Understanding the pH-responsive behavior of graphene oxide membrane in removing ions and organic micropollulants

Functionalized 2D membranes for separations at the 1-nm scale

The ongoing evolution of two-dimensional (2D) material-based membranes has prompted the realization of mass separations at the 1-nm scale due to their well-defined selective nano- and subnanochannels. Strategic membrane functionalization is further found to be key to augmenting channel accuracy and efficiency in distinguishing ions, gases and molecules within this range and is thus trending as a research focus in energy-, resource-, environment- and pharmaceutical-related applications. In this review, we present the fundamentals underpinning functionalized 2D membranes in various separations, elucidating the critical "method-interaction-property" relationship. Starting with an introduction to various functionalization strategies, we focus our discussion on functionalization-induced channel-species interactions and reveal how they shape the transport- and operation-related features of the membrane in different scenarios. We also highlight the limitations and challenges of current functionalized 2D membranes and outline the necessary breakthroughs needed to apply them as reliable and high-performance separation units across industries in the future.

Adsorption behavior of triclosan on polystyrene nanoplastics: The roles of particle size, surface functionalization, and environmental factors

Nanoplastics (NPs) contribute substantially to the transport of waterborne pollutants. Triclosan (TCS) has a high potential to contact with NPs because of their prevalence in natural waters. Herein, this study investigated the adsorption behavior of TCS on differently sized and functionalized polystyrene (PS) NPs. The effects of environmental factors such as pH, salinity, and dissolved organic matter (DOM) were also evaluated. Results suggest that the adsorption equilibrium constant (kd) of TCS in pristine PSNP suspensions followed the order as: PSNPs-50 nm (4.39 L·g-1) > PSNPs-100 nm (2.78 L·g-1) > PSNPs-200 nm (2.59 L·g-1) > PSNPs-500 nm (1.36 L·g-1) ≈ PSNPs-900 nm (1.36 L·g-1). For the functionalized PSNPs (i.e., PSNPs-COOH, PSNPs-NH2), the values of specific surface area normalized kd called kd, SSA were higher than those of pristine PSNPs. Meanwhile, TCS adsorption on two functionalized PSNPs remained stable and then decreased as salinity increased, while an opposite trend was observed toward pristine PSNPs. All these suggested that physicochemical properties of PSNPs (e.g., particle size and surface functional groups) are important factors influencing their adsorption capacity. When the solution pH raised, the adsorbed amounts of TCS on all tested PSNPs prone to decline. However, DOM only affected the adsorption behavior of PSNPs-50 nm, probably owing to its aggregation with tiny PSNPs and the induced secondary adsorption.

Extreme Ion-Transport Inorganic 2D Membranes for Nanofluidic Applications

Inorganic 2D materials offer a new approach to controlling mass diffusion at the nanoscale. Controlling ion transport in nanofluidics is key to energy conversion, energy storage, water purification, and numerous other applications wherein persistent challenges for efficient separation must be addressed. The recent development of 2D membranes in the emerging field of energy harvesting, water desalination, and proton/Li-ion production in the context of green energy and environmental technology is herein discussed. The fundamental mechanisms, 2D membrane fabrication, and challenges toward practical applications are highlighted. Finally, the fundamental issues of thermodynamics and kinetics are outlined along with potential membrane designs that must be resolved to bridge the gap between lab-scale experiments and production levels.

A Facile Way to Fabricate GO-EDA/Al2O3 Tubular Nanofiltration Membranes with Enhanced Desalination Stability via Fine-Tuning the pH of the Membrane-Forming Suspensions

Pristine graphene oxide (GO)-based membranes have proven promising for molecular and ion separation owing to efficient molecular transport nanochannels, but their separation ability in an aqueous environment is limited by the natural swelling tendency of GO. To obtain a novel membrane with anti-swelling behavior and remarkable desalination capability, we used the Al₂O₃ tubular membrane with an average pore size of 20 nm as the substrate and fabricated several GO nanofiltration ceramic membranes with different interlayer structures and surface charges by fine-tuning the pH of the GO-EDA membrane-forming suspension (pH = 7, 9, 11). The resultant membranes could maintain desalination stability, whether immersed in water for 680 h or operated under a high-pressure environment. When the pH of the membrane-forming suspension was 11, the prepared GE-11 membrane showed a rejection of 91.5% (measured at 5 bar) towards 1 mM Na₂SO₄ after soaking in water for 680 h. An increase in the transmembrane pressure to 20 bar resulted in an increase in the rejection towards the 1 mM Na₂SO₄ solution to 96.3%, and an increase in the permeance to 3.7 L·m^-2·h^-1·bar^-1. The proposed strategy in varying charge repulsion is beneficial to the future development of GO-derived nanofiltration ceramic membranes.

Tuneable molecular selective boron nitride nanosheet ultrafiltration lamellar membrane for dye exclusion to remediate the environment

Smart tuning of the membrane's porous nanostructures offers an effective strategy for creating state-of-the-art, high-performance separation membranes. In aqueous solution, polyethylene glycol (PEG) grafted boron nitride PEG_X-g-(f-BN) nanosheets exhibit high permeance and excellent molecular sieving. The molecular selectivity of the PEG_X-g-(f-BN) lamellar membrane is controlled by the nanopores, which can be tuned by modulating the interplanar spacing between the nanosheets. Herein, the interplanar spacing of h-BN nanosheets is enhanced in the range of 0.334-0.348 nm through grafting different molecular weight PEG. Moreover, the grafted PEG instigates a synergistic effect on the nanosheets in two ways. Firstly, through PEG intercalation, the interlayer spacing of the (002) plane could be adjusted without significant deterioration to the hexagonal crystallographic structure. Secondly, intercalated PEG in BN nanosheets reflects in terms of improved h-BN wettability through transformation to hydrophilic surface characteristics (small contact angle of 36-39°). The fabricated PEG_X-g-(f-BN) lamellar membrane acquires stable and interconnected nanopores and nanochannels with an average pore diameter of 1.36-2.19 nm. Permeance-exclusion trade-off manipulation through methodical approaches of PEG_X-g-(f-BN) decoration thickness and interplanar spacing is exploited to build a better understanding of water transport behavior. PEG_X-g-(f-BN) lamellar membranes show unprecedented permeance of ∼1253 L m^-2 h^-1 bar^-1 with a steady methyl blue (MB) exclusion of 98.9% even in different pH conditions.

Rejection of trace organic compounds by membrane processes: mechanisms, challenges, and opportunities

This work critically reviews the application of various membrane separation processes (MSPs) in treating water polluted with trace organic compounds (TOrCs) paying attention to nanofiltration (NF), reverse osmosis (RO), membrane bioreactor (MBR), forward osmosis (FO), and membrane distillation (MD). Furthermore, the focus is on loopholes that exist when investigating mechanisms through which membranes reject/retain TOrCs, with the emphasis on the characteristics of the model TOrCs which would facilitate the identification of all the potential mechanisms of rejection. An explanation is also given as to why it is important to investigate rejection using real water samples, especially when aiming for industrial application of membranes with novel materials. MSPs such as NF and RO are prone to fouling which often leads to lower permeate flux and solute rejection, presumably due to cake-enhanced concentration polarisation (CECP) effects. This review demonstrates why CECP effects are not always the reason behind the observed decline in the rejection of TOrCs by fouled membranes. To mitigate for fouling, researchers have often modified the membrane surfaces by incorporating nanoparticles. This review also attempts to explain why nano-engineered membranes have not seen a breakthrough at industrial scale. Finally, insight is provided into the possibility of harnessing solar and wind energy to drive energy intensive MSPs. Focus is also paid into how low-grade energy could be stored and applied to recover diluted draw solutions in FO mode.

Recent progress on fabrication methods of graphene-based membranes for water purification, gas separation, and energy sustainability

Graphene-based layered materials have got significant interest in membrane technology for water desalination, gas separation, organic nanofiltration, pervaporation, proton exchange applications, etc. and show remarkable results. Up to date, various methods have been developed for fabrication of high performance membrane. Most of them are only suitable for research purposes, but not appropriate for mass transport barrier and membrane applications that require large-area synthesis. In this comprehensive review, we summarized the current synthesis and fabrication methods of graphene-based membranes. Emphasis will be given on fabrication of both graphene-based nanoporous and lamellar membranes. Finally, we discuss the current engineering hurdles and future research directions yet to be explored for fabrication of such membranes.

D-spacing controllable GO membrane intercalated by sodium tetraborate pentahydrate for dye contamination wastewater treatment

Sodium tetraborate pentahydrate (STB) was intercalated into graphene oxide (GO) nanosheets to form a nanocomposite (STB@GO). Subsequently, it was self-assembled on a substrate membrane to prepare STB@GO nanofiltration membrane. The properties of the STB@GO powder samples and the nanofiltration membrane were studied using scanning electron microscopy (SEM), Fourier transform infrared spectroscopy (FTIR), X-ray photoelectron spectroscopy (XPS), X-ray diffraction (XRD), contact angle (CA), and zeta potential. When the STB concentration was 1.0 g/L in the cross-linking reaction, the membrane was described as the STB₂@GO membrane and exhibited a large interlayer space (d-spacing = 1.347 nm), high hydrophilicity (CA = 22.2°), and high negative potential (zeta = -18.0 mV). Meanwhile, the pure water flux of the membrane was significantly increased by 56.60% than that of the GO membrane. In addition, the STB₂@GO membrane exhibited a favorable capability for dye rejection,98.52% for Evans blue (EB), 99.26% for Victoria blue B (VB), 91.94% for Alizarin yellow (AY), and 93.21% for Neutral red (NR). Furthermore, the STB₂@GO membrane performed better in dye separation under various types and concentrations of dye, pH values, and ions in solution. Thus, this study provides a promising method for preparing laminated GO nanofiltration membranes for dye wastewater treatment.

Ultrafast and Selective Nanofiltration Enabled by Graphene Oxide Membranes with Unzipped Carbon Nanotube Networks

Carbon nanomaterials have proven their wide applicability in molecular separation and water purification techniques. Here, an unzipped carbon nanotubes (CNT) embedded graphene oxide (GO) membrane (uCNTm) is reported. The multiwalled CNTs were longitudinally cut into multilayer graphene oxide nanoribbons by a modified Hummer method. To investigate the varying effects of different bandwidths of unzipped CNTs on their properties, four uCNTms were prepared by a vacuum-assisted filtration process. Unzipped-CNTs with different bandwidths were made by unzipping multiwalled CNTs with outer diameters of 0-10, 10-20, 20-30, and 30-50 nm and named uCNTm-1, uCNTm-2, uCNTm-3, and uCNTm-4, respectively. The uCNTms exhibited good stability in different pH solutions, and the water permeability of the composite membranes showed an increasing trend with the increase of the inserted uCNTm's bandwidth up to 107 L·m^-2·h^-1·bar^-1, which was more than 10 times greater than that of pure GO membranes. The composite membranes showed decent dye screening performance with the rejection rate of methylene blue and rhodamine B both greater than 99%.

A porphyrin-based metal-organic framework with highly efficient adsorption and photocatalytic degradation of organic dyes

Organic dye pollution has become an urgent issue due to their toxicity to humans and potential for damage to the environment. However, achieving highly efficient adsorption and degradation materials for...

Tunable Graphene Oxide Nanofiltration Membrane for Effective Dye/Salt Separation and Desalination

Effective dye separation and desalination are critical for the treatment of highly saline textile wastewater with dye mixtures. In this study, a graphene oxide (GO) membrane with a tunable interlayer distance (d) was fabricated to generate clean water via two-stage filtration, namely, the dye/salt separation and desalination stages. In the first stage, under low pressure (e.g., 0.3 MPa), the membrane with a d value of ca. 7.60 Å was suitable for removing the dye from the saline wastewater. The dye and salt (i.e., Na₂SO₄) rejection rates of >99% and <6.5% were achieved, respectively, indicating the significant potential to recycle the dyes from the highly saline dye wastewater. In the second stage, under a higher pressure (e.g., 0.8 MPa), the d value was reduced to ca. 7.15 Å, bestowing the membrane with a desalination function. The desalination rate of a single filtration process could reach up to 51.8% from 1.0 g/L saline (i.e., Na₂SO₄) water. The as-prepared membrane also exhibited excellent practical advantages, including ultrahigh permeability, significant antifouling (against dye) performance, and excellent stability. Furthermore, with the stacking of multistage filtration systems, the proposed membrane technology will be capable of regenerating dye and producing clean water.

Planning of smart gating membranes for water treatment

The use of membranes in desalination and water treatment has been intensively studied in recent years. The conventional membranes however have various problems such as uncontrollable pore size and membrane properties, which prevents membranes from quickly responding to alteration of operating and environmental conditions. As a result the membranes are fouled, and their separation performance is lowered. The preparation of smart gating membranes inspired by cell membranes is a new method to face these challenges. Introducing stimuli-responsive functional materials into traditional porous membranes and use of hydrogels and microgels can change surface properties and membrane pore sizes under different conditions. This review shows potential of smart gating membranes in water treatment. Various types of stimuli-response such as those of thermo-, pH-, ion-, molecule-, UV light-, magnetic-, redox- and electro-responsive gating membranes along with various gel types such as those of polyelectrolyte, PNIPAM-based, self-healing hydrogels and microgel based-smart gating membranes are discussed. Design strategies, separation mechanisms and challenges in fabrication of smart gating membranes in water treatment are also presented. It is demonstrated that experimental and modeling and simulation results have to be utilized effectively to produce smart gating membranes.

Advanced graphene oxide-based membranes as a potential alternative for dyes removal: A review

Graphene oxide (GO) is one of the most well-known graphene derivatives which, due to its outstanding chemical, electrical and optical properties as well as its high oxygen content, has been recently applied in several fields such as in the construction of sensors, as antimicrobial agent for biomedical applications, as well as nanofiller material for membranes applied in wastewater treatment. In this last-mentioned field, the synthesis and functionalization of membranes with GO has proven to improve the performance of membranes applied in the treatment of wastewater containing dyes, regarding antifouling behavior, selectivity and flux. In this review, an overview of water pollution caused by effluents containing synthetic dyes, the advantages and limitations of GO-based membranes and the latest research advances on the use of GO-based membranes for dyes removal, including its impact on membrane performance, are discussed in detail. The future panorama of the applicability of GO-based membranes for the treatment of water contaminated by dyes is also provided.

Engineered two-dimensional nanomaterials: an emerging paradigm for water purification and monitoring

Water scarcity has become an increasingly complex challenge with the growth of the global population, economic expansion, and climate change, highlighting the demand for advanced water treatment technologies that can provide clean water in a scalable, reliable, affordable, and sustainable manner. Recent advancements on 2D nanomaterials (2DM) open a new pathway for addressing the grand challenge of water treatment owing to their unique structures and superior properties. Emerging 2D nanostructures such as graphene, MoS₂, MXene, h-BN, g-C₃N₄, and black phosphorus have demonstrated an unprecedented surface-to-volume ratio, which promises ultralow material use, ultrafast processing time, and ultrahigh treatment efficiency for water cleaning/monitoring. In this review, we provide a state-of-the-art account on engineered 2D nanomaterials and their applications in emerging water technologies, involving separation, adsorption, photocatalysis, and pollutant detection. The fundamental design strategies of 2DM are discussed with emphasis on their physicochemical properties, underlying mechanism and targeted applications in different scenarios. This review concludes with a perspective on the pressing challenges and emerging opportunities in 2DM-enabled wastewater treatment and water-quality monitoring. This review can help to elaborate the structure-processing-property relationship of 2DM, and aims to guide the design of next-generation 2DM systems for the development of selective, multifunctional, programmable, and even intelligent water technologies. The global significance of clean water for future generations sheds new light and much inspiration in this rising field to enhance the efficiency and affordability of water treatment and secure a global water supply in a growing portion of the world.

Interactions between carbon-based nanoparticles and steroid hormone micropollutants in water

The occurrence of micropollutants (MPs) including steroid hormones is a global environmental and health challenge. Carbon-based nanoparticles can be incorporated with water treatment processes to allow MP removal by adsorption. The aim was to compare the suitability of such nanoparticles (graphene, graphene oxide, carbon nanotubes and C₆₀) to adsorb steroid hormones for later incorporation in membrane composites. All nanoparticles displayed fast kinetics; carbon nanotubes and graphene showed high adsorption capacities for hormones undeterminable in isotherm studies (over 10 mg/g). External surface adsorption appears to be the most prominent factor impacting adsorption performance. Structure, conformation, geometry and surface charge of nanoparticles can influence the accessibility of surface area through colloidal instability in aqueous solution. Mechanism inspection shows that adsorption initiates at long ranges (up to 10 nm) through hydrophobic and electrostatic interactions. At relatively short ranges (0.2-0.5 nm), adsorption is enhanced by π/π stacking, XH / π (X = C, O) interactions, van der Waals forces and hydrogen bonding. Both long- and short-range forces transporting hormones from the liquid bulk into the adsorbed phase could control the rate. With relatively short residence time required and high adsorption capacity, carbon nanotubes and graphene are promising for incorporation in a membrane composite.

Bibliometric approach to the perspectives and challenges of membrane separation processes to remove emerging contaminants from water

The presence of contaminants in water is concerning due to the potential impacts on human health and the environment, and ingested contaminants cause harm in various ways. The conventional water treatment systems are not efficient to remove these contaminants. Therefore, novel techniques and materials for the removal of contaminants are increasingly being developed. The separation process using modified membranes can remove these micropollutants; therefore, they have attracted significant research attention. Among the materials used for manufacturing of these membranes, composites based on graphene oxide and reduced graphene oxide are preferred owing to their promising properties, such as mechanical resistance, thermal and chemical stability, antifouling capacity, water permeability, high thermal and electrical conductivity, high optical transmittance and high surface area. Membrane separation processes (MSP) can be used as secondary or tertiary treatment during the supply of wastewater. However, the efficient and accessible applications of these technologies are challenging. This study aims to demonstrate the main concepts of membrane separation processes and their application in the removal of emerging contaminants. This study reports bibliometric mapping, relevant data on studies using membranes as water treatment processes, and their viability in industrial applications. The main challenges and perspectives of these technologies are discussed in detail as well.

The Advances in the Special Microwave Effects of the Heterogeneous Catalytic Reactions

In the present, microwave field has been widely used in chemical processes as an important means of intensification. The heterogeneous catalysts coupling with microwave has been shown to have many advantages, such as high catalytic performance and stability. Our objective is to focus an up-to-date overview concerning the advances in the special microwave effects of the heterogeneous catalytic reactions including special thermal effect, microwave plasma, enhanced active groups, and the flexibility of structure. This review systematically states the action mechanism and some practical application of microwave-induced catalytic process. Finally, the potential research directions in the field of microwave-induced catalysis are prospected.

Reduced Holey Graphene Oxide Membranes for Desalination with Improved Water Permeance

Reduced graphene oxide (r-GO) membranes with narrow channels exhibit salt rejections comparable to conventional nanofiltration (NF) membranes. However, their water permeances are much lower because of the high tortuosity for water permeation. Herein, we report a facile solution-processable approach to create in-plane nanopores on GO nanosheets before reduction, dramatically decreasing the tortuosity and increasing water permeance while retaining the salt rejection. Specifically, holey GO (HGO) nanosheets were prepared via chemical etching using hydrogen peroxide followed by the deposition on a porous support by vacuum filtration and then reduction via exposure to hydriodic acid solutions to generate the reduced HGO (r-HGO) membrane. The generation of nanopores increases the water permeance from 0.4 L m^-2 h^-1 bar^-1 (LMH/bar) to 6.6 LMH/bar with Na₂SO₄ rejection greater than 98.5%, and the membranes were robust under strong cross-flow shearing force for 36 h. Both water permeance and Na₂SO₄ rejection of these r-HGO membranes for the first time simultaneously reach the level of the commercial polyamide-based NF membranes. Given their good antibacterial properties and resistance to aggressive chemical washing, the r-HGO membranes show promise as next-generation NF membranes for desalination.

Stimuli-responsive supramolecular polymer network based on bi-pillar[5]arene for efficient adsorption of multiple organic dye contaminants

A novel supramolecular polymer network gel has been successfully prepared via bi-pillar[5]arene and a tripodal guest, exhibiting multi-stimuli-responsiveness and efficient adsorption of organic dyes.

Stable Graphene-Based Membrane with pH-Responsive Gates for Advanced Molecular Separation

Graphene-based stable pH-responsive membranes (GPMs) were developed by alternative deposition of graphene oxide (GO) with polyethylenimine (PEI) in a layer-by-layer manner. Different from the conventional pore-blocking pH-responsive membranes, the size of the gaps among the GO sheets were first designed to respond to the surrounding pH. Atomic force microscopy was used to dynamically explore the internal structure alteration of GPM in the pH range from 3 to 11. It was found that the PEI molecules not only cross-linked the GO sheets through amide bonds to ensure the membrane stability but also reversibly altered the gate size of GPM in a certain extent according to the surrounding pH. In filtration, the gates of GPM were widened with the decreasing pH of the feed and vice versa. As a result, the permeate flux of GPM increased with the decreasing feed pH. More importantly, the molecular weight cutoff of GPM could be continuously regulated by the feed pH in a certain range; during the filtration of the polyvinylpyrrolidone (PVP) and polyethylene oxide (PEO) mixed solution, only PVP (58 kDa) could penetrate GPM at pH 11, while the left PEO (600 kDa) would penetrate GPM at pH 3. The controlled penetration through GPM led to a complete separation and recovery of the molecules in different sizes, which is highly desirable for advanced molecular separation in environmental applications.

Development of Graphene Oxide Framework Membranes via the "from" and "to" Cross-Linking Approach for Ion-Selective Separations

Graphene oxide (GO) membranes with well-defined nanochannels formed between the stacked GO nanosheets find great interest in molecular separations. However, GO membranes are unstable in aqueous solution environments because of weak interactions between the stacked nanosheets. Herein, we developed a preparation method by diminishing the self-contained oxidized functional groups in GO and subsequent cross-linking to form GO framework (GOF) membranes with excellent aqueous solution stability. GOF membranes were fabricated by alternate deposition of branched polyethylenimine (BPEI) and a mixed solution of GO and thiourea (TU). Structural elucidation illustrated that the TU partially reduced the GO molecules and acted as a "to" cross-linker by bridging adjacent GO nanosheets through in-plane and out-of-plane of interactions. During the GO deposition, BPEI performed the role as a "from" cross-linker by binding the TU-linked GO laminates to form stable GOF membranes with well-defined nanochannels. Morphological studies confirmed the formation of the tightly packed structure for BPEI/GO_TU membranes due to the high Π-Π interactions between the GO nanosheets and bridging effect of TU. The GOF membranes exhibited a rejection of 99.5% for anionic dye methyl orange and cationic dye rhodamine B. The BPEI/GO_TU membranes fabricated from 12 bilayers using 0.25 mg/mL of GO solution have a pure water flux of 24 L m^-2 h^-1 and a Na₂SO₄ rejection of 94%; this permeability is 2.5 times higher than that of commercial nanofiltration membranes. Moreover, (BPEI/GO_TU)₁₂ GOF membranes exhibited excellent aqueous solution stability in acidic and basic conditions. The excellent separation performance and aqueous solution stability of the BPEI/GO_TU membranes are intricately linked to the partial reduction and cross-linking of GO nanosheets in GOF membranes. Thus, the "from" and "to" cross-linking approach developed in this work can be extended for the fabrication of structurally stable membranes from other 2D materials.

Ion Transport Behavior through Thermally Reduced Graphene Oxide Membrane for Precise Ion Separation

The cation transport behavior of thermally treated reduced graphene oxide membranes (GOMs) is reported. The GOMs were reduced by heat treatment at 25, 80, and 120 °C and then characterized by Fourier transform infrared spectroscopy, X-ray powder diffraction, and X-ray photoelectron spectroscopy to determine oxygen group content, C/O ratio, and layer spacing. The permeation rates of various cations with different sizes and charge numbers through these membranes were measured to understand the effect of the cations on transport behavior. The results indicated that the cation transport through the membranes depended on the layer spacing of the membrane and ion size and charge. Cations of the same valence permeating through the same GOM could be differentiated by their hydration radius, whereas the same type of cation passing through different GOMs could be determined by the spacing of the GOM layers. The cation valence strongly affected permeation behavior. The GOM that was prepared at 120 °C exhibited a narrow layer spacing and high separation factors for Mg/Ca, Mg/Sr, K/Ca, and K/Fe. The cations moving through the membrane could insert into the membrane lamellas, which neutralized the negative charge of the membrane, enlarged the layer spacing of the GOMs, and affected cation permeation.

Non-Polyamide Based Nanofiltration Membranes Using Green Metal-Organic Coordination Complexes: Implications for the Removal of Trace Organic Contaminants

Polyamide-based thin film composite (TFC) membranes are generally optimized for salt rejection but not for the removal of trace organic contaminants (TrOCs). The insufficient rejection of TrOCs such as endocrine disrupting compounds (EDCs) by polyamide membranes can jeopardize product water safety in wastewater reclamation. In this study, we report a novel nonpolyamide membrane chemistry using green tannic acid-iron (TA-Fe) complexes to remove TrOCs. The nanofiltration membrane formed at a TA-Fe molar ratio of 1:3 (TA-Fe3) had a continuous thin rejection layer of 10-30 nm in thickness, together with a water permeability of 5.1 Lm^2-h^-1bar^-1 and a Na₂SO₄ rejection of 89.7%. Meanwhile, this membrane presented significantly higher rejection of EDCs (up to 99.7%) than that of polyamide membranes (up to 81.8%). Quartz crystal microbalance results revealed that the sorption amount of a model EDC, benzylparbaen, by TA-Fe3 layer was nearly 2 orders of magnitude less than that by polyamide, leading to reduced transmission and higher rejection. Further analysis of membrane revealed a much greater water/EDC selectivity of the TA-Fe3 membrane compared to the polyamide membranes.

Highly adsorptive chitosan/saponin-bentonite composite film for removal of methyl orange and Cr(VI)

Robust and simple composite films for the removal of methyl orange (MO) and Cr(VI) have been prepared by combining chitosan, saponin, and bentonite at a specific ratio. There are several composite films (chitosan-saponin-bentonite (CSB)) prepared; among them, the composite films CSB_2:3 and CSB_1:1 have the highest removal efficiency toward MO and Cr(VI) where the maximum removal is 70.4% (pH 4.80) and 92.3% (pH 5.30), respectively. It was found that different types of adsorbate have different thermodynamic properties of the adsorption process; the adsorption of MO onto CSB_2:3, chitosan, and acid-activated bentonite (AAB) proceeded endothermically, while the adsorption of Cr(VI) onto CSB_1:1, chitosan, and AAB proceeded exothermically. The parameters of the adsorption were modeled by using isotherm and kinetic equations. The models of Langmuir, Freundlich, Redlich-Peterson, Sips, and Toth were used for fitting the adsorption isotherm data at a temperature of 30, 45, and 60 °C; all of the isotherm models could represent the data well. The result indicates that CSB_2:3 has the highest adsorption capacity toward MO with q_m of 360.90 mg g^-1 at 60 °C; meanwhile, CSB_1:1 has the highest adsorption capacity toward Cr(VI) with q_m 641.99 mg g^-1 at 30 °C. The pseudo-second-order model could represent the adsorption kinetics data better than the pseudo-first-order equation. The adsorption mechanism was proposed, and the thermodynamic properties of the adsorption were also studied.

Ce-doped UiO-67 nanocrystals with improved adsorption property for removal of organic dyes

In this study, we report the synthesis of Ce-doped UiO-67 nanocrystals via one-step hydrothermal method and their potential use for waste water treatment to remove organic dyes. The as-prepared samples were characterized by using SEM, TEM, FT-IR, XRD, XPS and TG techniques and the results demonstrate the formation of the framework structure of the Ce-doped UiO-67. The adsorption study of the material shows that the Ce-doped UiO-67 nanoparticles preferentially adsorb the cationic dyes such as rhodamine B and malachite green rather than the anionic dyes such as methyl orange. Adsorption capacities are 754.4, 589.2 and 191.6 mg g⁻¹ for rhodamine B, malachite green and methyl orange respectively. Based on its zeta potential and adsorption isotherm, it is understood that Ce doping increases its electrostatic interactions, and as a consequence, improves the adsorption capacity for cationic dyes. In addition, it is found that a pseudo-second-order kinetics and the Langmuir isothermal model were suitable for describing the adsorption behavior of cationic dyes onto the Ce-doped UiO-67.

The Ce-doped UiO-67 nanocrystals were successfully synthesized via a one-step hydrothermal method. Ce doping increases the negative charge on the surface of the material, thus the adsorbent exhibits high adsorption capacity to cationic dyes.

Rightsizing Nanochannels in Reduced Graphene Oxide Membranes by Solvating for Dye Desalination

Membranes with high water permeance, near-zero rejection to inorganic salts (such as NaCl and Na₂SO₄), and almost 100% rejection to organic dyes are of great interest for the dye desalination (the separation of dyes and salts) of textile wastewater. Herein, we prepared reduced graphene oxide membranes in a solvation state (S-rGO) with nanochannel sizes rightly between the salt ions and dye molecules. The S-rGO membrane rejects >99.0% of Direct Red 80 (DR 80) and has almost zero rejection for Na₂SO₄. By contrast, conventional GO or rGO membranes often have channel sizes smaller than divalent ions (such as SO₄^2-) and thus high rejection for Na₂SO₄. More interestingly, high salinity in typical dye solutions decreases the channel size in the S-rGO membranes and thus increases the dye rejection, while the Na₂SO₄ rejection decreases because of the negatively charged surface on GO and the salt screening effect. The membranes also show pure water permeance as high as 80 L m^-2 h^-1 bar^-1, which is about 8 times that of commercial NF 90 membrane and 2 times that of a commercial ultrafiltration membrane (with a molecular weight cutoff of 2000 Da), rendering their promise for practical dye desalination.

Potable Water Reuse through Advanced Membrane Technology

Recycling water from municipal wastewater offers a reliable and sustainable solution to cities and regions facing shortage of water supply. Places including California and Singapore have developed advanced water reuse programs as an integral part of their water management strategy. Membrane technology, particularly reverse osmosis, has been playing a key role in producing high quality recycled water. This feature paper highlights the current status and future perspectives of advanced membrane processes to meet potable water reuse. Recent advances in membrane materials and process configurations are presented and opportunities and challenges are identified in the context of water reuse.

Potential-Dependent, Switchable Ion Selectivity in Aqueous Media Using Titanium Disulfide

The selective removal of ions by an electrochemical process is a promising approach to enable various water-treatment applications such as water softening or heavy-metal removal. Ion intercalation materials have been investigated for their intrinsic ability to prefer one specific ion over others, showing a preference for (small) monovalent ions over multivalent species. In this work, we present a fundamentally different approach: tunable ion selectivity not by modifying the electrode material, but by changing the operational voltage. We used titanium disulfide, which shows distinctly different potentials for the intercalation of different cations and formed binder-free composite electrodes with carbon nanotubes. Capitalizing on this potential difference, we demonstrated controllable cation selectivity by online monitoring the effluent stream during electrochemical operation by inductively coupled plasma optical emission spectrometry of aqueous 50 mm CsCl and MgCl₂ . We obtained a molar selectivity of Mg²⁺ over Cs⁺ of 31 (strong Mg preference) in the potential range between -396 mV and -220 mV versus Ag/AgCl. By adjusting the operational potential window from -219 mV to +26 mV versus Ag/AgCl, Cs⁺ was preferred over Mg²⁺ by 1.7 times (Cs preference).

2D graphene oxide channel for water transport

Proper crosslinking and reduction can effectively prevent graphene oxide from swelling, thus enabling the precise control of interlayer spacing.