Nanofiltration membranes via co-deposition of polydopamine/polyethylenimine followed by cross-linking

Recent advances in polymeric transdermal drug delivery systems

Transdermal delivery has proven to be one of the most favorable methods among novel drug delivery systems. Since drugs administered by transdermal delivery systems avoid the gastrointestinal tract, and thus avoid conversion by the liver, the likelihood of liver dysfunction and gastrointestinal tract irritation as side effects is low. Drug delivery through the skin has other advantages, such as maintaining an effective rate of drug delivery over time, a steady rate of circulation, and the benefits of a passive delivery system and diffusion. Transdermal drug delivery is achieved using patches which consist of different and specific layers. In the last few decades, many types of patches have been approved worldwide, such as medical plasters, which have been generally applied to the skin for localized diseases. Such patches can be traced back to ancient China (around 2000 BCE) and are the early precursors of today's transdermal patches. With the help of effective design, materials, manufacturing, and evaluation, a large number of drugs can now be administered using this valuable advanced technology. This study reviews different types of polymer patches, their advantages and disadvantages, and different studies related to transdermal drug delivery methods, and the advantages and disadvantages of each method. Different mechanisms of transdermal drug delivery system with patches are also discussed.

Sustainable Cellulose Aerogel from Waste Cotton Fabric for High-Performance Solar Steam Generation

The textile industry has been considered as one of the polluting industries, producing a large amount of textile waste and CO₂ emissions each year. Recycling of waste fabric has attracted more research interest in recent years. Herein, renewable polydopamine (PDA)-functionalized cellulose aerogels (CAs) have been designed by a feasible and green way for clean water generation. With the polymerization of PDA on the surface, which possesses excellent photothermal conversion performance and water purification ability, the resulting CA could achieve a high light absorption of 96.5% with the evaporation rate of 2.74 kg m^-2 h^-1 under 1 sun. Meanwhile, the solar steam generator with the increasing height can absorb energy from adjacent ambient air to strengthen the vapor generation. The features of renewable CAs can achieve efficient water evaporation, which combined with their low material cost and recycling, offer promise in reducing not only energy consumption but also the environmental footprint of cotton textiles.

A multifunctional green antibacterial rapid hemostasis composite wound dressing for wound healing

Rapid hemostasis and antibacterial properties are essential for novel wound dressings to promote wound healing. In particular, timely and rapid hemostasis could be of benefit to reduce the mortality caused by excessive bleeding loss. Herein, we present a novel strategy of combining electrospinning technology with post-modification technology to prepare a multifunctional wound dressing, cellulose diacetate-based composite wound dressing (CDCE), with rapid hemostasis and antibacterial activity. It is interesting that the CDCE wound dressing had superhydrophilicity, high water absorption, and strong absorbing capacity, which could eliminate the exudate around the wound in a timely manner and further promote rapid hemostasis. Additionally, its excellent antibacterial properties could inhibit severe infection in the wound and accelerate wound healing. Based on these advantages, the novel CDCE wound dressing could promote wound contraction and further accelerate wound healing compared with the common traditional wound dressing gauze. Taken together, the multifunctional CDCE wound dressing has high potential for clinical application in the future.

Polyethyleneimine impregnated alginate capsule as a high capacity sorbent for the recovery of monovalent and trivalent gold

For the first time, a polyethyleneimine-impregnated alginate capsule (PEIIAC) with a high adsorption capacity is developed for the recovery of monovalent and trivalent gold from an acidic solution. The strategy results in a new type of adsorbent, polyethyleneimine impregnated alginate capsule (PEIIAC) with a core-shell structure having a large number of amine groups as cationic binding site, facilitating maximum uptake of anionic auric chloride. The maximum uptake of PEIIAC was 3078 and 929 mg/g for Au (III) and Au (I), respectively, are recordable compared to other reported adsorbents to date. The as-prepared material was executed to check the sorption efficacy for Au (III) and Au (I) in the pH range of 1-12. With an increment in pH, the uptake capacity for Au (III) increased, while the uptake capacity for Au (I) decreased. The FTIR, XRD, and XPS studies revealed that the gold adsorption mechanism includes ionic interactions and reduction, wherein the amine, hydroxyl, and carboxyl groups are involved. The capsule showed a higher adsorption efficiency than other reported sorbents, making the material applicable in acidic solutions for the recovery of Au (I) and Au (III).

Nanofiltration-Inspired Janus Membranes with Simultaneous Wetting and Fouling Resistance for Membrane Distillation

Membranes with robust antiwetting and antifouling properties are highly desirable for membrane distillation (MD) of wastewater. Herein, we have proposed and demonstrated a highly effective method to mitigate wetting and fouling by designing nanofiltration (NF)-inspired Janus membranes for MD applications. The NF-inspired Janus membrane (referred to as PVDF-P-CQD) consists of a hydrophobic polyvinylidene fluoride (PVDF) membrane and a thin polydopamine/polyethylenimine (PDA/PEI) layer grafted by sodium-functionalized carbon quantum dots (Na⁺-CQDs) to improve its hydrophilicity. The vapor flux data have confirmed that the hydrophilic layer does not add extra resistance to water vapor transport. The PVDF-P-CQD membrane exhibits excellent resistance toward both surfactant-induced wetting and oil-induced fouling in direct contact MD (DCMD) experiments. The impressive performance arises from the fact that the nanoscale pore sizes of the PDA/PEI layer would reject surfactant molecules by size exclusion and lower the propensity of surfactant-induced wetting, while the high surface hydrophilicity resulted from Na⁺-CQDs would induce a robust hydration layer to prevent oil from attachment. Therefore, this study may provide useful insights and strategies to design novel membranes for next-generation MD desalination with minimal wetting and fouling propensity.

Cation Exchange Membranes Coated with Polyethyleneimine and Crown Ether to Improve Monovalent Cation Electrodialytic Selectivity

Developing monovalent cation permselective membranes (MCPMs) with high-efficient permselectivity is the core concern in specific industrial applications. In this work, we have fabricated a series of novel cation exchange membranes (CEMs) based on sulfonated polysulfone (SPSF) surface modification by polyethyleneimine (PEI) and 4′-aminobenzo-12-crown-4 (12C4) codeposited with dopamine (DA) successively, which was followed by the cross-linking of glutaraldehyde (GA). The as-prepared membranes before and after modification were systematically characterized with regard to their structures as well as their physicochemical and electrochemical properties. Particularly, the codeposition sequence of modified ingredients was investigated on galvanostatic permselectivity to cations. The modified membrane (M-12C4-0.50-PEI) exhibits significantly prominent selectivity to Li⁺ ions (PMg2+Li+ = 5.23) and K⁺ ions (PMg2+K+ = 13.56) in Li⁺/Mg²⁺ and K⁺/Mg²⁺ systems in electrodialysis (ED), which is far superior to the pristine membrane (M-0, PMg2+Li+ = 0.46, PMg2+K+ = 1.23) at a constant current density of 5.0 mA·cm⁻². It possibly arises from the synergistic effects of electrostatic repulsion (positively charged PEI), pore-size sieving (distribution of modified ingredients), and specific interaction effect (12C4 ~Li⁺). This facile strategy may provide new insights into developing selective CEMs in the separation of specific cations by ED.

Novel Positively Charged Metal-Coordinated Nanofiltration Membrane for Lithium Recovery

Nanofiltration (NF) with high water flux and precise separation performance with high Li⁺/Mg²⁺ selectivity is ideal for lithium brine recovery. However, conventional polyamide-based commercial NF membranes are ineffective in lithium recovery processes due to their undesired Li⁺/Mg²⁺ selectivity. In addition, they are constrained by the water permeance selectivity trade-off, which means that a highly permeable membrane often has lower selectivity. In this study, we developed a novel nonpolyamide NF membrane based on metal-coordinated structure, which exhibits simultaneously improved water permeance and Li⁺/Mg²⁺ selectivity. Specifically, the optimized Cu-m-phenylenediamine (MPD) membrane demonstrated a high water permeance of 16.2 ± 2.7 LMH/bar and a high Li⁺/Mg²⁺ selectivity of 8.0 ± 1.0, which surpassed the trade-off of permeance selectivity. Meanwhile, the existence of copper in the Cu-MPD membrane further enhanced anti-biofouling property and the metal-coordinated nanofiltration membrane possesses a pH-responsive property. Finally, a transport model based on the Nernst-Planck equations has been developed to fit the water flux and rejection of uncharged solutes to the experiments conducted. The model had a deviation below 2% for all experiments performed and suggested an average pore radius of 1.25 nm with a porosity of 21% for the Cu-MPD membrane. Overall, our study provides an exciting approach for fabricating a nonpolyamide high-performance nanofiltration membrane in the context of lithium recovery.

The Effect of the Oleophobicity Deterioration of a Membrane Surface on Its Rejection Capacity: A Computational Fluid Dynamics Study

In this work, the effects of the deteriorating affinity-related properties of membranes due to leaching and erosion on their rejection capacity were studied via computational fluid dynamics (CFD). The function of affinity-enhancing agents is to modify the wettability state of the surface of a membrane for dispersed droplets. The wettability conditions can be identified by the contact angle a droplet makes with the surface of the membrane upon pinning. For the filtration of fluid emulsions, it is generally required that the surface of the membrane is nonwetting for the dispersed droplets such that the interfaces that are formed at the pore openings provide the membrane with a criterion for the rejection of dispersals. Since materials that make up the membrane do not necessarily possess the required affinity, it is customary to change it by adding affinity-enhancing agents to the base material forming the membrane. The bonding and stability of these materials can be compromised during the lifespan of a membrane due to leaching and erosion (in crossflow filtration), leading to a deterioration of the rejection capacity of the membrane. In order to investigate how a decrease in the contact angle can lead to the permeation of droplets that would otherwise get rejected, a CFD study was conducted. In the CFD study, a droplet was released in a crossflow field that involved a pore opening and the contact angle was considered to decrease with time as a consequence of the leaching of affinity-enhancing agents. The CFD analysis revealed that the decrease in the contact angle resulted in the droplet spreading over the surface more. Furthermore, the interface that was formed at the entrance of the pore opening flattened as the contact angle decreased, leading the interface to advance more inside the pore. The droplet continued to pass over the pore opening until the contact angle reached a certain value, at which point, the droplet became pinned at the pore opening.

Novel Strategy of Mussel-Inspired Immobilization of Naringinase with High Activity Using a Polyethylenimine/Dopamine Co-deposition Method

Mussel-inspired surface chemistry is recognized as a simple, efficient, and mild surface modification method and has become a research hotspot in many fields. In this study, polyethylenimine/dopamine was coated on the surface of SBA-15 using a co-deposition method, making it possible to immobilize naringinase with high activity and operation stability. The optimal modification and immobilization conditions as well as enzyme properties were investigated. The naringinase activity can reach up to 753.78 U/g carrier, which was much higher than those of the previous works. Besides, the residual naringinase activity still kept 78.91% of the initial activity after one month of storage and maintained 60.79% after 8 cycles. Therefore, the strategy of mussel-inspired enzyme immobilization could be recognized as a promising and universal enzyme immobilization method, with the advantages of high relative enzyme activity, enzyme carrying rate, enzyme activity recovery rate, and good reusability and storage stability.

High-Throughput Zwitterion-Modified MoS2 Membranes: Preparation and Application in Dye Desalination

Although loose nanofiltration membranes have been extensively studied for dye desalination, high-throughput membranes with antifouling and antibacterial properties are still highly needed. In this study, a zwitterion-modified molybdenum disulfide (MoS₂) dual-layer loose nanofiltration membrane was prepared with the integration of antibacterial, antifouling, and high-flux properties. To be specific, MoS₂ nanosheets were loaded on a polyacrylonitrile ultrafiltration membrane through pressure-assisted self-assembly. Then, poly (sulfobetaine methacrylate) (PSBMA) was coated on the surface of the MoS₂ membrane via a simple polydopamine (PDA)-assisted one-step codeposition to prepare PSBMA/PDA/MoS₂ nanofiltration membranes. Elemental and morphological analyses confirmed the formation of the MoS₂ layer and PSBMA/PDA coating. In addition, the effect of the PSBMA amount and codeposition time on surface properties and membrane performances was investigated. Under optimum conditions, the as-prepared membrane showed excellent water permeance of 262 LMH/bar with good dye rejection (99.8% for methylene blue) and salt permeability, as well as excellent antifouling and antibacterial properties benefiting from the synergy of PSBMA/PDA coating layers and MoS₂ layers.

H2O2-Triggered Rapid Deposition of Poly(caffeic acid) Coatings: A Mechanism-Based Entry to Versatile and High-Efficient Molecular Separation

Plant-derived polyphenol coating offers a promising route to fabricate functional surfaces for different substrate materials. However, almost all of the deposition approaches are time-consuming and involve inefficient processes, and the mechanisms behind the coating deposition are rarely understood. Herein, we report a rational methodology to achieve the rapid deposition of poly(caffeic acid) (PCA) by using H₂O₂ as a trigger under the assistance of copper sulfate (CuSO₄). The comparative monomer structure of PCA oxidation polymerization has illustrated a significant distinction in the reaction path for PCA coating deposition which has never been reported before. Until now, the unprecedented fast velocity for polyphenol coating has been obtained, and the PCA coating exhibits excellent homogeneity, spatiotemporal tunability, and firm stability. Moreover, three different types of filtration membranes, poly(vinylidene fluoride) microfiltration membrane (PVDF MF membrane), poly(ether sulfone) (PES) ultrafiltration (UF) hollow fiber membrane, and PCA-coated PES nanofiltration (NF) membrane, are all successfully dip-coated using H₂O₂-triggered PCA coating. Without synthetic complexities and intricate procedures, the formation of hydrophilic and homogeneous PCA aggregates on the surface and/or inside pore walls resulted in various membranes. The as-prepared PCA-coated PVDF MF membrane demonstrates excellent oil/water separation efficiency of less than 150 ppm and a flux recovery rate of approximately 90% even after five cycles. By one-step co-deposition of PCA and poly(2-ethyl-2-oxazoline) (PEtOx) on the PES UF membrane surface, hydrophilicity and biofouling resistance are implemented for efficient protein filtration. The PES NF membrane formed by the PCA layer exhibits high mono-/divalent ion selectivity and excellent chlorine resistance. Overall, these results represent a rapid and sustainable approach to tailor PCA coatings for versatile liquid separation processes.

Enhanced Fouling Resistance and Antimicrobial Property of Ultrafiltration Membranes Via Polyelectrolyte-Assisted Silver Phosphate Nanoparticle Immobilization

Ultrafiltration (UF) is a low-pressure membrane that yields higher permeate flux and saves significant operating costs compared to high-pressure membranes; however, studies addressing the combined improvement of anti-organic and biofouling properties of UF membranes are lacking. This study investigated the fouling resistance and antimicrobial property of a UF membrane via silver phosphate nanoparticle (AgPNP) embedded polyelectrolyte (PE) functionalization. Negatively charged polyacrylic acid (PAA) and positively charged polyallylamine hydrochloride (PAH) were deposited on the membrane using a fluidic layer-by-layer assembly technique. AgPNPs were immobilized within the crosslinked "bilayers" (BL) of PAH/PAA. The effectiveness of AgPNP immobilization was confirmed by microprofile measurements on membrane surfaces using a solid contact Ag micro-ion-selective electrode. Upon stable and uniform BL formation on the membrane surface, the permeate flux was governed by a combined effect of PAH/PAA-derived hydrophilicity and surface/pore coverage by the BLs "tightening" of the membrane. When fouled by a model organic foulant (humic acid), the functionalized membrane exhibited a lower flux decline and a greater flux recovery due to the electrostatic repulsion imparted by PAA when compared to the unmodified membrane. The functionalization rendered antimicrobial property, as indicated by fewer attachments of bacteria that initiate the formation of biofilms leading to biofouling.

Simple Amphoteric Charge Strategy to Reinforce Superhydrophilic Polyvinylidene Fluoride Membrane for Highly Efficient Separation of Various Surfactant-Stabilized Oil-in-Water Emulsions

Long-term efficient separation of highly emulsified oily wastewater is challenging. Reported herein is the preparation of a reinforced superhydrophilic, underwater superoleophobic membrane with demulsification properties using active iron nanoparticles in situ generated on a polydopamine (PDA)/polyethylenimine (PEI)-modified polyvinylidene fluoride (PVDF) membrane surface. A stable zwitterionic structure is fabricated on the membrane surface and provides it with an excellent capability of binding a hydration layer, leading to enhanced superhydrophilic/underwater superoleophobic properties. The interaction between the membrane surface and water is quantified using the relaxation time of water. After iron nanoparticles in situ anchoring, the superhydrophilic, underwater superoleophobic PDA/PEI modified PVDF membrane shows more stable flux behaviors, higher oil separation efficiency, demulsification, and excellent antioil-fouling properties for various anionic, nonionic, and cationic surfactant-stabilized oil-in-water emulsions in a crossflow filtration system. The reinforced hydration layer and the amphoteric charged demusification properties of the membrane play important roles in enhancing the membrane separation performance. The reinforced membrane also exhibits excellent cleaning and reusability performance in long-term operations. The outstanding separation performance, as well as the simple and cost-effective fabrication process of the membrane with various favorable properties, highlight its promise in practical emulsified oily water applications.

Graphene oxide modified membrane for highly efficient wastewater treatment by dynamic combination of nanofiltration and catalysis

Stacked graphene oxide (GO) nano-sheets with plentiful nanopores incorporated onto polymeric membrane are promising for water purification. However, maintaining high water permeability without sacrificing separation efficiency remains a challenge. Delamination of the GO layer from the membrane surface is another bottleneck affecting the efficiency of the material. To solve those problems, we immobilized a chemically crosslinked GO composite layer with enlarged interlayer space on the surface of a novel catalytic membrane, which served as the support. The modified GO nanosheets-coated catalytic membranes showed excellent separation robustness with withstanding strong lateral shear force during 6 h filtration in a crossflow model. The as-prepared membrane showed high removal efficiencies to Congo red and Basic blue (99 % and 96 %, respectively) due to the surface-coated GO composite layer. Furthermore, while launching the catalytic function of the membrane with a trace amount of reducing agent (NaBH₄), the GO-coated composite membrane successfully purified 50 ppm Methyl orange, 50 ppm Methylene blue, 50 ppm Rhodamine B and 0.272 mmol/L 4-Nitrophenol to break through the membrane rejection limitation of 500 Mw. Combining nano-filtration and catalysis, the GO-coated composite membranes showed great potential for the continuous purification of chemically contaminated water.

Impact of MWCO and Dopamine/Polyethyleneimine Concentrations on Surface Properties and Filtration Performance of Modified Membranes

The mussel-inspired method has been investigated to modify commercial ultrafiltration membranes to induce antifouling characteristics. Such features are essential to improve the feasibility of using membrane processes in protein recovery from waste streams, wastewater treatment, and reuse. However, some issues still need to be clarified, such as the influence of membrane pore size and the polymer concentration used in modifying the solution. The aim of the present work is to study a one-step deposition of dopamine (DA) and polyethyleneimine (PEI) on ultrafiltration membrane surfaces. The effects of different membrane molecular weight cut-offs (MWCO, 20, 30, and 50 kDa) and DA/PEI concentrations on membrane performance were assessed by surface characterization (FTIR, AFM, zeta potential, contact angle, protein adsorption) and permeation of protein solution. Results indicate that larger MWCO membranes (50 kDa) are most benefited by modification using DA and PEI. Moreover, PEI is primarily responsible for improving membrane performance in protein solution filtration. The membrane modified with 0.5:4.0 mg mL^-1 (DA: PEI) presented a better performance in protein solution filtration, with only 15% of permeate flux drop after 2 h of filtration. The modified membrane can thus be potentially applied to the recovery of proteins from waste streams.

Constructing interlayer to tailor structure and performance of thin-film composite polyamide membranes: A review

Thin-film composite (TFC) structured membranes based on polyamide (PA) chemistry is the gold standard of nanofiltration and reverse osmosis-based technologies for water purification and desalination. Constructing interlayer between porous substrate and PA layer is a promising strategy to address the ubiquitous trade-off between permeability and selectivity, which is typically encountered by conventional TFC PA membranes. The progress in the interlayer benefits the precise control of interfacial polymerization process, which therefore can tailor the structure and performance of advanced TFC PA membranes. This review critically summarizes the recent advances in TFC PA membranes mediated by interlayer. The mechanisms of interlayer regulating the IP process and PA structure are first discussed based on available literature. Structure and performance of novel TFC PA membranes based on three kinds of interlayers, i.e., organic coatings, nanomaterial and nanocomposite interlayers, are systematically reviewed. Finally, perspectives and future efforts needed are proposed for interlayer based TFC PA membranes. This review offers comprehensive understanding and useful guidance on the rational design of advanced membranes mediated by interlayers for desalination and water purification.

Surface modification of polypropylene membrane for the removal of iodine using polydopamine chemistry

The development of stable and effective iodine removal systems would be highly desirable in addressing environmental issues relevant to water contamination. In the present research, a novel iodine adsorbent was synthesized by self-polymerization of dopamine (PDA) onto inert polypropylene (PP) membrane. This PP/PDA membrane was thoroughly characterized and its susrface propeties was analyzed by various analytical techniques indcluding field emission scanning electron microscopy (FESEM), atomic force microscopy (AFM), attenuated total reflectance Fourier transform infrared spectroscopy (ATR-FTIR), Brunauer-Emmett-Teller (BET) and Barrett-Joyner-Halenda (BJH), contact angle, and surface free energy measurement. The PP/PDA membranes were subsequently used for batchwise removal of iodine at different temperatures (25-70 °C), pH (2-7), and surface areas (1-10 cm²) to understand the underlying adsorption phenomena and to estimate the membrane capacity for iodine uptake. The increase in temperature and pH both led to higher adsorption of iodine. The present approach showed a removal efficiency of over 75% for iodine using 10 cm² PP/PDA membrane (18.87 m² g^-1) within 2 h at moderate temperatures (∼50 °C) and pH > 4, about 15 fold compared to the PP control membrane. The adsorption kinetics and isotherms were well fitted to the pseudo-second-order kinetic and Langmuir isotherm models (R² > 0.99). This adsorbent can be recycled and reused at least six times with stable iodine adsorption. These findings were attributed to the homogenous monolayer adsorption of the iodide on the surface due to the presence of catechol and amine groups in the PP/PDA membrane. This study proposes an efficient adsorbent for iodine removal.

Development of Polydopamine Forward Osmosis Membranes with Low Reverse Salt Flux

Application of forward osmosis (FO) is limited due to membrane fouling and, most importantly, high reverse salt fluxes that deteriorate the concentrated product. Polydopamine (PDA) is a widely used, easily applicable, hydrophilic, adhesive antifouling coating. Among the coating parameters, surprisingly, the effect of PDA coating temperature on the membrane properties has not been well studied. Polyethersulfone (PES) 30 kDa ultrafiltration membranes were PDA-coated with varying dopamine concentrations (0.5-3 g/L) and coating temperatures (4-55 °C). The quality of the applied coating has been determined by surface properties, water permeability and reverse salt flux using a 1.2 M MgSO₄ draw solution. The coating thickness increased both with the dopamine concentration and coating temperature, the latter having a remarkably stronger effect resulting in a higher PDA deposition speed and smaller PDA aggregates. In dead‑end stirred cell, the membranes coated at 55 °C with 2.0 g/L dopamine showed NaCl and MgSO₄ retentions of 41% and 93%, respectively. In crossflow FO, a low reverse MgSO₄ flux (0.34 g/m²·h) was found making a very low specific reverse salt flux (J_s/J_w) of 0.08 g/L, which outperformed the commercial CTA FO membranes, showing the strong benefit of high temperature PDA-coated PES membranes to assure high quality products.

Influence of the presence of cations on the water and salt dynamics inside layered graphene oxide (GO) membranes

Although over the past few years, graphene oxide (GO) has emerged as a promising membrane material, the applicability of layered GO membranes in water purification/seawater desalination is still a challenging issue because of the undesirable swelling of GO laminates in the aqueous environment. One of the ways to tune the interlayer spacing and to arrest the undesirable swelling of layered GO membranes in the aqueous environment is to intercalate the interlayer spacing of the GO laminates with cations. Although the cation intercalation imparts stabilization to GO laminates in the aqueous environment, their effect on the performance of the membrane is yet to be addressed in detail. In the present study we have investigated the effect of cation intercalation on the performance of layered GO membranes using molecular dynamics simulation. For the same interlayer spacing, the cation intercalated layered GO membranes have a higher water flux as compared to the corresponding pristine layered GO membranes. In the presence of the cations, the water molecules inside the interlayer gallery get more compactly packed. The presence of the cations also increases the stability of the hydrogen bond network among the water molecules inside the membrane. This can be attributed to slow water reorientation dynamics inside the interlayer gallery in the presence of the cations. The synergistic effect of all these changes is that the water permeability through the cation intercalated layered GO membranes is higher as compared to that through the corresponding pristine layered GO membranes. On the other hand, the intercalation of the cations (K+, Mg2+) leads to higher rejection of Na+ ions whereas the rejection of Cl- ions slightly decreases.

Highly Efficient Removal of Methylene Blue Dye from an Aqueous Solution Using Cellulose Acetate Nanofibrous Membranes Modified by Polydopamine

A new type of deacetylated cellulose acetate (DA)@polydopamine (PDA) composite nanofiber membrane was fabricated by electrospinning and surface modification. The membrane was applied as a highly efficient adsorbent for removing methylene blue (MB) from an aqueous solution. The morphology, surface chemistry, surface wettability, and effects of operating conditions on MB adsorption ability, as well as the equilibrium, kinetics, thermodynamics, and mechanism of adsorption, were systematically studied. The results demonstrated that a uniform PDA coating layer was successfully developed on the surface of DA nanofibers. The adsorption capacity of the DA@PDA nanofiber membrane reached up to 88.2 mg/g at a temperature of 25 °C and a pH of 6.5 after adsorption for 30 h, which is about 8.6 times higher than that of DA nanofibers. The experimental results showed that the adsorption behavior of DA@PDA composite nanofibers followed the Weber's intraparticle diffusion model, pseudo-second-order model, and Langmuir isothermal model. A thermodynamic analysis indicated that endothermic, spontaneous, and physisorption processes occurred. Based on the experimental results, the adsorption mechanism of DA@PDA composite nanofibers was also demonstrated.

Hierarchically Active Poly(vinylidene fluoride) Membrane Fabricated by In Situ Generated Zero-Valent Iron for Fouling Reduction

Sodium hypochlorite (NaClO) solution is a typical cleaning agent for membrane fouling. However, it can damage membrane chemical structures and produce toxic disinfection byproducts, which in turn reduces the membrane performance. This study focuses on the fabrication of active membranes thereby overcoming the limitations of chemical cleaning. A hierarchical active poly(vinylidene fluoride) membrane with polydopamine/polyethyleneimine (PEI) co-supported iron nanoparticle (Fe NP) catalysts was successfully constructed and denoted as a Fe-HP-membrane. The Fe-HP-membrane exhibited excellent advanced oxidation activity with maximum flux recoveries (∼85% with bovine serum albumin [BSA] and ∼95% with humic acid [HA] solutions). After the static experiment of ∼30 days, the BSA proteins and HA successfully desorbed from the membrane surface. Especially, with a trace amount of hydrogen peroxide (H₂O₂) flowing over the surface of the Fe-HP-membrane, highly exposed active sites were observed. Membrane cleaning showed that the "outside-to-in" active surfaces generated considerable amounts of ^•OH radicals at the interface of BSA or HA and the fouled membrane. As a result, the unwanted foulants were successfully removed from the membrane interface, enabling multiple use of the Fe-HP-membrane. Therefore, backwashing with a small amount of H₂O₂ (0.33 wt %) covered ∼20% of the flux. In contrary, backwashing with NaClO (1 wt %) can only achieve a flux recovery of ∼10% after six consecutive BSA filtration cycles. The Fe-HP-membrane exhibited better HA foulant removal (a flux recovery of ∼51%) after backwashing with H₂O₂ than using NaClO (a flux recovery of ∼43%). Our findings demonstrate a new platform for water treatment and regeneration of fouled membranes.

A nanofiltration membrane prepared by PDA-C3N4 for removal of divalent ions

In this study, a positively charged nanofiltration (NF) membrane was prepared by interfacial polymerization for separation of divalent cations, whereby a nanomaterial (modified graphitic carbon nitride (g-C₃N₄) with poly(dopamine), PDA-C₃N₄) was incorporated into the active layer of the NF membrane. PDA-C₃N₄ sheets were synthesized from g-C₃N₄ sheets prepared by thermal oxidation of melamine, and the preparation conditions of NF membrane were also optimized. The results show that the roughness of PDA-C₃N₄ embedded NF membrane decreases, and the hydrophilicity and the permeation increase. The membrane also shows high rejection for divalent cations (Mg²⁺, Ca²⁺, Ba²⁺, Cu²⁺ and Zn²⁺) but low rejection (36.8%) for monovalent cation (Li⁺), as well as good fouling resistance performance. The fabricated membrane has the potential for treatment of industrial wastewater.

The performance of UiO-66-NH2/graphene oxide (GO) composite membrane for removal of differently charged mixed dyes

The dye wastewater treatment by membrane separation technology has obtained extensive attention in recent years. Nevertheless, it was rare for research on the removal of differently charged mixed dyes. In this study, several UiO-66-NH₂ composite membranes were prepared and optimization experiments were conducted. The performance of composite membranes were evaluated by the removal of cationic (Methylene blue, MB), neutral (Rhodamine B, RB), and anionic (Congo red, CR) dyes. The optimization results demonstrated that the UiO-66-NH₂/graphene oxide (UNG) composite membrane (PUF/PDA/UNG) which was loaded on polyurethane foam modified with polydopamine (PUF/PDA) had the best properties. In filtration experiments, the solution pH exhibited greater effect on the removal efficiency of MB and CR than RB. When NaCl, KCl, CaCl₂ and Na₂SO₄ coexisted in the dye solution, the removal efficiency of MB by PUF/PDA/UNG membrane were 96.62%, 98.17%, 86.39% and 99.34% respectively. The presence of humic acid showed slight inhibitory effect on the removal of MB by PUF/PDA/UNG membrane (71.93%). The experimental results for mixed dyes filtration showed that PUF/PDA/UNG membrane could effectively remove MB, RB and CR in binary (i.e., MB/RB and RB/CR) and ternary (i.e., MB/RB/CR) systems through secondary filtration. And PUF/PDA/UNG membrane could remove MB and CR simultaneously through one-time filtration in MB/CR binary system. The removal mechanism was mainly attributed to the aggregation of mixed dyes, electrostatic interaction between dye molecules and the membrane surface, and hydrogen bonding. All results suggested that the as-prepared PUF/PDA/UNG membrane have great potential in practical treatment of dye wastewater.