Polyvinylidene fluoride membrane blended with quaternary ammonium compound for enhancing anti-biofouling properties: Effects of dosage

Deciphering the behavior and potential mechanism of biochar at different pyrolysis temperatures to alleviate membrane biofouling

Biofouling limits applications of membrane technology in wastewater treatment, but dosing additives to membrane tanks is an effective method to alleviate biofouling. In this study, biochar derived from corncob and pyrolyzed at 300, 500, and 700°C was dosed to determine the underlying anti-biofouling mechanism. The effects of the biochar on the membrane properties and foulant behavior were systematically investigated. The results showed that biochar delayed the occurrence of the fouling transition (0.5-3.0 h), and decreased the flux decline rate, thus achieving a higher water flux (3.1-3.7 times of the control group). Biochar altered membrane surface properties, and increased the membrane surface charge, roughness, and hydrophilicity, which all contributed to higher membrane permeability. Moreover, adding biochar reduced the number of foulants in the fouling layer, particularly protein substances. The flux model fit and the XDLVO theory further revealed the mitigating effect of biochar on membrane biofouling. At the initial intermediate-blocking stage, the effect of biochar on membrane fouling was determined by its properties, and adsorption capacity to the foulants, BC500 presented the best mitigation performance. At the later cake-filtration stage, the role of biochar in membrane fouling was strongly associated with protein content in the fouling layer, and the minimum rate of flux decline occurred in BC300. This study promotes the understanding and development of biochar to alleviate membrane biofouling.

Leveraging the Dielectric Barrier Discharge Plasma Process to Create Regenerative Biocidal ePTFE Membranes

The utilization of dielectric barrier discharge (DBD) plasma treatment for modifying substrate surfaces constitutes an easy and simple approach with a potential for diverse applications. This technique was used to modify the surface of a commercial porous expanded poly(tetrafluoroethylene) (ePTFE) film with either dimethylaminoethyl methacrylate (DMAEMA) or (trimethylamino)ethyl methacrylate chloride (TMAEMA) monomers, aiming to obtain antibacterial ePTFE. Physicochemical analyses of the membranes revealed that DBD successfully enhanced the surface energy and surface charge of the membranes while maintaining high porosity (>75%) and large pore size (>1.0 μm). Evaluation of the bacteria killing-releasing (K-R) function revealed that both DMAEMA and TMAEMA endowed ePTFE with the ability to kill Escherichia coli bacteria. However, only TMAEMA-grafted ePTFE allowed for the release of dead bacteria from the surface upon washing with sodium hexametaphosphate (SHMP) saline solution, owing to its cationic charge derived from the quaternary amine. Washing with SHMP disturbed the electrostatic force between the polymer brushes and dead bacteria, which caused the release of the dead bacteria. Lastly, dead-end bacteria filtration showed that the TMAEMA-grafted ePTFE was able to kill 99.78% of the bacteria, while approximately 61.55% of bacteria were killed upon contact. The present findings support the feasibility of using DBD plasma treatment for designing surfaces that target bacteria and aid in the containment of disease-causing pathogens.

From Antibacterial to Antibiofilm Targeting: An Emerging Paradigm Shift in the Development of Quaternary Ammonium Compounds (QACs)

In a previous development stage, mostly individual antibacterial activity was a target in the optimization of biologically active compounds and antiseptic agents. Although this targeting is still valuable, a new trend has appeared since the discovery of superhigh resistance of bacterial cells upon their aggregation into groups. Indeed, it is now well established that the great majority of pathogenic germs are found in the environment as surface-associated microbial communities called biofilms. The protective properties of biofilms and microbial resistance, even to high concentrations of biocides, cause many chronic infections in medical settings and lead to serious economic losses in various areas. A paradigm shift from individual bacterial targeting to also affecting more complex cellular frameworks is taking place and involves multiple strategies for combating biofilms with compounds that are effective at different stages of microbiome formation. Quaternary ammonium compounds (QACs) play a key role in many of these treatments and prophylactic techniques on the basis of both the use of individual antibacterial agents and combination technologies. In this review, we summarize the literature data on the effectiveness of using commercially available and newly synthesized QACs, as well as synergistic treatment techniques based on them. As an important focus, techniques for developing and applying antimicrobial coatings that prevent the formation of biofilms on various surfaces over time are discussed. The information analyzed in this review will be useful to researchers and engineers working in many fields, including the development of a new generation of applied materials; understanding biofilm surface growth; and conducting research in medical, pharmaceutical, and materials sciences. Although regular studies of antibacterial activity are still widely conducted, a promising new trend is also to evaluate antibiofilm activity in a comprehensive study in order to meet the current requirements for the development of highly needed practical applications.

Induced mazEF-mediated programmed cell death contributes to antibiofouling properties of quaternary ammonium compounds modified membranes

Functionalized antibiofouling membranes have attracted increasing attention in water and wastewater treatment. Among them, contact-killing antibiofouling membranes deliver a long-lasting effect with no leaching or release, thus providing distinctive advantages. However, the antibiofouling mechanism especially in the vicinity of the membrane surface remains unclear. Herein, we demonstrate that mazEF-mediated programmed cell death (PCD) is critical for the antibiofouling behaviors of quaternary ammonium compounds modified membranes (QM). The viability of wild type Escherichia coli (WT E. coli) upon exposure to QM for 1 h was decreased dramatically (31.5 ± 1.4% of the control). In contrast, the bacterial activity of E. coli with the knockout of mazEF gene (KO E. coli) largely remained (85.8 ± 5.2%). Through addition of quorum sensing factor, i.e., extracellular death factor (EDF), the antibacterial activity was significantly enhanced in a dilute culture, indicating that the density-dependent bacterial communication played an important role in the mazEF-mediated PCD system in biofouling control. Long-term study further showed that QM exhibited a better antibiofouling performance to treat feedwater containing WT E. coli, especially when EDF was dosed. Results of this study suggested that the bacteria on the membrane surface subject to contact killing could modulate the population growth in the vicinity via quorum-sensing mazEF-mediated PCD, paving a way to develop efficient antibiofouling materials based on contact-killing scenarios.

Biofouling-Resistant Ultrafiltration Membranes via Codeposition of Dopamine and Cetyltrimethylammonium Bromide with Retained Size Selectivity and Water Flux

Biofouling is a serious problem in ultrafiltration (UF) membrane applications. Modifying the surface of membranes with low molecular weight, commercially available antibacterial chemistries is an excellent strategy to mitigate biofouling. Herein, we report a new strategy to impart antibacterial and anti-biofouling behavior without changing the support membrane's size selectivity and pure water permeance (PWP). To this end, a strong antibacterial agent, cetyltrimethylammonium bromide (CTAB), was codeposited with dopamine onto commercial polyethersulfone (PES) UF membranes in the presence of nitrogen (N₂) gas backflow. The PWP and pore size of the support membrane did not change with codeposition, confirming the benefit of N₂ backflow in mitigating the solution intrusion phenomenon. X-ray photoelectron spectroscopy (XPS), surface ζ potentials, and contact angle measurements confirmed the successful codeposition of polydopamine (PDA) and CTAB onto the membrane. Among three different CTAB concentrations systematically investigated, the membrane functionalized with CTAB at the critical micelle concentration (CMC) provided the best anti-biofouling activity against Gram-positive (Staphylococcus aureus) and Gram-negative (Escherichia coli) bacteria and retained its surface ζ potential after being stored in 1 M NaCl (pH = 6.8) for 3 months. Our results demonstrate the potential of using a facile, one-step approach to modify commercial UF membranes without compromising their pore size or flux, while simultaneously endowing antibacterial activity.

Efficient removal of water bacteria and viruses using electrospun nanofibers

Pathogenic contamination has been considered as a significant worldwide water quality concern. Due to providing promising opportunities for the production of nanocomposite membranes with tailored porosity, adjustable pore size, and scaled-up ability of biomolecules incorporation, electrospinning has become the center of attention. This review intends to provide a detailed summary of the recent advances in the fabrication of antibacterial and antiviral electrospun nanofibers and discuss their application efficiency as a water filtration membrane. The current review attempts to give a functionalist perspective of the fundamental progress in construction strategies of antibacterial and antiviral electrospun nanofibers. The review provides a list of antibacterial and antiviral agents commonly used as water membrane filters and discusses the challenges in the incorporation process. We have thoroughly studied the recent application of functionalized electrospun nanofibers in the water disinfection process, with an emphasis on their efficiency. Moreover, different antibacterial and antiviral assay techniques for membranes are discussed, the gaps and limitations are highlighted and promising strategies to overcome barriers are studies.

Antifouling and antibacterial behavior of membranes containing quaternary ammonium and zwitterionic polymers

To overcome the organic-/bio- fouling of the membrane, a dual-functional ultrafiltration membrane containing quaternary ammonium and zwitterionic polymers via quaternization and surface radical polymerization was designed, and its antifouling and antibacterial behavior was studied. In this work, poly(vinylidene fluoride)/poly(methyl methacrylate-co-dimethylamino-2-ethyl methacrylate) (PVDF/P(MMA-co-DMAEMA)) blend membrane was quaternized by p-chloromethyl styrene (p-CMS), and the double bonds were introduced onto the membrane surface, which further participated in the polymerization of zwitterionic monomers on the membrane surface. The results indicated that the resultant membrane exhibited obviously improved hydrophilicity and weak positive charge (isoelectric point, 7.49). The membrane presented higher flux recovery ratio and lower protein adhesion compared with the pure PVDF membrane. Meanwhile, the membrane showed high-efficiency broad-spectrum antibacterial performance, that is, the bacteria killing efficiency of S. aureus and E. coli reached 98.2% and 97.0%, respectively. Moreover, the membrane effectively inhibited bacterial adhesion, which is important for the long-term antibacterial properties of membrane. This antifouling and antibacterial PVDF membrane may have potential in the long-term filtration process, especially when dealing with microbiologically contaminated water.

Permanent Antimicrobial Poly(vinylidene fluoride) Prepared by Chemical Bonding with Poly(hexamethylene guanidine)

Biofouling is one of the major obstacles in the application of poly(vinylidene fluoride) (PVDF) membrane in water and wastewater treatment. Developing antimicrobial PVDF could kill the attached microbe in the initial stage, thus theoretically inhibiting the formation of biofilm and delaying the occurrence of biofouling. However, the leaching of the antimicrobial component and deterioration of antimicrobial properties remain a concern. In this work, an antimicrobial PVDF (PVDF-g-AGE-PHMG) was developed by chemical bonding PVDF with poly(hexamethylene guanidine hydrochloride) (PHMG). The obtained PVDF-g-AGE-PHMG was blended with pristine PVDF to prepare an antimicrobial PVDF membrane. The results of Fourier transform infrared spectroscopy (FT-IR) and X-ray photoelectron spectroscopy (XPS) confirmed that PHMG was successfully grafted into the PVDF membrane. The morphologies, membrane porosity, water contact angles, antimicrobial properties, mechanical properties, and thermostability of the as-prepared membranes were investigated. When the content of PVDF-g-AGE-PHMG reached 10.0 wt %, the inhibition rates of both antimicrobial PVDF membrane against Escherichia coli and Staphylococcus aureus were above 99.99%. Due to the increased hydrophilicity, excellent antimicrobial activity, nonleaching of antimicrobial component, good mechanical properties, and thermostability, the as-prepared PVDF membrane has promising applications in the field of water treatment.

The Recent Progress in Modification of Polymeric Membranes Using Organic Macromolecules for Water Treatment

For decades, the water deficit has been a severe global issue. A reliable supply of water is needed to ensure sustainable economic development in population growth, industrialization and urbanization. To solve this major challenge, membrane-based water treatment technology has attracted a great deal of attention to produce clean drinking water from groundwater, seawater and brackish water. The emergence of nanotechnology in membrane science has opened new frontiers in the development of advanced polymeric membranes to enhance filtration performance. Nevertheless, some obstacles such as fouling and trade-off of membrane selectivity and permeability of water have hindered the development of traditional polymeric membranes for real applications. To overcome these issues, the modification of membranes has been pursued. The use of macromolecules for membrane modification has attracted wide interests in recent years owing to their interesting chemical and structural properties. Membranes modified with macromolecules have exhibited improved anti-fouling properties due to the alteration of their physiochemical properties in terms of the membrane morphology, porosity, surface charge, wettability, and durability. This review provides a comprehensive review of the progress made in the development of macromolecule modified polymeric membranes. The role of macromolecules in polymeric membranes and the advancement of these membrane materials for water solution are presented. The challenges and future directions for this subject are highlighted.

Roles of surfactants in pressure-driven membrane separation processes: a review

Surfactants widely exist in various kinds of wastewaters which could be treated by pressure-driven membrane separation (PDMS) techniques. Due to the special characteristics of surfactants, they may affect the performance of membrane filtration. Over the last two decades, there are a number of studies on treating wastewaters containing surfactants by PDMS. The current paper gives a review of the roles of surfactants in PDMS processes. The effects of surfactants on membrane performance were discussed via two aspects: influence of surfactants on membrane fouling and enhanced removal of pollutants by surfactants. The characteristics of surfactants in solution and at solid-liquid interface were summarized. Surfactants in membrane filtration processes cause membrane fouling mainly through adsorption, concentration polarization, pore blocking, and cake formation, and fouling degree may be influenced by various factors (feed water composition, membrane properties, and operation conditions). Furthermore, surfactants may also have a positive effect on membrane performance. Enhanced removal of various kinds of pollutants by PDMS in the presence of surfactants has been summarized, and the removal mechanism has been revealed. Based on the current reports, further studies on membrane fouling caused by surfactants and enhanced removal of pollutants by surfactant-aided membrane filtration were also proposed.

Optimized anti-biofouling performance of bactericides/cellulose nanocrystals composites modified PVDF ultrafiltration membrane for micro-polluted source water purification

The covalently functionalized cellulose nanocrystal (CNC) composites were synthesized by bonding common bactericides, such as dodecyl dimethyl benzyl ammonium chloride (DDBAC), ZnO and graphene oxide (GO) nanosheets, onto the CNC's surface. Then, the DDBAC/CNC, ZnO/CNC and GO/CNC nanocomposites modified polyvinylidene fluoride (PVDF) ultrafiltration membranes were fabricated by a simple one-step non-solvent induced phase separation (NIPS) process. The resultant hybrid membranes possessed porous and rough surfaces with more finger-like macropores that even extended through the entire cross-section. The hydrophilicity, permeability, antibacterial and antifouling performance and mechanism of the hybrid ultrafiltration membranes were evaluated and compared in detail, aiming at screening a superior hybrid membrane for practical application in micro-polluted source water purification. Among these newly-developed hybrid membranes, GO/CNC/PVDF exhibited an enhanced perm-selectivity with a water flux of 230 L/(m² h bar) and humic acid rejection of 92%, the improved antibacterial activity (bacteriostasis rate of 93%) and antifouling performance (flux recovery rate (FRR) of >90%) being due to the optimized pore structure, higher surface roughness, incremental hydrophilicity and electronegativity. A lower biofouling level after three weeks' filtration of the actual micro-polluted source water further demonstrated that embedding the hydrophilic and antibacterial GO/CNC nanocomposite into the polymer matrix is an effective strategy to improve membrane anti-biofouling ability.

Impacts of sodium hydroxide and sodium hypochlorite aging on polyvinylidene fluoride membranes fabricated with different methods

This study compared the effects of chemical aging on the polyvinylidene fluoride (PVDF) membranes fabricated with the methods of non-solvent induced phase separation (NIPS) (named NIPS-PVDF) and thermally induced phase separation (TIPS) (named TIPS-PVDF). The chemical solutions of sodium hypochlorite (NaClO) and sodium hydroxide (NaOH) were chosen at the concentration of 5000mg/L. The equivalence of 5 and 10years was respectively selected as the time of aging. The physicochemical evolutions of membrane aging are characterized on the base of morphology analysis, chemical components, permeation ability and mechanical properties. The aging of NIPS-PVDF membrane led to the elimination of surface hydrophilic additives, while NaOH focused on the dehydrofluorination process resulting in the formation of conjugated chains of polyene on the skeleton structure. The chemical components of the surface of TIPS-PVDF membrane were removed continuously during the aging processes of both NaClO and NaOH, which was caused by the saponification of surface additives and the chain scissions of skeleton structure, but without producing any obvious conjugated chains of polyene. All the aging processes led to the increase of contact angle and the decrease of mechanical properties, and the permeability was reduced first and increased later due to the enlargement of surface membrane pores and membrane block. With the influence of membrane aging, selectivity of membrane was decreased (except coliform bacteria). At the beginning of filtration, the turbidity and particle count were at relatively high levels and declined with the filtration process.