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

The superior catalytic property of single-atom catalysts (SACs) renders them highly desirable in the energy and environmental fields. However, using SACs for water decontamination is hindered by their limited spatial distribution and density on engineered surfaces and low stability in complex aqueous environments. Herein, we present copper SACs (Cu

Despite decades of dominance in separation technology, progress in the design and development of high-performance polymer-based membranes has been incremental. Recent advances in materials science and chemical synthesis provide opportunities for molecular-level design of next-generation membrane materials. Such designs necessitate a fundamental understanding of transport and separation mechanisms at the molecular scale. Molecular simulations are important tools that could lead to the development of fundamental structure-property-performance relationships for advancing membrane design. In this Perspective, we assess the application and capability of molecular simulations to understand the mechanisms of ion and water transport across polymeric membranes. Additionally, we discuss the reliability of molecular models in mimicking the structure and chemistry of nanochannels and transport pathways in polymeric membranes. We conclude by providing research directions for resolving key knowledge gaps related to transport phenomena in polymeric membranes and for the construction of structure-property-performance relationships for the design of next-generation membranes.

Conventional phototrophic cultivation for microalgae production suffers from low and unstable biomass productivity due to limited and unreliable light transmission outdoors. Alternatively, the use of a renewable lignocellulose-derived carbon source, cellulosic hydrolysate, offers a cost-effective and sustainable pathway to cultivate microalgae heterotrophically with high algal growth rate and terminal density. In this study, we evaluate the feasibility of cellulosic hydrolysate-mediated heterotrophic cultivation (Cel-HC) for microalgae production by performing economic and environmental comparisons with phototrophic cultivation through techno-economic analysis and life cycle assessment. We estimate a minimum selling price (MSP) of 4722 USD/t for producing high-purity microalgae through Cel-HC considering annual biomass productivity of 300 t (dry weight), which is competitive with the conventional phototrophic raceway pond system. Revenues from the lignocellulose-derived co-products, xylose and fulvic acid fertilizer, could further reduce the MSP to 2976 USD/t, highlighting the advantages of simultaneously producing high-value products and biofuels in an integrated biorefinery scheme. Further, Cel-HC exhibits lower environmental impacts, such as cumulative energy demand and greenhouse gas emissions, than phototrophic systems, revealing its potential to reduce the carbon intensity of algae-derived commodities. Our results demonstrate the economic and environmental competitiveness of heterotrophic microalgae production based on renewable bio-feedstock of lignocellulose.

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.

Dissolved silica is a major concern for a variety of industrial processes owing to its tendency to form complex scales that severely deteriorate system performance. In this work, we present a pretreatment technology using a Joule-heated sponge to rapidly remove silica from saline waters through adsorption, thereby effectively mitigating silica scaling in subsequent membrane desalination processes. The adsorbent sponge is fabricated by functionalizing two-dimensional layered double hydroxide (LDH) nanosheets on a porous, conductive stainless-steel sponge. With the application of an external voltage of 4 V, the Joule-heated sponge achieves 85% silica removal and 95% sponge regeneration within 15 min, which is much more efficient than its counterpart without Joule-heating (360 min for silica adsorption and 90 min for sponge regeneration). Material characterization and reaction kinetics analysis reveal that electrostatic interactions and "memory effect"-induced intercalation are the primary mechanisms for silica removal by the LDH nanosheets. Moreover, Joule-heating reduces the boundary layer resistance on nanosheets and facilitates intraparticle diffusion of dissolved silica, thereby increasing silica removal kinetics. Joule-heating also enhances the release of silicate ions during the regeneration stage through exchange with the surrounding anions (OH- or CO32-), resulting in a more efficient sponge regeneration. Pretreatment of silica-rich feedwaters by the Joule-heated sponge effectively reduces reverse osmosis membrane scaling by amorphous silica scale, demonstrating great potential for silica scaling control in a broad range of engineered processes.

Conventional water disinfection methods such as chlorination typically involve the generation of harmful disinfection byproducts and intensive chemical consumption. Emerging electroporation disinfection techniques using nanowire-enhanced local electric fields inactivate microbes by damaging their outer structures without byproduct formation or chemical dosing. However, this physical-based method suffers from a limited inactivation efficiency under high water flux due to an insufficient contact time. Herein, we integrate electrochlorination with nanowire-enhanced electroporation to achieve a synergistic flow-through process for efficient water disinfection targeting bacteria and viruses. Electroporation at the cathode induces sub-lethal damages on the microbial outer structures. Subsequently, electrogenerated active chlorine at the anode aggravates these electroporation-induced injuries to the level of lethal damage. This sequential flow-through disinfection system achieves complete disinfection (6.0-log) under a very high water flux of 2.4 × 10

Despite recent advancements in photocatalysis enabled by materials science innovations, the application of photocatalysts in water treatment is still hampered due to low overall efficiency. Herein, we present a TiO2 photocatalytic process with significantly enhanced efficiency by the introduction of micro-nano bubbles (MNBs). Notably, the removal rate of a model organic contaminant (methylene blue, MB) in an air MNB-assisted photocatalytic degradation (PCD) process was 41-141% higher than that obtained in conventional macrobubble (MaB)-assisted PCD under identical conditions. Experimental observations and supporting mechanistic modeling suggest that the enhanced photocatalytic degradation is attributed to the combined effects of increased dissolution of oxygen, improved colloidal stability and dispersion of the TiO2 nanocatalysts, and interfacial photoelectric effects of TiO2/MNB suspensions. The maximum dissolved oxygen (DO) concentration of the MNB suspension (i.e., 11.7 mg/L) was 32% higher than that of an MaB-aerated aqueous solution (i.e., 8.8 mg/L), thus accelerating the hole oxidation of H2O on TiO2. We further confirmed that the MNBs induced unique light-scattering effects, consequently increasing the optical path length in the TiO2/MNB suspension by 7.6%. A force balance model confirmed that a three-phase contact was formed on the surface of the bubble-TiO2 complex, which promoted high complex stability and PCD performance. Overall, this study demonstrates the enhanced photocatalytic water decontamination by MNBs and provides the underlying mechanisms for the process.

We discuss a recent publication in Energy & Environmental Science that presented a techno-economic analysis of electrochemical water desalination technologies.

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.

Inorganic scaling caused by precipitation of sparingly soluble salts at supersaturation is a common but critical issue, limiting the efficiency of membrane-based desalination and brine management technologies as well as other engineered systems. A wide range of minerals including calcium carbonate, calcium sulfate, and silica precipitate during membrane-based desalination, limiting water recovery and reducing process efficiency. The economic impact of scaling on desalination processes requires understanding of its sources, causes, effects, and control methods. In this Critical Review, we first describe nucleation mechanisms and crystal growth theories, which are fundamental to understanding inorganic scale formation during membrane desalination. We, then, discuss the key mechanisms and factors that govern membrane scaling, including membrane properties, such as surface roughness, charge, and functionality, as well as feedwater characteristics, such as pH, temperature, and ionic strength. We follow with a critical review of current characterization techniques for both homogeneous and heterogeneous nucleation, focusing on the strengths and limitations of each technique to elucidate scale-inducing mechanisms, observe actual crystal growth, and analyze the outcome of scaling behaviors of desalination membranes. We conclude with an outlook on research needs and future research directions to provide guidelines for scale mitigation in water treatment and desalination.

Nanoscale catalysts that can enable Fenton-like chemistry and produce reactive radicals from hydrogen peroxide activation have been extensively studied in order to overcome the limitations of homogeneous Fenton processes. Despite several advantageous features, limitation in mass transfer of short-lived radical species is an inherent drawback of the heterogeneous system. Here, we present a mechanistic foundation for the way spatial confinement of Fenton chemistry at the nanoscale can significantly enhance the kinetics of radical-mediated oxidation reactions-pollutant degradation in particular. We synthesized a series of Fe3O4-functionalized nanoreactors with precise pore dimensions, based on an anodized aluminum oxide template, to enable quantitative analysis of nanoconfinement effects. Combined with computational simulation of spatial distribution of radicals, we found that hydroxyl radical concentration was strongly dependent on the distance from the surface of Fenton catalysts. This distance dependency significantly influences the gross reaction kinetics and accounts for the observed nanoconfinement effects. We further found that a length scale below 25 nm is critical to avoid the limitation of short-lived species diffusion and achieve kinetics that are orders of magnitude faster than those obtained in a batch suspension of heterogeneous catalysts. These findings suggest a new strategy to develop an innovative heterogeneous catalytic system with the most effective use of hydroxyl radicals in oxidation treatment scenarios.

Increased demand for highly selective and energy-efficient separations processes has stimulated substantial interest in emerging two-dimensional (2D) nanomaterials as a potential platform for next-generation membranes. However, persistently poor separation performance continues to hinder the viability of many novel 2D-nanosheet membranes in desalination applications. In this study, we examine the role of the lamellar structure of 2D membranes on their performance. Using self-fabricated molybdenum disulfide (MoS2) membranes as a platform, we show that the separation layer of 2D nanosheet frameworks not only fails to demonstrate water-salt selectivity but also exhibits low rejection toward dye molecules. Moreover, the MoS2 membranes possess a molecular weight cutoff comparable to its underlying porous support, implying negligible selectivity of the MoS2 layer. By tuning the nanochannel size through intercalation with amphiphilic molecules and analyzing mass transport in the lamellar structure using Monte Carlo simulations, we reveal that small imperfections in the stacking of MoS2 nanosheets result in the formation of catastrophic microporous defects. These defects lead to a precipitous reduction in the selectivity of the lamellar structure by negating the interlayer sieving mechanism that prevents the passage of large penetrants. Notably, the imperfect stacking of nanosheets in the MoS2 membrane was further verified using 2D X-ray diffraction measurements. We conclude that developing a well-controlled fabrication process, in which the lamellar structure can be carefully tuned, is critical to achieving defect-free and highly selective 2D desalination membranes.

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.

Manganese(III/IV) oxide minerals are known to spontaneously degrade organic pollutants in nature. However, the kinetics are too slow to be useful for engineered water treatment processes. Herein, we demonstrate that nanoscale Mn3O4 particles under nanoscale spatial confinement (down to 3-5 nm) can significantly accelerate the kinetics of pollutant degradation, nearly 3 orders of magnitude faster compared to the same reaction in the unconfined bulk phase. We first employed an anodized aluminum oxide scaffold with uniform channel dimensions for experimental and computational studies. We found that the observed kinetic enhancement resulted from the increased surface area of catalysts exposed to the reaction, as well as the increased local proton concentration at the Mn3O4 surface and subsequent acceleration of acid-catalyzed reactions even at neutral pH in bulk. We further demonstrate that a reactive Mn3O4-functionalized ceramic ultrafiltration membrane, a more suitable scaffold for realistic water treatment, achieved nearly complete removal of various phenolic and aniline pollutants, operated under a common ultrafiltration water flux. Our findings mark an important advance toward the development of catalytic membranes that can degrade pollutants in addition to their intrinsic function as a physical separation barrier, especially since they are based on accelerating natural catalytic pathways that do not require any chemical addition.

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

Minimum and zero liquid discharge (MLD/ZLD) are emerging brine management strategies that attract heightened attention. Although conventional reverse osmosis (RO) can improve the energy efficiency of MLD/ZLD processes, its application is limited by the maximum hydraulic pressure (ΔPmax) that can be applied in current membrane modules. To overcome such limitation, novel RO-based technologies, including osmotically assisted RO (OARO) and low-salt-rejection RO (LSRRO), have been proposed. Herein, we utilize process modeling to systematically compare the energy consumption of OARO and LSRRO for MLD/ZLD applications. Our modeling results show that the specific energy consumption (SEC) of LSRRO is lower (by up to ∼30%) than that of OARO for concentrating moderately saline feed waters ( ∼70,000 mg/L TDS). However, by implementing more stages and/or an elevated ΔPmax, LSRRO has the potential to outperform OARO energetically for treating high-salinity feed waters. Notably, the SEC of both OARO and LSRRO could be 50% lower than that of mechanical vapor compressor, the commonly used brine concentrator in MLD/ZLD applications. We conclude with a discussion on the practicability of OARO and LSRRO based on membrane module availability and capital cost, suggesting that LSRRO could potentially be more feasible than OARO.

Surface functionalization using two-dimensional (2D) graphene oxide (GO) materials is a promising technique to enhance the biofouling resistance of membranes used in water purification and reuse. However, the role of GO exposure, which is crucial for the contact-mediated toxicity mechanism, has not been well evaluated or elucidated in previous studies. Herein, we employ bioinspired polydopamine chemistry to fabricate GO-functionalized membranes through two strategies: coating and blending. The two types of GO-functionalized membranes displayed comparable roughness, hydrophilicity, water permeability, and solute retention properties but different degrees of GO nanosheet exposure on the membrane surface. When in contact with the model bacterium, Escherichia coli, the GO-coated membrane exhibited enhanced biofouling resistance compared to that of the GO-blended membrane, as evidenced by lower viable cells in static adsorption experiments, and lower water flux decline and higher flux recovery in dynamic biofouling experiments. Moreover, the development of biofilm on the GO-coated membrane was also inhibited to a greater extent than on the GO-blended membrane. Taken together, our findings indicate the paramount importance of GO exposure on the membrane surface in conferring antibacterial activity and biofouling resistance, which should be considered in the future design of antibiofouling membranes using 2D nanomaterials.

Composite magnetic aluminum hydroxide at iron oxide nanomaterials, Al(OH)3@Fe3O4, with a well-defined core–shell structure, were used as pretreatment adsorbents for the removal of silica in brackis...

Forward osmosis (FO) can potentially treat textile wastewaters with less fouling than pressure-driven membrane processes such as reverse osmosis and nanofiltration. However, conventional FO membranes with asymmetric architecture experience severe flux decline caused by internal concentration polarization and fouling as dye molecules accumulate on the membrane surface. In this study, we present a new strategy for concentrating dye by using a self-standing, support-free FO membrane with a symmetric structure. The membrane was fabricated by a facile solution-casting approach based on a poly(triazole- co-oxadiazole- co-hydrazine) (PTAODH) skeleton. Due to its dense architecture, ultrasmooth surface, and high negative surface charge, the PTAODH membrane exhibits excellent FO performance with minimal fouling, low reverse salt flux, and negligible dye passage to the draw solution side. Cleaning with a 40% alcohol solution, after achieving a concentration factor of ∼10, resulted in high flux recovery ratio (98.7%) for the PTAODH membrane, whereas significant damage to the active layers of two commercial FO membranes was observed. Moreover, due to the existence of cytotoxic oxadiazole and triazole moieties in the polymer structure, our PTAODH membrane exhibited an outstanding antibacterial property with two model bacteria. Our results demonstrate the promising application of the symmetric PTAODH membrane for the concentration of textile wastewaters and its superior antifouling performance compared to state-of-the-art commercial FO membranes.

Surface modification with bactericides is a promising approach to imparting membrane materials with biofouling resistance. However, chemical modification of membranes made from inert materials, such as polyvinylidene fluoride (PVDF) and polysulfone, is challenging because of the absence of reactive functional groups on these materials. In this study, we develop a facile procedure using benzophenone as an anchor to graft biocidal graphene oxide (GO) to chemically inactive membrane materials. GO nanosheets are first functionalized with benzophenone through an amide coupling reaction. Then, benzophenone-functionalized GO nanosheets are irreversibly grafted to the inert membrane surfaces via benzophenone-initiated cross-linking under ultraviolet irradiation. The binding of GO to the membrane surface is confirmed by scanning electron microscopy and Raman spectroscopy. When exposed to a model bacterium (Escherichia coli), GO-functionalized PVDF and polysulfone membranes exhibit strong antibacterial activity, re...

Heterogeneous advanced oxidation processes (AOPs) allow for the destruction of aqueous organic pollutants via oxidation by hydroxyl radicals (•OH). However, practical treatment scenarios suffer from the low availability of short-lived •OH in aqueous bulk, due to both mass transfer limitations and quenching by water constituents, such as natural organic matter (NOM). Herein, we overcome these challenges by loading iron oxychloride catalysts within the pores of a ceramic ultrafiltration membrane, resulting in an internal heterogeneous Fenton reaction that can degrade organics in complex water matrices with pH up to 6.2. With •OH confined inside the nanopores (∼ 20 nm), this membrane reactor completely removed various organic pollutants with water fluxes of up to 100 L m-2 h-1 (equivalent to a retention time of 10 s). This membrane, with a pore size that excludes NOM (>300 kDa), selectively exposed smaller organics to •OH within the pores under confinement and showed excellent resiliency to representative water matrices (simulated surface water and sand filtration effluent samples). Moreover, the membrane exhibited sustained AOPs (>24 h) and could be regenerated for multiple cycles. Our results suggest the feasibility of exploiting ultrafiltration membrane-based AOP platforms for organic pollutant degradation in complex water scenarios.

Global depletion of natural resources provides an impetus for developing low-cost, environmentally benign technologies for the recovery of valuable resources from wastewater. In this study, we present an autonomous photo-electrochemical osmotic system (PECOS) that can recover a wide range of metals from simulated metal-laden wastewater with sunlight illumination while generating electricity. The PECOS comprises a draw solution chamber with a nickel nanoparticle-functionalized titanium nanowire (Ni-TiNA) photoanode, a feed solution chamber containing synthetic wastewater with an immersed carbon fiber cathode, and a forward osmosis (FO) membrane mounted between the chambers as a separator. Using a Na2-EDTA anolyte as a draw solution at neutral pH, we demonstrate that a sunlit PECOS achieves copper recovery at a rate of 51 g h-1 per m-2 of membrane area from simulated copper-laden wastewater while simultaneously producing a maximum power density of 228 mW m-2. Moreover, because of the osmotic pressure difference generated by the photo-electrochemical reactions, the PECOS reduces the wastewater volume by extracting fresh water through the FO membrane at a water flux of 0.84 L m-2 h-1. We further demonstrate the feasibility of the PECOS in recovering diverse metals from a simulated metal-laden industrial wastewater under sunlight irradiation. Our proof-of-concept PECOS prototype provides a sustainable technological solution that leverages sunlight in an electrochemical osmotic system to recover multiple resources from wastewater.

A highly-efficient, autonomous electrochemical-osmotic system (EOS) is developed for simultaneous recovery of electric energy, water, and metals from wastewater. We demonstrate that the system can generate a maximum electric power density of 10.5 W m-2 using a spontaneous Fe/Cu2+ galvanic cell, while simultaneously achieving copper recovery from wastewater. With an osmotic pressure difference generated by the deployed electrochemical reactions, water is osmotically extracted from the feed solution with the EOS at a water flux of 5.1 L m-2 h-1. A scaled-up EOS realizes a power density of 105.8 W per m-3 of treated water to light an LED over 24 h while also enhancing water extraction and metal recovery. The modularized EOS obtains ultrahigh (>97.5%) Faradaic efficiencies under variable operating conditions, showing excellent system stability. The EOS is also versatile: it can recover Au, Ag, and Hg from wastewaters with simultaneous electricity and water coproduction. Our study demonstrates a promising pathway for realizing multiresource recycling from wastewater by coupling electrochemical and osmosis-driven processes.

Reactive membranes based on hydroxyl radical generation are hindered by the need for chemical dosing and complicated module and material design. Herein, we utilize an electrochemical approach featuring in situ generation of reactive (radical) chlorine species (RCS) through anodization of chloride ions for membrane self-cleaning. A hybridized carbon nanotube (CNT)-functionalized ceramic membrane (h-CNT/CM), possessing high hydrophilicity, permeability, and conductivity, was fabricated. Using carbamazepine (CBZ) as a probe, we confirmed the presence of RCS in the electrified h-CNT/CM. The rapid and complete degradation of CBZ in a single-pass ultrafiltration indicates a high localized RCS concentration within the three-dimensional porous CNT interwoven layer. We further demonstrate that the electrogeneration of RCS is a critical prestep for free chlorine (HClO and ClO-) formation. The self-cleaning efficiency of the membrane after fouling with a model organic foulant (alginate) was assessed using an electrified cross-flow membrane filtration system. The fouled h-CNT/CM exhibits a near complete water flux recovery following a short (1 min) self-cleaning with an applied voltage of 3 or 4 V and feed solutions of 100 or 10 mM sodium chloride, respectively. Considering the superior performance of the RCS-mediated self-cleaning compared to conventional membrane chemical cleaning using sodium hypochlorite, our results exemplify an effective strategy for in situ electrogeneration of RCS to achieve a highly efficient membrane self-cleaning.

Reverse Osmosis (RO) is a membrane-based technology for water desalination. Of paramount importance is the understanding of ion selectivity in mixtures of salts, i.e., to what extent the membrane retains one ion more than another in a multicomponent salt solution. We apply continuum transport theory to describe a large set of data for the ion selectivity of RO membranes treating brackish groundwater with more than 10 different monovalent and divalent ions. The model is based on the Donnan steric partitioning pore model extended to include ions of multiple charge states, such as bicarbonate/carbonic acid, ammonia/ammonium, and hydroxyl/hydronium ions and the acid-base reactions between them and with the membrane charge. By adjusting for each ion the ratio of ion size over pore size, we can fit the model to the data. We note that the fitted ion sizes do not always follow a logical order based on the ionic or hydrated size of the ions and that rejection of divalent cations is overestimated in some cases. We discuss possible theoretical improvements to address these discrepancies. Our results highlight the potential of continuum transport theory to describe in detail multicomponent ion transport in RO membranes. The development of a detailed and validated physics-based model is an important step toward achieving improved operation and design of RO-based desalination systems.

Enhancing the chemical and physical properties of the polyamide active layer of thin-film composite (TFC) membranes by surface coating is a goal long-pursued. Atomic layer deposition (ALD) has been proposed as an innovative approach to deposit chemically robust metal oxides onto membrane surfaces due to its unique capability to control coating conformality and thickness with atomic scale precision. This study examined the potential to coat the surface of TFC reverse osmosis (RO) and nanofiltration (NF) membranes via ALD of TiO2. Our results suggest that the optimal ALD conditions, the film growth kinetics, and the depth of deposition are different for RO and NF membranes due to the different diffusive transport of ALD precursors through the membrane pores. The TiO2 coating mainly located at the surface of the RO membrane; in contrast, the TiO2 coating extended to the depth of the NF membrane. The TiO2 coating degraded membrane water permeability and salt rejection beyond 10 cycles of ALD, the condition commonly employed in previous ALD-based membrane modification studies. Instead, this study showed that with fewer than 10 cycles, the TiO2 coating of RO membrane increased the membrane surface charge without negatively impacting water permeability and salt rejection. For the NF membranes, the coating of TiO2 inside their pores led to the tuning of pore sizes and increased the rejection of selected solutes.

Discovering electrode materials with exceptional capacitance, an indicator of the ability of a material to hold charge, is critical for developing capacitive deionization devices for water desalina...

Reverse osmosis (RO) has become a premier technology for desalination and water purification. The need for increased selectivity has incentivized research into novel membranes, such as biomimetic membranes that incorporate the perfectly selective biological water channel aquaporin or synthetic water channels like carbon nanotubes. In this study, we consider the performance of composite biomimetic membranes by projecting water permeability, salt rejection, and neutral-solute retention based on the permeabilities of the individual components, particularly the water channel, the amphiphilic bilayer matrix, and potential support layers that include polymeric RO, nanofiltration (NF), and porous ultrafiltration membranes. We find that the support layer will be crucial in the overall performance. Selective, relatively low-permeability supports minimize the negative impact of defects in the biomimetic layer, which are currently the main performance-limiting factor for biomimetic membranes. In particular, RO membranes as support layers would enable99.85% salt rejection at ∼10000-fold greater biomimetic-layer defect area than for porous supports. By fundamentally characterizing neutral-solute permeation through RO and NF membranes, we show that RO membranes as support layers would enable high rejection of organic pollutants based on molecular size, overcoming the rapid permeation of hydrophobic solutes through the biomimetic layer. A biomimetic membrane could also achieve exceptionally high boron rejections of ∼99.7%, even with 1% defect area in the biomimetic layer. We conclude by discussing the implications of our findings for biomimetic membrane design.

Biofouling is a major obstacle for the efficient and reliable operation of membrane-based desalination processes. Innovations in membrane materials and fabrication processes are therefore needed to develop antibiofouling strategies. In this study, we utilize the alignability of an emerging two-dimensional nanomaterial, graphene oxide (GO), to fabricate a desalination membrane with enhanced bacterial resistance. GO nanosheets are dispersed in a polymer solution to form a homogeneous mixture, which undergoes slow solvent evaporation in a magnetic field to create a thin nanocomposite membrane with vertically aligned GO nanosheets. The structural characteristics of the fabricated membranes confirm the enhanced exposure of nanosheet edges on the surface through the vertical alignment of GO. Notably, the addition and alignment of GO do not compromise membrane water permeability and water–salt selectivity. When contacted with bacterial cells, membranes with vertically aligned GO nanosheets exhibit enhanced antim...

We investigated the relationship between silica scaling and protein fouling in reverse osmosis (RO). Flux decline caused by combined scaling and fouling was compared with those by individual scaling or fouling. Bovine serum albumin (BSA) and lysozyme (LYZ), two proteins with opposite charges at typical feedwater pH, were used as model protein foulants. Our results demonstrate that water flux decline was synergistically enhanced when silica and protein were both present in the feedwater. For example, flux decline after 500 min was far greater in combined silica scaling and BSA fouling experiments (55 ± 6% decline) than those caused by silica (11 ± 2% decline) or BSA (9 ± 1% decline) alone. Similar behavior was observed with silica and LYZ, suggesting that this synergistic effect was independent of protein charge. Membrane characterization by scanning electron microscopy and Fourier transform infrared spectroscopy revealed distinct foulant layers formed by BSA and LYZ in the presence of silica. A combination of dynamic light scattering, transmission electron microscopy , and energy dispersive X-ray spectroscopy analyses further suggested that BSA and LYZ facilitated the formation of aggregates with varied chemical compositions. As a result, BSA and LYZ were likely to play different roles in enhancing flux decline in combined scaling and fouling. Our study suggests that the coexistence of organic foulants, such as proteins, largely alters scaling behavior of silica, and that accurate prediction of RO performance requires careful consideration of foulant-scalant interactions.

Self-cleaning, antifouling ultrafiltration membranes are critically needed to mitigate organic fouling in water and wastewater treatment. In this study, we fabricated a novel polyvinylidene fluoride (PVDF) composite ultrafiltration membrane coated with FeOCl nanocatalysts (FeOCl/PVDF) via a facile, scalable thermal-treatment method, for the synergetic separation and degradation of organic pollutants. The structure, composition, and morphology of the FeOCl/PVDF membrane were extensively characterized. Results showed that the as-prepared FeOCl/PVDF membrane was uniformly covered with FeOCl nanoparticles with an average diameter of 1–5 nm, which greatly enhanced membrane hydrophilicity. The catalytic self-cleaning and antifouling properties of the FeOCl/PVDF membrane were evaluated in the presence of H2O2 at neutral pH. Using a facile H2O2 cleaning process, we showed that the FeOCl/PVDF membrane can achieve an excellent water flux recovery rate of ∼100%, following organic fouling with a model organic foulant...

Biological wastewater treatment is not effective in removal of antibiotic resistant bacteria (ARB) and antibiotic resistance genes (ARGs). In this study, we fabricated a photocatalytic reactive membrane by functionalizing polyvinylidene fluoride (PVDF) ultrafiltration (UF) membrane with titanium oxide (TiO2) nanoparticles for the removal of ARB and ARGs from a secondary wastewater effluent. The TiO2-modified PVDF membrane provided complete retention of ARB and effective photocatalytic degradation of ARGs and integrons. Specifically, the total removal efficiency of ARGs (i.e., plasmid-mediated floR, sul1, and sul2) with TiO2-modified PVDF membrane reached ∼98% after exposure to UV irradiation. Photocatalytic degradation of ARGs located in the genome was found to be more efficient than those located in plasmid. Excellent removal of integrons (i.e., intI1, intI2, and intI3) after UV treatment indicated that the horizontal transfer potential of ARGs was effectively controlled by the TiO2 photocatalytic reacti...

Water scarcity, expected to become more widespread in the coming years, demands renewed attention to freshwater protection and management. Critical to this effort are the minimization of freshwater withdrawals and elimination of wastewater discharge, both of which can be achieved via zero liquid discharge (ZLD), an aggressive wastewater management approach. Because of the high energetic cost of thermal desalination, ZLD is particularly challenging for high-salinity wastewaters. In this review, we discuss the potential of high-pressure reverse osmosis (HPRO) (i.e., reverse osmosis operated at a hydraulic pressure greater than ∼100 bar) to efficiently desalinate hypersaline brines. We first discuss the inherent energy efficiency of membrane processes compared to that of conventional thermal processes for brine desalination. We then highlight the opportunity of HPRO to reduce energy requirements for desalination of key high-salinity industrial wastewaters. The current state of membrane materials and processe...

We demonstrate the fabrication of a loose, negatively charged nanofiltration (NF) membrane with tailored selectivity for the removal of perfluoroalkyl substances with reduced scaling potential. A selective polyamide layer was fabricated on top of a poly(ether sulfone) support via interfacial polymerization of trimesoyl chloride and a mixture of piperazine and bipiperidine. Incorporating high molecular weight bipiperidine during the interfacial polymerization enables the formation of a loose, nanoporous selective layer structure. The fabricated NF membrane possessed a negative surface charge and had a pore diameter of ∼1.2 nm, much larger than a widely used commercial NF membrane (i.e., NF270 with pore diameter of ∼0.8 nm). We evaluated the performance of the fabricated NF membrane for the rejection of different salts (i.e., NaCl, CaCl2, and Na2SO4) and perfluorooctanoic acid (PFOA). The fabricated NF membrane exhibited a high retention of PFOA (∼90%) while allowing high passage of scale-forming cations (i...

Conventional asymmetric or thin-film composite forward osmosis (FO) membranes suffer from severe internal concentration polarization, which significantly hinders process performance and practical applications. Here we report the synthesis of the COOH-derived polyoxadiazole copolymer for the fabrication of a self-standing selective thin film without a support layer. The thickness of the membrane was controlled at merely a few micrometers to achieve a high rate of rejection of the Na2SO4 draw solution, while maintaining acceptable water permeability. Because of the symmetric architecture, the membrane exhibited excellent and identical FO performance at both of its sides. The structural parameter of the fabricated membranes was zero because of the absence of internal concentration polarization in the symmetric FO membranes. Our results highlight the potential of support-free membranes for the further development of FO technology.

This study intends to reinvent classical Fenton chemistry by enabling the Fe(II)/Fe(III) redox cycle to occur on a newly developed FeOCl nanosheet catalyst for facile hydroxyl radical (•OH) generation from H2O2 activation. This approach overcomes challenges such as low operating pH and large sludge production that have prevented a wider use of otherwise attractive Fenton chemistry for practical water treatment, in particular, for the destruction of recalcitrant pollutants through nonselective oxidation by •OH. We demonstrate that FeOCl catalysts exhibit the highest performance reported in the literature for •OH production and organic pollutant destruction over a wide pH range. We further elucidate the mechanism of rapid conversion between Fe(III) and Fe(II) in FeOCl crystals based on extensive characterizations. Given the low-cost raw material and simple synthesis and regeneration, FeOCl catalysts represent a critical advance toward application of iron-based advanced oxidation in real practice.

Recent studies have documented the existence of discrete voids in the thin polyamide selective layer of composite reverse osmosis membranes. Here we present compelling evidence that these nanovoids are formed by nanosized gas bubbles generated during the interfacial polymerization process. Different strategies were used to enhance or eliminate these nanobubbles in the thin polyamide film layer to tune its morphology and separation properties. Nanobubbles can endow the membrane with a foamed structure within the polyamide rejection layer that is approximately 100 nm in thickness. Simple nanofoaming methods, such as bicarbonate addition and ultrasound application, can result in a remarkable improvement in both membrane water permeability and salt rejection, thus overcoming the long-standing permeability–selectivity trade-off of desalination membranes.

Low-grade heat from geothermal sources and industrial plants is a significant source of sustainable power that has great potential to be converted to electricity. The two main approaches that have been extensively investigated for converting low-grade heat to electrical energy, organic Rankine cycles and solid-state thermoelectrics, have not produced high power densities or been cost-effective for such applications. Newer, alternative liquid-based technologies are being developed that can be categorized by how the heat is used. Thermoelectrochemical cells (TECs), thermo-osmotic energy conversion (TOEC) systems, and thermally regenerative electrochemical cycles (TRECs) all use low-grade heat directly in a device that generates electricity. Other systems use heat sources to prepare solutions that are used in separate devices to produce electrical power. For example, low-temperature distillation methods can be used to produce solutions with large salinity differences to generate power using membrane-based systems, such as pressure-retarded osmosis (PRO) or reverse electrodialysis (RED); or highly concentrated ammonia solutions can be prepared for use in thermally regenerative batteries (TRBs). Among all these technologies, TRECs, TOEC, and TRBs show the most promise for effectively converting low-grade heat into electrical power mainly due to their high power productions and energy conversion efficiencies.

The interaction of two-dimensional (2D) nanomaterials with biological membranes has important implications for ecotoxicity and human health. In this study, we use a dye-leakage assay to quantitatively assess the disruption of a model phospholipid bilayer membrane (i.e., lipid vesicles) by five emerging 2D nanomaterials: graphene oxide (GO), reduced graphene oxide (rGO), molybdenum disulfide (MoS2), copper oxide (CuO), and iron oxide (α-Fe2O3). Leakage of dye from the vesicle inner solution, which indicates loss of membrane integrity, was observed for GO, rGO, and MoS2 nanosheets but not for CuO and α-Fe2O3, implying that 2D morphology by itself is not sufficient to cause loss of membrane integrity. Mixing GO and rGO with lipid vesicles induced aggregation, whereas enhanced stability (dispersion) was observed with MoS2 nanosheets, suggesting different aggregation mechanisms for the 2D nanomaterials upon interaction with lipid bilayers. No loss of membrane integrity was observed under strong oxidative condi...

Innovation in urban water systems is required to address the increasing demand for clean water due to population growth and aggravated water stress caused by water pollution, aging infrastructure, and climate change. Advances in materials science, modular water treatment technologies, and complex systems analyses, coupled with the drive to minimize the energy and environmental footprints of cities, provide new opportunities to ensure a resilient and safe water supply. We present a vision for enhancing efficiency and resiliency of urban water systems and discuss approaches and research needs for overcoming associated implementation challenges.

We investigated the relationship between membrane surface properties and silica scaling in reverse osmosis (RO). The effects of membrane hydrophilicity, free energy for heterogeneous nucleation, and surface charge on silica scaling were examined by comparing thin-film composite polyamide membranes grafted with a variety of polymers. Results show that the rate of silica scaling was independent of both membrane hydrophilicity and free energy for heterogeneous nucleation. In contrast, membrane surface charge demonstrated a strong correlation with the extent of silica scaling (R2 > 0.95, p < 0.001). Positively charged membranes significantly facilitated silica scaling, whereas a more negative membrane surface charge led to reduced scaling. This observation suggests that deposition of negatively charged silica species on the membrane surface plays a critical role in silica scale formation. Our findings provide fundamental insights into the mechanisms governing silica scaling in reverse osmosis and highlight th...

1,4-Dioxane has historically been used to stabilize chlorinated solvents and more recently has been found as a contaminant of numerous consumer and food products. Once discharged into the environment, its physical and chemical characteristics facilitate migration in groundwater, resulting in widespread contamination of drinking water supplies. Over one-fifth of U.S. public drinking water supplies contain detectable levels of 1,4-dioxane. Remediation efforts using common adsorption and membrane filtration techniques have been ineffective, highlighting the need for alternative removal approaches. While the data evaluating human exposure and health effects are limited, animal studies have shown chronic exposure to cause carcinogenic responses in the liver across multiple species and routes of exposure. Based on this experimental evidence, the U.S. Environmental Protection Agency has listed 1,4-dioxane as a high priority chemical and classified it as a probable human carcinogen. Despite these health concerns, there are no federal or state maximum contaminant levels for 1,4-dioxane. Effective public health policy for this emerging contaminant requires additional information about human health effects, chemical interactions, environmental fate, analytical detection, and treatment technologies. This review highlights the current state of knowledge, key uncertainties, and data needs for future research on 1,4-dioxane.

Two-dimensional nanomaterial (2-D NM) frameworks, especially those comprising graphene oxide, have received extensive research interest for membrane-based separation processes and desalination. However, the impact of horizontal defects in 2-D NM frameworks, which stem from nonuniform deposition of 2-D NM flakes during layer build-up, has been almost entirely overlooked. In this work, we apply Monte Carlo simulations, under idealized conditions wherein the vertical interlayer spacing allows for water permeation while perfectly excluding salt, on both the formation of the laminate structure and molecular transport through the laminate. Our simulations show that 2-D NM frameworks are extremely tortuous (tortuosity ≈10

Structural tunability and surface functionality of layered two-dimensional (2-D) iron oxychloride (FeOCl) nanosheets are critical for attaining exceptional adsorption properties. In this study, we combine computational and experimental tools to elucidate the distinct adsorption nature of Pb(II) on 2-D FeOCl nanosheets. After finding promising Pb(II) adsorption characteristics by bulk FeOCl sheets (B-FeOCl), we applied computational quantum mechanical modeling to mechanistically explore Pb(II) adsorption on representative FeOCl facets. Results indicate that increasing the exposure of FeOCl oxygen and chlorine sites significantly enhances Pb(II) adsorption. The (110) and (010) facets of FeOCl possess distinct orientations of oxygen and chlorine, resulting in different Pb(II) adsorption energies. Consequently, the (110) facet was found to be more selective toward Pb(II) adsorption than the (010) facet. To exploit this insight, we exfoliated B-FeOCl to obtain ultrathin FeOCl nanosheets (U-FeOCl) possessing unique chlorine- and oxygen-enriched surfaces. As we surmised, U-FeOCl nanosheets achieved excellent Pb(II) adsorption capacity (709 mg g-1 or 3.24 mmol g-1). Moreover, U-FeOCl demonstrated rapid adsorption kinetics, shortening adsorption equilibration time to one-third of the time for B-FeOCl. Extensive characterization of FeOCl-Pb adsorption complexes corroborated the simulation results, illustrating that increasing the number of Pb-O and Pb-Cl interaction sites led to the improved Pb(II) adsorption capacity of U-FeOCl.