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

Polymer membranes have been used extensively for Angstrom-scale separation of solutes and molecules. However, the pore size of most polymer membranes has been considered an intrinsic membrane property that cannot be adjusted in operation by applied stimuli. In this work, we show that the pore size of an electrically conductive polyamide membrane can be modulated by an applied voltage in the presence of electrolyte via a mechanism called electrically induced osmotic swelling. Under applied voltage, the highly charged polyamide layer concentrates counter ions in the polymer network via Donnan equilibrium and creates a sizeable osmotic pressure to enlarge the free volume and the effective pore size. The relation between membrane potential and pore size can be quantitatively described using the extended Flory-Rehner theory with Donnan equilibrium. The ability to regulate pore size via applied voltage enables operando modulation of precise molecular separation in-situ. This study demonstrates the amazing capability of electro-regulation of membrane pore size at the Angstrom scale and unveils an important but previously overlooked mechanism of membrane-water-solute interactions.

Achieving controllable fine-tuning of defects in catalysts at the atomic level has become a zealous pursuit in catalysis-related fields. However, the generation of defects is quite random, and their flexible manipulation lacks theoretical basis. Herein, we present a facile and highly controllable thermal tuning strategy that enables fine control of nanodefects via subtle manipulation of atomic/lattice arrangements in electrocatalysts. Such thermal tuning endows common carbon materials with record high efficiency in electrocatalytic degradation of pollutants. Systematic characterization and calculations demonstrate that an optimal thermal tuning can bring about enhanced electrocatalytic efficiency by manipulating the N-centered annulation–volatilization reactions and C-based sp3/sp2 configuration alteration. Benefiting from this tuning strategy, the optimized electrocatalytic anodic membrane successfully achieves >99% pollutant (propranolol) degradation during a flow-through (~2.5 s for contact time), high-flux (424.5 L m−2 h−1), and long-term (>720 min) electrocatalytic filtration test at a very low energy consumption (0.029 ± 0.010 kWh m−3 order−1). Our findings highlight a controllable preparation approach of catalysts while also elucidating the molecular level mechanisms involved.

Formation of mineral scale on a material surface has profound impact on a wide range of natural processes as well as industrial applications. However, how specific material surface characteristics affect the mineral-surface interactions and subsequent mineral scale formation is not well understood. Here we report the superior resistance of hexagonal boron nitride (hBN) to mineral scale formation compared to not only common metal and polymer surfaces but also the highly scaling-resistant graphene, making hBN possibly the most scaling resistant material reported to date. Experimental and simulation results reveal that this ultrahigh scaling-resistance is attributed to the combination of hBN’s atomically-smooth surface, in-plane atomic energy corrugation due to the polar boron-nitrogen bond, and the close match between its interatomic spacing and the size of water molecules. The latter two properties lead to strong polar interactions with water and hence the formation of a dense hydration layer, which strongly hinders the approach of mineral ions and crystals, decreasing both surface heterogeneous nucleation and crystal attachment.

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

Chlorination is a common practice to prevent biofouling in municipal water supplies, wastewater reuse and seawater desalination. However, polyamide thin-film composite reverse osmosis membranes—the premier technology for desalination and clean-water production—structurally deteriorate when continually exposed to chlorine species. Here, we use layer-by-layer interfacial polymerization of 3,5-dihydroxybenzoic acid with trimesoyl chloride to fabricate a polyester thin-film composite reverse osmosis membrane that is chlorine-resistant in neutral and acidic conditions. Strong steric hindrance and an electron-withdrawing group effectively prevent direct aromatic chlorination, and residual OH groups capped with isophthaloyl dichloride preclude reaction with active chlorine. The poly(isophthalester) membrane exhibits high salt rejection (99.1 ± 0.2%) and water permeability (2.97 ± 0.13 l m−2 h−1 bar−1), even after demonstrating biofouling prevention with chlorine (50 mg l−1 of NaOCl for 15 min). We anticipate that our chlorine-resistant membrane will greatly advance reverse osmosis desalination as a sustainable technology to meet the global challenge of water supply. Reverse osmosis membranes are the primary technology used for desalination and wastewater recycling, but they are prone to biofouling and subsequent performance deterioration due to poor tolerance to disinfecting agents such as chlorine. Here a chlorine-resistant polyester reverse osmosis membrane is developed to prevent biofouling and increase the sustainability of desalination and wastewater reuse.

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.

Creating well-defined single-crystal textures in materials requires the biaxial alignment of all grains into desired orientations, which is challenging to achieve in soft materials. Here we report the formation of single crystals with rigorously controlled texture over macroscopic areas (>1 cm2) in a soft mesophase of a columnar discotic liquid crystal. We use two modes of directed self-assembly, physical confinement and magnetic fields, to achieve control of the orientations of the columnar axes and the hexagonal lattice along orthogonal directions. Field control of the lattice orientation emerges in a low-temperature phase of tilted discogens that breaks the field degeneracy around the columnar axis present in non-tilted states. Conversely, column orientation is controlled by physical confinement and the resulting imposition of homeotropic anchoring at bounding surfaces. These results extend our understanding of molecular organization in tilted systems and may enable the development of a range of new materials for distinct applications. Macroscopic single crystals with controlled texture are formed from a self-assembled columnar discotic liquid crystal.

Three-dimensional macrostructures (3DMs) of graphene and graphene oxide are being developed for fast and efficient removal of contaminants from water and air. The large specific surface area, versatile surface chemistry and exceptional mechanical properties of graphene-based nanosheets enable the formation of robust and high-performance 3DMs such as sponges, membranes, beads and fibres. However, little is known about the relationship between the materials properties of graphene-based 3DMs and their environmental performance. In this Review, we summarize the self-assembly and environmental applications of graphene-based 3DMs in removing contaminants from water and air. We also develop the critical link between the materials properties of 3DMs and their environmental performance, and identify the key parameters that influence their capacities for contaminant removal.

Current technologies for water purification are limited by their contaminant-specific removal capability, requiring multiple processes to meet water quality objectives. Here we show an innovative biomimetic micellar nanocoagulant that imitates the structure of Actinia, a marine predator that uses its tentacles to ensnare food, for the removal of an array of water contaminants with a single treatment step. The Actinia-like micellar nanocoagulant has a core-shell structure and readily disperses in water while maintaining a high stability against aggregation. To achieve effective coagulation, the nanocoagulant everts its configuration, similar to Actinia. The shell hydrolyses into 'flocs' and destabilizes and enmeshes colloidal particles while the core is exposed to water, like the extended tentacles of Actinia, and adsorbs the dissolved contaminants. The technology, with its ability to remove a broad spectrum of contaminants and produce high-quality water, has the potential to be a cost-effective replacement for current water treatment processes.

Sustainable provision of safe, clean and adequate water supply is a global challenge. Water treatment and desalination technologies remain chemically and energy intensive, ineffective in removing key trace contaminants, and poorly suited to deployment in decentralized (distributed) water treatment systems globally. Several recent efforts have sought to leverage the reactive and tunable properties of nanomaterials to address these technological shortcomings. This Review assesses the potential applications of nanomaterials in advancing sustainable water treatment systems and proposes ways to evaluate the environmental risks and social acceptance of nanotechnology-enabled water treatment processes. Future areas of research necessary to realize safe deployment of promising nanomaterial applications are also identified. Despite recent technological progress, providing safe, clean and sufficient water sustainably for all remains challenging. This Review assesses the potential applications of nanomaterials in advancing the sustainability of water treatment systems, and their associated barriers.

Membrane desalination is a leading technology for treating saline waters to augment fresh water supply. The need for high-performance desalination membranes, particularly with high water/salt selectivity, has stimulated research into the fundamental structure-property-performance relationship of state-of-the-art membranes. In this study, we utilize a facile method for tuning properties of a polymeric desalination membrane to shed light on water and salt transport mechanisms of such membranes. A desalination membrane made of cellulose triacetate is treated in a plasticizer solution, followed by water rinsing. The modified membranes exhibit reduced salt flux without compromising water flux, indicating enhanced water/salt selectivity. An inspection of material characteristics using a model film system reveals a plasticizing-extracting process in changing the polymeric structure, which leads to the reduction of crystallite size in the polymer matrix, consequently affecting the transport properties of the membranes. Our findings highlight the potential of the plasticizing-extracting process in fabricating membranes with desired desalination performance., Increasing the selectivity of desalination membranes is of crucial importance for advancing membrane desalination technologies. Here the authors demonstrate a plasticizing-extracting process for tuning the permselectivity of polymeric membranes and achieving desired desalination performance.

Applying high-potential alternating current to a carbon-nanotube–polymer composite film provides a self-heating membrane that enhances desalination performance of high-salinity brines by membrane distillation.

Low-grade heat from sources below 100 ∘C offers a vast quantity of energy. The ability to extract this energy, however, is limited with existing technologies as they are not well-suited to harvest energy from sources with variable heat output or with a small temperature difference between the source and the environment. Here, we present a process for extracting energy from low-grade heat sources utilizing hydrophobic, nanoporous membranes that trap air within their pores when submerged in a liquid. By driving a thermo-osmotic vapour flux across the membrane from a hot reservoir to a pressurized cold reservoir, heat energy can be converted to mechanical work. We demonstrate operation of air-trapping membranes under hydraulic pressures up to 13 bar, show that power densities as high as 3.53 ± 0.29 W m−2 are achievable with a 60 ∘C heat source and a 20 ∘C heat sink, and estimate the efficiency of a full-scale system. The results demonstrate a promising process to harvest energy from low-temperature differences (

Membranes have an increasingly important role in alleviating water scarcity and the pollution of aquatic environments. Promising molecular-level design approaches are reviewed for membrane materials, focusing on how these materials address the urgent requirements of water treatment applications.

Water has always been crucial to combustion and hydroelectric processes, but it could become the source of power in membrane-based systems that capture energy from natural and waste waters. Two processes are emerging as sustainable methods for capturing energy from sea water: pressure-retarded osmosis and reverse electrodialysis. These processes can also capture energy from waste heat by generating artificial salinity gradients using synthetic solutions, such as thermolytic salts. A further source of energy comes from organic matter in waste waters, which can be harnessed using microbial fuel-cell technology, allowing both wastewater treatment and power production.