Membranes with Surface-Enhanced Antifouling Properties for Water Purification

Enhanced Antifouling Capability of In Situ-Grown Hydrophilic-Hydrophobic Nanodomains on Membrane Surface in the Ultralow Pressurized Ultrafiltration Process

Although hydrophilic modification of the membrane surface is widely adopted, polymeric membranes still suffer from irreversible fouling caused by hydrophilic components in surface water. Here, an ultrathin hydrogel layer (40 nm) with hydrophilic-hydrophobic textures was in situ grown onto the polysulfone ultrafiltration membrane surface using an organic-radical-initiated interfacial polymerization technique. The interfacial polymerization of hydrophilic and hydrophobic monomers ensured the molecular-scale distribution of hydrophilic and hydrophobic nanodomains on the membrane surface. These nanodomains, with their molecular lengths, facilitated dynamic repulsion interactions between the uniformly textured surface and foulant components with different degrees of hydrophilicity. Chemical force characterization confirmed that the adhesion force between the hydrophilic-hydrophobic textured membrane surface and foulants (dodecane, bovine serum albumin, and humic acid) was greatly reduced. Dynamic filtration experiments showed that a hydrophilic-hydrophobic textured membrane always possessed the largest water flux and the best antifouling performance. Furthermore, the foulant coverage ratio on the membrane surface was first evaluated by measuring changes in surface streaming potentials, which demonstrated a 69% reduction in the amount of foulant adhering to the hydrophilic-hydrophobic textured membrane surface. Therefore, the construction of hydrophilic-hydrophobic nanodomains on the membrane surface provides a promising strategy for alleviating membrane fouling caused by both hydrophobic and hydrophilic components during ultralow pressurized ultrafiltration processes.

State-of-the-Art Advances and Current Applications of Gel-Based Membranes

Gel-based membranes, a fusion of polymer networks and liquid components, have emerged as versatile tools in a variety of technological domains thanks to their unique structural and functional attributes. Historically rooted in basic filtration tasks, recent advancements in synthetic strategies have increased the mechanical strength, selectivity, and longevity of these membranes. This review summarizes their evolution, emphasizing breakthroughs that have positioned them at the forefront of cutting-edge applications. They have the potential for desalination and pollutant removal in water treatment processes, delivering efficiency that often surpasses conventional counterparts. The biomedical field has embraced them for drug delivery and tissue engineering, capitalizing on their biocompatibility and tunable properties. Additionally, their pivotal role in energy storage as gel electrolytes in batteries and fuel cells underscores their adaptability. However, despite monumental progress in gel-based membrane research, challenges persist, particularly in scalability and long-term stability. This synthesis provides an overview of the state-of-the-art applications of gel-based membranes and discusses potential strategies to overcome current limitations, laying the foundation for future innovations in this dynamic field.

Nano-clay modified membranes: A promising green strategy for microalgal antifouling filtration

Membrane fouling is a major challenge which limits the sustainable application of membrane filtration-based microalgal harvesting at industrial level. Membrane fouling leads to increased operational and maintenance costs and represents a major obstacle to microalgal downstream processing. Nano-clays are promising naturally occurring nanoparticles in membrane fabrication due to their low-cost, facile preparation, and their superior properties in terms of surface hydrophilicity, mechanical stability, and resistance against chemicals. The membrane surface modification using nano-clays is a sustainable promising approach to improve membranes mechanical properties and their fouling resistance. However, the positive effects of nano-clay particles on membrane fouling are often limited by aggregation and poor adhesion to the base polymeric matrix. This review surveys the recent efforts to achieve anti-fouling behavior using membrane surface modification with nano-clay fillers. Further, strategies to achieve a better incorporation of nano-clay in the polymer matrix of the membrane are summarised, and the factors that govern the membrane fouling, stability, adhesion, agglomeration and leaching are discussed in depth.

Polyethylene glycol-grafted graphene oxide nanosheets in tailoring the structure and reverse osmosis performance of thin film composite membrane

Introducing hydrophilic polymers such as polyethylene glycol (PEG) within the polyamide (PA) layer of thin film composite (TFC) membranes helps achieve high water desalination performance. Here, PEGs of different molecular weights (X: 1500, 6000, 16,000 g/mol) are effectively introduced into the PA layer of TFC membranes utilizing PEG-grafted graphene oxide (GOPX) nanosheets and their effects on the physicochemical properties and reverse osmosis (RO) performance of the thin film nanocomposite (TFN) membranes are investigated. Among the TFNs prepared the GOP16000/TFN exhibits the best performance with 68% improvement in water flux and almost constant salt rejection compared to those of the bare TFC. The influence of PEG molecular weight on the RO performance of the membranes is interpreted by different surface and bulk hydrophilicity as well as thickness and surface roughness of PA layers of GOPX/TFNs. Furthermore, TFNs with thinner and smoother PA layers and thus higher water flux are obtained by dispersing GOPXs in the aqueous phase of the PA interfacial polymerization reaction than by dispersing them in the organic phase of the reaction. Finally, the high antifouling potential of TFNs containing PEG-grafted GOs is demonstrated.

Nanofiltration Membranes for the Removal of Heavy Metals from Aqueous Solutions: Preparations and Applications

Water shortages are one of the problems caused by global industrialization, with most wastewater discharged without proper treatment, leading to contamination and limited clean water supply. Therefore, it is important to identify alternative water sources because many concerns are directed toward sustainable water treatment processes. Nanofiltration membrane technology is a membrane integrated with nanoscale particle size and is a superior technique for heavy metal removal in the treatment of polluted water. The fabrication of nanofiltration membranes involves phase inversion and interfacial polymerization. This review provides a comprehensive outline of how nanoparticles can effectively enhance the fabrication, separation potential, and efficiency of NF membranes. Nanoparticles take the form of nanofillers, nanoembedded membranes, and nanocomposites to give multiple approaches to the enhancement of the NF membrane's performance. This could significantly improve selectivity, fouling resistance, water flux, porosity, roughness, and rejection. Nanofillers can form nanoembedded membranes and thin films through various processes such as in situ polymerization, layer-by-layer assembly, blending, coating, and embedding. We discussed the operational conditions, such as pH, temperature, concentration of the feed solution, and pressure. The mitigation strategies for fouling resistance are also highlighted. Recent developments in commercial nanofiltration membranes have also been highlighted.

Advanced (bio)fouling resistant surface modification of PTFE hollow-fiber membranes for water treatment

Membrane surface treatment to modify anti-(bio)fouling resistivity plays a key role in membrane technology. This paper reports on the successful use of air-stimulated surface polymerization of dopamine hydrochloride incorporated ZnO nanoparticles (ZnO NPs) for impeding the intrinsic hydrophobicity and low anti-(bio)fouling resistivity of polytetrafluoroethylene (PTFE) hollow-fiber membranes (HFMs). The study involved the use of pristine and polydopamine (Pdopa) coated PTFE HFMs, both with and without the presence of an air supply and added ZnO NPs. Zeta potential measurements were performed to evaluate the dispersion stability of ZnO NPs prior to immobilization, while morphological characterization and time-dependency of the Pdopa growth layer were illustrated through scanning electron microscopy. Pdopa surface polymerization and ZnO NPs immobilization were confirmed using FT-IR and EDX spectroscopy. Transformation of the PTFE HFM surface features to superhydrophilic was demonstrated through water contact angle analysis and the stability of immobilized ZnO NPs assessed by ICP analysis. Anti-fouling criteria and (bio)fouling resistivity performance of the surface-modified membranes were assessed through flux recovery determination of bovine serum albumin in dead-end filtration as well as dynamic-contact-condition microbial evaluation against Staphylococcus spp. and Escherichia coli, respectively. The filtration recovery ratio and antimicrobial results suggested promising surface modification impacts on the anti-fouling properties of PTFE HFM. As such, the method represents the first successful use of air-stimulated Pdopa coating incorporating ZnO NPs to induce superhydrophilic PTFE HFM surface modification. Such a method can be extended to the other membranes associated with water treatment processes.

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.

A Review on Membrane Biofouling: Prediction, Characterization, and Mitigation

Water scarcity is an increasing problem on every continent, which instigated the search for novel ways to provide clean water suitable for human use; one such way is desalination. Desalination refers to the process of purifying salts and contaminants to produce water suitable for domestic and industrial applications. Due to the high costs and energy consumption associated with some desalination techniques, membrane-based technologies have emerged as a promising alternative water treatment, due to their high energy efficiency, operational simplicity, and lower cost. However, membrane fouling is a major challenge to membrane-based separation as it has detrimental effects on the membrane's performance and integrity. Based on the type of accumulated foulants, fouling can be classified into particulate, organic, inorganic, and biofouling. Biofouling is considered the most problematic among the four fouling categories. Therefore, proper characterization and prediction of biofouling are essential for creating efficient control and mitigation strategies to minimize the damage associated with biofouling. Moreover, the use of artificial intelligence (AI) in predicting membrane fouling has garnered a great deal of attention due to its adaptive capability and prediction accuracy. This paper presents an overview of the membrane biofouling mechanisms, characterization techniques, and predictive methods with a focus on AI-based techniques, and mitigation strategies.

Mussel-Inspired Clickable Antibacterial Peptide Coating on Ureteral Stents for Encrustation Prevention

Long-term indwelling catheters or stents often cause complications like infection, encrustation, hematuria, pain, and so on. The source of these problems is bacteria, which can form biofilms on the stents to reduce antibiotic sensitivity and produce urease to form encrustation by increasing the urine pH. Urinary tract infection (UTI) can aggravate the body damage and even seriously endanger lives, and the encrustation will block the stents, which can cause hydronephrosis and renal function damage. Therefore, the prevention of UTI and encrustation represents a great challenge in clinical ureteral stent uses. In this work, a clickable mussel-inspired peptide and antimicrobial peptide (AMP) were used to functionalize the commercial stents' surfaces to inhibit long-term infection and encrustation caused by bacteria. Copper (Cu) ions were used to coordinate the mussel-inspired peptide to improve the stability. The AMP with an azido group was clicked to the mussel-inspired Cu-coordinated peptide coating through click chemistry. The bio-inspired antibacterial coating was constructed with excellent stability, bactericidal properties, and improved biological compatibility. In in vitro and in vivo experiments, it was further found that the coating showed bactericidal and encrustation reduction abilities. This study thus developed an effective, safe, and stable AMP coating on urinary stents/catheters capable of long-term antibacterial and encrustation inhibition.

Functional Nanohybrids and Nanocomposites Development for the Removal of Environmental Pollutants and Bioremediation

World population growth, with the consequent consumption of primary resources and production of waste, is progressively and seriously increasing the impact of anthropic activities on the environment and ecosystems. Environmental pollution deriving from anthropogenic activities is nowadays a serious problem that afflicts our planet and that cannot be neglected. In this regard, one of the most challenging tasks of the 21st century is to develop new eco-friendly, sustainable and economically-sound technologies to remediate the environment from pollutants. Nanotechnologies and new performing nanomaterials, thanks to their unique features, such as high surface area (surface/volume ratio), catalytic capacity, reactivity and easy functionalization to chemically modulate their properties, represent potential for the development of sustainable, advanced and innovative products/techniques for environmental (bio)remediation. This review discusses the most recent innovations of environmental recovery strategies of polluted areas based on different nanocomposites and nanohybrids with some examples of their use in combination with bioremediation techniques. In particular, attention is focused on eco-friendly and regenerable nano-solutions and their safe-by-design properties to support the latest research and innovation on sustainable strategies in the field of environmental (bio)remediation.

Highly selective and antifouling reverse osmosis membrane by crosslinkers induced surface modification

Post treatment is a very competent and scalable approach to develop the higher water-flux and salt-rejection membrane since it does not require any change in existing manufacturing process. The virgin Thin Film Composite Reverse Osmosis (TFC-RO) membrane was exposed to various concentrations of Poly (ethylene Glycol) diacrylate (PEGDA) and Ethylene glycol dimethacrylate (EGDMA) after activation with sodium hypochlorite for 1 h. Crosslinkers modified membranes were characterized for degree of wettability by contact angle analyses, surface morphology and surface roughness study by Scanning electron micrographs and Atomic force micrographs, chemical structural modifications by Attenuated total reflectance Fourier transform Infrared spectroscopy. These treatments resulted in improved membrane performance. 3500 mg/l PEGDA-treated membrane permeate flux increased by 48.7% and salt-rejection by 3.43%. 2000 mg/l EGDMA treated membrane demonstrated 46.13% increase in water-flux and 3.08% increase in salt-rejection as compared with virgin membrane. Organic fouling study indicated that fouling in PEGDA-treated membrane was significantly lower than virgin membrane. Heavy metal ion removal performances for Zinc and Chromium were also higher for modified membranes. Thus, the surface modification by crosslinkers led to increase in selectivity for certain metal ions and better antifouling performance as compared to virgin membrane.

Ultrafiltration Membranes Functionalized with Copper Oxide and Zwitterions for Fouling Resistance

Polymeric membrane fouling is a long-standing challenge for water filtration. Metal/metal oxide nanoparticle functionalization of the membrane surface can impart anti-fouling properties through the reactivity of the metal species and the generation of radical species. Copper oxide nanoparticles (CuO NPs) are effective at reducing organic fouling when used in conjunction with hydrogen peroxide, but leaching of copper ions from the membrane has been observed, which can hinder the longevity of the CuO NP activity at the membrane surface. Zwitterions can reduce organic fouling and stabilize NP attachment, suggesting a potential opportunity to combine the two functionalizations. Here, we coated polyethersulfone (PES) ultrafiltration membranes with polydopamine (PDA) and attached the zwitterionic compound, thiolated 2-methacryloyloxyethyl phosphorylcholine (MPC-SH), and CuO NPs. Functionalized membranes resulted in a higher flux recovery ratio (0.694) than the unfunctionalized PES control (0.599). Copper retention was high (>96%) for functionalized membranes. The results indicate that CuO NPs and MPC-SH can reduce organic fouling with only limited copper leaching.

The synthesis of an amended membrane coated with graphene oxide and dopamine and guanidyl-based modifier and its antifouling properties

The membrane fouling issue has aroused great concern. To improve their antifouling properties, surface grafting with oxidative deposition were employed to amend a polyvinylidene fluoride (PVDF) membrane. The modifiers were amino-modified graphene oxide (AMGO), dopamine (DPA) and 1,3-diaminoguanidine hydrochloride (DAGH). To take bovine serum albumin (BSA, 1 g/l) as an example of organic materials, BSA interception rate and pure water flux recovery rate increased to 93.65% and 66.74%, respectively, while the corresponding values for the original membrane were much lower (72.82% and 31.72%). The optimum synthesis conditions were found to be 1.5 mg/ml of DPA, 1 wt% of DAGH, 2 mg/ml of AMGO, 4 h of DPA oxidation deposition time and 1 h of AMGO grafting time. Many functional groups like C = N, -NH₂, C = O and -OH improved the membrane surface hydrophilicity leading to a higher resistance to organic pollution. Dopamine and guanidyl facilitated the antimicrobial performance of the modified membrane, whose antimicrobial rate was up to 96%, while the raw membrane had no antimicrobial activity. The amended membrane possessed 40% higher mechanical strength than the initial one. It could withstand a high pumping suction force. The noteworthy property was that the irreversible fouling rate decreased by 55%. Therefore, the amended membrane could restore its flux much more easily.

Dopamine Assisted Self-Cleaning, Antifouling, and Antibacterial Coating via Dynamic Covalent Interactions

The rapid accumulation of dead bacteria or protein on a bactericidal surface can reduce the effectiveness of the modified surface and alter its biocidal activity by shielding the surface biocide functional groups, promoting microbial attachment and subsequent biofilm formation. Thus, the alteration of biocidal activity due to biofilm formation can cause serious trouble including severe infection or implant or medical device failure leading to death. Therefore, developing a smart self-cleaning surface is of great interest. Ideally, such a surface can not only kill the attached microbials but also release the dead cells and foulants from the surface under a particular incitement on demand. In this project, a sugar-responsive self-cleaning coating has been developed by forming covalent boronic ester bonds between catechol groups from polydopamine and a benzoxaborole pendant from zwitterionic and cationic polymers. To incorporate antifouling properties and enhance the biocompatibility of the coating, bioinspired zwitterionic compound 2-methacryloyloxyethyl phosphorylcholine (MPC) was chosen and benzoxaborole pendant containing zwitterionic polymer poly(MPC-st-MAABO) (MAABO: 5-methacrylamido-1,2-benzoxaborole) was synthesized. Additionally to impart antibacterial properties to the surface, a quaternary ammonium containing cationic polymer poly(2-(methacryloyloxy)ethyl trimethylammonium (META)-st-MAABO)) was synthesized. These synthesized polymers were covalently grafted to a polydopamine (PDA) coated surface by forming a strong cyclic boronic ester complex with a catechol group of the PDA layer endowing the surface with bacteria contact-killing properties and capturing specific protein. After the addition of cis-diol containing competitive molecules, i.e., saccharides/sugars, this boronic ester complex with a catechol group of PDA was replaced and the attached polymer layer was cleaved from the surface, resulting in the release of both absorbed protein and live/killed bacteria electrostatically attached to the polymer layer. This dynamic self-cleaning surface can be a promising material for biomedical applications avoiding the gathering of dead cells and debris that are typically encountered on a traditional biocidal surface.

Biomolecule-Enabled Liquid Separation Membranes: Potential and Recent Progress

The implementation of membrane surface modification to enhance the performance of membrane-based separation has become a favored strategy due to its promise to address the trade-off between water permeability and salt rejection as well as to improve the durability of the membranes. Tremendous work has been committed to modifying polymeric membranes through physical approaches such as surface coating and ontology doping, as well as chemical approaches such as surface grafting to introduce various functional groups to the membrane. In the context of liquid separation membranes applied for desalination and water and wastewater treatment, biomolecules have gained increasing attention as membrane-modifying agents due to their intriguing structural properties and chemical functionalities. Biomolecules, especially carbohydrates and proteins, exhibit attractive features, including high surface hydrophilicity and zwitterionic and antimicrobial properties that are desired for liquid separation membranes. In this review, we provide an overview of the recent developments in biomolecule-enabled liquid separation membranes. The roles and potentials of some commonly explored biomolecules in heightening the performance of polymeric membranes are discussed. With the advancements in material synthesis and the need to answer the call for more sustainable materials, biomolecules could serve as attractive alternatives for the development of high-performance composite membranes.

Bio-inspired antibacterial coatings on urinary stents for encrustation prevention

Urinary tract infection (UTI) represents one of the most common nosocomial infections, which is mainly related to indwelling catheters or stents. In addition to the formation of biofilms to reduce antibiotic sensitivity, the urease-producing bacteria can also increase urine pH, causing Ca²⁺ and Mg²⁺ deposition and finally catheter obstruction. The prevention of UTIs and its complication (i.e., encrustation) thus is a great challenge in design of catheters and ureteral stents. In this work, a metal-catechol-assisted mussel chemistry (i.e., dopamine self-polymerization) was employed for surface functionalization of commercially available catheters with antimicrobial peptides (AMP), for the purpose of long-term anti-infection and encrustation prevention. To improve the stability of the polydopamine coating on polymeric stents, we used Cu²⁺-coordinated dopamine self-polymerization. Then, a cysteine-terminated AMP was introduced on the polydopamine coating through Michael addition. We found that the Cu²⁺-coordinated polydopamine coating showed improved stability and antibacterial effect. The cytotoxicity test confirmed that the bioinspired antibacterial coating showed good biocompatibility and no obvious toxicity. The results confirmed that the stents with AMP could in situ inhibit bacterial growth and biofilm formation, and finally reduce the deposition of struvite and hydroxyapatite crystals both in vitro and in vivo. We anticipate that this bioinspired strategy would develop a safe, stable and effective antibacterial coating on urinary tract medical devices for long-term bacterial inhibition and encrustation prevention.

Zwitterionic hydrogel modified reduced graphene oxide/ZnO nanocomposite blended membrane with high antifouling and antibiofouling performances

Membrane fouling and biofouling are major challenges in the application of membrane technology for wastewater treatment. The synthesis of antifouling and antibiofouling dual functionality membranes is a promising approach to tackling these problems. In this work, we fabricated a high-efficiency dual functionality polyethersulfone (PES) ultrafiltration membrane by blending an antibacterial reduced graphene oxide-ZnO nanocomposite into a PES matrix (rGO/ZnO-PES) followed by surface grafting of a low-fouling polyampholyte hydrogel (rGO/ZnO-z-PES). The antibacterial activity of the blended membrane was optimized by changing the nanocomposite fraction in the PES dope solution. Surface characterizations (SEM-EDS, XPS, ATR-FTIR, contact angle, and Zeta potential) confirmed the successful grafting of the zwitterionic hydrogel on the rGO/ZnO-PES membrane surface. Contact killing assays revealed that the polyampholyte hydrogel grafting did not affect the high antibacterial activity of the rGO/ZnO-PES membrane. Dynamic filtration experiments demonstrated the very high antifouling and antibiofouling of the rGO/ZnO-z-PES membrane, and significantly higher than those of the rGO/ZnO-PES and pristine PES membranes. The measured concentration of zinc ions in the permeate was low. Overall, our results demonstrate that the rGO/ZnO-z-PES membrane has excellent antifouling and antibiofouling performance and is stable and safe, and therefore very promising for wastewater treatment.

Fouling Prevention in Polymeric Membranes by Radiation Induced Graft Copolymerization

The application of membrane processes in various fields has now undergone accelerated developments, despite the presence of some hurdles impacting the process efficiency. Fouling is arguably the main hindrance for a wider implementation of polymeric membranes, particularly in pressure-driven membrane processes, causing higher costs of energy, operation, and maintenance. Radiation induced graft copolymerization (RIGC) is a powerful versatile technique for covalently imparting selected chemical functionalities to membranes' surfaces, providing a potential solution to fouling problems. This article aims to systematically review the progress in modifications of polymeric membranes by RIGC of polar monomers onto membranes using various low- and high-energy radiation sources (UV, plasma, γ-rays, and electron beam) for fouling prevention. The feasibility of the modification method with respect to physico-chemical and antifouling properties of the membrane is discussed. Furthermore, the major challenges to the modified membranes in terms of sustainability are outlined and the future research directions are also highlighted. It is expected that this review would attract the attention of membrane developers, users, researchers, and scientists to appreciate the merits of using RIGC for modifying polymeric membranes to mitigate the fouling issue, increase membrane lifespan, and enhance the membrane system efficiency.

Development of Antibacterial and Antifouling Innovative and Eco-Sustainable Sol-Gel Based Materials: From Marine Areas Protection to Healthcare Applications

Bacterial colonization of surfaces is the leading cause of deterioration and contaminations. Fouling and bacterial settlement led to damaged coatings, allowing microorganisms to fracture and reach the inner section. Therefore, effective treatment of surface damaged material is helpful to detach bio-settlement from the surface and prevent deterioration. Moreover, surface coatings can withdraw biofouling and bacterial colonization due to inherent biomaterial characteristics, such as superhydrophobicity, avoiding bacterial resistance. Fouling was a past problem, yet its untargeted toxicity led to critical environmental concerns, and its use became forbidden. As a response, research shifted focus approaching a biocompatible alternative such as exciting developments in antifouling and antibacterial solutions and assessing their antifouling and antibacterial performance and practical feasibility. This review introduces state-of-the-art antifouling and antibacterial materials and solutions for several applications. In particular, this paper focuses on antibacterial and antifouling agents for concrete and cultural heritage conservation, antifouling sol-gel-based coatings for filtration membrane technology, and marine protection and textile materials for biomedicine. In addition, this review discusses the innovative synthesis technologies of antibacterial and antifouling solutions and the consequent socio-economic implications. The synthesis and the related physico-chemical characteristics of each solution are discussed. In addition, several characterization techniques and different parameters that influence the surface finishing coatings deposition were also described.

Cross-Linking Combined with Surfactant Bilayer Assembly Enhances the Hydrophilic and Antifouling Properties of PTFE Microfiltration Membranes

The inherent strong hydrophobicity of Polytetrafluoroetylene (PTFE) microfiltration membranes results in low separation efficiency and easy contamination. In order to enhance its hydrophilic and antifouling properties, we first modified the PTFE microfiltration membrane by using Polyethylene glycol laurate (PEGML) for first layer deposition and then used Polyvinyl alcohol (PVA)/citric acid (CA) cross-linked coatings for second layer deposition. The Scanning Electron Microscope (SEM) results showed that the fibers and nodes of the modified PTFE microfiltration membrane were coated with PVA/CA hydrophilic coating. FT-IR Spectromete and X-ray photoelectron spectrometer (XPS) analysis results confirmed that crosslinking of PVA and CA occurred and that PEGML and PVA/CA were successfully deposited onto the membrane surface. The modification conditions were optimized by hydrophilicity testing, and the best hydrophilicity of the modified membrane was achieved when the crosslinking content of PEGML was 2 g·L−1, PVA was 5 g·L−1, and CA was 2 g·L−1. PTFE microfiltration membranes modified by the optimal conditions achieved a water flux of 396.9 L·m−2·h−1 (three times that of the original membrane) at low operating pressures (0.05 MPa), and the contact angle decreased from 120° to 40°. Meanwhile, the modified PTFE microfiltration membrane has improved contamination resistance and good stability of the hydrophilic coating.

Fouling mitigation in an anaerobic membrane bioreactor via membrane surface modification with tannic acid and copper

Anaerobic membrane bioreactors (AnMBRs) have recently received a great amount of attention as an alternative anaerobic treatment process due to their superior capability for sludge retention with high effluent quality. Nevertheless, membrane fouling in AnMBRs has been a major concern. In this study, the surfaces of polyvinylidene fluoride (PVDF) ultrafiltration membranes were modified with tannic acid (TA) and Cu(II) at various molar ratios of TA to Cu(II), including 3:1, 2:1, 1:1, 1:2, and 1:3. The hydrophilicity, morphology, chemical structure, elemental composition, and antibacterial properties of the unmodified and modified membranes were analyzed using water contact angle measurements, scanning electron microscopy (SEM), Fourier transform infrared spectroscopy (FT-IR), SEM-energy dispersive X-ray spectrometry (SEM-EDX), and the clear zone method, respectively. The modified membrane with a TA-to-Cu(II) molar ratio of 1:3 had high hydrophilicity with certain antibacterial properties; therefore, it was selected to be further tested in an AnMBR along with an unmodified membrane. The chemical oxygen demand (COD) removal efficiencies of the unmodified membrane and modified membrane were 92.2 ± 3.6% and 91.8 ± 4.0%, respectively. The modified membrane had higher permeability after backwashing with less chemical cleaning (CC) than the unmodified membrane. Surface modification with TA and Cu(II) appeared to reduce irreversible fouling on the membranes.

A Review on Promising Membrane Technology Approaches for Heavy Metal Removal from Water and Wastewater to Solve Water Crisis

Due to the impacts of water scarcity, the world is looking at all possible solutions for decreasing the over-exploitation of finite freshwater resources. Wastewater is one of the most reliable and accessible water supplies. As the population expands, so do industrial, agricultural, and household operations in order to meet man’s enormous demands. These operations generate huge amounts of wastewater, which may be recovered and used for a variety of reasons. Conventional wastewater treatment techniques have had some success in treating effluents for discharge throughout the years. However, advances in wastewater treatment techniques are required to make treated wastewater suitable for industrial, agricultural, and household use. Diverse techniques for removing heavy metal ions from various water and wastewater sources have been described. These treatments can be categorized as adsorption, membrane, chemical, or electric. Membrane technology has been developed as a popular alternative for recovering and reusing water from various water and wastewater sources. This study integrates useful membrane technology techniques for water and wastewater treatment containing heavy metals, with the objective of establishing a low-cost, high-efficiency method as well as ideal production conditions: low-cost, high-efficiency selective membranes, and maximum flexibility and selectivity. Future studies should concentrate on eco-friendly, cost-effective, and long-term materials and procedures.

Facile Fabrication of Multi-Hydrogen Bond Self-Assembly Poly(MAAc-co-MAAm) Hydrogel Modified PVDF Ultrafiltration Membrane to Enhance Anti-Fouling Property

In this work, a facile preparation method was proposed to reduce natural organics fouling of hydrophobic membrane via UV grafting polymerization with methacrylic acid (MAAc) and methyl acrylamide (MAAm) as hydrophilic monomers, followed by multihydrogen bond self-assembly. The resulting poly(vinylidene fluoride)-membranes were characterized with respect to monomer ratio, chemical structure and morphology, surface potential, and water contact angle, as well as water flux and organic foulants ultrafiltration property. The results indicated that the optimal membrane modified with a poly(MAAc-co-MAAm) polymer gel layer derived from a 1:1 monomer ratio exhibited superior hydrophilicity and excellent gel layer stability, even after ultrasonic treatment or soaking in acid or alkaline aqueous solution. The initial water contact angle of modified membranes was only 36.6° ± 2.9, and dropped to 0° within 13 s. Moreover, flux recovery rates (FRR) of modified membranes tested by bovine serum albumin (BSA), humic acid (HA), and sodium alginate (SA) solution, respectively, were all above 90% after one-cycle filtration (2 h), significantly higher than that of the pure membrane (70–76%). The total fouling rates (R_t) of the pure membrane for three foulants were as high as 47.8–56.2%, while the R_t values for modified membranes were less than 30.8%. Where R_t of BSA dynamic filtration was merely 10.7%. The membrane designed through grafting a thin-layer hydrophilic hydrogel possessed a robust antifouling property and stability, which offers new insights for applications in pure water treatment or protein purification.

Combined Nanofiltration and Thermocatalysis for the Simultaneous Degradation of Micropollutants, Fouling Mitigation and Water Purification

Due to progressive limitation of access to clean drinkable water, it is nowadays a priority to find an effective method of water purification from those emerging organic contaminants, which might have potentially harmful and irreversible effects on living organisms and environment. This manuscript reports the development of a new strategy for water purification, which combines a novel and recently developed Al2O3-doped silica nanofiltration membrane with a thermocatalytic perovskite, namely cerium-doped strontium ferrate (CSF). The thermocatalytic activity of CSF offers the opportunity to degrade organic pollutants with no light and without input of chemical oxidants, providing simplicity of operation. Moreover, our studies on real samples of secondary effluent from wastewater treatment showed that the thermocatalyst has the ability to degrade also part of the non-toxic organic matter, which allows for reducing the chemical oxygen demand of the retentate and mitigating membrane fouling during filtration. Therefore, the new technology is effective in the production of clean feed and permeate and has a potential to be used in degradation of micropollutants in water treatment.

A Comprehensive Review on Membrane Fouling: Mathematical Modelling, Prediction, Diagnosis, and Mitigation

Membrane-based separation has gained increased popularity over the past few decades, particularly reverse osmosis (RO). A major impediment to the improved performance of membrane separation processes, in general, is membrane fouling. Fouling has detrimental effects on the membrane’s performance and integrity, as the deposition and accumulation of foulants on its surface and/or within its pores leads to a decline in the permeate flux, deterioration of selectivity, and permeability, as well as a significantly reduced lifespan. Several factors influence the fouling-propensity of a membrane, such as surface morphology, roughness, hydrophobicity, and material of fabrication. Generally, fouling can be categorized into particulate, organic, inorganic, and biofouling. Efficient prediction techniques and diagnostics are integral for strategizing control, management, and mitigation interventions to minimize the damage of fouling occurrences in the membranes. To improve the antifouling characteristics of RO membranes, surface enhancements by different chemical and physical means have been extensively sought after. Moreover, research efforts have been directed towards synthesizing membranes using novel materials that would improve their antifouling performance. This paper presents a review of the different membrane fouling types, fouling-inducing factors, predictive methods, diagnostic techniques, and mitigation strategies, with a special focus on RO membrane fouling.

Influence of Deposition Parameters of TiO2 + CuO Coating on the Membranes Surface Used in the Filtration Process of Dairy Wastewater on Their Functional Properties

A novel approach of the deposition of two-component coating consisting of TiO₂ and CuO on polymer membranes by MS-PVD method was presented in this work. This confirmed the possibility of using thin functional coatings for the modification of polymer membranes. The influence of technological parameters of the coating deposition on the membrane’s structure, chemical composition and functional properties (hydrophilic, photocatalytic and bactericidal properties) were analyzed using SEM. Model microorganism such as Escherichia coli and Bacillus subtilis have been used to check the antibacterial properties. The results indicated that doping with CuO highlights the potential of bactericidal efficiency. The surface properties of the membranes were evaluated with the surface free energy. For evaluating photocatalytic properties, the UV and visible light were used. The filtration tests showed that polymer membranes treated with two-component TiO₂ + CuO coatings have a permeate flux similar to the reference material (non-coated membrane). The obtained results constitute a very promising perspective of the potential application of magnetron sputtering for deposition of TiO₂ + CuO coatings in the prevention of biofouling resulted from the membrane filtration of dairy wastewater.

Fouling control in reverse osmosis for water desalination & reuse: Current practices & emerging environment-friendly technologies

Reverse Osmosis (RO) is becoming increasingly popular for seawater desalination and wastewater reclamation. However, fouling of the membranes adversely impacts the overall process efficiency and economics. To date, several strategies and approaches have been used in RO plants and investigated at the laboratory-scale for their effectiveness in the control of different fouling types. Amid growing concerns and stringent regulations for the conservation of environment, there is an increasing trend to identify technologies that are effective in fouling mitigation as well as friendly to the environment. The present review elaborates on the different types of environment-friendly technologies for membrane fouling control that are currently being used or under investigation. It commences with a brief introduction to the global water crisis and the potential of membrane-based processes in overcoming this problem. This is followed by a section on membrane fouling that briefly describes the major fouling types and their impact on the membrane performance. Section 3 discusses the predominant fouling control/prevention strategies including feedwater pretreatment, membrane and spacer surface modification and membrane cleaning. The currently employed techniques are discussed together with their drawbacks, with some light being shed on the emerging technologies that have the ability to overcome the current limitations. The penultimate section provides a detailed discussion on a variety of eco-friendly/chemical free techniques investigated to control different fouling types. These include both control and prevention strategies, for example, bioflocculation and electromagnetic fields, as well as remediation techniques such as osmotic backwashing and gas purging. In addition, quorum sensing has been specifically discussed for biofouling remediation. The promising findings from different studies are presented followed by a discussion on their drawbacks and limitations. The review concludes with a need for carrying out fundamental studies to develop better understanding of the eco-friendly processes discussed in the penultimate section and their optimization for possible integration into the RO plants.

Antifouling and Flux Enhancement of Reverse Osmosis Membrane by Grafting Poly (3-Sulfopropyl Methacrylate) Brushes

A commercial thin film composite (TFC) polyamide (PA) reverse osmosis membrane was grafted with 3-sulfopropyl methacrylate potassium (SPMK) to produce PA-g-SPMK by atom transfer radical polymerization (ATRP). The grafting of PA was done at varied concentrations of SPMK, and its effect on the surface composition and morphology was studied by Fourier-Transform Infrared Spectroscopy (FTIR), Scanning Electron Microscopy (SEM), optical profilometry, and contact angle analysis. The grafting of hydrophilic ionically charged PSPMK polymer brushes having acrylate and sulfonate groups resulted in enhanced hydrophilicity rendering a reduction of contact angle from 58° of pristine membrane sample labeled as MH0 to 10° for a modified membrane sample labeled as MH3. Due to the increased hydrophilicity, the flux rate rises from 57.1 L m-2 h-1 to 71.2 L m-2 h-1, and 99% resistance against microbial adhesion (Escherichia coli and Staphylococcus aureus) was obtained for MH3 after modification.

Treatment of Electroplating Wastewater Using NF pH-Stable Membranes: Characterization and Application

Industrial adoption of nanofiltration (NF) for treatment of low-pH wastewater is hindered by the limited membrane lifetime at strongly acidic conditions. In this study, the electroplating wastewater (EPWW) filtration performance of a novel pH-stable NF membrane is compared against a commercial NF membrane and a reverse osmosis (RO) membrane. The presented membrane is relatively hydrophobic and has its isoelectric point (IEP) at pH 4.1, with a high and positive zeta potential of +10 mV at pH 3. A novel method was developed to determine the molecular weight cut-off (MWCO) at a pH of 2, with a finding that the membrane maintains the same MWCO (~500 Da) as under neutral pH operating conditions, whereas the commercial membrane significantly increases it. In crossflow filtration experiments with simulated EPWW, rejections above 75% are observed for all heavy metals (compared to only 30% of the commercial membrane), while keeping the same pH in the feed and permeate. Despite the relatively lower permeance of the prepared membrane (~1 L/(m2·h·bar) versus ~4 L/(m2·h·bar) of the commercial membrane), its high heavy metals rejection coupled with a very low acid rejection makes it suitable for acid recovery applications.

Chlorine Rechargeable Biocidal N-Halamine Nanofibrous Membranes Incorporated with Bifunctional Zwitterionic Polymers for Efficient Water Disinfection Applications

An intrinsically hydrophilic nanofibrous membrane with chlorine rechargeable biocidal and antifouling functions was prepared by using a combination of chemically bonded N-halamine moieties and zwitterionic polymers (PEI-S). The designed nanofibrous membrane, named as PEI-S@BNF-2 h, can exhibit integrated features of reduced bacterial adhesion, rechargeable biocidal activity, and easy release of killed bacteria by using mild hydrodynamic forces. The representative functional performances of the PEI-S@BNF-2 h membrane include high active chlorine capacity (>4000 ppm), large specific surface area, ease of chlorine rechargeability, long-term stability, and exceptional biocidal activity (99.9999% via contact killing). More importantly, the zwitterionic polymer moieties (PEI-S) brought robust antifouling properties to this biocidal membrane, therefore reducing the biofouling-biofilm effect and prolonging the lifetime of the filtration membrane. These attributes enable the PEI-S@BNF-2 h nanofibrous membrane to effectively disinfect the microbe-contaminated water with high fluxes (10,000 L m^-2 h^-1) and maintain itself clean for a long-term application.

Effect of the Zwitterion, p(MAO-DMPA), on the Internal Structure, Fouling Characteristics, and Dye Rejection Mechanism of PVDF Membranes

The zwitterion poly-(maleic anhydride-alt-1-octadecene-3-(dimethylamino)-1-propylamine) (p(MAO-DMPA)) synthesized using a ring-opening reaction was used as a poly(vinylidene fluoride) (PVDF) membrane modifier/additive during phase inversion process. The zwitterion was characterized using proton nuclear magnetic resonance (¹HNMR) and attenuated total reflectance Fourier transform infrared spectroscopy (ATR-FTIR). Atomic force microscopy (AFM), field emission scanning electron microscope (SEM), FTIR, and contact angle measurements were taken for the membranes. The effect of the zwitterionization content on membrane performance indicators such as pure water flux, membrane fouling, and dye rejection was investigated. The morphology of the membranes showed that the increase in the zwitterion amount led to a general decrease in pore size with a concomitant increase in the number of membrane surface pores. The surface roughness was not particularly affected by the amount of the additive; however, the internal structure was greatly influenced, leading to varying rejection mechanisms for the larger dye molecule. On the other hand, the wettability of the membranes initially decreased with increasing content to a certain point and then increased as the membrane homogeneity changed at higher zwitterion percentages. Flux and fouling properties were enhanced through the addition of zwitterion compared to the pristine PVDF membrane. The high (>90%) rejection of anionic dye, Congo red, indicated that these membranes behaved as ultrafiltration (UF). In comparison, the cationic dye, rhodamine 6G, was only rejected to <70%, with rejection being predominantly electrostatic-based. This work shows that zwitterion addition imparted good membrane performance to PVDF membranes up to an optimum content whereby membrane homogeneity was compromised, leading to poor performance at its higher loading.

Grafting Activated Graphene Oxide Nanosheets onto Ultrafiltration Membranes Using Polydopamine to Enhance Antifouling Properties

Graphene oxide (GO) nanosheets are negatively charged and exhibit excellent antifouling properties. However, their hydrophilicity makes it challenging for their grafting onto membrane surfaces to improve antifouling properties for long-term underwater operation. Herein, we demonstrate a versatile approach to covalently graft GO onto ultrafiltration membrane surfaces in aqueous solutions at ≈22 °C. The membrane surface is first primed using dopamine and then reacted with activated GO (aGO) containing amine-reactive esters. The aGO grafting improves the membrane surface hydrophilicity without decreasing water permeance. When the membranes are challenged with 1.0 g/L sodium alginate in a constant-flux crossflow system, the aGO grafting increases the critical flux by 20% and reduces the fouling rate by 63% compared with the pristine membrane. The modified membranes demonstrate stability for 48 h operation and interval cleanings using NaOH solutions.