%, promoting anammox-based nitrogen removal in suspended phase. Nitrospira propagated to 2.3% in flocs at <0.4 mgO₂/L and reserved high nitrite oxidation activities of 90.3±18.2 mgN/L/d over the entire mainstream PN/A operation period. Apart from low-oxygen facilitated suppression of Nitrospira, nitrate-to-nitrite reducing bacteria (NRB), i.e., Quisquiliibacterium, Diaphorobacter, and Aridibacter occupying 1.5%−6.8% in flocs and 0.8%−6.1% in granules, were deemed to offset Nitrospira's activity by reducing nitrate to nitrite so as to underpin the metabolism of anammox bacteria and process stability. Our study certified that the mainstream PN/A process exhibited resilience to the recurrent seawater intrusions and was able to sustain competent nitrogen removal comparable to that of the conventional activated sludge process. Knowledge gleaned from this study underscored the vital role of NRB in reinforcing the PN/A process under operation variables, which is currently being overlooked.

The growing demand for energy storage technologies has led to a rapid rise in battery consumption, generating large volumes of hazardous spent batteries that pose serious environmental and health risks. While conventional recycling focuses on metal recovery, the emerging strategy of upcycling battery waste into functional photocatalysts offers a unique value-added pathway that goes beyond resource recovery to tackle global environmental challenges. To the best of our knowledge, this is the first comprehensive review dedicated to the upcycling of both primary and secondary battery components into advanced photocatalytic materials. We systematically summarize recent progress in transforming battery-derived transition metals, metal oxides, and carbonaceous materials into photocatalysts for diverse applications including carbon dioxide reduction, hydrogen evolution, volatile organic compound degradation, organic transformations, and wastewater treatment. Emphasis is placed on synthesis strategies, structure–property relationships, and the underlying photocatalytic mechanisms. Finally, we critically discuss the challenges, knowledge gaps, and future opportunities for scaling up this green and circular approach. This timely review not only underscores the potential of battery waste as a sustainable feedstock but also aims to guide the rational design of low-cost, efficient, and environmentally friendly photocatalysts for different energy and environmental applications.

Ion dehydration has been shown to strongly influence separation performance in membrane systems and ion transport in nanoscale channels. However, the molecular details of ion dehydration in membranes are not well understood, in particular under the high pressures and concentrations required for brine treatment. In this study, we define de-coordination as the process by which an ion decreases its total coordination number, including both water molecules and counterions. We estimate the de-coordination free energies in bulk solution for a range of different ions at high pressure and salinity relevant to brine treatment using molecular simulation. We also propose alternative features to the coordination number as the size constraint for traversing nanoscale constrictions, such as the maximum cross-sectional area of the complexed ion. We show that high operating pressures do not significantly change cation hydration shell stability nor the shell size, while high ionic concentrations lower the free energy barrier to reduce the cation coordination number. For anions, however, we find that de-coordination free energies are largely unaffected by elevated salinity and pressure conditions, presumably due to a lack of ion pairing upon de-coordination. Finally, we discuss the implications on ion-ion selectivity in membrane separation (e.g. extracting lithium from salt-lake brines) due to the effects of elevated pressure and salinity on ion de-coordination.

Fabricating polymeric membranes with ion-specific selectivity has been targeted in recent years to address the growing challenges of water and resource scarcity. Inspired by discoveries of the selectivity mechanisms in biological channels, ion dehydration has been increasingly recognized as a key phenomenon governing the transport and selectivity in dense polymeric membranes and other synthetic nanochannels. However, understanding the molecular details of this phenomenon and leveraging and controlling it to increase the selectivity between ions in state-of-the-art membranes remain elusive. In this Perspective, we discuss the foundations of ion dehydration and explore opportunities to study and leverage this phenomenon for improving ion–ion selectivity in membranes. We first introduce the fundamentals and measurements of ion’s hydration properties in solution, distinguishing between static and dynamic hydration properties. Next, we discuss simulation and experimental techniques to study ion dehydration under confinement, highlighting critical knowledge gaps that impede our understanding of this phenomenon. We then discuss effects of ion dehydration on the energy landscape of ion transport and analyze attempts in the literature to improve ion selectivity by promoting dehydration of specific ions. We conclude by proposing research directions to enhance our understanding of ion dehydration and fabricate sustainable and robust membranes with ion-specific selectivity.

Surface charge critically affects ion-selective membrane performance, particularly in separating ions with similar size and charge, the key challenge in water treatment. Herein, we investigate the permeation of alkali chlorides (LiCl, KCl, and CsCl) through steric hindrance–free nanoporous membranes with tunable surface charge densities. Supported by molecular dynamics simulations, we confirm that electrostatic effects promote the dehydration of Cl⁻, the counterions to the membrane charge, at the positively charged membrane surface. This dehydration leads to a great tendency of Cl⁻ to absorb to the membrane surface and be retained, compromises Cl⁻ partitioning, and impedes salt cotransport. For negatively charged membranes, Cs⁺ with its lower hydration energy undergoes greater electrostatic-driven dehydration and partition hindrance than K⁺, resulting in selective KCl transport. Our findings provide both theoretical and experimental proofs of ionic dehydration and transport impediments driven by electrostatic interactions at charged membrane surfaces, presenting an in-depth perspective for designing ion-selective membranes to separate similar ions based on charge effects.

Crude oil contains multiple valuable hydrocarbons used for material synthesis, but their separation involves a laborious and energy-intensive multistep distillation process. Now, scientists introduce a simple two-step separation of important hydrocarbons from crude oil under ambient conditions using selective chemistry.

Polyamide (PA) reverse osmosis (RO) membranes are crucial for water desalination and purification, where salt ion transport is governed by partitioning and diffusion through the PA film. Despite extensive research, decoupling these two steps and quantifying their relative contributions remain challenging due to the lack of reliable characterization methods. Here, we develop a rapid, reproducible electrochemical impedance spectroscopy (EIS) protocol incorporating advanced electrical equivalent circuits to directly quantify partitioning and diffusion resistance. Its validity is verified through membrane filtration experiments and activation energy analysis. Our findings reveal that diffusion dominates ion transport resistance, with values 4.5 to 6.0 times higher than partitioning resistance across diverse monovalent cations. However, we discovered a critical concentration-dependent behavior where partitioning resistance becomes increasingly significant at lower electrolyte concentrations, eventually equaling diffusion resistance near 0.1 mM. We also uncovered that the anomalously low rejection of NH₄⁺ of RO membranes stemmed from significantly reduced diffusion resistance, likely due to moderate hydrogen-bonding interactions with membrane pores or its tetrahedral geometry. This quantitative insight into transport resistance mechanisms establishes new design principles for next-generation RO membranes, enabling tailored strategies for applications ranging from high-salinity desalination to the removal of low-concentration micropollutants.

Understanding the mechanisms of molecular transport in polyamide membranes is imperative to improve their solute-specific selectivity. We explored the partitioning behaviors of water and salts in polyamide membranes to elucidate the role of ion-membrane interactions in the transport. Quartz crystal microbalance (QCM) was employed to quantify the mass uptake at different temperatures and determine partition energies (E_k) for water and salts under two different pH values. Zeta potential and permeability tests were conducted to support the ion-membrane affinity trends observed with QCM and link these trends to ion-ion selectivity. Our results demonstrate a high affinity of water to the polyamide membrane (E_k < 0), with a significant swelling effect attributed to dipole interactions and hydrogen bonding. Ion partitioning revealed distinct differences between monovalent and divalent cations, as well as between kosmotropic and chaotropic anions. Specifically, divalent cations (Ca²⁺ and Mg²⁺) exhibited considerably lower partition energies (-0.99 and 0.29 kcal mol^-1, respectively) and more efficient charge neutralization, indicating stronger interactions with the membrane compared to monovalent cations (∼2.2 kcal mol^-1). The partition energies of the chaotropic iodide and kosmotropic sulphate anions were substantially different (-5.5 and 4.0 kcal mol^-1, respectively), likely due to the different tendency of these anions to shed their hydration shell and stick to the polymer. Last, our permeability tests indicate the potential existence of an intrinsic tradeoff between ion partitioning and intrapore diffusion, presumably due to the opposite effects that ion-membrane interactions have on these transport steps. Overall, our work underscores the role of ion-specific interactions in membrane transport and selectivity.

Understanding the transport mechanisms in salt-rejecting membranes is critical for improving their separation efficiency and selectivity. Examining transmembrane permeation in terms of energy barriers using the Arrhenius or Eyring approach provides valuable insights into molecular transport within the membrane and at the solution-membrane interfaces. Although useful insights have been gained using the energy barriers framework, which is based on measuring permeability at different temperatures, the method can sometimes show counterintuitive and inconsistent results. In this study, we examine methods to improve the reliability of experimentally obtained energy barriers for transport in salt-rejecting membranes. We first compile energy barrier results for the transport of various solutes in loose and tight salt-rejecting membranes, observing data variability across studies and a weak correlation between energy barriers and membrane type. Next, we demonstrate the importance of thermally stabilizing membranes prior to experimentally evaluating energy barriers, showing that membranes equilibrated at high temperatures and tested with descending temperature produce more stable and reliable trends. In addition to thermal stabilization, we identify that comparing energy barrier values based on a similar concentration polarization modulus is critical when analyzing trends between different solutes and membranes. Following these recommendations, we obtain energy barriers for ion permeation that align with the performance of loose and tight salt-rejecting membranes. We conclude by demonstrating consistent and rational energy barrier measurements in two independent laboratories using the principles discussed. Overall, this study provides important guidelines for the experimental quantification of energy barriers for transport in salt-rejecting membranes.

Water scarcity presents a pressing global challenge, profoundly impacting both human societies and environmental ecosystems. Brackish water and seawater desalination serve as a crucial solution to alleviate water stress. While pressure-driven membrane processes — reverse osmosis (RO) and nanofiltration (NF) — dominate the desalination market, electrified desalination processes have experienced substantial development in recent years, attributed to their improved scalability and reduced fouling tendencies compared to the pressure-driven technologies. However, their energy efficiency compared to RO remains a subject of debate, as reflected by the conclusions of numerous recent studies that have performed such comparisons. To settle this debate and provide more direct conclusions regarding the use of electrified technologies for desalination, this study conducts a comprehensive survey of electrified desalination processes reported in the literature over the last decades, focusing on key desalination parameters such as feed salinity, salt removal, water recovery, and water productivity. Based on the literature survey, we identify membrane capacitive deionization (MCDI), electrodialysis (ED), and flow-electrode capacitive deionization (FCDI) as the most promising electrified technologies. Employing mechanistic process modeling, we rigorously compare the energy consumption of electrified technologies with RO. Furthermore, we integrate this modeling with techno-economic analysis to evaluate the economic viability of various desalination technologies. Results demonstrate promising prospects for electrified technologies, demonstrating lower energy consumption and comparable economics of ED and FCDI relative to RO in brackish water desalination. Using our framework, we explore the impacts of material advancements on performance enhancement, emphasizing the importance of reducing manufacturing material costs rather than solely focusing on fabricating high-performance materials. Overall, our study highlights the significance of integrated mechanistic modeling and techno-economic analysis in assessing and guiding the future of electrified desalination technologies. Importantly, these insights extend beyond desalination to the broader water-energy nexus, offering valuable implications for sustainable water resource management.

To improve the solute-specific selectivity of nanofiltration (NF) membranes, a fundamental understanding of the transport mechanisms in these membranes is required. In this study, we explored the selectivity trends of common anionic pollutants (i.e., nitrate and perchlorate) compared to the common chloride anion and examined the underlying transport mechanisms for these trends in loose polyamide NF membranes. Permeation experiments show that nitrate and perchlorate, despite being polyatomic and larger, permeate the membrane faster than the monoatomic, spherical chloride, suggesting that other mechanisms beyond size and charge exclusion govern the separation. Significantly higher enthalpic barriers measured for the transport of chloride compared to nitrate and perchlorate elucidated the important role of ion dehydration in the selectivity observed. To further support the influence of dehydration, we systematically altered the ions’ hydration by introducing various organic aliphatic alcohols of different hydrophobicity into the feed solution. The inclusion of aliphatic alcohols intensified the chaotropic characteristics of nitrate and perchlorate, augmenting their capacity to dehydrate, as reflected by their enhanced permeation and reduced enthalpic barrier in the presence of alcohols. We also demonstrated that this effect is boosted when a strong kosmotropic anion like sulfate is added to the system due to its counter effect on water structuring. We conclude with proposing mechanisms for the anion behavior in water-alcohol solutions that highlight the critical importance of ionic hydration in transmembrane permeation.

Pressure-driven membrane desalination (PMD), such as reverse osmosis or nanofiltration, is an energy-efficient technology that addresses water shortages by using saline waters to augment freshwater supplies. This Primer describes several key methodological aspects of PMD, including membrane fabrication, characterization and performance evaluation; system modelling; process configurations; and applications. Thin-film composite polyamide membranes represent the state of the art in reverse osmosis and nanofiltration membranes and are the focus of the membrane development discussion. First, thin-film composite polyamide membrane fabrication using interfacial polymerization and alternative methods is discussed, followed by an exploration of techniques for characterizing the morphological, structural and interfacial properties. Experimental procedures and model frameworks for evaluating membrane performance are introduced, noting caveats in data collection, interpretation and reproducibility, with best practices recommended. Additionally, the general method for modelling the module-scale behaviour of PMD processes is introduced, alongside process configurations for existing and emerging applications. Finally, an outlook for the development of PMD is provided, highlighting the most meaningful directions for future research to further advance PMD beyond the current state of the art.

Major efforts in recent years have been directed towards understanding molecular transport in polymeric membranes, in particular reverse osmosis and nanofiltration membranes. Transition-state theory is an increasingly common approach to explore mechanisms of transmembrane permeation with molecular details, but most applications of this theory treat all free energy barriers to transport within the membrane as equal. This assumption neglects the inherent structural and chemical heterogeneity in polymeric membranes. In this work, we expand the transition-state theory framework to include distributions of membrane free energy barriers. Our mathematical framework is mechanism-agnostic, such that it generalizes to transport through any membrane for molecular separation. However, we focus our analysis on dense nanofiltration and reverse osmosis membranes. We show that the highest free energy barriers along the most permeable paths, rather than typical paths, provide the largest contributions to the experimentally-observed effective free energy barrier. We show that even moderate, random heterogeneity in molecular barriers will significantly impact how we interpret the mechanisms of transport through these membranes. Our study suggests that experimentally-measured barriers are not easily related to the underlying mechanisms governing transport, and simplified interpretations of these barriers will likely miss the mechanisms most relevant to the overall permeability.

Considering growing efforts to understand and improve the solute-specific selectivity of nanofiltration (NF) membranes, we explored the ion-specific effects that govern the charge and performance of a loose polyamide NF membrane that is commonly used for solute-solute separations. Specifically, we systematically evaluated the zeta potential of the membrane under different conditions of pH, salinity, and ionic composition, and correlated the obtained data with membrane performance tested under similar conditions. Our results identify the pK_a of both carboxylic and amine groups bonded to the membrane surface and suggest that the highly polarizable chloride anions in the solution adsorb to the polyamide, increasing its negative charge. We also show that monovalent cations of different “stickiness” can neutralize the negative membrane charge to different extents due to their varying tendency to sorb to the polymer matrix or screen the fixed carboxyl groups on the membrane surface. Notably, our correlation between zeta potential measurements and permeability experiments indicates the substantial contribution of solution ions to Donnan exclusion in NF membranes.

Synthetic membranes featuring confined nanostructures have emerged as a prominent category of leading materials that can selectively separate target ions from complex water matrices. Further advancements in these membranes will pressingly rely on the ability to elucidate the inherent connection between transmembrane ion permeation behaviors and the ion-selective nanostructures. In this review, we first abstract state-of-the-art nanostructures with a diversity of spatial confinements in current synthetic membranes. Next, the underlying mechanisms that govern ion permeation under the spatial nanoconfinement are analyzed. We then proceed to assess ion-selective membrane materials with a focus on their structural merits that allow ultrahigh selectivity for a wide range of monovalent and divalent ions. We also highlight recent advancements in experimental methodologies for measuring ionic permeability, hydration numbers, and energy barriers to transport. We conclude by putting forth the future research prospects and challenges in the realm of high-performance ion-selective membranes.

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.

Isomer conversion and separation are crucial in the chemical, pharmaceutical, and food industries. However, this process remains challenging due to the reversed conversion of isomers and their similar boiling points and kinetic diameters. Herein, an enzyme-catalyzed nanochannel membrane with a hierarchical structure is fabricated for selective and directional isomeric conversion and separation. The enzyme is embedded in a confined polymer network that shows high stability and retains 95% of its initial activity after 50 reaction cycles. Studying the transport mechanisms between isomers through the membrane shows that the hierarchical layers of the membrane impose unequal energy barriers to the forward and backward transport of products, promoting unidirectional diffusion of products and interrupting the equilibrium of the isomerization reaction to increase the conversion rate by 43%. Overall, this work opens a new avenue for the design of biocatalytic nanochannel membranes and the directional production and in situ separation of isomers.

While the detrimental effect of concentration polarization (CP) on water flux and solute rejection in pressure-driven membrane processes has been extensively explored, the impact of CP on the selectivity between solutes in these processes has been somewhat overlooked. Considering the growing interest in solute-solute selectivity, in this study, we explored the effect of CP on ion-ion selectivity in nanofiltration (NF) membranes. We first show and discuss the “reversed” observed rejection trend of monovalent cations in NF, which is opposite to the trend of the ions’ hydrated size and mobility in solution. Next, we apply the film theory using three independent approaches to evaluate the extent of CP in the boundary layer adjacent to the membrane surface, from which the real rejection of the ions can be calculated. Our calculated real rejections of monovalent cations, which were in higher correspondence with the ions’ hydration properties and mobility in solution, suggest that CP played a major role in the “reversed” selectivity observed. Last, we demonstrate how CP adversely affects the commonly pursued monovalent-divalent ion separation in NF. Overall, our results highlight the necessity to rigorously account for CP in future studies on NF and suggest minimizing CP as a primary step to improve the selectivity between solutes.

Water and ion transport in salt-rejecting membranes, such as nanofiltration (NF) and reverse osmosis (RO), are exposed to different hindrance effects that result in selectivity between species. In this study, we explored systematically the hindrance effects that govern water-salt and ion-ion selectivity in NF and RO membranes. More specifically, we measured the permeability of different species at varying temperatures and applied an Eyring-type equation to quantify the enthalpic and entropic barriers for their transport in three types of salt-rejecting membranes. We found that water-salt selectivity is entropically driven, where water molecules gain entropy in the membrane, while the salt ions face a substantial entropic barrier due to their larger size that reduces their possible configurations in the membrane. As the enthalpic barriers of the water and salt did not show a prominent difference (with minor effect on the water-salt selectivity), most ion-ion separations were restricted by an enthalpy-entropy compensation (EEC) effect; that is, an ion with a higher entropic barrier experienced a lower enthalpic barrier compared to the competing ion, resulting in minor variations in the free-energy barriers between ions that limit the ion-ion selectivity. We conclude by proposing possible mechanisms that promote EEC during ion transport in salt-rejecting membranes.

Ion-selective removal is an important frontier in water purification technologies. For many emerging applications, removing all ions indiscriminately can lead to excessive energy consumption, high levelized cost of water, poor effluent water quality, and increased waste brine volume. Electrodialysis and capacitive deionization are two electrochemical water purification technologies which are promising toward tunable, ion-selective purification. These technologies have fundamentally different ion removal mechanisms, as electrodialysis leverages electrodiffusion through ion-exchange membranes while capacitive deionization utilizes electrosorption into charged electrodes. We here provide a direct comparison of ion selectivity achieved by these two technologies, focusing on several important ion pairs. We highlight distinct differences in achieved selectivity between these technologies and provide theory results to connect such observations to ion removal mechanisms. Based on the experimental literature, we find that capacitive deionization demonstrates a wider range of achieved ion selectivities than electrodialysis for competing cations such as Na⁺vs Ca²⁺and Li⁺vs Na⁺, while a wider range is observed for electrodialysis when separating anion pairs such as Cl^-vs SO4_2-and Cl^-vs NO3_-. We conclude with reviewing "knobs" that can be adjusted to tune the achieved selectivities by both technologies, and emphasize important questions that should be answered in future studies to improve the selectivity of both technologies.

Membrane technologies using reverse osmosis (RO) and nanofiltration (NF) have been widely implemented in water purification and desalination processes. Separation between species at the molecular level is achievable in RO and NF membranes due to a complex and poorly understood combination of transport mechanisms that have attracted the attention of researchers within and beyond the membrane community for many years. Minimizing existing knowledge gaps in transport through these membranes can improve the sustainability of current water-treatment processes and expand the use of RO and NF membranes to other applications that require high selectivity between species. Since its establishment in 1949, and with growing popularity in recent years, Eyring's transition-state theory (TST) for transmembrane permeation has been applied in numerous studies to mechanistically explore molecular transport in membranes including RO and NF. In this review, we critically assess TST applied to transmembrane permeation in salt-rejecting membranes, focusing on mechanistic insights into transport under confinement that can be gained from this framework and the key limitations associated with the method. We first demonstrate and discuss the limited ability of the commonly used solution-diffusion model to mechanistically explain transport and selectivity trends observed in RO and NF membranes. Next, we review important milestones in the development of TST, introduce its underlying principles and equations, and establish the connection to transmembrane permeation with a focus on molecular-level enthalpic and entropic barriers that govern water and solute transport under confinement. We then critically review the application of TST to explore transport in RO and NF membranes, analyzing trends in measured enthalpic and entropic barriers and synthesizing new data to highlight important phenomena associated with the temperature-dependent measurement of the activation parameters. We also discuss major limitations of the experimental application of TST and propose specific solutions to minimize the uncertainties surrounding the current approach. We conclude with identifying future research needs to enhance the implementation and maximize the benefit of TST application to transmembrane permeation.

Recent insights into the permeation of small ions through polyamide membranes highlighted the role of ion dehydration in the transport and selectivity observed. However, such insights were a by-product of studies exploring different transport phenomena without a systematic methodology that focuses on ion dehydration itself. In this study, we quantified the intrinsic permeability and its underlying Eyring's enthalpy and entropy of activation for a systematic set of cations and anions in polyamide nanofiltration membranes to gain better understanding of ion dehydration. Our results in a diffusion-only system expose a distinct correlation between the hydration strength of the ions and the enthalpic barrier they experience during permeation, with a permeability order that is similar to the mobility order in solution. Smaller entropic losses are observed for smaller bare ions, indicating their higher freedom of motion after dehydration compared to larger bare ions. Comparing the measured activation enthalpies to quantum chemical calculations for the ions' hydration in solution suggests that ions are only partially dehydrated. Last, our simulated ion permeability at higher temperatures and experimental results in a pressure-driven system expose higher permeation rates with reversed ion-ion selectivity, indicating the activation of ion transport in the form of ion dehydration when additional energy is supplied.

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.

Typical product water from desalination plants is depleted of salts and important minerals that are essential for its final consumption as well as its conveyance in the distribution system. In this study, we propose a hybrid nanofiltration (NF)-reverse osmosis (RO) filtration scheme that can produce desalinated brackish water with improved mineral composition. Here, the monovalent-divalent ion selectivity of NF membranes is used in a preliminary step to remove and “store” divalent minerals in the NF retentate stream, which are later dosed back to the mineral-free RO permeate to replenish the product water. We combine experimental data from pilot-scale NF with RO simulation to demonstrate the feasibility and economic viability of the proposed treatment scheme. In particular, we show the potential to operate the hybrid filtration scheme at high recovery ratios (>85%) and minimum saturation indices of potential scaling solids. A cost assessment further supports the applicability of the proposed treatment scheme, estimating the total capital and operating cost at $0.24/m³ product, which is on par with existing brackish-water desalination costs.

While polyamide reverse osmosis and nanofiltration membranes have been extensively utilized in water purification and desalination processes, the molecular details governing water and solute permeation in these membranes are not fully understood. In this study, we apply transition-state theory for transmembrane permeation to systematically break down the intrinsic permeabilities of water and small ions in loose and tight polyamide nanofiltration membranes into enthalpic and entropic components using an Eyring-type equation. We analyze trends in these components to elucidate molecular phenomena that induce water-salt, monovalent-divalent, and monovalent-monovalent selectivity at different pH values. Our results suggest that in pores that are either too small or contain an electrostatically repelling mouth, the thermal activation of ions in the form of ion dehydration is less likely, promoting entropically driven selectivity with steric exclusion of hydrated ions. Instead, larger uncharged pores enable ion dehydration, inducing enthalpic selectivity that is driven by differences in the ion hydration properties. We also demonstrate that electrostatic interactions between cations and intrapore carboxyl groups hinder salt permeability, increasing the enthalpic barrier of the transport. Last, permeation tests of monovalent cations in the loose and tight polyamide membranes expose opposite rejection trends that further support the phenomenon of ion dehydration in large subnanopores.

Synthesizing nanopores which mimic the functionality of ion-selective biological channels has been a challenging yet promising approach to advance technologies for precise ion-ion separations. Inspired by the facilitated fluoride (F-) permeation in the biological fluoride channel, we designed a highly fluoride-selective TiO2 film using the atomic layer deposition (ALD) technique. The subnanometer voids within the fabricated TiO2 film (4 Å < d < 12 Å, with two distinct peaks at 5.5 and 6.5 Å), created by the hindered diffusion of ALD precursors (d = 7 Å), resulted in more than eight times faster permeation of sodium fluoride compared to other sodium halides. We show that the specific Ti-F interactions compensate for the energy penalty of F- dehydration during the partitioning of F- ions into the pore and allow for an intrapore accumulation of F- ions. Concomitantly, the accumulation of F- ions on the pore walls also enhances the transport of sodium (Na+) cations due to electrostatic interactions. Molecular dynamics simulations probing the ion concentration and mobility within the TiO2 pore further support our proposed mechanisms for the selective F- transport and enhanced Na+ permeation in the TiO2 film. Overall, our work provides insights toward the design of ion-selective nanopores using the ALD technique.

A pressurized hydrogenotrophic denitrification reactor to remove nitrate from groundwater was recently presented. To enable treatment of nitrate-concentrated brines, we demonstrate here the removal of high nitrate concentrations without nitrogen gas accumulation in the reactor's headspace over time. In order to eliminate nitrogen gas build up in the pressurized reactor's headspace, the main unsaturated flow pressurized reactor was connected to an external degassing unit via liquid recirculation. Pressurized liquid from the main reactor was intermittently discharged to the degassing unit where dissolved N₂ gas effervesced and escaped to the atmosphere. The degassed effluent water was continuously recirculated back to the main reactor to maintain steady-state nitrogen and hydrogen gas partial pressures. Denitrifying rates of up to 6 g N/L-reactor/d were achieved in the reactor system under freshwater and brine conditions. Nitrate removal was over 97 %, with nitrate effluent concentrations lower than 10 mg N/L for influent concentrations of about 400 mg N/L. The reactor system maintained steady-state nitrogen and hydrogen gas partial pressures using low external recirculation rates at low reactor total pressures (< 5 atm). At high denitrification rates, dissolved nitrogen concentrations in the reactor were super saturated (125–150%) while on the other hand, dissolved hydrogen was undersaturated (30–60%) due to intense bacteria uptake from the water. These dynamic conditions reduced the external recirculation flow requirement for degassing and resulted in much better hydrogen utilization than expected (greater than 93 %).

State-of-the-art desalination membranes exhibit high water-salt selectivity, but their ability to discriminate between ions is limited. Elucidating the fundamental mechanisms underlying ion transport and selectivity in subnanometer pores is therefore imperative for the development of ion-selective membranes. Here, we compare the overall energy barrier for salt transport and energy barriers for individual ion transport, showing that cations and anions traverse the membrane pore in an independent manner. Supported by density functional theory simulations, we demonstrate that electrostatic interactions between permeating counterion and fixed charges on the membrane substantially hinder intrapore diffusion. Furthermore, using quartz crystal microbalance, we break down the contributions of partitioning at the pore mouth and intrapore diffusion to the overall energy barrier for salt transport. Overall, our results indicate that intrapore diffusion governs salt transport through subnanometer pores due to ion-pore wall interactions, providing the scientific base for the design of membranes with high ion-ion selectivity.

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.

We investigated the relative contributions of intra-pore diffusion (via membrane thickness) and partitioning into nanofiltration (NF) membrane pores (via membrane pore size and ion hydration energy) to the apparent energy barriers for ion transport in NF membranes. Using polyelectrolyte layer-by-layer assembly, we independently altered NF membrane thickness as well as membrane pore size and then determined the apparent energy barriers to bromide and fluoride transport through the fabricated membranes. Membrane thickness and pore size were estimated using an AFM scratch technique and the hydrodynamic pore transport model, respectively. By increasing the number of polyelectrolyte bilayers from four to ten, the polyelectrolyte film thickness increased from 28 to 77 nm, while the apparent energy barriers to bromide transport through the membranes with four, seven, and ten bilayers were negligibly affected (4.4, 3.4, and 3.9 kcal mol⁻¹, respectively, at 1.7 bar). Instead, we found that solute flux and the apparent energy barriers to ion transport were significantly affected by both membrane pore size and ion hydration energy. Overall, our results support the traditional energy barrier-based models for ion transport in membranes and the recently proposed notion that ion dehydration at the solution-membrane interface is the rate-limiting step during transport through NF membranes.

Capacitive deionization (CDI) has been solely employed for the removal of charged ions from water, showing limited feasibility compared to other conventional technologies such as reverse osmosis (RO). In this work, we propose to use CDI with activated carbon electrodes for simultaneous removal of inorganic salt and trace organic contaminants (TOrCs). This approach is based on the inherent sorption potential of activated carbon CDI electrodes towards organic species. We show that salt removal by CDI is only slightly affected by the presence of different TOrCs (bisphenol A, carbamazepine, estrone, and phentoxifylline). Sorption and removal of TOrCs (taking place concomitantly) was most effective for the hydrophobic compounds (bisphenol A and estrone) and was not affected by the presence of salt or the applied electric field. Sequential desorption of salt and TOrCs into two separated streams was achieved by short-circuiting the two electrodes and washing the electrodes with water and ethanol, respectively. Notably, the described process produces separate waste streams for salts (i.e., water) and organics (i.e, ethanol), which can facilitate their disposal or further treatment. Altogether, the study shows the high potential of the proposed CDI application, which may be valuable for treating water or wastewater streams contaminated with both salt and TOrCs.

Improving solute selectivity is critical to enhancing the efficiency and sustainability of membrane separation processes. Doing so, however, requires understanding the molecular-level processes that culminate in solute transport through semipermeable membranes. Existing experimental techniques lack the spatiotemporal resolution necessary for probing such molecular-level events, and conventional simulation techniques cannot probe timescales relevant to the transport of unwanted solutes through ultra-selective membranes. Here, we use path-sampling molecular simulations to circumvent both these limitations. We not only accurately and efficiently compute arbitrarily long solute passage times but also identify induced charged anisotropy as a hidden variable affecting ion transport through nanopores. Our approach provides a scalable paradigm for computational studies of selectivity in applications such as desalination, chemical separation, and biological membrane transport.

The authors regret that an error was made in the title of the right y axis of Figure 6 (A and B) and Figure S9. The title of the right y axis in these figures should be “ln (Pre-Exponential Factor)” instead of “Pre-Exponential Factor (min⁻¹)”. The captions of these figures should be corrected as well to reflect this change: “pre-exponential factor” should be “natural logarithm of the pre-exponential factor”. The error has not affected any of the calculations or the results presented in this study. The authors would like to apologise for any inconvenience caused.

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.

Understanding ion solvation in liquid water is critical in optimizing materials for a wide variety of emerging technologies, including water desalination and purification. In this work, we report a systematic investigation and comparison of solvated K⁺ and NH₄⁺ using first-principles molecular dynamics simulations. Our simulations reveal a strong analogy in the solvation properties of the two ions, including the size of the solvation shell as well as the solvation strength. On the other hand, we find that the local water structure in the ion solvation is significantly different; specifically, NH₄⁺ yields a smaller number of water molecules and a more ordered water structure in the first solvation shell due to the formation of hydrogen bonds between the ion and water molecules. Finally, our simulations indicate that a comparable solvation strength of the two ions is a result of an interplay between the nature of ion-water interaction and number of water molecules that can be accommodated in the ion solvation shell.

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 Digit Nanopores (SDNs) and defined as those with regular diameters less than 10 nm, have only recently been accessible experimentally for precision transport measurements. This Review summarizes recent experiments on pores in this size range that yield surprising results, pointing toward extraordinary transport efficiencies and selectivities for SDN systems. These studies have identified critical gaps in our understanding of nanoscale hydrodynamics, molecular sieving, fluidic structure, and thermodynamics. These knowledge gaps are, in turn, an opportunity to discover and understand fundamentally new mechanisms of molecular and ionic transport at the nanometer scale that may inspire a host of new technologies, from novel membranes for separations and water purification to new gas-permeable materials and energy storage devices. Here we highlight seven critical knowledge gaps in the study of SDNs and identify the need for new approaches to address these topics.

The authors regret that errors were made in the units of electrical resistance and therefore some reported values. During the revision process, we were asked to present the resistance as area-normalized resistance. To normalize the electrical resistance, we divided the resistance by the projected electrode area (giving units of Ω m ⁻² ), which was erroneous. Rather the resistance should have been multiplied by projected electrode area to give units of Ω m ² . The incorrect normalization was only performed when presenting the values in Table 2, Section 4.1, and the captions for Fig. 10 in the main manuscript. Importantly, the normalization error was not propagated to the Python code developed for this project. Consequently, the errors have not affected any of the calculations or the results presented in this study. The corrected Table 2 should be:[Table presented] In addition, in the first paragraph of Section 4.1, “1.8 Ω m ⁻² ” should be changed to “0.02 Ω m ² ” and “7.1 Ω m ⁻² ” should be changed to “0.09 Ω m ² ”. The corrections for the resistance values presented in the caption of Fig. 10 are listed in the following table:[Table presented] In the caption of Fig. 4A, “Cell voltage” should be changed to “Voltage”.

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.

Capacitive deionization (CDI), which is based on the electrosorption of ions by porous electrodes, is an emerging technology for brackish water desalination. Understanding the key drivers of energy consumption in CDI and benchmarking CDI with reverse osmosis (RO), the current state-of-the-art for brackish and seawater desalination, is crucial to guide the future development of desalination technologies. In this study, we develop system-scale models to analyze the energy consumption and energy efficiency of CDI and RO over a wide range of material properties and operating conditions. Using our models, we explore how the energetic performance of CDI and RO compare as a function of feed salinity, water recovery, salt rejection, and average water flux, which is normalized by electrode and membrane area in CDI and RO, respectively. Our analysis shows that RO is significantly more energy efficient than CDI, particularly when targeting higher salinity feed streams and higher salt rejection values. For brackish water with a salt concentration of 2000 mg L⁻¹, achieving 50% water recovery and 75% salt rejection, with an average water flux of 10 L m⁻² h⁻¹ using CDI requires a specific energy consumption of 0.85 kWh m⁻³, more than eight times that of RO (0.09 kWh m⁻³). Importantly, our results also indicate that current efforts to improve electrode materials can only marginally reduce the energy consumption of CDI. We conclude with a discussion highlighting other important factors, such as capital cost, electrode stability, and membrane fouling, which affect the efficacy of CDI and RO for low-salinity desalination.

We fabricated polyelectrolyte multilayer (PEM) nanofiltration (NF) membranes using a layer-by-layer (LbL) method for effective removal of scale-forming divalent cations (Mg²⁺, Ca²⁺, Sr²⁺, and Ba²⁺) from feedwaters with different salinities. Two polymers with opposite charges, polycation (poly(diallyldimethylammonium chloride), PDADMAC) and polyanion (poly(sodium 4-styrenesulfonate), PSS), were sequentially deposited on a commercial polyamide NF membrane to form a PEM. Compared to pristine and PSS-terminated membranes, PDADMAC-terminated membranes demonstrated much higher rejection of divalent cations and selectivity for sodium transport over divalent cations (Na⁺/X²⁺) due to a combination of both Donnan- and size-exclusion effects. A PDADMAC-terminated membrane with 5.5 bilayers exhibited 97% rejection of Mg²⁺ with selectivity (Na⁺/Mg²⁺) greater than 30. We attribute the order of cation rejection (Mg²⁺ > Ca²⁺ > Sr²⁺ > Ba²⁺) to the ionic size, which governs both the hydration radius and hydration energy of the cations. The ionic strength (salinity) of the feed solution had a significant influence on both water flux and cation rejection of PEM membranes. In feed solutions with high ionic strength, abundant NaCl salt screened the charge of the polyelectrolytes and led to swelling of the multilayers, resulting in decreased selectivity (Na⁺/X²⁺) and increased water permeability. The fabricated PEM membranes can be potentially applied to the pretreatment of mild-salinity brackish waters to reduce membrane scaling in the main desalination stage.

The implementation of hydrogenotrophic denitrification is limited due to safety concerns, poor H₂ utilization and low solubility of H₂ gas with the resulting low transfer rate. The current paper presents the main research work conducted on a pressurized hydrogenotrophic reactor for denitrification that was recently developed. The reactor is based on a new concept suggesting that a gas-liquid equilibrium is achieved in the closed headspace of denitrifying reactor, further produced N₂ gas is carried out by the effluent and gas purging is not required. The feasibility of the proposed reactor was shown for two effluent concentrations of 10 and 1 mg NO₃ ⁻-N/L. Hydrogen gas utilization efficiencies of 92.8% and 96.9% were measured for the two effluent concentrations, respectively. Reactor modeling predicted high denitrification rates above 4 g NO₃ ⁻-N/(L_reactor·d) at reasonable operational conditions. Hydrogen utilization efficiency was improved up to almost 100% by combining the pressurized reactor with a following open-to-atmosphere polishing unit. Also, the potential of the reactor to remove ClO₄ ⁻ was shown.

The main objective of this study is to examine how the charge densities of four monovalent anions - fluoride (F^-), chloride (Cl^-), bromide (Br^-), and nitrate (NO₃^-) - influence their Donnan (charge) exclusion by a charged nanofiltration (NF) membrane. We systematically studied the rejection behavior of ternary ion solutions containing sodium cation (Na⁺) and two of the monovalent anions as a function of the pH with a polyamide NF membrane. In the solutions containing F^- and Cl^- or F^- and Br^-, F^- rejection was higher than Cl^- or Br^- rejection only when the solution pH was higher than 5.5, suggesting that F^- (which has a higher charge density) was repelled more strongly by the negatively charged membrane. The order of change in the activation energy for the transport of the four anions through the polyamide membrane as a response to the increase of the membrane negative charge was the following: F^- > Cl^- > NO₃^- > Br^-. This order corroborates our main hypothesis that an anion with a smaller ionic radius, and hence a higher charge density, is more affected by the Donnan (charge)-exclusion mechanism in NF. We conclude with a proposed mechanism for the role of ionic charge density in the rejection of monovalent anions in NF.

We used layer-by-layer (LbL) self-assembly to fabricate a polyelectrolyte (PE) nanofiltration membrane for salt rejection and to immobilize trypsin on the membrane outer layer for biocatalytic activity. Poly(ethylene imine) (PEI) and poly(diallyl dimethyl ammonium chloride) (PDADMAC) were used as cationic PE while poly(acrylic acid) (PAA) and poly(styrene sulfonate) (PSS) were used as anionic PE. The impact of PE type, number of PE bilayers, and PE concentration on the rejection of inorganic salts (NaCl, MgCl₂, Na₂SO₄, and MgSO₄) and protein (bovine serum albumin, BSA) was systematically investigated. A maximum rejection of 12.7%, 45.2%, 85.5%, 94.0%, and 100% of MgCl₂, NaCl, MgSO₄, Na₂SO₄, and BSA, respectively, was obtained by the PDADMAC-PSS membrane with four bilayers. Trypsin (TRY) was immobilized on the membrane surface by electrostatic attraction or covalent bonding to produce a biocatalytic membrane and to alleviate protein fouling. Important parameters for enzymatic activity, such as immobilization time, pH, temperature, salt concentration and type, as well as the reuse number and storage time were investigated to expound the mechanism of enzyme activity in the presence of salt and BSA. BSA was used as a model protein for organic fouling experiments, and flux decline rate of the membranes was determined. Our results show that LbL-modified membranes with covalent enzyme immobilization had the lowest protein fouling rate, which we attribute to the biocatalytic activity of the immobilized trypsin.

Despite the strong similarity between chloride (Cl^-) and nitrate (NO₃^-) anions in terms of their hydrated radius and charge, Cl^- is rejected more favorably than NO₃^- by nanofiltration (NF) membranes. The main goal of this study is to provide a better understanding of the removal mechanisms favoring the higher rejection of Cl^- over NO₃^- in NF. A series of experiments with polyamide (NF270) and cellulose acetate (CK) NF membranes at different pH values, followed by calculation of the activation energies for Cl^- and NO₃^- passage through the membranes, showed that the higher Cl^- than NO₃^- rejection is attributed to both size-exclusion and Donnan (charge)-exclusion mechanisms. At a neutral membrane charge, a size exclusion mechanism dominates the rejection of both anions. In this case, we observe higher rejection of Cl^- over NO₃^- due to the lower hydration energy of NO₃^-, which corresponds to higher degree of dehydration and thus higher rate of passage through the NF membrane pores. At a negative membrane charge, the smaller volume of Cl^- compared to NO₃^-, corresponding to higher surface charge density, results in a stronger electrostatic repulsion of Cl^- by the negatively charged membrane and therefore higher Cl^- rejection than NO₃^-. The coupling of size- and Donnan-exclusion mechanisms with the NF270 membrane results in a maximum Cl^- to NO₃^- rejection ratio at near the isoelectric pH where the membrane is slightly negatively charged. At a positive membrane charge, the sodium (Na⁺) counter ions dictate salt rejection independently of the anion type, resulting in almost similar rejections of Cl^- and NO₃^-. Based on the insight gained from these experiments, a layer-by-layer (LbL) polyelectrolyte modification was applied to the NF270 membrane to control its surface charge. This modification showed that shifting the isoelectric point of the NF270 membrane from its original value (pH 4–5) to higher values (pH 6–9) increased the Cl^- to NO₃^- rejection ratio at near neutral pH conditions, thus providing further support for our proposed mechanism for the difference between Cl^- and NO₃^- rejection by NF membranes.

A novel pressurized hydrogenotrophic reactor operating at high rates was recently developed specifically for the removal of nitrate (NO₃⁻) from drinking water. The reactor is characterized by safe and economical operation since hydrogen (H₂) purging intrinsic to conventional H₂-based denitrifying systems is not required and H₂ loss occurs only through the effluent, resulting in H₂ utilization efficiency above 90%. In this research, a new treatment scheme to remove NO₃⁻ and perchlorate (ClO₄⁻) combining the pressurized reactor with a following open-to-atmosphere polishing unit is presented. In the pressurized reactor, NO₃⁻ and ClO₄⁻ are simultaneously removed. In the polishing unit, the residual dissolved H₂ from the pressurized reactor serves to further reduce ClO₄⁻ to trace concentrations below recommended levels. First, ClO₄⁻ reduction together with denitrification was demonstrated in the pressurized reactor without special inoculation and a maximal ClO₄⁻ volumetric removal rate of 1.83 g/(L_reactor·d) was achieved. Microbial population analyses before and after the addition of ClO₄⁻ were similar with a large fraction of the genus Dechloromonas. Results show that the combined treatment scheme consisting of the pressurized reactor and the polishing unit allowed for the reduction of ClO₄⁻ concentration down to a minimal value of 2 µg/L with a simultaneous increase of the H₂ utilization efficiency from 95% up to almost 100%.

A novel unsaturated-flow pressurized reactor (UFPR) for hydrogenotrophic denitrification was recently developed. The reactor is characterized by safe and economic operation since gas purging intrinsic to conventional H₂-based systems is not required and H₂ loss is limited only to the dissolved H₂ in the effluent. Additionally, high denitrification rates are achieved by high water recirculation over plastic carriers with high surface area. This paper focuses on mathematical modeling of the novel reactor, based on its unique and specific characteristics. The continuously stirred hydraulic regime formed due to the relatively high recirculation flow rate required for efficient media wetting and the homogeneous gas phase in the closed reactor headspace, simplified the model design for the UFPR. The reaction rate constant and the overall volumetric gas (H₂)-liquid mass transfer coefficient (k_La) were determined for different recirculation flow rates at steady state. A rate constant correction factor β was developed to compensate for pH changes within the biofilm, deviation from intrinsic zero-order degradation kinetics and non-homogeneity of the biofilm. Model validation tests showed a high correlation between experimental and model results for various combinations of operational parameters. Results from the model showed that high denitrification rates of up to 7.5 g NO₃⁻-N/(L_reactor·d) together with H₂ utilization efficiencies above 90% can be achieved by the UFPR.

The paper compares the main features of a submerged bed reactor (SuBR) with bubbling and recirculation of gas to those of an unsaturated flow reactor (uSFR) with liquid recirculation. A novel pressurized closed-headspace hydrogenotrophic denitrification system characterized by safe and economic utilization of H₂ gas was used for the comparison. Under similar conditions, denitrification rates were lower in the SuBR as a result of a lower effective biofilm surface area and overall gas-liquid mass transfer coefficient k_La. Similar values of effluent DOC were achieved for both reactors, although effluent suspended solids concentration of the SuBR were substantially higher. On the other hand, the required cleaning frequency in the SuBR was 2.5 times lower. Moreover, the SuBR is expected to reduce the recirculation energy consumption by 0.35 kWh/m³ treated.

Most conventional hydrogenotrophic denitrification reactors based on packed- or fluidized-bed present a similar H₂ delivery scheme of continuous gas purging to the atmosphere in order to improve H₂ transfer rates and enable discharge of N₂ gas produced during denitrification. This operation results in a significant release of H₂ gas to atmosphere with its related economic and safety concerns. The current research proposes a novel pressurized high-rate hydrogenotrophic reactor for denitrification without gas purging. The investigation performed refutes a prevalent notion that N₂ gas accumulates in the headspace of a closed reactor during denitrification. Instead, this research shows that during continuous operation a gas-liquid equilibrium is established in the reactor according to Henry's law and excess N₂ gas is carried out by the effluent in dissolved form. Therefore, no gas purging is required and H₂ loss is limited only to the dissolved H₂ in the effluent. As a consequence, a simple low-cost and high-rate reactor with closed headspace can be designed for denitrification. The proposed reactor is operated as a trickling filter where water is recirculated over biofilm carriers with high surface area.The feasibility of the proposed reactor was shown for two effluent concentrations of 10 and 1mgNO₃^--N/L. Average denitrification rates of 2.1±0.2 and 1.06±0.06gNO₃^--N/(L_reactord) with H₂ utilization efficiencies of 92.8% and 96.9% were measured for the two effluent concentrations, respectively. Higher denitrification rates of up to 5gNO₃^--N/(L_reactord) were observed at higher recirculation flow rates and higher partial pressures of H₂.

A novel and potentially cost effective filtration scheme for removal of nitrate from groundwater, characterized by production of low salinity waste brine that can be easily discharged to sewerage systems and high product-water recovery, is proposed. The inherent preference of particular NF membranes for rejecting chloride and sodium over nitrate ions is utilized in a preliminary NF stage to remove Na⁺, Cl⁺, Ca²⁺ and Mg²⁺ to a side stream. In a second stage, RO is applied to remove NO₃^- and the RO permeate is mixed with the side stream of the NF stage to create product water low in nitrate, yet with a balanced composition consisting all the required species and minerals. The number of NF stages depends mainly on the rejection efficiency of the NF membrane. Based on Israeli regulations for both drinking water and required composition of brines discharged to the sewage, a treatment scheme composed of a single and double NF stages followed by RO is shown to reach water recoveries of 91.6% and 94.3%, respectively. Each NF stage raises the energy cost by approximately 0.5cent/m³ product water. However, this cost is easily paid back by the inherent additional advantages of the combined scheme, i.e., less water treated by the RO, significant increase in total recovery ratio, no need in re-mineralization of the product water and minimization of calcium carbonate precipitation potential on the RO membrane. The principles for process design are described, making the specific treatment scheme proposed here easily adjustable to other regulatory requirements and other water characteristics. A provisional patent has been filed.

In-line coagulation with aluminum or iron salts and ultrafiltration (UF) or microfiltration (MF) membranes is a valuable treatment option. The efficiency of the treatment is often evaluated by the achieved separation degree. That separation-oriented approach implies the coagulation with doses that are prohibitively high for many operations including the tertiary effluent treatment. The main purpose of the advanced wastewater treatment however is the retention of microorganisms and suspended solids, and that goal can be achieved even without coagulants. Thus the in-line coagulation can pursue the prevention or minimization of the irreversible fouling as an ultimate goal not related to the maximal separation of organic and inorganic impurities. Pilot experiments at conventional activated sludge (CAS) municipal wastewater treatment plant confirmed that the addition of 1 mg/L Fe³⁺ prevents the irreversible fouling as efficiently as the addition of 5 and 10 mg/L Fe³⁺. The economic impact of the suggested alteration is significant. Estimated operational expenses (OPEX) of a filtration at 60 LMH with 45 min cycles and 1 chemical - enhanced backwash per day is around 2 cents (€)/m³, almost a half of an OPEX of the separation-oriented treatment. Intermittent in-line coagulation down to first 2.5 min of 30 and 45 min filtration cycles is another cost-effective method to successfully depress the fouling. The success is explained by two-stage kinetics of a cake formation. At ripening stage, a layer of flocks restricted by a membrane gradually covers its surface and forms an initial dynamic cake. At operable stage, the cake entraps fresh solutes and prevents their contact with a membrane surface even without a coagulant. A superposition of two approaches reduces the consumption of ferric chloride coagulant by 94%.

Retraction: The following article from the Journal of Chemical Technology and Biotechnology, Efficient separation of Nannochloropsis salina using minerals to optimize algae sedimentation by Razi Epsztein, Amichai Felder, Alex Mishelevitz and Vitaly Gitis, published online on 20 March, 2012 in WileyOnlineLibrary ( http://onlinelibrary.wiley.com; doi: 10.1002/jctb.3754), has been retracted by agreement between the authors, the journal Editor-in-Chief, Jack Melling and John Wiley & Sons. The retraction has been agreed following an investigation by the academic ethics code committee of Ben-Gurion University which decided that the corresponding author had no right to publish the results of the study without permission from the other authors (decision from 21 June 2012).

BACKGROUND: Due to limited reservoirs of fossil fuels, biofuels have become one of the common alternatives to fill the gap between energy demand and available resources. Algae with high growth rate and significant lipid content can be produced even in marginal conditions and do not need a large footprint, typical for the majority of biodiesel plants. The main problem of biodiesel from algae is the absence of a fast and efficient process for separation of biomass from the growing media to produce initial stocks with high solids content. RESULTS: Successful harvesting of a lipid-rich algae strain Nannochloropsis salina was achieved with a thorough screening of commercial coagulants, polymers and minerals. It was found that kaolin-enhanced flocculation with ferric chloride resulted in 98.5% algae separation. The initial solution that contained 2000-4000 mg L^-1 dry matter was concentrated eight times to slurry that contained 2-3% solids. The sedimentation profile was improved with the addition of kaolin and Flok1. The potential of a simple belt filter to further concentrate the slurry was evaluated by dewatering potential. Specific resistance to filtration (SRF) displayed a potential for algae slurry with more than 20% solids mass by weight. CONCLUSION: A combination of 50 mg L^-1 kaolin and 85 mg L^-1 FeCl₃ as a first stage, and a belt press as a second stage produced a slurry that contained 20% N. salina by dry mass.

Secondary effluents of municipal wastewater are the major pollutant and the most significant irrigation reservoir at the same time. Although secondary effluents are widely used for irrigation of some crops, the unrestricted irrigation and groundwater recharge require implementation of membrane technologies. A combination of coagulation, ultrafiltration (UF) and nanofiltration (NF) stages has a potential to turn secondary effluents to treasure from waste. The current study focuses on the optimization of the coagulation of secondary effluents from Sede Teiman wastewater treatment plant (Southern Israel). The study was performed with ferric chloride, and the optimal dose was determined in jar tests by parallel measurement of total suspended solids (TSS), UV absorption at 254 nm (UV₂₅₄) and conductivity. The optimal dose of 130 mg/l FeCl₃•6H₂O (equivalent to 27 mg/l Fe³⁺) was then applied for pretreatment of secondary effluents prior to UF membrane. The combined coagulation-UF treatment reduced the total organic carbon (TOC) by more than 50%. Moreover, it increased the average flow rate during 30 min filtration cycles by up to 90% when compared without coagulation. The most significant improvement in UF performance due to coagulation was observed in membranes with a molecular weight cut-off (MWCO) greater than 30 kDa.