Molecular layer-by-layer assembled thin-film composite membranes for water desalination

Tailored design of nanofiltration membranes for water treatment based on synthesis-property-performance relationships

Tailored design of high-performance nanofiltration (NF) membranes is desirable because the requirements for membrane performance, particularly ion/salt rejection and selectivity, differ among the various applications of NF technology ranging from drinking water production to resource mining. However, this customization greatly relies on a comprehensive understanding of the influence of membrane fabrication methods and conditions on membrane properties and the relationships between the membrane structural and physicochemical properties and membrane performance. Since the inception of NF, much progress has been made in forming the foundation of tailored design of NF membranes and the underlying governing principles. This progress includes theories regarding NF mass transfer and solute rejection, further exploitation of the classical interfacial polymerization technique, and development of novel materials and membrane fabrication methods. In this critical review, we first summarize the progress made in controllable design of NF membrane properties in recent years from the perspective of optimizing interfacial polymerization techniques and adopting new manufacturing processes and materials. We then discuss the property-performance relationships based on solvent/solute mass transfer theories and mathematical models, and draw conclusions on membrane structural and physicochemical parameter regulation by modifying the fabrication process to improve membrane separation performance. Next, existing and potential applications of these NF membranes in water treatment processes are systematically discussed according to the different separation requirements. Finally, we point out the prospects and challenges of tailored design of NF membranes for water treatment applications. This review bridges the long-existing gaps between the pressing demand for suitable NF membranes from the industrial community and the surge of publications by the scientific community in recent years.

Comparative Degradation Kinetics Study of Polyamide Thin Films in Aqueous Solutions of Chlorine and Peracetic Acid Using Quartz Crystal Microbalance

Polyamide thin film composite membranes are widely used in water reclamation. Peracetic acid (PAA) is an emerging wastewater disinfectant with a potential for membrane cleaning and disinfection; however, its interaction with polyamide remains poorly understood. This study employs quartz crystal microbalance with dissipation (QCM-D) to determine the PAA-induced degradation kinetics of polyamide thin films, in comparison with the conventional disinfectant-free chlorine (HOCl). Polyamide films showed a sorption phase followed by a degradation phase when exposed to PAA (1000 mg L^-1) and HOCl (100 mg L^-1) solutions. While the sorption phase in HOCl experiments was short (1.4-3.5 min) and followed a Boltzmann-sigmoidal model, it spanned over 3-33 h in PAA experiments and displayed a two-stage behavior. The latter kinetics are attributed to sequential processes of the physical sorption of PAA in polyamide films followed by PAA-induced polyamide oxidation. In the degradation phase, the HOCl-exposed films followed a rapid, two-step exponential decay reaching an equilibrium mass of ∼50% of the initial (wet) mass after ∼5 h of exposure. In contrast, the PAA-exposed films followed a Boltzmann-sigmoidal decay, with ∼80% of the initial (wet) mass remaining intact after >10 h of exposure. Fast force maps generated using atomic force microscopy showed a progressive increase in the morphological heterogeneity of the polyamide films in HOCl solution due to pitting, cracking, bulging, and eventual delamination under both flow and no-flow conditions. In contrast, PAA only formed small pits on the polyamide film under flow; in a stagnant PAA solution, the film had no visible changes even after ∼148 h of exposure. This is the first comparative study on the chemical and morphological changes in polyamide films induced by PAA and HOCl. The much higher compatibility of polyamide with PAA than with chlorine supports the potential of PAA being used as a halogen-free membrane cleaning/disinfecting agent.

Non-destructive determination of functionalized polyelectrolyte placement in layer-by-layer films by IR ellipsometry

Layer-by-layer (LbL) assembly facilitates controlled coatings on a variety of surfaces with the ability to manipulate the composition through the thickness by selection of the complementary pairs. However, the characterization of these composition profiles tends to be destructive and requires significant compositional differences that can limit their utility. Here, we demonstrate the ability to non-destructively quantify the depth dependence of the allyl content associated with the selective incorporation of poly(sodium acrylate-co-allylacrylamide) (84 : 16 mol : mol) (allyl-PAA) in LbL films based on the assembly of poly(diallyldimethylammonium chloride) (PDAC)/poly(acrylic acid) (PAA) and PDAC/allyl-PAA. Although the atomic composition of the film is not dramatically influenced by the change between PAA and allyl-PAA, the absorption in the IR near 1645 cm^-1 by the allyl group provides sufficient optical contrast to distinguish the LbL components with spectroscopic ellipsometry. The use of IR spectroscopic ellipsometry can determine the thickness of layers that contain allyl-PAA and also gradients that develop due to re-arrangements during the LbL process. With multiple films fabricated simultaneously, the location of the gradient between the 1st and 2nd series of multilayers (e.g., first PDAC/PAA bilayers and then PDAC/allyl-PAA bilayers) can be readily assessed. The results from a variety of different multilayer architectures indicate that the gradient is located within the thickness expected for the 1st deposited bilayer stack (PDAC/PAA or PDAC/allyl-PAA). These results are indicative of a dynamic dissolution-deposition process (in- and out- diffusion) during the fabrication of these LbL films. These results provide additional evidence into the mechanisms for exponential growth in LbL assemblies. The ability to quantify a gradient with the low contrast system examined indicates that spectroscopic IR ellipsometry should be able to non-destructively determine compositional gradients for most polymer films where such gradients exist.

Facile Modification of NF Membrane by Multi-Layer Deposition of Polyelectrolytes for Enhanced Fouling Resistance

Fouling not only deteriorates the membrane structure but also compromises the quality of the permeate and has deleterious consequences on the membrane operation. In the current study, a commercial thin film composite nanofiltration membrane (NF90) was modified by sequentially depositing oppositely charged polycation (poly(allylamine hydrochloride)) and polyanion (poly(acrylic acid)) polyelectrolytes using the layer-by-layer assembly method. The water contact angle was decreased by ~10° after the coating process, indicating increased hydrophilicity. The surface roughness of the prepared membranes decreased from 380 nm (M-0) to 306 nm (M-10) and 366 nm (M-20). M-10 membrane showed the highest permeate flux of 120 L m-2 h-1 with a salt rejection of >98% for MgSO4 and NaCl. The fabricated membranes M-20 and M-30 showed 15% improvement in fouling resistance and maintained the initial permeate flux longer than the pristine membrane.

Stitch and copolymerization of thin-film composite membranes to enhance hydrophilicity and organics resistance for the separation of glycerol-based wastewater

Many industries produce large amounts of glycerol-based wastewater, which always contains hazardous organic chlorides. Compared with complicated biological treatments or physical adsorption, membrane separation decreases the cost and saves energy. Strong swelling of traditional thin-film composite (TFC) membranes influence the performance in the separation of organic molecules. Here we prepared TFC membranes with an acrylamide-grafted PAN support layer to copolymerize with m-phenylenediamine (MPD) and trimesoyl chloride (TMC). The link of separative layer and support layer was created like a zipper stitching to enhance the stability and resistance for the removal of organic molecules. An aquatic grass-like layer of acrylamide enlarges the surface area and hydrophilicity with superior separation performances (15.8 LMH bar^-1 flux, 72.0% rejection of dichloropropanol (DCP) and 64.6% rejection of glycerol (Gl)). The trade-off upper bound was improved to a high level. We also established the simulations of evaporation using Aspen Plus and mathematical models of reverse osmosis to calculate the energy consumption corresponding to the recycle of glycerol-based wastewater. The experimental and theoretical results illustrate the advantages of acrylamide-grafted TFC membranes in the ap-plications to concentrate organic solutes and treat wastewater.

Fabrication of desalination membranes by interfacial polymerization: history, current efforts, and future directions

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

Thin-Film Composite Nanofiltration Membranes for Non-Polar Solvents

Organic solvent nanofiltration (OSN) has been recognized as an eco-friendly separation system owing to its excellent cost and energy saving efficiency, easy scale-up in the narrow area and mild operation conditions. Membrane properties are the key part in terms of determining the separation efficiency in the OSN system. In this review paper, the recently reported OSN thin-film composite (TFC) membranes were investigated to understand insight of membrane materials and performance. Especially, we highlighted the representative study concepts and materials of the selective layer of OSN TFC membranes for non-polar solvents. The proper choice of monomers and additives for the selective layer forms much more interconnected voids and the enhanced microporosity, which can improve membrane performance of the OSN TFC membrane with reducing the transport resistance. Therefore, this review paper could be an important bridge to connect with the next-generation OSN TFC membranes for non-polar solvents.

Thickness-Dependent Permeance of Molecular Layer-By-Layer Polyamide Membranes

We present the thickness-dependent permeance of highly cross-linked polyamide (PA) membranes formed by a molecular layer-by-layer (mLbL) deposition process. The deposition allows for the synthesis of extremely smooth, uniform PA films of tunable thickness, which is counter to the less controlled interfacial polymerization process used commercially. The ability to control and measure the membrane thickness allows us to elucidate the relationships among network structure, transport properties, and separation performance. In this work, a series of large-area mLbL PA membranes is prepared with thickness ranging from less than 5 nm to greater than 100 nm, which can be transferred defect-free via a film floating technique onto a macroporous support layer and challenged with salt solutions. A critical thickness of 15 nm is identified for efficient desalination, and water permeance is described using a multi-layer solution diffusion model that allows for the extraction of material properties relevant to transport. Finally, the model demonstrates the existence of two distinct layers in the mLbL films, one layer comprised of a (5 to 10) nm graded or less cross-linked layer at the surface and a more densely cross-linked layer in the interior of the film. This graded layer appears inherent to the mLbL deposition process and is observed at all film thicknesses.

Highly Permeable Reverse Osmosis Membranes via Molecular Layer-by-Layer Deposition of Trimesoyl Chloride and 3,5-Diaminobenzoic Acid

We present a series of polyamide membranes synthesized via molecular layer-by-layer (mLbL) deposition of trimesoyl chloride (TMC) and 3,5-diaminobenzoic acid (BA). These membranes exhibit superior NaCl rejection compared to previously reported TMC-BA membranes prepared via interfacial polymerization, with the improved performance of the mLbL films attributable to higher cross-link density facilitated by the stepwise deposition process in good solvents. We compare the TMC-BA series with membranes synthesized from TMC and m-phenylenediamine (MPD), a conventional reverse osmosis membrane chemistry. At the minimum thickness capable of 90 % NaCl rejection, mLbL TMC-BA membranes exhibit 50 % greater water permeance than mLbL TMC-MPD.

Unveiling the Molecular Mechanisms of Thickness-Dependent Water Dynamics in an Ultrathin Free-Standing Polyamide Membrane

Aromatic polyamide (PA) membranes fabricated from interfacial polymerization are widely used for desalination and water treatment. The fabrication of the high-flux PA membrane requires a fundamental understanding of the molecular mechanisms of water dynamics in the PA, which is still obscure due to the limited experimental methods. Herein, molecular dynamics (MD) simulations were employed to establish an atomic model of ultrathin free-standing PA membranes with various thickness and to explore the thickness-dependent dynamics of water molecules in the PA membrane. Simulation results illustrate that the simulated PA membrane has an average pore radius of 3 Å similar to the free volume size of the experimental PA membrane measured by PALS. The PA could be identified as the swelling layer (SL) and the confined layer (CL) based on their water diffusion rates. The diffusivity of water in the confined layer of PA membrane was much lower than that in the swelling layer and thus determined the water flux of the PA membrane. The water diffusivity in the sub-8 nm PA membrane is greatly enhanced due to a very thin confined layer thickness, illustrating the mechanism of the experimentally fabricated sub-8 nm PA membrane having the dramatically enhanced water permeability. Furthermore, results show that water molecules tend to transport rapidly in the free space inside the PA membrane. Our results provide some insights into the thickness-dependent water dynamics in the PA on a molecular level and may help to design the next generation of high-flux PA membranes.

A Critical Review on Thin-Film Nanocomposite Membranes with Interlayered Structure: Mechanisms, Recent Developments, and Environmental Applications

The separation properties of polyamide reverse osmosis and nanofiltration membranes, widely applied for desalination and water reuse, are constrained by the permeability-selectivity upper bound. Although thin-film nanocomposite (TFN) membranes incorporating nanomaterials exhibit enhanced water permeance, their rejection is only moderately improved or even impaired due to agglomeration of nanomaterials and formation of defects. A novel type of TFN membranes featuring an interlayer of nanomaterials (TFNi) has emerged in recent years. These novel TFNi membranes show extraordinary improvement in water flux (e.g., up to an order of magnitude enhancement) along with better selectivity. Such enhancements can be achieved by a wide selection of nanomaterials, ranging from nanoparticles, one-/two-dimensional materials, to interfacial coatings. The use of nanostructured interlayers not only improves the formation of polyamide rejection layers but also provides an optimized water transport path, which enables TFNi membranes to potentially overcome the longstanding trade-off between membrane permeability and selectivity. Furthermore, TFNi membranes can potentially enhance the removal of heavy metals and micropollutants, which is critical for many environmental applications. This review critically examines the recent developments of TFNi membranes and discusses the underlying mechanisms and design criteria. Their potential environmental applications are also highlighted.

Asymmetric polyamide nanofilms with highly ordered nanovoids for water purification

Tailor-made structure and morphology are critical to the highly permeable and selective polyamide membranes used for water purification. Here we report an asymmetric polyamide nanofilm having a two-layer structure, in which the lower is a spherical polyamide dendrimer porous layer, and the upper is a polyamide dense layer with highly ordered nanovoids structure. The dendrimer porous layer was covalently assembled in situ on the surface of the polysulfone (PSF) support by a diazotization-coupling reaction, and then the asymmetric polyamide nanofilm with highly ordered hollow nanostrips structure was formed by interfacial polymerization (IP) thereon. Tuning the number of the spherical dendrimer porous layers and IP time enabled control of the nanostrips morphology in the polyamide nanofilm. The asymmetric polyamide membrane exhibits a water flux of 3.7−4.3 times that of the traditional monolayer polyamide membrane, showing an improved divalent salt rejection rate (more than 99%), which thus surpasses the upper bound line of the permeability−selectivity performance of the existing various structural polyamide membranes. We estimate that this work might inspire the preparation of highly permeable and selective reverse osmosis (RO), organic solvent nanofiltration (OSNF) and pervaporation (PV) membranes.

Structure and morphology are critical to the performance of permeable and selective polyamide membranes in water purification. Here, the authors report a two layer asymmetric polyamide nanofilm in which a spherical polyamide dendrimer porous lower and a polyamide dense upper layer form hierarchical nanovoids.

Can Composite Janus Membranes with an Ultrathin Dense Hydrophilic Layer Resist Wetting in Membrane Distillation?

Tackling membrane wetting is an ongoing challenge for large-scale applications of membrane distillation (MD). Herein, composite Janus MD membranes comprising an ultrathin dense hydrophilic layer are developed by layer-by-layer assembling cationic polyethyleneimine and anionic poly(sodium 4-styrenesulfonate) polyelectrolytes on a hydrophobic polyvinylidene fluoride substrate. Using surfactant-containing saline water as the feed with low surface tension, experiments reveal that the number of polyelectrolyte layers, rather than surface wettability or surface charge, determines the anti-wetting performance of the composite Janus membranes. More deposited layers yield higher wetting resistance. With the aid of positron annihilation spectroscopy, this study, for the first time, demonstrates the origin of the excellent wetting resistance of the composite Janus membranes. The effective pore size of the polyelectrolyte multilayer decreases with an increase in the number of the deposited layer. The membrane with an ultrathin hydrophilic multilayer of 48 nm has a sufficiently small pore size to sieve out surfactant molecules from the feed solution via a size exclusion mechanism, thus protecting the hydrophobic substrate from being wetted by the low-surface-tension feed water. This study may pave the way for developing next-generation anti-wetting Janus membranes for robust membrane distillation.

Applications of tannic acid in membrane technologies: A review

Today, membrane technologies play a big role in chemical industry, especially in separation engineering. Tannic acid, one of the most famous polyphenols, has attracted widespread interest in membrane society. In the past several years, researches on the applications of tannic acid in membrane technologies have grown rapidly. However, there has been lack of a comprehensive review for now. Here, we summarize the recent developments in this field for the first time. We comb the history of tannic acid and introduce the properties of tannic acid firstly, and then we turn our focus onto the applications of membrane surface modification, interlayers and selective layers construction and mixed matrix membrane development. In those previous works, tannic acid has been demonstrated to be capable of making a great contribution to the membrane science and technology. Especially in membrane surface/interface engineering (such as the construction of superhydrophilic and antifouling surfaces and polymer/nanoparticle interfaces with high compatibility) and development of thin film composite membranes with high permselectivity (such as developing thin film composite membranes with ultrahigh flux and high rejection), tannic acid can play a positive and great role. Despite this, there are still many critical challenges lying ahead. We believe that more exciting progress will be made in addressing these challenges in the future.

Ultrafast Ion Sieving from Honeycomb-like Polyamide Membranes Formed Using Porous Protein Assemblies

Despite the commercial success of thin film composite polyamide membranes, further improvements to the water permeation of polyamide membranes without degradation in product water quality remain a great challenge. Herein, we report the fabrication of an interfacially polymerized polyamide nanofiltration membrane with a novel 3D honeycomb-like spatial structure, which is formed from a tobacco mosaic virus (TMV) porous protein nanosheet-coated microfiltration membrane support. TMV nanosheets with uniform pores and appropriate hydrophilicity deposited inside the support membrane pores facilitate the construction of a localized water-oil reaction interface with evenly distributed monomers and guide the formation of a defect-free polyamide layer with a spatial structure that copies the geometry of the membrane cavities. Such a 3D morphology possesses ultrahigh specific surface area, leading to unprecedented membrane water permeance as high as 84 L m^-2 h^-1 bar^-1, high MgSO₄ rejection of 98%, and monovalent/divalent ion sieving selectivity up to 89.

Recent Development in Composite Membranes for Flow Batteries

Flow batteries (FBs) are one of the most attractive candidates for stationary energy storage and vital in realizing the wide application of renewable energies. Membranes play an important role in isolating redox couples while transporting ions to close the internal electrical circuit. Therefore, membranes with high selectivity and conductivity are highly important. Among different membranes, a composite membrane with independent design of support layer and thin selective top layer becomes one of the most promising candidates to break the trade-off between selectivity and conductivity. In this Review, recent studies on composite membranes for FBs and the principles of membrane design in different systems are discussed and summarized. Finally, the future direction on membrane design for different FBs is presented, which will provide an extensive, comprehensive reference to design and construct high-performance composite membranes for FBs.

Water-polyamide chemical interplay in desalination membranes explored by ambient pressure X-ray photoelectron spectroscopy

Reverse osmosis using aromatic polyamide membranes is currently the most important technology for seawater desalination. The performance of reverse osmosis membranes is highly dependent on the interplay of their surface chemical groups with water and water contaminants. In order to better understand the underlying mechanisms of these membranes, we study ultrathin polyamide films that chemically resemble reverse osmosis membranes, using ambient pressure X-ray photoelectron spectroscopy. This technique can identify the functional groups at the membrane-water interface and allows monitoring of small shifts in the electron binding energy that indicate interaction with water. We observe deprotonation of free acid groups and formation of a 'water complex' with nitrogen groups in the polymer upon exposure of the membrane to water vapour. The chemical changes are reversed when water is removed from the membrane. While the correlation between functional groups and water uptake is an established one, this experiment serves to understand the nature of their chemical interaction, and opens up possibilities for tailoring future materials to specific requirements.

Dual Monitoring of Surface Reactions in Real Time by Combined Surface-Plasmon Resonance and Field-Effect Transistor Interrogation

By combining surface plasmon resonance (SPR) and electrolyte gated field-effect transistor (EG-FET) methods in a single analytical device we introduce a novel tool for surface investigations, enabling simultaneous measurements of the surface mass and charge density changes in real time. This is realized using a gold sensor surface that simultaneously serves as a gate electrode of the EG-FET and as the SPR active interface. This novel platform has the potential to provide new insights into (bio)adsorption processes on planar solid surfaces by directly relating complementary measurement principles based on (i) detuning of SPR as a result of the modification of the interfacial refractive index profile by surface adsorption processes and (ii) change of output current as a result of the emanating effective gate voltage modulations. Furthermore, combination of the two complementary sensing concepts allows for the comparison and respective validation of both analytical techniques. A theoretical model is derived describing the mass uptake and evolution of surface charge density during polyelectrolyte multilayer formation. We demonstrate the potential of this combined platform through the observation of layer-by-layer assembly of PDADMAC and PSS. These simultaneous label-free and real-time measurements allow new insights into complex processes at the solid-liquid interface (like non-Fickian ion diffusion), which are beyond the scope of each individual tool.

Clean Water through Nanotechnology: Needs, Gaps, and Fulfillment

Sustainable nanotechnology has made substantial contributions in providing contaminant-free water to humanity. In this Review, we present the compelling need for providing access to clean water through nanotechnology-enabled solutions and the large disparities in ensuring their implementation. We also discuss the current nanotechnology frontiers in diverse areas of the clean water space with an emphasis on applications in the field and provide suggestions for future research. Extending the vision of sustainable and affordable clean water to environment in general, we note that cities can live and breathe well by adopting such technologies. By understanding the global environmental challenges and exploring remedies from emerging nanotechnologies, sustainability in clean water can be realized. We suggest specific pointers and quantify the impact of such technologies.