Mitigation of membrane fouling by nano/microplastics via surface chemistry control

Resilient forward osmosis membranes against microplastics fouling enhanced by MWCNTs/UiO-66-NH2 hybrid nanoparticles

The escalating presence of microplastics (MPs) in wastewater necessitates the investigation of effective tertiary treatment process. Forward osmosis (FO) emerges as an effective non-pressurized membrane process, however, for the effective implementation of FO systems, the development of fouling-resistance FO membranes with high-performance is essential. This study focuses on the integration of MWCNT/UiO-66-NH2 as metal-organic frameworks (MOFs) and multi-wall carbon nanotubes (MWCNT) nanocomposites in thin film composite (TFC) FO membranes, harnessing the synergistic power of hybrid nanoparticles in FO membranes. The results showed that the addition of MWCNT/UiO-66-NH2 in the aqueous phase during polyamide formation changed the polyamide surface structure, and enhanced membranes' hydrophilicity by 44%. The water flux of the modified FO membrane incorporated with 0.1 wt% MWCNTs/UiO-66-NH2 increased by 67% and the reverse salt flux decreased by 22% as in comparison with the control membrane. Moreover, the modified membrane showed improved antifouling behavior against both organic foulant and MPs. The MWCNT/UiO-66-NH2 membrane experienced 35% flux decline while the control membrane experienced 65% flux decline. This proves that the integration of MWCNT/UiO-66-NH2 nanoparticles into TFC FO membranes is a viable approach in creating advanced FO membranes with high antifouling propensity with potential to be expanded further to other membrane applications.

Nano-based remediation strategies for micro and nanoplastic pollution

Due to rapid urbanization, there have been continuous environmental threats from different pollutants, especially from microplastics. Plastic products rapidly proliferate significantly contributing to the occurrence of micro-plastics, which poses a significant environmental risk. These microplastics originated from diverse sources and are characterized by their persistent and widespread occurrence; human health and the entire ecosystem are adversely affected by them. The removal of microplastics not only requires innovative technologies but also efficient materials capable of effectively eliminating them from our environment. The progress made so far has highlighted the advantages of utilizing the dimensional and structural properties of nanomaterials to increase the effectiveness of existing methods for micro-plastic treatment, aiming for a more sustainable approach to their removal. In the current review, we demonstrate a thorough overview of the sources, occurrences, and potential harmful effects of microplastics, followed by a further discussion of promising technologies used for their removal. An in-depth examination of both advantages and a few limitations of all these given technologies, including physical, chemical, and biological approaches, has been discussed. Additionally, the review explores the use of nanomaterials as an effective means to overcome obstacles and improve the efficiency of microplastic elimination methods. n conclusion, this review addresses, current challenges in this field and outlines the future perspectives for further research in this domain.

Kinetic trapping of nanoparticles by solvent-induced interactions

Theoretical analysis based on mean field theory indicates that solvent-induced interactions (i.e. structural forces due to the rearrangement of wetting solvent molecules) not considered in DLVO theory can induce the kinetic trapping of nanoparticles at finite nanoscale separations from a well-wetted surface, under a range of ubiquitous physicochemical conditions for inorganic nanoparticles of common materials (e.g., metal oxides) in water or simple molecular solvents. This work proposes a simple analytical model that is applicable to arbitrary materials and simple solvents to determine the conditions for direct particle-surface contact or kinetic trapping at finite separations, by using experimentally measurable properties (e.g., Hamaker constants, interfacial free energies, and nanoparticle size) as input parameters. Analytical predictions of the proposed model are verified by molecular dynamics simulations and numerical solution of the Smoluchowski diffusion equation.

Multifunctional sodium alginate/chitosan-modified graphene oxide reinforced membrane for simultaneous removal of nanoplastics, emulsified oil, and dyes in water

Membrane technology is widely recognized as an efficient and advanced approach for wastewater treatment. However, the development of environmentally friendly and versatile membranes capable of effectively removing multiple contaminants remains a significant challenge. Inspired by natural magnets, we developed a heterostructured membrane using biomass materials to achieve the efficient removal of multiple contaminants from wastewater. Specifically, a bionic three-layer SA/GO/CS composite membrane was prepared by using sodium alginate (SA) and chitosan (CS) to modify graphene oxide (GO), respectively, and then assembled to both sides of the glass fiber (GF) membrane. The composite membranes achieved 99.87 % and 97.10 % removal of NPs with particle sizes of 500 nm and 50 nm. Moreover, the membrane demonstrated superior separation performance for mixed wastewater, enabling effective treatment of a broad spectrum of contaminants. Additionally, the membrane exhibited excellent stability when exposed to strong acid and alkali environments and demonstrated good recyclability throughout the multiple contaminants removal process. The bionic membrane, prepared using a straightforward method proposed in this study, provides an effective approach for enhanced removal of multiple contaminants in water. These findings contribute to the advancement of eco-friendly and versatile wastewater treatment membranes, opening new possibilities for sustainable water purification technologies.

Recent advances on micro/nanoplastic pollution and membrane fouling during water treatment: A review

Effluent from sewage treatment plant, as an important source of microplastics (MPs) in receiving water, has attracted extensive attention. Membrane separation process shows good microplastic removal performance in the existing tertiary water treatment process. Problematically, membrane fouling and insufficient removal of small organic molecules are still the key obstacles to its further extensive application. Dissolved organics, extracellular polymers and suspended particles in the influent are deposited on the membrane surface and internal structure, reducing the number and pore diameter of effective membrane aperture, and increasing the resistance of membrane filtration. Exploring the mechanism and approach of membrane fouling caused by micro/nanoplastics is the key to alleviate fouling and allow membranes to operate longer. In this paper, removal performance of micro/nanoplastics by current membrane filtration and the contribution to membrane fouling during water treatment are thoroughly reviewed. The coupling mechanisms between micro/nanoplastics and other pollutants and mechanism of membrane fouling caused by composite micro/nanoplastics are discussed. Additionally, on this basis, the prospect of combined process for micro/nanoplastic removal and membrane fouling prevention is also proposed and discussed, which provides a valuable reference for the preferential removal of micro/nanoplastics and development of antifouling membrane.

Quantitative Assessment of Interfacial Interactions Governing Ultrafiltration Membrane Fouling by the Mixture of Silica Nanoparticles (SiO2 NPs) and Natural Organic Matter (NOM): Effects of Solution Chemistry

Mixtures of silica nanoparticles (SiO₂ NPs) and natural organic matter (NOM) are ubiquitous in natural aquatic environments and pose risks to organisms. Ultrafiltration (UF) membranes can effectively remove SiO₂ NP-NOM mixtures. However, the corresponding membrane fouling mechanisms, particularly under different solution conditions, have not yet been studied. In this work, the effect of solution chemistry on polyethersulfone (PES) UF membrane fouling caused by a SiO₂ NP-NOM mixture was investigated at different pH levels, ionic strengths, and calcium concentrations. The corresponding membrane fouling mechanisms, i.e., Lifshitz-van der Waals (LW), electrostatic (EL), and acid-base (AB) interactions, were quantitatively evaluated using the extended Derjaguin-Landau-Verwey-Overbeek (xDLVO) theory. It was found that the extent of membrane fouling increased with decreasing pH, increasing ionic strength, and increasing calcium concentration. The attractive AB interaction between the clean/fouled membrane and foulant was the major fouling mechanism in both the initial adhesion and later cohesion stages, while the attractive LW and repulsive EL interactions were less important. The change of fouling potential with solution chemistry was negatively correlated with the calculated interaction energy, indicating that the UF membrane fouling behavior under different solution conditions can be effectively explained and predicted using the xDLVO theory.

Designing sustainable membrane-based water treatment via fouling control through membrane interface engineering and process developments

Membrane-based water treatment processes have been established as a powerful approach for clean water production. However, despite the significant advances made in terms of rejection and flux, provision of sustainable and energy-efficient water production is restricted by the inevitable issue of membrane fouling, known to be the major contributor to the elevated operating costs due to frequent chemical cleaning, increased transmembrane resistance, and deterioration of permeate flux. This review provides an overview of fouling control strategies in different membrane processes, such as microfiltration, ultrafiltration, membrane bioreactors, and desalination via reverse osmosis and forward osmosis. Insights into the recent advancements are discussed and efforts made in terms of membrane development, modules arrangement, process optimization, feed pretreatment, and fouling monitoring are highlighted to evaluate their overall impact in energy- and cost-effective water treatment. Major findings in four key aspects are presented, including membrane surface modification, modules design, process integration, and fouling monitoring. Among the above mentioned anti-fouling strategies, a large part of research has been focused on membrane surface modifications using a number of anti-fouling materials whereas much less research has been devoted to membrane module advancements and in-situ fouling monitoring and control. At the end, a critical analysis is provided for each anti-fouling strategy and a rationale framework is provided for design of efficient membranes and process for water treatment.

Reduction of Ultrafiltration Membrane Fouling by the Pretreatment Removal of Emerging Pollutants: A Review

Ultrafiltration (UF) processes exhibit high removal efficiencies for suspended solids and organic macromolecules, while UF membrane fouling is the biggest obstacle affecting the wide application of UF technology. To solve this problem, various pretreatment measures, including coagulation, adsorption, and advanced oxidation, for application prior to UF processes have been proposed and applied in actual water treatment processes. Previously, researchers mainly focused on the contribution of natural macromolecular pollutants to UF membrane fouling, while the mechanisms of the influence of emerging pollutants (EPs) in UF processes (such as antibiotics, microplastics, antibiotic resistance genes, etc.) on membrane fouling still need to be determined. This review introduces the removal efficiency and separation mechanism for EPs for pretreatments combined with UF membrane separation technology and evaluates the degree of membrane fouling based on the UF membrane's materials/pores and the structural characteristics of the cake layer. This paper shows that the current membrane separation process should be actively developed with the aim of overcoming specific problems in order to meet the technical requirements for the efficient separation of EPs.

Particulate plastics in drinking water and potential human health effects: Current knowledge for management of freshwater plastic materials in Africa

Plastic materials have contributed to the release of environmentally relevant particulate plastics which can be found almost everywhere and may be present in drinking water. Human exposure to these materials is diverse and our understanding of their internalization in the human body is incipient. This review discusses the state of knowledge of particulate plastics exposure in drinking water and the potential risks of adverse health in the human body. Particulate plastics have problematized water systems worldwide, and about 4,000,000 fine plastics may be ingested from drinking water annually by an individual. Testing methods for these materials in environmental media are presently inconsistent and standard protocols do not exist. Their potential ecotoxicological consequences are recognised to be linked to their physicochemical diversity, biological transpositions, and cytological tolerance in living organisms. It is observed that toxicological endpoints are varied and lack properly defined modes of action. In particular, fine particulate plastics have been observed to translocate into body tissues and cells where they are capable of provoking endocrine disruption, genetic mutations, and cancer responses. We propose a reclassification of particulate plastics to cater for their biological deposition and attributable risks of adverse health. Environmental management of particulate plastics in many developing countries is weak and their potential releases into drinking water have received limited research. Given that large populations are exposed to fresh surface water and plastic packaged drinking water worldwide, and that the risk assessment pathways are unvalidated at the moment, we argue for developing countries to increase their capacity for the environmental monitoring and circular management of plastic materials. Large-scale epidemiological cohort studies and surrogate assessment pathways are also recommended to provide a better understanding of the hazard characterization of particulate plastics exposure.

The impact of PET microplastic fibres on PVDF ultrafiltration performance - A short-term assessment of MP fouling in simple and complex matrices

Wastewater treatment plants (WWTPs) are key components for the capture of microplastics (MPs) before they are released into natural waterways. Removal efficiencies as high as 99% may be achieved but sub-micron MPs as well as nanoplastics have been overlooked because of analytical limitations. Furthermore, short MP fibres are of concern because of their low capture rate as well as the lack of understanding of their influence on purification system efficiency. This study has investigated the impact of poly(ethylene terephthalate) (PET) short nanofibres on the performance of polyvinylidene fluoride (PVDF) ultrafiltration membranes during cross-flow operation. Model MP fibres with an average length of 10 ± 7 μm and a diameter of 142 ± 40 nm were prepared via a combination of electrospinning and fine cutting using a cryomicrotome. The manufactured MPs were added to both pure and synthetic domestic wastewater at a concentration of 1 mg.L^-1 to determine their impact on the performance of PVDF ultrafiltration membranes. The results show that PET fibres attach to the membrane in a disorganised manner with low pore coverage. The water flux was decreased by 8% for MPs in pure water and no noticeable effect in wastewater after 3 days of filtration. Additionally, the nutrient removal efficiency of the membrane was not altered by the presence of PET MPs. These findings show that MP fibres do not significantly influence the early stages of filtration for a standard concentration of MPs in wastewater treatment plant studies.

Synergistic effects of microplastics and organic foulants on the performance of forward osmosis membranes

Microplastics (MPs) are emerging contaminants that are abundantly present in the influent and effluent of wastewater treatment plants (WWTPs). Forward osmosis (FO) is an advanced treatment technology with potential applications in WWTPs. The presence of MPs in WWTP effluents can contribute to FO fouling and performance deterioration. This study focuses on FO membrane fouling by MPs of different sizes, and the interactional impacts of MPs and Humic acid (HA) (as the most common organic foulant in WWTPs) on FO membrane performance. The synergistic effect of combined MPs and HA fouling is shown to cause higher flux decline for FO membranes than that of HA or MPs alone. Reverse salt flux increased in the presence of MPs, and decreased when HA was present. Further, full flux recovery was obtained for all fouled membranes after hydraulic cleaning. This indicates the efficiency of FO systems for treating wastewater with high fouling potential. This study highlights the necessity of considering MPs in studying fouling behaviour, and for mitigation strategies of membranes used in WWT. The fundamentals created here can be further extended to other membrane-assisted separation processes.

Novel metallic stainless-steel mesh-supported conductive membrane and its performance in the electro-filtration process

In this study, we demonstrate the fabrication of a thoroughly metallic electro-conductive membrane by using simple filtration to uniformly coat AgNWs dispersion through stainless steel (SUS)-mesh, which functions both as filter and a flexible conductive substrate. The as-prepared AgNWs networks layer on the SUS-mesh was further strengthened by electroplating Ag layers (P-SUS membrane); exhibiting an overall electrical conductivity of 9.2 × 10⁴ S/m, which is up to 42 times greater than the conductivity of pristine SUS-mesh. The P-SUS membrane exhibited adequate physical durability against chemical and mechanical stresses under prolonged filtration, and high pure water flux of 534 ± 54 LMH/bar. This electro-membrane displayed the anticipated flux recovery in harvesting microalgae (Chlorella sp. HS-2) when filtration was done with the membrane used as a cathode: micro-sized bubbles, generated from the cathodic membrane, functioned to detach the foulants and recover the relative flux to a significant level. The P-SUS membrane indeed possesses necessary traits that the polymer-support membrane lacks, in terms of not only electrical conductivity and mechanical strength but also filtration performance with anti-fouling capability, all of which are of necessity to be considered workable electroconductive membrane.

Optimization of coagulation and sedimentation conditions by turbidity measurement for nano- and microplastic removal

Plastic pollution has emerged as a potential threat to drinking water quality. Coagulation and sedimentation processes are suggested to be an effective way of removing small plastic particles from water. However, a conventional jar test consumes a large volume of plastic specimens, producing secondary wastes and hindering a quick optimization of the treatment processes for nano/microplastic removal. This study investigates the monitoring of nano- and microplastic behavior (0.1, 1, and 10 μm-sized polystyrene) in coagulation and sedimentation processes by a simple turbidity measurement. The coagulation and the subsequent floc sedimentation of the plastic particles could be observed in a turbidity vial with a small volume (∼15 mL), allowing a fast screening of coagulant type, dose, sedimentation time, and water environment. In particular, the physicochemical properties of coagulants could be screened in the turbidity monitoring, where the hydrophobic interaction is identified to be important for the coagulation of nano-sized plastic particles. The optimal coagulation/sedimentation conditions from the turbidity monitoring could be applied to the jar test, achieving the high removal efficiencies of nano/microplastic particles. The plastic removal after the coagulation/sedimentation process could be estimated based on linearity between the plastic concentration and the turbidity. The turbidity-driven removal efficiency well corresponds to that derived from the mass-based calculation of the jar test when with a complete floc settling. Our findings suggest that the turbidity measurement can provide a rational optimization of the water treatment processes for the effective removal of nano- and microplastics.

Ecotoxic effects of microplastics and contaminated microplastics - Emerging evidence and perspective

The high prevalence and persistence of microplastics (MPs) in pristine habitats along with their accumulation across environmental compartments globally, has become a matter of grave concern. The resilience conferred to MPs using the material engineering approaches for outperforming other materials has become key to the challenge that they now represent. The characteristics that make MPs hazardous are their micro to nano scale dimensions, surface varied wettability and often hydrophobicity, leading to non-biodegradability. In addition, MPs exhibit a strong tendency to bind to other contaminants along with the ability to sustain extreme chemical conditions thus increasing their residence time in the environment. Adsorption of these co-contaminants leads to modification in toxicity varying from additive, synergistic, and sometimes antagonistic, having consequences on flora, fauna, and ultimately the end of the food chain, human health. The resulting environmental fate and associated risks of MPs, therefore greatly depend upon their complex interactions with the co-contaminants and the nature of the environment in which they reside. Net outcomes of such complex interactions vary with core characteristics of MPs, the properties of co-contaminants and the abiotic factors, and are required to be better understood to minimize the inherent risks. Toxicity assays addressing these concerns should be ecologically relevant, assessing the impacts at different levels of biological organization to develop an environmental perspective. This review analyzed and evaluated 171 studies to present research status on MP toxicity. This analysis supported the identification and development of research gaps and recommended priority areas of research, accounting for disproportionate risks faced by different countries. An ecological perspective is also developed on the environmental toxicity of contaminated MPs in the light of multi-variant stressors and directions are provided to conduct an ecologically relevant risk assessment. The presented analyses will also serve as a foundation for developing environmentally appropriate remediation methods and evaluation frameworks.

Nanomaterials for microplastic remediation from aquatic environment: Why nano matters?

The contamination of microplastics in aquatic environment is regarded as a serious threat to ecosystem especially to aquatic environment. Microplastic pollution associated problems including their bioaccumulation and ecological risks have become a major concern of the public and scientific community. The removal of microplastics from their discharge points is an effective way to mitigate the adverse effects of microplastic pollution, hence has been the central of the research in this realm. Presently, most of the commonly used water or wastewater treatment technologies are capable of removing microplastic to certain extent, although they are not intentionally installed for this reason. Nevertheless, recognizing the adverse effects posed by microplastic pollution, more efforts are still desired to enhance the current microplastic removal technologies. With their structural multifunctionalities and flexibility, nanomaterials have been increasingly used for water and wastewater treatment to improve the treatment efficiency. Particularly, the unique features of nanomaterials have been harnessed in synthesizing high performance adsorbent and photocatalyst for microplastic removal from aqueous environment. This review looks into the potentials of nanomaterials in offering constructive solutions to resolve the bottlenecks and enhance the efficiencies of the existing materials used for microplastic removal. The current efforts and research direction of which studies can dedicate to improve microplastic removal from water environment with the augmentation of nanomaterial-enabled strategies are discussed. The progresses made to date have witnessed the benefits of harnessing the structural and dimensional advantages of nanomaterials to enhance the efficiency of existing microplastic treatment processes to achieve a more sustainable microplastic cleanup.

Review of New Approaches for Fouling Mitigation in Membrane Separation Processes in Water Treatment Applications

This review investigates antifouling agents used in the process of membrane separation (MS), in reverse osmosis (RO), ultrafiltration (UF), nanofiltration (NF), microfiltration (MF), membrane distillation (MD), and membrane bioreactors (MBR), and clarifies the fouling mechanism. Membrane fouling is an incomplete substance formed on the membrane surface, which will quickly reduce the permeation flux and damage the membrane. Foulant is colloidal matter: organic matter (humic acid, protein, carbohydrate, nano/microplastics), inorganic matter (clay such as potassium montmorillonite, silica salt, metal oxide, etc.), and biological matter (viruses, bacteria and microorganisms adhering to the surface of the membrane in the case of nutrients) The stability and performance of the tested nanometric membranes, as well as the mitigation of pollution assisted by electricity and the cleaning and repair of membranes, are reported. Physical, chemical, physico-chemical, and biological methods for cleaning membranes. Biologically induced biofilm dispersion effectively controls fouling. Dynamic changes in membrane foulants during long-term operation are critical to the development and implementation of fouling control methods. Membrane fouling control strategies show that improving membrane performance is not only the end goal, but new ideas and new technologies for membrane cleaning and repair need to be explored and developed in order to develop future applications.

Fabrication and Performance of Low-Fouling UF Membranes for the Treatment of Isolated Soy Protein Solutions

Consumers are becoming more conscious about the need to include functional and nutritional foods in their diet. This has increased the demand for food extracts rich in proteins and peptides with physiological effects that are used within the food and pharmaceutical industries. Among these protein extracts, soy protein and its derivatives are highlighted. Isolated soy protein (ISP) presents a protein content of at least 90%. Wastewaters generated during the production process contain small proteins (8–50 kDa), and it would be desirable to find a recovery treatment for these compounds. Ultrafiltration membranes (UF) are used for the fractionation and concentration of protein solutions. By the appropriate selection of the membrane pore size, larger soy proteins are retained and concentrated while carbohydrates and minerals are mostly recovered in the permeate. The accumulation and concentration of macromolecules in the proximity of the membrane surface generates one of the most important limitations inherent to the membrane technologies. In this work, three UF membranes based on polyethersulfone (PES) were fabricated. In two of them, polyethylene glycol (PEG) was added in their formulation to be used as a fouling prevention. The membrane fouling was evaluated by the study of flux decline models based on Hermia’s mechanisms.

Microplastics fouling and interaction with polymeric membranes: A review

The emergence and accumulation of microplastics (MPs) in various aquatic environments have recently raised significant concerns. Wastewater treatment plants (WWTPs) have been identified as one of the major sources of MPs discharge to the environment, implying a substantial need to improve advanced techniques for more efficient removal of MPs. Polymeric membranes have been proven effective in MPs removal. However, fouling is the main drawback of membrane processes and MPs can foul the membranes due to their small size and specific surface properties. Hence, it is important to investigate the impacts of MPs on membrane fouling to develop efficient membrane-based techniques for MPs removal. Although membrane technologies have a high potential for MPs removal, the interaction of MPs with membranes and their fouling effects have not been critically reviewed. The purpose of this paper is to provide a state-of-the-art review of MPs interaction with membranes and facilitate a better understanding of the relevant limitations and prospects of the membrane technologies. The first section of this paper is dedicated to a review of recent studies on MPs occurrence in WWTPs aiming to determine the most frequent MPs. This is followed by a summary of recent studies on MPs removal using membranes and discussions on the impact of MPs on membrane fouling and other probable issues (abrasion, concentration polarisation, biofouling, etc.). Finally, some recommendations for further research in this area are highlighted. This study serves as a valuable reference for future research on the development of anti-fouling membranes considering these new emerging contaminates.

Mitigation of membrane particulate fouling by nano/microplastics via physical cleaning strategies

Membrane fouling by nano/microplastics (NP/MPs) is an emerging concern threatening the performance of water and wastewater treatment facilities. The NP/MPs can lead to surface adsorption, fouling and potential mechanical abrasion of the membranes. In this work, periodic gas scouring was applied during the filtration of nano/microplastics across ultrafiltration membranes to investigate the impact of shear forces on the adsorption of nano/microplastics. A series of surface energy and chemistry-modified membranes were also used including acrylic acid, cyclopropylamine and hexamethyldisiloxane plasma-modified membranes, allowing for a set of materials with controlled hydrophilicity, roughness and surface charge. Bubbling gas within the system at a gas flow rate of 0.5 to 1 L·min^-1 and a water flow rate of 2 L·min^-1 was found to limit the water flux decline across the pristine and hydrophobic membranes compared to the filtration experiments performed without cleaning from 38 to 22 and 23%, respectively. The adsorption of nano/microplastics onto the surface of the membranes was also simultaneously decreased from 40 to 25 and 19%, respectively. Interestingly, for the hydrophilised membranes no enhancement in permeance was observed when performing gas scouring due to the already low tendency for selective adsorption of the nano/microplastics onto their surface. The correlation of a dimensionless fouling number to the shear stress number suggested that the shear forces induced by gas scouring reduced nano/microplastics adsorption up to a gas injection ratio (volume fraction of gas) of 0.3, where the wall shear stress at the surface of the membrane was limited. This work offers an advanced physical strategy to reduce and control membrane fouling by nano/microplastics, with potential for this strategy to be adapted for more complex water matrices and plastic particles.