Bio-Inspired Polymer Membrane Surface Cleaning

Reagent-Free Immobilization of Industrial Lipases to Develop Lipolytic Membranes with Self-Cleaning Surfaces

Biocatalytic membrane reactors combine the highly efficient biotransformation capability of enzymes with the selective filtration performance of membrane filters. Common strategies to immobilize enzymes on polymeric membranes are based on chemical coupling reactions. Still, they are associated with drawbacks such as long reaction times, high costs, and the use of potentially toxic or hazardous reagents. In this study, a reagent-free immobilization method based on electron beam irradiation was investigated, which allows much faster, cleaner, and cheaper fabrication of enzyme membrane reactors. Two industrial lipase enzymes were coupled onto a polyvinylidene fluoride (PVDF) flat sheet membrane to create self-cleaning surfaces. The response surface methodology (RSM) in the design-of-experiments approach was applied to investigate the effects of three numerical factors on enzyme activity, yielding a maximum activity of 823 ± 118 U m⁻² (enzyme concentration: 8.4 g L⁻¹, impregnation time: 5 min, irradiation dose: 80 kGy). The lipolytic membranes were used in fouling tests with olive oil (1 g L⁻¹ in 2 mM sodium dodecyl sulfate), resulting in 100% regeneration of filtration performance after 3 h of self-cleaning in an aqueous buffer (pH 8, 37 °C). Reusability with three consecutive cycles demonstrates regeneration of 95%. Comprehensive membrane characterization was performed by determining enzyme kinetic parameters, permeance monitoring, X-ray photoelectron spectroscopy, FTIR spectroscopy, scanning electron microscopy, and zeta potential, as well as water contact angle measurements.

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.

Highly Efficient One-Step Protein Immobilization on Polymer Membranes Supported by Response Surface Methodology

Immobilization of proteins by covalent coupling to polymeric materials offers numerous excellent advantages for various applications, however, it is usually limited by coupling strategies, which are often too expensive or complex. In this study, an electron-beam-based process for covalent coupling of the model protein bovine serum albumin (BSA) onto polyvinylidene fluoride (PVDF) flat sheet membranes was investigated. Immobilization can be performed in a clean, fast, and continuous mode of operation without any additional chemicals involved. Using the Design of Experiments (DoE) approach, nine process factors were investigated for their influence on graft yield and homogeneity. The parameters could be reduced to only four highly significant factors: BSA concentration, impregnation method, impregnation time, and electron beam irradiation dose. Subsequently, optimization of the process was performed using the Response Surface Methodology (RSM). A one-step method was developed, resulting in a high BSA grafting yield of 955 mg m−2 and a relative standard deviation of 3.6%. High efficiency was demonstrated by reusing the impregnation solution five times consecutively without reducing the final BSA grafting yield. Comprehensive characterization was conducted by X-ray photoelectron spectroscopy (XPS), Fourier-transform infrared spectroscopy (FTIR), and measurements of zeta potential, contact angle and surface free energy, as well as filtration performance. In addition, mechanical properties and morphology were examined using mercury porosimetry, tensile testing, and scanning electron microscopy (SEM).

Biocatalytic membranes for combating the challenges of membrane fouling and micropollutants in water purification: A review

Over the last few years, the list of water contaminants has grown tremendously due to many anthropogenic activities. Various conventional technologies are available for water and wastewater treatment. However, micropollutants of emerging concern (MEC) are posing a great threat due to their activity at trace concentration and poor removal efficiency by the conventional treatment processes. Advanced technology like membrane technology can remove MEC to some extent. However, issues like the different chemical properties of MEC, selectivity, and fouling of membranes can affect the removal efficiency. Moreover, the concentrate from the membrane filtration may need further treatment. Enzymatic degradation of pollutants and foulants is one of the green approaches for removing various contaminants from the water as well as mitigating membrane fouling. Biocatalytic membranes (BCMs), in which enzymes are immobilized on membranes, combines the advantages of membrane separation and enzymatic degradation. This review article discussed various commonly used enzymes in BCMs for removing MEC and fouling. The majorly used enzymes were oxidoreductases and hydrolases for removing MEC, antifouling, and self-cleaning ability. The various BCM synthesis processes based on entrapment, crosslinking, and binding have been summarized, along with the effects of the addition of the nanoparticles on the performances of the BCMs. The scale-up, commercial viability, challenges, and future direction for improving BCMs have been discussed and shown bright possibilities for these new generation membranes.

Protein Removal from Hydrogels through Repetitive Surface Degradation

Suppression of protein adsorption is a necessary property for materials used in the living body. In this study, thermoresponsive and degradable hydrogels were prepared by the radical polymerization of 2-methylene-1,3-dioxepane, 2-hydroxyethyl acrylate (HEA), and poly(ethylene glycol) monomethacrylate (PEGMA). The prepared hydrogels re-exposed PEG-grafted chains to the interface through surface degradation, which was confirmed by the maintenance of the chemical composition of the hydrogel surfaces after hydrolysis. Notably, adsorbed proteins can be removed from the hydrogel surfaces through hydrogel surface degradation at least thrice.

Dual Functional Ultrafiltration Membranes with Enzymatic Digestion and Thermo-Responsivity for Protein Self-Cleaning

Controlling surface⁻protein interaction during wastewater treatment is the key motivation for developing functionally modified membranes. A new biocatalytic thermo-responsive poly vinylidene fluoride (PVDF)/nylon-6,6/poly(N-isopropylacrylamide)(PNIPAAm) ultrafiltration membrane was fabricated to achieve dual functionality of protein-digestion and thermo-responsive self-cleaning. The PVDF/nylon-6,6/PNIPAAm composite membranes were constructed by integrating a hydrophobic PVDF cast layer and hydrophilic nylon-6,6/PNIPAAm nanofiber layer on to which trypsin was covalently immobilized. The enzyme immobilization density on the membrane surface decreased with increasing PNIPAAm concentration, due to the decreased number of amine functional sites. An ultrafiltration study was performed using the synthetic model solution containing BSA/NaCl/CaCl2, where the PNIPAAm containing biocatalytic membranes demonstrated a combined effect of enzymatic and thermo-switchable self-cleaning. The membrane without PNIPAAm revealed superior fouling resistance and self-cleaning with an RPD of 22%, compared to membranes with 2 and 4 wt % PNIPAAm with 26% and 33% RPD, respectively, after an intermediate temperature cleaning at 50 °C, indicating that higher enzyme density offers more efficient self-cleaning than the combined effect of enzyme and PNIPAAm at low concentration. The conformational volume phase transition of PNIPAAm did not affect the stability of immobilized trypsin on membrane surfaces. Such novel surface engineering design offer a promising route to mitigate surface⁻protein contamination in wastewater applications.

Surface-Engineered Biocatalytic Composite Membranes for Reduced Protein Fouling and Self-Cleaning

A new biocatalytic nanofibrous composite ultrafiltration membrane was developed to reduce protein fouling interactions and self-clean the membrane surface. The dual-layer poly(vinylidenefluoride)/nylon-6,6/chitosan composite membrane contains a hydrophobic poly(vinylidenefluoride) cast support layer and a hydrophilic functional nylon-6,6/chitosan nanofibrous surface layer where enzymes were chemically attached. The intrinsic surface chemistry and high surface area of the nanofibers allowed optimal and stable immobilization of trypsin (TR) and α-chymotrypsin enzymes via direct covalent binding. The enzyme immobilization was confirmed by X-ray photoelectron spectroscopy and visualized by confocal microscopy analysis. The prepared biocatalytic composite membranes were nanoporous with superior permeability offering stable protein antiadhesion and self-cleaning properties owing to the repulsive mechanism and digestion of proteins into peptides and amino acids, which was quantified by the gel electrophoresis technique. The TR-immobilized composite membranes exhibited 2.7-fold higher permeance and lower surface protein contamination with 3-fold greater permeance recovery, when compared to the pristine membrane after two ultrafiltration cycles with the model feed solution containing bovine serum albumin/NaCl/CaCl₂. The biocatalytic membranes retained about 50% of the enzyme activity after six reuse cycles but were regenerated to 100% activity after enzyme reloading, leading to a simple and cost-effective water remediation operation. Such surface- and pore-engineered membranes with self-cleaning properties offer a viable solution for severe surface protein contamination in food and water applications.

Electron Beam Immobilization of Novel Antimicrobial, Short Peptide Motifs Leads to Membrane Surfaces with Promising Antibacterial Properties

In this study, the efficacy of electron beam irradiation versus chemical coupling for yielding polyethersulfone (PES) membranes with antibacterial properties was investigated. For the surface coating, a recently discovered lead compound, IL-KKA, comprising a short peptide sequence functionalized with imidazolium groups, was used. For better integration within the membrane, several novel variants of IL-KKA were generated. Membrane immobilization was achieved using different doses of electron beam irradiation and NHS/EDC chemical coupling. Physicochemical characterization of the coated membranes was performed by water contact angle measurements, X-ray photoelectron spectroscopy, and scanning electron microscopy. Our results show that electron beam irradiation is as effective and gentle as chemical coupling using the NHS/EDC method. Moreover, it was demonstrated that the obtained membranes exhibit promising antibacterial activity against B. subtilis. In summary, the technique presented herein might be promising as a template for developing future anti-biofilm devices.

Colour-Value Based Method for Polydopamine Coating-Stability Characterization on Polyethersulfone Membranes

Porous polyethersulfone membranes as used in oenology were investigated in order to evaluate temperature-dependent permeances in a temperature range from 10 to 35 °C. A temperature correction factor was determined for this type of membrane to get accurate and comparable results for further developments. Moreover, the membranes were modified with a bio-inspired polydopamine coating in order to reduce fouling. The performance of the membranes could be increased with respect to permeance and flux recovery under cross-flow conditions. In order to test the applicability and stability of the coating layer, they were treated with basic and acidic cleaning agents as used in industry for fouled membranes. The chemical stability of the coating layer was studied under basic and acidic conditions, by systematic observation of the colour change of the coated membranes over treatment time.

Preparation of Fouling-Resistant Nanofibrous Composite Membranes for Separation of Oily Wastewater

A facile and low-cost method has been developed for separation of oily wastewater. Polyvinylidene fluoride/polyacrylonitrile (PVDF/PAN) nanofibers laminated on a supporting layer were tested. In order to create highly permeable and fouling-resistant membranes, surface modifications of both fibers were conducted. The results of oily wastewater separation showed that, after low vacuum microwave plasma treatment with Argon (Ar) and chemical modification with sodium hydroxide (NaOH), the membranes had excellent hydrophilicity, due to the formation of active carboxylic groups. However, the membrane performance failed during the cleaning procedures. Titanium dioxide (TiO₂) was grafted onto the surface of membranes to give them highly permeable and fouling-resistance properties. The results of the self-cleaning experiment indicated that grafting of TiO₂ on the surface of the membranes after their pre-treatment with Ar plasma and NaOH increased the permeability and the anti-fouling properties. A new surface modification method using a combination of plasma and chemical treatment was introduced.

Facile Preparation of Crosslinked PAN Membranes Based on Thiol-Ene Photopolymerization

To improve the mechanical strength and antipollution properties of membranes, this research presents a facile method to prepare crosslinked polyacrylonitrile (PAN) membranes. This was achieved firstly by radical copolymerization with acrylonitrile, allyl methacrylate and sulfobetaine methacrylamide. Then, the copolymer was crosslinked by a thiol-ene click reaction under UV irradiation. Finally, the crosslinked membranes were prepared by traditional immersion precipitation phase inversion. These prepared membranes showed excellent water-pressure resistance and solvent swelling, owing to their crosslinked structure. This research will help in preparing crosslinked membranes through facile crosslinking under mild reaction conditions. The betaine structure also considerably improved the antifouling properties of the membranes.

A novel strategy to immobilize enzymes on microporous membranes via dicarboxylic acid halides

A simple and efficient enzyme immobilization strategy on microporous membrane surfaces using dicarboxylic acid halides as a spacer offers a tool to design membranes used in enzymatic membrane reactors.