Constructing zwitterionic surface of nanofiltration membrane for high flux and antifouling performance

Zwitterionic-enhanced hyperbranched polysiloxane membrane with advanced anti-crude oil fouling for high-efficient oil-in-water emulsion separation

Membrane fouling, attributed to the adhesion of oil droplets, presents a significant challenge to membrane separation technology. The hydrophilic surface modification of polymer membranes has been proven as an effective antifouling strategy. Here, a novel zwitterionic hyperbranched polysiloxane modificated membrane (ZHBPSi@PDA@M) was synthesized through oxidative self-polymerization to form the polydopamine (PDA) adhesion layer, Michael addition, and ring-opening reaction. The surface of ZHBPSi@PDA@M achieved the superhydrophilic modification, acquiring an underwater oil contact angle of 158.6°. Furthermore, the membrane had a high water permeance of 13,924 L m-2 h-1 bar-1, representing a 29-fold increase compared to the nascent membrane, and achieved excellent separation of various oil-in-water emulsions, with permeance and rejection efficiency of 5534 L m-2 h-1 bar-1 and 99.9 %, respectively. Additionally, the ZHBPSi@PDA@M not only achieved excellent acid-base stability, but also possessed good self-cleaning and anti-fouling properties. Moreover, the composite membrane effectively separated crude oil emulsions, achieving a permeance of 3986 L m-2 h-1 bar-1 and a separation efficiency of 98.3 %, thereby offering a novel approach for the treatment of high-viscosity oil-containing wastewater.

Aminolysis-Based Zwitterionic Immobilization on Polyethersulfone Membranes for Enhanced Hemocompatibility: Experimental, Computational, and Ex Vivo Investigations

Dialysis membranes are not hemocompatible with human blood, as the patients are suffering from the blood-membrane interactions' side effects. Zwitterionic structures have shown improved hemocompatibility; however, their complicated synthesis hinders their commercialization. The goal of the study is to achieve fast functionalization for carboxybetaine and sulfobetaine zwitterionic immobilization on PES membranes while comparing the stability and the targeted hemocompatibility. The chemical modification approach is based on an aminolysis reaction. Characterization, computational simulations, and clinical analysis were conducted to study the modified membranes. Atomic force microscopy (AFM) patterns showed a lower mean roughness for carboxybetaine-modified (6.3 nm) and sulfobetaine-modified (7.7 nm) membranes compared to the neat membrane (52.61 nm). The pore size of the membranes was reduced from values above 50 nm for the neat PES to values between 2 and 50 nm for zwitterionized membranes, using Brunauer-Emmett-Teller (BET) analysis. More hydrophilic surfaces led to a growth equilibrium water content (EWC) of nearly 6% for carboxybetaine and 10% for sulfobetaine-modified membranes. Differential scanning calorimetry (DSC) measurements were 12% and 16% stable water for carboxybetaine- and sulfobetaine-modified membranes, respectively. Sulfobetaine membranes showed better compatibility with blood with respect to C5a, IL-1a, and IL-6 biomarkers. Aminolysis-based zwitterionization was found to be suitable for the improvement of hemodialysis membranes. The approach introduced in this paper could be used to modify the current dialysis membranes with minimal change in the production facilities.

Tuning nanofiltration membrane performance: OH-MoS2 nanosheet engineering and divalent cation influence on fouling and organic removal

Natural organic matter (NOM) present in surface water causes severe organic fouling of nanofiltration (NF) membranes employed for the production of potable water. Calcium (Ca2+) and magnesium (Mg2+) are alkaline earth metals present in natural surface water and severely exacerbate organic fouling owing to their ability to cause charge neutralization, complexation, and bridging of NOM and the membrane surface. Hence, it is of practical significance to engineer membranes with properties suitable for addressing organic fouling in the presence of these cations. This study employed OH-functionalized molybdenum disulphide (OH-MoS2) nanosheets as nanofillers via the interfacial polymerization reaction to engineer NF membranes for enhanced removal of NOM and fouling mitigation performance. At an optimized concentration of 0.010 wt.% of OH-MoS2 nanosheet, the membrane was endowed with higher hydrophilicity, negative charge and rougher membrane morphology which enhanced the pure water permeance by 46.33% from 11.2 to 16.39 L m-2 h-1 bar-1 while bridging the trade-off between permeance and salt selectivity. The fouling performance was evaluated using humic acid (HA) and sodium alginate (SA), which represent the hydrophobic and hydrophilic components of NOM in the presence of 0, 0.5, and 1 mM Ca2+ and Mg2+, respectively, and the performance was benchmarked with control and commercial membranes. The modified membrane exhibited normalized fluxes of 95.09% and 93.26% for HA and SA, respectively, at the end of the 6 h filtration experiments, compared to the control membrane at 89.71% and 74.25%, respectively. This study also revealed that Ca2+ has a more detrimental effect than Mg2+ on organic fouling and NOM removal. The engineered membrane outperformed the commercial and the pristine membranes during fouling tests in the presence of 1 mM Ca2+ and Mg2+ in the feed solution. In summary, this study has shown that incorporating OH-MoS2 nanosheets into membranes is a promising strategy for producing potable water from alternative water sources with high salt and NOM contents.

Removal of natural organic matter from surface water sources by nanofiltration and surface engineering membranes for fouling mitigation - A review

Given that surface water is the primary supply of drinking water worldwide, the presence of natural organic matter (NOM) in surface water presents difficulties for water treatment facilities. During the disinfection phase of the drinking water treatment process, NOM aids in the creation of toxic disinfection by-products (DBPs). This problem can be effectively solved using the nanofiltration (NF) membrane method, however NOM can significantly foul NF membranes, degrading separation performance and membrane integrity, necessitating the development of fouling-resistant membranes. This review offers a thorough analysis of the removal of NOM by NF along with insights into the operation, mechanisms, fouling, and its controlling variables. In light of engineering materials with distinctive features, the potential of surface-engineered NF membranes is here critically assessed for the impact on the membrane surface, separation, and antifouling qualities. Case studies on surface-engineered NF membranes are critically evaluated, and properties-to-performance connections are established, as well as challenges, trends, and predictions for the field's future. The effect of alteration on surface properties, interactions with solutes and foulants, and applications in water treatment are all examined in detail. Engineered NF membranes containing zwitterionic polymers have the greatest potential to improve membrane permeance, selectivity, stability, and antifouling performance. To support commercial applications, however, difficulties related to material production, modification techniques, and long-term stability must be solved promptly. Fouling resistant NF membrane development would be critical not only for the water treatment industry, but also for a wide range of developing applications in gas and liquid separations.

Designing sustainable membrane-based water treatment via fouling control through membrane interface engineering and process developments

Membrane-based water treatment processes have been established as a powerful approach for clean water production. However, despite the significant advances made in terms of rejection and flux, provision of sustainable and energy-efficient water production is restricted by the inevitable issue of membrane fouling, known to be the major contributor to the elevated operating costs due to frequent chemical cleaning, increased transmembrane resistance, and deterioration of permeate flux. This review provides an overview of fouling control strategies in different membrane processes, such as microfiltration, ultrafiltration, membrane bioreactors, and desalination via reverse osmosis and forward osmosis. Insights into the recent advancements are discussed and efforts made in terms of membrane development, modules arrangement, process optimization, feed pretreatment, and fouling monitoring are highlighted to evaluate their overall impact in energy- and cost-effective water treatment. Major findings in four key aspects are presented, including membrane surface modification, modules design, process integration, and fouling monitoring. Among the above mentioned anti-fouling strategies, a large part of research has been focused on membrane surface modifications using a number of anti-fouling materials whereas much less research has been devoted to membrane module advancements and in-situ fouling monitoring and control. At the end, a critical analysis is provided for each anti-fouling strategy and a rationale framework is provided for design of efficient membranes and process for water treatment.

Enhancing Stability of Tannic Acid-FeIII Nanofiltration Membrane for Water Treatment: Intercoordination by Metal-Organic Framework

Tannic acid (TA)-Fe^III nanofiltration (NF) membrane has been demonstrated to possess more favorable removal of trace organic contaminants (TrOCs) over the conventional polyamide NF membrane. However, the drawback of acid instability severely hinders the practical application of TA-Fe^III NF membrane in the treatment of (weak) acidic wastewater containing TrOCs (e.g., pharmaceutical wastewater, surface water, and drinking water). Herein, we introduced the MIL-101(Cr) nanoparticle, a kind of metal-organic framework (MOF), into the TA-Fe^III selective layer to enhance the membrane acid stability. The acid-tolerance parameter of MIL-101(Cr)-stabilized TA-Fe^III membrane (TA-Fe^III-MOF membrane, 12,000 ppm/s^-1) was two orders of magnitude larger than that of the TA-Fe^III membrane (50 ppm/s^-1), and the TA-Fe^III-MOF membrane can withstand acid treatment at pH = 4 for more than 30 days. Meanwhile, the TA-Fe^III-MOF membrane displayed increased water permeance from 9.5 to 12.7 L/(m²·h·bar) after the MOF addition, without compromising the selectivity. The enhanced acid stability for the TA-Fe^III-MOF membrane was ascribed to an intercoordination mechanism, where Fe^III centers (from TA-Fe^III complex) coordinated with -COOH groups (from terephthalic acid of MOF) and Cr^III centers (from MOF) coordinated with -OH groups (from TA of TA-Fe^III complex), which was verified by the density functional theory calculation. This study highlights a new approach for the development of a TA-Fe^III-based NF membrane with markedly enhanced acid stability, which is important for its real application in wastewater treatment and water reuse.

Removal of perfluorooctanoic acid via polyethyleneimine modified graphene oxide: Effects of water matrices and understanding mechanisms

This research aimed to evaluate the adsorption behaviors and mechanisms of perfluorooctanoic acid (PFOA) onto polyethyleneimine modified graphene oxide (GO-PEI) from aqueous solutions. The adsorption capacity was significantly improved by doping polyethyleneimine (PEI) onto graphene oxide (GO). The Brunauer-Emmett-Teller (BET) isotherm model was considered as the best isotherm model in describing the PFOA adsorption onto GO-PEI3 (w_PEI/w_GO = 3). GO-PEI3 exhibited high adsorption capacity (q_e = 368.2 mg/g, calculated from BET isotherm model) and excellent stability. The maximum monolayer amount of PFOA adsorption onto GO-PEI3 (q_m = 231.2 mg/g) was successfully evaluated. The calculated saturated concentration (C_s = 169.9 mg/L) of PFOA on GO-PEI3 closely agrees with its critical micelle concentration (CMC = 157.0 mg/L), suggesting the formation of multilayer hemi-micelles or micelles PFOA structures on the surface of GO-PEI3. PFOA adsorption onto GO-PEI3 was inhibited by several factors including: the presence of humic acid (HA) by competing with the adsorption sites, background salts through the double-layer compression effect, and the competition from soluble ions for the amine or amide functional groups on GO-PEI3. Finally, both the FT-IR and XPS results confirmed that the adsorption of PFOA onto GO-PEI3 was through electrostatic attraction and hydrophobic interaction (physical adsorption), but not chemical adsorption. This work provides fundamental knowledge both in understanding the adsorption behavior through the BET isotherm model and in developing a stable adsorbent for PFOA adsorption. In addition, the findings highlight the potential of PFOA remediation from wastewater systems using GO-PEI in engineering applications.

Single-walled carbon nanotube gutter layer supported ultrathin zwitterionic microporous polymer membrane for high-performance lithium-sulfur battery

Development of efficient lithium-sulfur (Li-S) battery requires the need to develop an appropriate functional separator that allows strong facilitation and transport of lithium ions together with limited passage of polysulfides. In this work, a multifunctional separator (TB-BAA/SWCNT/PP) is developed through spin coating of a novel zwitterionic microporous polymer (TB-BAA) on the gutter layer constructed from single-walled carbon nanotubes (SWCNT), where commercially available polypropylene (PP) separator is used to act as the mechanical support. SWCNT in this study serves as the first modification layer to decrease the size of the macropores in the PP separator, while the ultrathin TB-BAA top barrier layer with the presence of zwitterionic side chains allows the creation of confined ionic channels with both lithiophilic and sulfophilic groups. Due to the presence of available chemical interactions with lithium polysulfides, selective ion transport can be foreseen through such separator. In this regard, shuttle effect that is frequently encountered in Li-S battery can be suppressed effectively via implementing the as-obtained functional separator, resulting in the creation of credible and stable sulfur electrochemistry. The TB-BAA/SWCNT/PP-based Li-S battery has been investigated to possess high cycling ability (capacity fading per cycle of 0.055% over 500 cycles at 1 C) together with decent rate capability (736.6 mAh g^-1 at 3 C). In addition, a high areal capacity retention of 5.03 mAh cm^-2 after 50 cycles can be also obtained under raised sulfur loading (5.4 mg cm^-2).

In Situ Chemical Modification with Zwitterionic Copolymers of Nanofiltration Membranes: Cure for the Trade-Off between Filtration and Antifouling Performance

Breaking the trade-off between filtration performance and antifouling property is critical to enabling a thin-film nanocomposite (TFC) nanofiltration (NF) membrane for a wide range of feed streams. We proposed a novel design route for TFC NF membranes by grafting well-defined zwitterionic copolymers of [2-(methacryloyloxy)ethyl]dimethyl-(3-sulfopropyl)ammonium hydroxide (SBMA) and 2-aminoethyl methacrylate hydrochloride (AEMA) on the polyamide surfaces via an in situ surface chemical modification process. The successful grafting of a zwitterionic copolymer imparted the modified NF membranes with better surface hydrophilicity, a larger actual surface area (i.e., nodular structures), and a thinner polyamide layer. As a result, the water permeability of the modified membrane (i.e., TFC-10) was triple that of the pristine TFC membrane while maintaining high Na₂SO₄ rejection. We further demonstrated that the TFC-10 membrane possessed exceptional antifouling properties in both static adsorption tests and three cycles of dynamic protein and humic acid fouling tests. To recap, this work provides valuable insights and strategies for the fabrication of TFC NF membranes with simultaneously enhanced filtration performance and antifouling property.

Bioinspired Graphene Oxide Membranes with pH-Responsive Nanochannels for High-Performance Nanofiltration

Tunable gating graphene oxide (GO) membranes with high water permeance and precise molecular separation remain highly desired in smart nanofiltration devices. Herein, bioinspired by the filtration function of the renal glomerulus, we report a smart and high-performance graphene oxide membrane constructed via introducing positively charged polyethylenimine-grafted GO (GO-PEI) to negatively charged GO nanosheets. It was found that the additional GO-PEI component changed the surface charge, improved the hydrophilicity, and enlarged the nanochannels. The glomerulus-inspired graphene oxide membrane (G-GOM) shows a water permeance up to 88.57 L m^-2 h^-1 bar^-1, corresponding to a 4 times enhancement compared with that of a conventional GO membrane due to the enlarged confined nanochannels. Meanwhile, owing to the electrostatic interaction, it can selectively remove positively charged methylene blue at pH 12 and negatively charged methyl orange at pH 2, with a removal rate of over 96%. The high and cyclic water permeance and highly selective organic removal performance can be attributed to the synergic effect of controlled nanochannel size and tunable electrostatic interaction in responding to the environmental pH. This strategy provides insight into designing pH-responsive gating membranes with tunable selectivity, representing a great advancement in smart nanofiltration with a wide range of applications.

Liquid-to-Solid Phase Transitions of Imidazolium-Based Zwitterionic Polymers Induced by Hofmeister Anions

In this study, we compared the responses of two different types of zwitterionic polymers (ZPs), polyvinylimidzole sulfobetaine (poly(SBVI)) and polymethacrylate sulfobetaine (poly(SBMA)) to Hofmeister anions. Although the anions of the two ZPs were the same as the sulfonate anions and only the types of their cations were different from each other, the aggregation behavior of each in the salt aqueous solution was remarkably different. Consequently, poly(SBVI) exhibited both salting-in and salting-out effects depending on the type and concentration of salt, while poly(SBMA) only exhibited the anti-polyelectrolyte effect. The results of this study provide a deeper understanding of the behavior of zwitterionic polymers in salt solutions and will greatly expand their applications.

Enhanced Water Permeability and Antifouling Property of Coffee-Ring-Textured Polyamide Membranes by In Situ Incorporation of a Zwitterionic Metal-Organic Framework

Modulation of the polyamide structure is critically important for the reverse-osmosis performance of thin-film composite (TFC) membranes in the field of water reuse and desalination. Herein, zwitterionic nanoparticles of zeolitic imidazolate framework-8 (PZ@ZIF-8) were fabricated and incorporated into the polyamide active layer through the interfacial polymerization method. A hydrophilic, zwitterionic coffee-ring structure was formed on the surface of polyamide thin-film nanocomposite (TFN) membranes due to the adjusted diffusion rate of m-phenylenediamine (MPD) from the aqueous phase into the organic phase during the interfacial polymerization process. Surface characterization demonstrated that the coffee-ring structure increased the amounts of water transport channels on the membrane surface and the intrinsic pores of PZ@ZIF-8 maintained the salt rejection. Antifouling and bactericidal activities of TFN membranes were enhanced remarkably owing to the bacterial-"defending" and bacterial-"attacking" behaviors of hydrophilic and zwitterionic groups from PZ@ZIF-8 nanoparticles. This work would provide a promising method for the application of MOFs to enhance the bio-/organic-fouling resistance of TFN membranes with high water permeation and salt rejection.

Incorporation of Core-Shell-Structured Zwitterionic Carbon Dots in Thin-Film Nanocomposite Membranes for Simultaneously Improved Perm-Selectivity and Antifouling Properties

The development of highly efficient thin-film nanocomposite (TFN) membranes with superior water permeability, maintained rejection performance, and excellent antifouling capacity is critical to meeting the ever-escalating demand for fresh water. Herein, carbon dots (CDs) grafted with hyperbranched zwitterions, denoted as CDs-ZPEI_0.6-10k, were first prepared by the hydrothermal treatment of citric acid in the presence of zwitterionic hyperbranched polyethylenimine (ZPEI_0.6-10k) with different molecular weights (0.6, 1.8, and 10 kDa). Subsequently, the synthesized nanoparticles were introduced in membrane fabrication to form CDs-ZPEI_0.6-10k-embedded TFN (TFN-CDs-ZPEI_0.6-10k) membranes. The grafted shells of superhydrophilic ZPEI not only increased the chemical compatibility of CDs in the polyamide layer to suppress the formation of nonselective voids but also created a densely packed network for efficient water transportation and effective divalent salt rejection. The TFN-CDs-ZPEI_10k membrane demonstrated a 2.8-fold enhancement in the permeate flux with an increased Na₂SO₄ rejection rate of 98.1% and improved antifouling properties than the pristine thin-film composite (TFC) membrane. This work provides an insight into the development of functionalized core-shell structured nanoparticles to effectively overcome the permeability-selectivity trade-off limitations and fouling problems in TFC membranes.

Sharpening Nanofiltration: Strategies for Enhanced Membrane Selectivity

Nanofiltration plays an increasingly large role in many industrial applications, such as water treatment (e.g., desalination, water softening, and fluoride removal) and resource recovery (e.g., alkaline earth metals). Energy consumption and benefits of nanofiltration processes are directly determined by the selectivity of the nanofiltration membranes, which is largely governed by pore-size distribution and Donnan effects. During operation, the separation performance of unmodified nanofiltration membranes will also be impacted (deleteriously) upon unavoidable membrane fouling. Many efforts, therefore, have been directed toward enhancing the selectivity of nanofiltration membranes, which can be classified into membrane fabrication method improvement and process intensification. This review summarizes recent developments in the field and provides guidance for potential future approaches to improve the selectivity of nanofiltration membranes.

Carbodiimide-assisted zwitterionic modification of poly(piperazine amide) thin-film composite membrane for enhanced separation and anti-depositing performances to cationic/anionic dye aqueous solutions

In this work, a novel method of carbodiimide-assisted zwitterionic modification was proposed and implemented to incorporate zwitterionic moieties onto poly(piperazine amide) membrane for improved water permeability and anti-depositing property, which are crucial for highly efficient nanofiltration of dye-contained effluents. Carboxyl groups of polyamide layer were firstly transferred into N-acylurea using excess l-ethyl-3-(3-(dimethylamino)propyl)-carbodiimide. Zwitterions were then incorporated through ring-opening reaction between tertiary amine groups of N-acylurea and 1, 4-butanesultone. Carbodiimide-assisted zwitterionic modification was verified by ATR-IR and XPS analyses and was found to not affect membrane pore size but significantly enhance membrane's permeation and anti-dye-deposition performances. Compared with those of virgin membrane, water permeabilities of the desired zwitterionic membrane to pure water, Congo red aqueous solution and Victoria blue B aqueous solution were higher by 42.9, 62.3 and 95.2 %, respectively, hydraulic resistances from irreversible deposition of Congo red and Victoria blue B molecules were dramatically lowered by 68.4 and 91.8 %, respectively. Furthermore, the perm-selectivity performance of the desired zwitterionic membrane in terms of molecular weight cut-off and pure water permeability was better than most of the reported zwitterionic membranes, and the separation and anti-depositing performances to both anionic and cationic dye aqueous solutions were better than commercial membrane NF270.

Superhydrophilic Sub-1-nm Porous Membrane with Electroneutral Surface for Nonselective Transport of Small Organic Molecules

The study of traditional Chinese medicines (TCMs) is receiving increasing attention worldwide because of their contribution to human health. Developing an effective and sustainable method for screening TCMs is highly desired to accelerate the modernization of TCMs. In this work, we report a neutrally charged membrane made of a positively charged polyelectrolyte electrostatically assembled on a negatively charged superhydrophilic nanoporous membrane. The composite membrane possesses stable electroneutrality in a wide pH range and can precisely and nonselectively separate various charged molecules in TCMs with a transmittance higher than 90% for molecules with molecular weight (M_w) < 400 and a high rejection of 90% for molecules with M_w > 800. In addition, the membrane exhibits a superior antifouling performance, and the recovery ratio observed during a continuous cycling test of a simulated TCM solution was more than 93%. The combination of superhydrophilicity and electroneutrality in a nanoporous membrane provides a new route for designing nanofiltration membranes for highly efficient molecule separation and is promising for screening TCMs.

High-Performance Zwitterionic Nanofiltration Membranes Fabricated via Microwave-Assisted Grafting of Betaine

The thin-film composite (TFC) nanofiltration (NF) membrane is a very important method in solving the water crisis. However, the fabrication and industrialization of high-performance NF membranes still remains challenging. In this work, zwitterionic NF membranes via microwave-assisted grafting of betaine was first proposed. The resulting polyamide layer showed leaflike nanostructures after modification. Because of the enlarged permeation area and enhanced hydrophilicity derived from the unique leaflike structure, the optimal membrane permeability reached 40.8 L m^-1 h^-1 bar^-1. This water permeance was 2.2 times as high as the original polypiperazine-amide membrane, with a Na₂SO₄ rejection maintained at 97.0%. More importantly, the membrane demonstrated excellent selectivity to monovalent and divalent anions. This zwitterionic membrane fabricated by microwave-assisted grafting of betaine provides new insight for industrial scalable NF membranes with great potentials.

Superhydrophilicity and strong salt-affinity: Zwitterionic polymer grafted surfaces with significant potentials particularly in biological systems

In recent years, zwitterionic polymers have been frequently reported to modify various surfaces to enhance hydrophilicity, antifouling and antibacterial properties, which show significant potentials particularly in biological systems. This review focuses on the fabrication, properties and various applications of zwitterionic polymer grafted surfaces. The "graft-from" and "graft-to" strategies, surface grafting copolymerization and post zwitterionization methods were adopted to graft lots type of the zwitterionic polymers on different inorganic/organic surfaces. The inherent hydrophilicity and salt affinity of the zwitterionic polymers endow the modified surfaces with antifouling, antibacterial and lubricating properties, thus the obtained zwitterionic surfaces show potential applications in biosystems. The zwitterionic polymer grafted membranes or stationary phases can effectively separate plasma, water/oil, ions, biomolecules and polar substrates. The nanomedicines with zwitterionic polymer shells have "stealth" effect in the delivery of encapsulated drugs, siRNA or therapeutic proteins. Moreover, the zwitterionic surfaces can be utilized as wound dressing, self-healing or oil extraction materials. The zwitterionic surfaces are expected as excellent support materials for biosensors, they are facing the severe challenges in the surface protection of marine facilities, and the dense ion pair layers may take unexpected role in shielding the grafted surfaces from strong electromagnetic field.