Enhancing the performance of PVDF membranes by hydrophilic surface modification via amine treatment

Cyclodextrin-modified PVDF membranes with improved anti-fouling performance

The design of hydrophilic polyvinylidene fluoride (PVDF) membranes with anti-fouling properties has been explored for decades. Surface modification and blending are typical strategies to tailor the hydrophilicity of PVDF membranes. Herein, cyclodextrin was used to improve the antifouling performance of PVDF membranes. Cyclodextrin-modified PVDF membranes were prepared by coupling PVDF amination (blending with branched polyethyleneimine) and activated cyclodextrin grafting. The blending of PEI in the PVDF casting solution preliminarily aminated the PVDF, resulting in PEI-crosslinked/grafted PVDF membranes after phase inversion. Aldehydes groups on cyclodextrin, introduced by oxidation, endow cyclodextrin to be grafted on the aminated PVDF membrane by the formation of imines. Borch reduction performed on the activated cyclodextrin-grafted PVDF membrane converted the imine bonds to secondary amines, ensuring the membrane stability. The resulting membranes possess excellent antifouling performance, with a lower protein adsorption capacity (5.7 μg/cm2, indicated by Bovine Serum Albumin (BSA)), and a higher water flux recovery rate (FRR = 96%). The proposed method provides a facial strategy to prepare anti-fouling PVDF membranes.

Advances in Membrane Separation for Biomaterial Dewatering

Biomaterials often contain large quantities of water (50-98%), and with the current transition to a more biobased economy, drying these materials will become increasingly important. Contrary to the standard, thermodynamically inefficient chemical and thermal drying methods, dewatering by membrane separation will provide a sustainable and efficient alternative. However, biomaterials can easily foul membrane surfaces, which is detrimental to the performance of current membrane separations. Improving the antifouling properties of such membranes is a key challenge. Other recent research has been dedicated to enhancing the permeate flux and selectivity. In this review, we present a comprehensive overview of the design requirements for and recent advances in dewatering of biomaterials using membranes. These recent developments offer a viable solution to the challenges of fouling and suboptimal performances. We focus on two emerging development strategies, which are the use of electric-field-assisted dewatering and surface functionalizations, in particular with hydrogels. Our overview concludes with a critical mention of the remaining challenges and possible research directions within these subfields.

Preparation and characterization of superior hydrophilic PVDF/DA membranes by the self-polymerization approach of dopamine

Polyvinylidene fluoride (PVDF) membranes are favored for their excellent properties. However, the inherent strong hydrophobicity of PVDF membranes limits their development in the field of water treatment. The objective of this study was to improve the performance of PVDF membranes using the self-polymerization, strong adhesion properties, and biocompatible effects of dopamine (DA). The PVDF/DA membrane modification conditions were simulated and optimized using response surface methodology (RSM), and the experimental design was used to investigate three main parameters. The results showed that the DA solution concentration of 1.65 g/L, the coating time of 4.5 h, the post-treatment temperature of 25°C, the contact angle decreased from 69° to 33.9°, and the pure water flux on the PVDF/DA membrane was higher than that on the original membrane. The absolute value of the relative error between the actual and predicted values is only 3.36 %. In the MBR parallel comparison test, compared with the PVDF/DA membrane, the total amount of extracellular polymers (EPS) of the PVDF membrane increased by 1.46 times and the polysaccharide increased by 1.56 times, which further showed that the PVDF/DA modified membrane had the excellent anti-pollution ability. Through Alpha diversity analysis, the biodiversity detected on PVDF/DA membranes was higher than that of PVDF membranes, which further proved its good bio-adhesion ability. These findings could offer a reference for the hydrophilicity, antifouling, and stability of PVDF/DA membranes, which would establish the foundation for the comprehensive applications in MBR.

Calcium Sulfate and Calcium Carbonate Scaling of Thin-Film Composite Polyamide Reverse Osmosis Membranes with Surface-Tethered Polyacrylic Acid Chains

The gypsum and calcite scaling propensities of the thin-film composite polyamide (PA-TFC) reverse osmosis (RO) membrane, modified with a tethered surface layer of polyacrylic acid (PAA) chains, was evaluated and compared to the scaling of selected commercial RO membranes. The tethered PAA layer was synthesized onto a commercial polyamide membrane (i.e., base-PA) via atmospheric pressure plasma-induced graft polymerization (APPIGP). The PAA nano-structured (SNS) base-PA membrane (SNS-PAA-PA) was scaled to a lesser degree, as quantified by a lower permeate flux decline and surface imaging, relative to the tested commercial membranes (Dow SW30, Toray SWRO, and BWRO). The cleaning of gypsum-scaled membranes with D.I. water flushing achieved 100% water permeability recovery for both the SNS-PAA-PA and Dow SW30 membranes, relative to 92-98% permeability restoration for the Toray membranes. The calcium carbonate scaling of SNS-PAA-PA membranes was also lower relative to the commercial membranes, but permeability recovery after D.I. water cleaning was somewhat lower (94%) but consistent with the level of surface scale coverage. In contrast, the calcite and gypsum-scaled membrane areas of the commercial membranes post-cleaning were significantly higher than for the SNS-PAA-PA membrane but with 100% permeability recovery, suggesting the potential for membrane damage when mineral scaling is severe.

Surface Modification of PVDF Membrane Using Formic Acid for Enhance the Hydrophobicity for Desalination

Membrane distillation (MD) is a thermally driven membrane separation process. In recent years, MD is considered as a key technology for desalination applications. It has been proven that it has numerous advantages compared to other desalination processes such as reverse osmosis. The thermal driving force, no osmotic pressure effect on membrane fluxes are among the advantages of this MD process. Membranes fabricated from hydrophobic polymers such as polyvinylidene fluoride (PVDF) can be a great choice for MD for the desalination process. However, MD membranes are still held back by some problems, most notably due to pore wetting effects, thereby limiting their use in treating seawater for desalination propose. This study proposes the use of carboxylic acid to modify the PVDF membrane to increase its hydrophobicity as a solution to avoid wetting in MD for water desalination. As a first step, membrane produced using PVDF pellets under the phase inversion method. Then the membrane underwent modification using 5wt%, 10wt% and 15wt% of formic acid via immersion technique. The characteristics of membrane produced observed by the contact angle, Fourier-transform infrared spectroscopy (FTIR) and membrane performance test. The impact of the concentration of formic acid solution and feed temperature on the membrane was evaluated. The result showed that by modifying the membrane with formic acid resulted in the increase in contact angle from unmodified PVDF membrane, 83.03° to 100.88° when the membrane modified with formic acid with concentration was 10 wt%. It was subsequently shown through FTIR peaks that formic acid successfully modified the PVDF membranes. Highest salt rejection percentage was also obtained with 68.9% when the feed temperature is 60 °C.

Recent Progress in Microfiltration/Ultrafiltration Membranes for Separation of Oil and Water Emulsions

Oily wastewater has become one of the leading causes of environmental pollution. A massive quantity of oily wastewater is released from industries, oil spills, and routine activities, endangering the ecosystem's sustainability. Due to the enormous negative impact, researchers put strenuous efforts into developing a sustainable solution to treat oily wastewater. Microfiltration/ultrafiltration membranes are considered an efficient solution to treat oily wastewater due to their low cost, small footprint, facile operation, and high separation efficiencies. However, membranes severely fouled during the separation process due to oil's adsorption and cake layer formation, which shortens the membranes' life. This review has critically discussed the microfiltration/ultrafiltration membrane synthesizing methods and their emulsion's separation performance. In the end, key challenges and their possible solutions are highlighted to provide future direction to synthesize next-generation membranes.

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.

Green Synthesis of Silver Nanoparticles as an Effective Antibiofouling Material for Polyvinylidene Fluoride (PVDF) Ultrafiltration Membrane

Silver nanoparticles (AgNPs) were successfully synthesized using the aqueous extract of the Paronychia argentea Lam (P. argentea) wild plant. The results showed that the conversion of Ag⁺ to Ag⁰ nanoparticles ratio reached 96.5% as determined by Inductively Coupled Plasma Atomic Emission Spectroscopy (ICP-AES), with a negative zeta potential (ζ) of −21.3 ± 7.68 mV of AgNPs expected to improve the stability of synthesized AgNPs. AgNP antibacterial activity has been examined against Streptococcus aureus (S. aureus) and Escherichia coli (E. coli) bacteria. The minimum inhibition concentration (MIC) was 4.9 µL/mL for both E. coli and S. aureus bacteria, while the minimum bactericidal concentrations (MBC) were 19.9 µL/mL and 4.9 µL/mL for S. aureus and E. coli, respectively. The synthesized AgNPs were incorporated in ultrafiltration polyvinylidene Fluoride (PVDF) membranes and showed remarkable antibiofouling behavior against both bacterial strains. The membranes were characterized using Fourier transform infrared spectroscopy (FTIR), scanning electron microscope (SEM), and X-ray diffraction (XRD). The contact angle and porosity of the membrane were also determined. The efficiency of the membranes regarding rejection rate was assessed using bovine serum albumin (BSA). It was found in the flux experiments that membranes BSA rejection was 99.4% and 98.7% with and without AgNPs, respectively.

Superwetting materials for hydrophilic-oleophobic membrane in oily wastewater treatment

The vast amount of oily wastewater released to the environment through industrialization has worsened the water quality in recent years, posing adverse impacts on general human health. Oil emulsified in water is one of the most difficult mixtures to be treated, making it imperative for new technology to be explored to address this issue. The use of conventional water treatment such as flotation, coagulation, precipitation, adsorption, and chemical treatment have low separation efficiencies and high energy costs, and are not applicable to the separation of oil/water emulsions. Therefore, there is a demand for more efficient methods and materials for the separations of immiscible oil/water mixtures and emulsions. Superwetting materials that can repel oil, while letting water pass through have been widely explored to fit into this concern. These materials usually make use of simultaneous hydrophilic/oleophobic mechanisms to allow a solid surface to separate oily emulsion with little to no use of energy. Also, by integrating specific wettability concepts with appropriate pore scale, solid surfaces may achieve separation of multifarious oil/water mixtures namely immiscible oil/water blends and consolidated emulsions. In this review, materials used to impart superwetting in solid surfaces by focusing on superhydrophilic/superoleophobic wetting properties of the materials categorized into fluorinated and non-fluorinated surface modification are summarized. In each material, its background, mechanism, fabricating processes, and their effects on solid surface's wetting capability are elaborated in detail. The materials reviewed in this paper are mainly organic and green, suggesting the alternative material to replace the fluorine group that is widely used to achieve oleophobicity in oily wastewater treatment.

Impacts of Sulfuric Acid on the Stability and Separation Performance of Polymeric PVDF-Based Membranes at Mild and High Concentrations: An Experimental Study

This study investigated the effects of an aqueous acidic solution at typical concentrations on polymeric polyvinylidene fluoride (PVDF)-based membranes. Flat-sheet PVDF-based membranes were completely embedded in sulfuric acid at varying concentrations. The effect of the acid concentration after a prolonged exposure time on the chemical, mechanical and physical properties of the membrane were checked via FE-SEM, EDX (Energy-Dispersive Spectrometer), FTIR, XRD, tensile strength, zeta potential, contact angle, porosity, pure water flux measurement and visual observation. The result reveals prompt initiation of reaction between the PVDF membrane and sulfuric acid, even at a mild concentration. As the exposure time extends with increasing concentration, the change in chemical and mechanical properties become more pronounced, especially in the morphology, although this was not really noticeable in either the crystalline phase or the functional group analyses. The ultimate mechanical strength decreased from 46.18 ± 0.65 to 32.39 ± 0.22 MPa, while the hydrophilicity was enhanced due to enlargement of the pores. The flux at the highest concentration and exposure period increased by 2.3 times that of the neat membrane, while the BSA (Bovine Serum Albumin) rejection dropped by 55%. Similar to in an alkaline environment, the stability and performance of the PVDF-based membrane analyzed in this study manifested general deterioration.

PVDF-Modified Nafion Membrane for Improved Performance of MFC

Low power production and unstable power supply are important bottlenecks restricting the application of microbial fuel cells (MFCs). It is necessary to explore effective methods to improve MFC performance. By using molasses wastewater as fuel, carbon felt as an electrode, and the mixture of K3[Fe(CN)6] and NaCl as a catholyte, an MFC experimental system was set up to study the performance of MFCs with three different proton exchange membranes. A Nafion membrane was used as the basic material, and polyvinylidene fluoride (PVDF) and acetone-modified PVDF were used to modify it, respectively. The experimental results show that a PVDF-modified membrane can improve the water absorption effectively and, thus, make the MFC have greater power generation and better wastewater treatment effect. The acetone-modified PVDF can further improve the stability of output power of the MFC. When the acetone-modified PVDF was used to modify the Nafion membrane, the steady output voltage of the MFC was above 0.21 V, and the Chemical Oxygen Demand (COD) removal rate for molasses wastewater was about 66.7%, which were 96.3% and 75.1% higher than that of the MFC with the ordinary Nafion membrane. Membrane modification with acetone-modified PVDF can not only increase the output voltage of the MFC but also improve the stability of its output electrical energy.

The Construction of a Hydrophilic Inorganic Layer Enables Mechanochemically Robust Super Antifouling UHMWPE Composite Membrane Surfaces

In this study, a facile and effective method is adopted to prepare mechanochemically robust super antifouling membrane surfaces. During the process, vinyl trimethoxy silane (VTMS) was used as the reactive intermediate for coupling the hydrophilic inorganic SiO₂ nanoparticle layer on to the organic ultra-high-molecular-weight polyethylene (UHMWPE) membrane surface, which created hierarchical nanostructures and lower surface energy simultaneously. The physical and chemical properties of the modified UHMWPE composite membrane surface were investigated. FTIR and XPS showed the successful chemical grafting of VTMS and SiO₂ immobilization, and this modification could effectively enhance the membrane’s surface hydrophilicity and filtration property with obviously decreased surface contact angle, the pure water flux and bovine serum albumin (BSA) rejection were 805 L·m⁻²·h⁻¹ and 93%, respectively. The construction of the hydrophilic nano-SiO₂ layer on the composite membrane surface for the improvement of membrane antifouling performance was universal, water flux recovery ratio values of BSA, humic acid (HA), and sodium alginate (SA) were all up to 90%. The aim of this paper is to provide an effective approach for the enhancement of membrane antifouling performance by the construction of a hydrophilic inorganic layer on an organic membrane surface.

Preparation and characterization of PVDF/CaCO3 composite membranes etched by hydrochloric acid

This study aimed to improve the pore size, porosity, and hydrophobicity of polyvinylidene fluoride (PVDF) membranes for desalination by vacuum membrane distillation (VMD). New membranes were prepared via etching PVDF/calcium carbonate (CaCO₃) composite membranes using hydrochloric acid (HCl), depending on the chemical reaction of CaCO₃ and HCl. Etched membranes were characterized by Fourier transform infrared spectroscopy (FT-IR), X-ray diffraction (XRD), differential scanning calorimetry (DSC), contact angle (CA), atomic force microscope (AFM), and scanning electron microscopy (SEM). The results showed that CaCO₃ of composite membranes was completely reacted by 1.5 mol/L HCl after composite membranes had been etched 90 min. The crystallinity of etched membranes was the same as that of PVDF/CaCO₃ composite membranes, and no new functional groups appeared in etched membranes, which indicated that etched membranes had good chemical stability. The surface roughness increased and led to the increase of contact angle, which means the hydrophobicity of etched membranes was enhanced. As a result, the increment of permeation flux had been improved in a VMD process. It was found that the maximum flux of etched membrane was enhanced and up to 1.65 times of composite membrane when the concentration of sodium chloride (NaCl) solution was 5.0 wt%, and the maximum flux reached up to 30.9 kg m^-2 h^-1.

Fabrication of Tween-20 coated PVDF membranes for wastewater treatment: optimization of preparation parameters, removal and membrane fouling control performance

In the present study, polyoxyethylene (20) sorbitan monolaurate (Tween-20) was employed as a surface coating agent for hydrophilic modification of poly(vinylidene fluoride) microfiltration membranes. The optimized parameters for membrane preparation (i.e., coating temperature, coating concentration, coating time and drying time) were systematically investigated. Contact angle and transmembrane pressure were employed to evaluate the efficiency of the modified membranes, and the optimized parameters were proposed. The removal of chemical oxygen demand (COD) and suspended solids (SS), as well as fouling control performance, was further evaluated. The results showed that the optimized parameters were 40 °C, 4.5 mmol L-1, 45 min and 45 min for coating temperature, coating concentration, coating time and drying time, respectively. Under these conditions, a hydration layer on the surface was formed, resulting in a more hydrophilic membrane surface. During domestic wastewater treatment in membrane bioreactor (MBR), the Tween-20 modified membrane exhibited better performance with rejection efficiencies of 94.56% and 97.53% for COD and SS, respectively. Tween-20 coating could mitigate the increase of transmembrane pressure and reduce the concentration of proteins accumulated on the membrane surface, which was effective for membrane fouling control. Simultaneously, the operation time of MBR was extended from 25 to 46 days. Furthermore, the stability of Tween-20 coated PVDF membrane was also verified. The results indicated that surface coating with Tween-20 is efficient and easy to be carried out, showing a great potential for application in MBR during wastewater treatment.

Multicomponent metal-organic framework membranes for advanced functional composites

Several strategies are presented for combining different metal–organic frameworks (MOFs) into composite mixed-matrix membranes. Some membranes are shown to be component for multistep organic catalytic transformations.

The diverse chemical and structural properties of metal–organic frameworks (MOFs) make them attractive for myriad applications, but their native powder form is limiting for industrial implementation. Composite materials of MOFs hold promise as a means of exploiting MOF properties in engineered forms for real-world applications. While interest in MOF composites is growing, research to date has largely focused on utilization of single MOF systems. The vast number of different MOF structures provides ample opportunity to mix and match distinct MOF species in a single composite to prepare multifunctional systems. In this work, we describe the preparation of three types of multi-MOF composites with poly(vinylidene fluoride) (PVDF): (1) co-cast MOF MMMs, (2) mixed MOF MMMs, and (3) multilayer MOF MMMs. Finally, MOF MMMs are explored as catalytic membrane reactors for chemical transformations.

Poly(vinylidene Fluoride-Hexafluoropropylene) Porous Membrane with Controllable Structure and Applications in Efficient Oil/Water Separation

Poly(vinylidene fluoride-hexafluoropropylene) (PVDF-HFP) porous membranes are fabricated via thermally induced phase separation (TIPS) with mixed diluent (dibutyl phthalate (DBP)/dioctyl phthalate (DOP)). The effects of mixed diluent are discussed in detail in term of morphology, mean pore size, selective wettability, etc. The results show that the membrane structure changes from spherulitic to bicontinuous with the change of DBP/DOP ratio. It is also found that the degree of crystallization decreases with the decrease of DBP/DOP ratio in mixed diluent. When liquid–liquid (L-L) phase separation precedes solid–liquid (S-L) phase separation, the obtained membranes have outstanding hydrophobicity and lipophilicity, excellent mechanical property. Additionally, the PVDF-HFP hybrid membranes are prepared with silica (SiO₂) particles and the effect of SiO₂ content on structure and properties is discussed. It is found that the PVDF-HFP hybrid membrane with 2 wt % SiO₂ (M3-S2) has better properties and higher filtration rate and separation efficiency for surfactant-stabilized water-in-oil emulsion separation. Moreover, the membrane M3-S2 also exhibits excellent antifouling performance for long-running.