Modification of microfiltration membranes by alkoxysilane polycondensation induced quaternary ammonium compounds grafting for biofouling mitigation

Biosorption of Escherichia coli Using ZnO-Trimethyl Chitosan Nanocomposite Hydrogel Formed by the Green Synthesis Route

In this study, we tested the biosorption capacity of trimethyl chitosan (TMC)-ZnO nanocomposite (NC) for the adsorptive removal of Escherichia coli (E. coli) in aqueous suspension. For the formation of ZnO NPs, we followed the green synthesis route involving Terminalia mantaly (TM) aqueous leaf extract as a reducing agent, and the formed ZnO particles were surface-coated with TMC biopolymer. On testing of the physicochemical characteristics, the TM@ZnO/TMC (NC) hydrogel showed a random spherical morphology with an average size of 31.8 ± 2.6 nm and a crystal size of 28.0 ± 7.7 nm. The zeta potential of the composite was measured to be 23.5 mV with a BET surface area of 3.01 m² g^-1. The spectral profiles of TM@ZnO/TMC NC hydrogel on interaction with Escherichia coli (E. coli) revealed some conformational changes to the functional groups assigned to the stretching vibrations of N-H, C-O-C, C-O ring, and C=O bonds. The adsorption kinetics of TM@ZnO/TMC NC hydrogel revealed the pseudo-second-order as the best fit mechanism for the E. coli biosorption. The surface homogeneity and monolayer adsorption of the TM@ZnO/TMC NC hydrogel reflects majorly the entire adsorption mechanism, observed to display the highest correlation for Jovanovic, Redlich-Peterson, and Langmuir's isotherm models. Further, with the use of TM@ZnO/TMC NC hydrogel, we measured the highest adsorption capacity of E. coli to be 4.90 × 10 mg g^-1, where an in-depth mechanistic pathway was proposed by making use of the FTIR analysis.

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.

Highly Permeable Polylactic Acid Membrane Grafted with Quaternary Ammonium Salt for Effective and Durable Water Disinfection

Given the increasing usage of drinking water purifiers, highly permeable membranes with strong antimicrobial functions are desperately desirable for effective and durable water disinfection. Hereby, we prepared such antimicrobial membranes by chemical grafting of quaternary ammonium salt (QAS) molecules, 3-(trimethoxysilyl)propyldimethyloctadecyl ammonium chloride (TPMMC), onto air plasma pretreated biodegradable polylactic acid (PLA) substrates. The high chemical grafting density promoted very strong and positive zeta potential charge of the resulted PLA-QAS membrane, contributing to effective and broad-spectrum antimicrobial efficiencies (>99.99%) against different microbes, including fungi and conventional and drug-resistant bacteria. The solid grafting of QAS molecules produced a durable antimicrobial performance of the PLA-QAS membrane. In addition, the pleated filter (0.45 m²) of PLA-QAS membrane showed outstanding bacteria rejection properties (>99.99%) and excellent washing durability (up to 20 m³ water) even at very high water filtration rates (up to 4 L/min). The disinfection mechanism was clarified that negatively charged bacteria could be rapidly adsorbed to positively charged PLA-QAS spinnings, followed by devastating cell membrane damage to bacterial debris, leaving a clean environment without significant biofilm and biofouling formation.

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.

Modification of ultrafiltration membrane with antibacterial agent intercalated layered nanosheets: Toward superior antibiofouling performance for water treatment

Membrane fouling, especially biofouling induced by biofilm formation on membranes, can result in frequent cleaning or even replacement of membranes. Fabrication of membrane with excellent antibiofouling property is quite attractive due to its effectiveness and low-impact on the operation of membrane-based process. Herein, a cationic antibacterial agent, quaternary ammonium compound (QAC), was intercalated into the interlayer spaces of the MgAl layered double hydroxide (QAC/LDH) by self-assembly. The QAC/LDH composite was incorporated into polyethersulfone (PES) ultrafiltration (UF) membrane (PES-QLDH). The QAC/LDH enhanced the hydrophilicity, water flux, and resistance to organic fouling for the PES-QLDH membrane. The PES-QLDH membrane exhibited superior antibiofouling performance than the control PES membrane, with deposition of a thinner biofilm layer consisted of almost dead cells. The superior antibacterial activity inhibits the adhesion and growth of bacteria on the membrane surface, effectively retarding the formation of biofilms. Importantly, the synergistic effect of QAC and LDH in the PES-QLDH membrane resulted in a high biocidal activity based on both direct and indirect killing mechanisms. The PES-QLDH membrane maintained a stable and high antibacterial activity after several fouling-cleaning cycles. These results imply that the PES-QLDH membrane provides an effective and promising strategy for its long-term application in wastewater treatment.

Effect of Filter Types on Physicochemical Properties, Volatile Compounds, and Sensory Evaluations of Purified Water by Point-of-Use Water Treatment

This study investigated purified water from four different filter types for removing minerals, anions, and volatile organic compounds (VOCs), and affecting sensory perception and consumer acceptability. Ultrafiltration (UF), CSM-ultrafiltration (CU), alumina nanofiber (AN), and reverse osmosis (RO) filters were used for a point-of-use water treatment system with a pre-carbon filter (PR) and post-carbon filter (PO). Filters efficiently removed VOCs, which could negatively affect the sensory perception of water. The total VOC concentration of tap water (TW) (14.97 µg/Kg) was reduced by 70% by the PR, 75.3-88.7% by the PR-main filter, and >97% by the PR-RO-PR. Using the polarized sensory position test, the subjects clearly discriminated TW from the samples; however, most of the purified water was not. The difference in the mean ratings of consumer acceptability among the purified samples was <1 except for PR-RO-PO in consumer testing. These results suggested that although there are differences in the capability of different filter types to eliminate minerals, anions, and VOCs, overall consumers did not identify sensory differences among them, and demonstrated similar consumer acceptability of the purified water produced. Simply applying a pre-carbon filter for TW treatment is enough to minimize VOCs, which negatively influence consumer acceptability.

Antimicrobial resistance of Staphylococcus spp. isolated from organic and conventional Minas Frescal cheese producers in São Paulo, Brazil

The genus Staphylococcus is recognized worldwide as a cause of bacterial infections in humans and animals. Antibiotics used in dairy cattle combined with ineffective control can increase antimicrobial resistance. The objective of this study was to characterize 95 Staphylococcus strains isolated from organic and conventional Minas Frescal cheese production regarding antibiotic resistance (phenotype and genotype), presence of sanitizer-resistant genes and biofilm-formation genes, and SCCmec typing. Most strains (25.3%) showed higher resistance to penicillin, followed by oxacillin (21.1%) and clindamycin (11.6%). Among antibiotic resistance genes, the most prevalent were blaZ (25.3%), mecA (13.7%), lsaB (6.3%), msrA (4.2%), ant4 (3.2%), and tetM (2.1%); among sanitizer-resistance genes they were qacA/B (5.3%) and qacC (6.3%); and among biofilm, bap (4.2%), icaA (29.5%), icaD (41.1%). However, there was no statistically significant difference between organic and conventional dairy products, possibly due to the lack of synthetic antibiotic use on conventional farms during the sample collection period. Methicillin-resistant Staphylococcus aureus (MRSA) had their SCCmec identified as types I and IVc, and the methicillin-resistant coagulase-negative staphylococci had nontypeable SCCmec. These results suggest that there are antibiotic-resistant strains in both organic and conventional Minas Frescal cheese production in the state of São Paulo, Brazil. This supports the idea that improved quality control is needed from the milking stage up to the final product.

Robust immobilization of anionic silver nanoparticles on cellulose filter paper toward a low-cost point-of-use water disinfection system with improved anti-biofouling properties

Silver nanoparticle (AgNP)-decorated cellulose filter paper (FP), a low-cost point-of-use (POU) water disinfection system, can supply affordable and safe drinking water for people in desperate need, especially in rural areas in developing countries. However, owing to the unstable immobilization of AgNPs, silver can leach into the treated drinking water from the FP and exceed the World Health Organization (WHO) drinking water limit (<100 μg L-1), which is a potential threat to both human health and the environment. In this work, in order to robustly immobilize anionic silver nanoparticles (GA@AgNPs), we facilely prepared lipoic acid-modified cellulose filter paper (LA-FP), in which GA@AgNPs were robustly immobilized onto filter paper (GA@AgNPs-LA-FP) by strong chelation via the disulfide bond of LA with the surface of the silver nanoparticles. GA@AgNPs-LA-FP exhibited both excellent bacterial anti-adhesion activity and strong bactericidal activity, which can synergistically mitigate biofouling by inhibiting biofilm formation on the paper surface. Moreover, employed as a gravity-driven bactericidal filter, the GA@AgNPs-LA-FP membrane treated 100 mL of river water within 10 min, and the resulting water quality met the WHO drinking water standards, indicating this material's practical application for POU water disinfection.

Increased E. coli bio-adsorption resistance of microfiltration membranes, using a bio-inspired approach

Cells have inherent anti-fouling properties. The mechanisms underpinning these natural properties inform the design of an anti-biosorption coating for a polyethersulfone microfiltration membrane, which includes polydopamine and chitosan layers. This tri-layered membrane is created using quick and easy synthesis method. Its ability to resist bio-adsorption and membrane extracellular polymeric substances (EPS) formation is investigated using the bacterium E. coli (ATCC 11775, 1.5 × 10⁷ CFU/mL). In addition, the proliferative bio-adsorption process is explored on the microfiltration membrane surface, using natural water under static and shaken conditions, while monitoring the bio-adsorption kinetics and EPS dynamic changes. The characterization results show that the modification by polydopamine and chitosan change the membrane surface morphology and increase its hydrophilicity. After 10 min dipping in 5 g/L chitosan solution, the pure water flux of the modified membrane is 5469 ± 30 L/(m²·h) (0.2 bar) and the contact angle decreases to 36.7 ± 1.0°, compared with 9889 ± 23 L/(m²·h) (0.2 bar) and 60.3 ± 1.5° for the unmodified polyethersulfone membrane, respectively. In proliferative bio-adsorption tests, the modified membrane is shown to decrease bio-adsorption by 0.4-2.3 orders of magnitude. However, no antimicrobial function is observed, probably due to the alkaline environment and insufficient functional amino groups. A series of linear and non-linear kinetic models is applied to fit the proliferative bio-adsorption process. The pseudo-second-order model is found to describe the proliferative bio-adsorption process best. Neither total organic carbon (TOC) nor protein is detected on the modified membrane surface. In contrast, on the unmodified PES membrane the ratios of protein/TOC (%), TOC/abundance ((μg/cm²)/CFU (log)) and protein/abundance ((μg/cm²)/CFU(log)) are 10%-16%, 0.17-0.28 and 0.02-0.04, respectively. No significant difference (p > 0.05) is found between static and shaken conditions. All these results point to improved anti-biosorption properties for water treatment applications, encouraging further studies on this membrane.

Membrane Biofouling Control by Surface Modification of Quaternary Ammonium Compound Using Atom-Transfer Radical-Polymerization Method with Silica Nanoparticle as Interlayer

A facile approach to fabricate antibiofouling membrane was developed by grafting quaternary ammonium compounds (QACs) onto polyvinylidene fluoride (PVDF) membrane via surface-initiated activators regenerated by electron transfer atom-transfer radical-polymerization (ARGET ATRP) method. During the modification process, a hydrophilic silica nanoparticle layer was also immobilized onto the membrane surface as an interlayer through silicification reaction for QAC grafting, which imparted the membrane with favorable surface properties (e.g., hydrophilic and negatively charged surface). The QAC-modified membrane (MQ) showed significantly improved hydrophilicity and permeability mainly due to the introduction of silica nanoparticles and exposure of hydrophilic quaternary ammonium groups instead of long alkyl chains. Furthermore, the coverage of QAC onto membrane surface enabled MQ membrane to have clear antibacterial effect, with an inhibition rate ~99.9% of Escherichia coli (Gram-negative) and Staphylococcus aureus (Gram-positive), respectively. According to the batch filtration test, MQ had better antibiofouling performance compared to the control membrane, which was ascribed to enhanced hydrophilicity and antibacterial activity. Furthermore, the MQ membrane also exhibited impressive stability of QAC upon suffering repeated fouling–cleaning tests. The modification protocols provide a new robust way to fabricate high-performance antibiofouling QAC-based membranes for wastewater treatment.

Biological Activity of Quaternary Ammonium Salts and Their Derivatives

Besides their positive role, microorganisms are related to a number of undesirable effects, including many diseases, biodeterioration and food spoilage, so when their presence is undesired, they must be controlled. Numerous biocides limiting the development of microorganisms have been proposed, however, in this paper the biocidal and inhibitory activity of quaternary ammonium salts (QASs) and their zwitterionic derivatives is addressed. This paper presents the current state of knowledge about the biocidal activity of QAS and their derivatives. Moreover, the known mechanisms of antimicrobial activity and the problem of emerging resistance to QAS are discussed. The latest trends in the study of surfactants and their potential use are also presented.

Magnetic field assisted preparation of PES-Ni@MWCNTs membrane with enhanced permeability and antifouling performance

Multiple wall carbon nanotubes (MWCNTs), as an excellent material, have been used in various applications including preparation of polymer-MWCNTs composite membranes. However, few reports have combined the magnetic Ni@MWCNTs with polyether sulfone (PES) membrane to improve its antifouling performance to humic acid (HA), sodium alginate (SA), bovine serum albumin (BSA) and yeast (YE) solutions. In this study, the Ni@MWCNTs was generated by immersing MWCNTs into Ni²⁺ solution where in-situ reduction reaction was launched by the adsorbed Ag⁺ on MWCNTs. Since the loaded Ni endowed magnetism to MWCNTs, the Ni@MWCNTs can be easily attracted onto the membrane surface by an external magnetic field during the phase inversion process. The morphology measurements confirmed that the Ni@MWCNTs headed out of the PES-Ni@MWCNTs membrane surface. Because the MWCNTs played a role of free channels for water molecules, the composite membrane water flux reached to threefold flux of the pristine membrane. Moreover, the PES-Ni@MWCNTs membranes displayed the obviously enhanced antifouling ability during all the three alternative filtration cycles of water and BSA, SA, YE and HA solutions. In addition, the optimal PES-Ni@MWCNTs membrane demonstrated a flux recovery rate (FRR) of 67.89%, 85.53%, 60.28 and 90.12% for BSA, SA, YE and HA, respectively, which were not only much higher than that of the pristine membrane, but also exhibited significant improvements comparing with the previous studies. Further results of extended Derjaguin-Landau-Verwey-Overbeek (XDLVO) theory indicated that the modified membrane possessed advantageous interaction energies with contaminant molecules over the pristine membrane.

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.

Physico-chemical processes

The review scans research articles published in 2018 on physico-chemical processes for water and wastewater treatment. The paper includes eight sections, that is, membrane technology, granular filtration, flotation, adsorption, coagulation/flocculation, capacitive deionization, ion exchange, and oxidation. The membrane technology section further divides into six parts, including microfiltration, ultrafiltration, nanofiltration, reverse osmosis/forward osmosis, and membrane distillation. PRACTITIONER POINTS: Totally 266 articles on water and wastewater treatment have been scanned; The review is sectioned into 8 major parts;  Membrane technology has drawn the widest attention from the research community.

Versatile Surface Modification of TFC Membrane by Layer-by-Layer Assembly of Phytic Acid-Metal Complexes for Comprehensively Enhanced FO Performance

Polyamide TFC membranes are widely applied in membrane-based water treatment but generally suffer various fouling problems. In this work, the layer-by-layer assembly of phytic acid (PA) and metal ions (M) is constructed on the surface TFC membrane for the first time, to improve the bio/organic fouling resistances and separation performance of TFC membranes simultaneously. The PA molecule with six phosphonic acid groups of strong chelation ability acts as the organic ligand, and the metal ion acts as the inorganic cross-linker, inducing the assembly of hydrophilic and antibacterial PA-M (Ag or Cu) complexes on the TFC membrane surface. Various characterizations including FTIR, XPS, SEM, AFM, and EDX are employed to confirm the successful and uniform modification of PA-M. FO performance of the PA-M modified TFC membranes, i.e., TFC_PA-Ag and TFC_PA-Cu, is optimized by varying PA concentration and assembly cycles, where the water flux can be improved by 57% and 68%, respectively, without compromising the membrane selectivity. Additionally, the PA-M modification improves the biofouling and organic fouling resistances of the TFC membrane remarkably, owing to the enhanced antibacterial ability and hydrophilicity. The modified TFC membranes are also proven to show the excellent stability by the quantitative release test.