Strong improvement of nanofiltration performance on micropollutant removal and reduction of membrane fouling by hydrolyzed-aluminum nanoparticles

Deciphering membrane biofouling induced by micro-/nano-plastics in nanofiltration: Metagenomic insights and spacer-driven mitigations

Nanofiltration (NF) is an effective process for micro-/nano-plastics (MNPs) interception, but the impact of accumulated MNPs on the microbial community structure and metabolic pathways of biofilms on NF membranes remains unclear. This provides uncertainty with respect to membrane biofouling behavior and the risks to efficient NF operations. In this study, the size-dependent (20 nm-25 μm) and concentration-dependent (0.1-50 mg·L-1) effects of MNPs on the biofouling of a NF membrane treating secondary wastewater effluent were studied. Three MNPs-tolerant, hypermetabolic and polystyrene-degradable genera (i.e., Acinetobacter, Novosphingobium and Asticcacaulis) were detected in biofilms as dominant taxonomic compositions. MNPs led to an increase of 19.3 %-76.7 % in biomass contents and a more rapid decrease in permeate flux, with 0.1 mg·L-1 of 80 nm NPs causing the most severe membrane biofouling. Metagenomic analysis revealed that MNPs upregulated enzymes involved in exopolysaccharide (ExoA/L/M/P/Q/X/Y/Z) and tyrosine (COMT, FeaB and AOC3) biosynthesis and quorum sensing (PhzF and CiaH/R), and suppressed cell motility pathways including flagellar assembly and bacterial chemotaxis. Novel types of perforated column spacer (PCS) enhanced the hydrodynamics of the membrane feed with a lower pressure drop and higher fluid velocity, introduced micro-jets and greater mass transfer inside feed channels, thus eliminating the deposition of MNPs and mitigating membrane biofouling. Overall, a greater understanding of the interaction mechanisms between MNPs and membrane biofouling in secondary effluent filtration will help develop more effective MNPs management strategies and achieve more sustainable NF operations.

One-step construction of carbon nanoparticle/graphene oxide nanofiltration membranes with uniform sandwich structure for enhanced water purification

Graphene-based membranes have great potential for water purification. However, it is still a challenge to achieve high solute rejection at high water flow by controlling the water permeation channel. Herein, carbon nanoparticles (CNPs) were uniformly sandwiched between graphene oxide (GO) sheets by one-step vacuum-assisted filtration of CNPs and GO mixed solution, resulting in the formation of CNPs/GO composite nanofiltration membranes with uniform sandwich structure. The addition of CNPs in the composite membrane could help to form a continuous transverse channel of water permeation and greatly increase the water flow. The results showed that the CNPs/GO composite membrane with a mass ratio of 20% exhibited the best performance. The pure water flow rate was 49.9 L m-2 h-1, which was 21 times higher than that of the pure GO membrane. The rejection rate for four different organic dyes exceeded 97%. The rejection rate for methylene blue (MB) was still 94.7% after 8 recycling cycles. In addition, the membranes allow the penetration of salts, which makes them promising for dye wastewater desalination. This study provides a simple and effective strategy to tune the channel microstructure of the composite membranes and increases the understanding of the important role of the sandwich particles in achieving a better performance of the membranes.

Esterified Chlorine-Resistant Nanofiltration Membranes with Enhanced Removal of Disinfection Byproducts for Efficient Water Purification

The permeance-selectivity trade-off and chlorine sensitivity of conventional polyamide membranes limit further efficiency improvement and cost reduction of nanofiltration (NF) processes for drinking water treatment. To overcome these challenges, this study proposed a reconstruction-esterification strategy for the development of advanced NF membranes. Results showed that the combination of Na3PO4 solution post-treatment and polyol molecule grafting generated a thinner active layer with smaller and more uniform pores. More importantly, the critical role of alkaline post-treatment in reducing the residual amine groups of polyamide layers was revealed, which enhanced the chlorine resistance of membranes jointly with the effect of surface esterification. In comparison with the surface water purification performance of several commercial NF membranes, the obtained esterified membrane showed excellent selectivity between natural organic matter and salts, along with a reasonable water permeance. Moreover, the higher and stable removal capacity of the esterified membrane for disinfection byproducts and their precursors demonstrated its application advantage in the potential chlorination-NF-coupled process. The developed chlorine-resistant membrane and initially attempted NF filtration of chlorinated water in this study can help promote process innovation and highlight more benefits of NF technology for drinking water treatment.

Bisphenol A (BPA) toxicity assessment and insights into current remediation strategies

Bisphenol A (BPA) raises concerns among the scientific community as it is one of the most widely used compounds in industrial processes and a component of polycarbonate plastics and epoxy resins. In this review, we discuss the mechanism of BPA toxicity in food-grade plastics. Owing to its proliferation in the aqueous environment, we delved into the performance of various biological, physical, and chemical techniques for its remediation. Detailed mechanistic insights into these removal processes are provided. The toxic effects of BPA unravel as changes at the cellular level in the brain, which can result in learning difficulties, increased aggressiveness, hyperactivity, endocrine disorders, reduced fertility, and increased risk of dependence on illicit substances. Bacterial decomposition of BPA leads to new intermediates and products with lower toxicity. Processes such as membrane filtration, adsorption, coagulation, ozonation, and photocatalysis have also been shown to be efficient in aqueous-phase degradation. The breakdown mechanism of these processes is also discussed. The review demonstrates that high removal efficiency is usually achieved at the expense of high throughput. For the scalable application of BPA degradation technologies, removal efficiency needs to remain high at high throughput. We propose the need for process intensification using an integrated combination of these processes, which can solve multiple associated performance challenges.

Enhanced Antifouling Capability of In Situ-Grown Hydrophilic-Hydrophobic Nanodomains on Membrane Surface in the Ultralow Pressurized Ultrafiltration Process

Although hydrophilic modification of the membrane surface is widely adopted, polymeric membranes still suffer from irreversible fouling caused by hydrophilic components in surface water. Here, an ultrathin hydrogel layer (40 nm) with hydrophilic-hydrophobic textures was in situ grown onto the polysulfone ultrafiltration membrane surface using an organic-radical-initiated interfacial polymerization technique. The interfacial polymerization of hydrophilic and hydrophobic monomers ensured the molecular-scale distribution of hydrophilic and hydrophobic nanodomains on the membrane surface. These nanodomains, with their molecular lengths, facilitated dynamic repulsion interactions between the uniformly textured surface and foulant components with different degrees of hydrophilicity. Chemical force characterization confirmed that the adhesion force between the hydrophilic-hydrophobic textured membrane surface and foulants (dodecane, bovine serum albumin, and humic acid) was greatly reduced. Dynamic filtration experiments showed that a hydrophilic-hydrophobic textured membrane always possessed the largest water flux and the best antifouling performance. Furthermore, the foulant coverage ratio on the membrane surface was first evaluated by measuring changes in surface streaming potentials, which demonstrated a 69% reduction in the amount of foulant adhering to the hydrophilic-hydrophobic textured membrane surface. Therefore, the construction of hydrophilic-hydrophobic nanodomains on the membrane surface provides a promising strategy for alleviating membrane fouling caused by both hydrophobic and hydrophilic components during ultralow pressurized ultrafiltration processes.

Removal of Heavy Metals from Wastewaters and Other Aqueous Streams by Pressure-Driven Membrane Technologies: An Outlook on Reverse Osmosis, Nanofiltration, Ultrafiltration and Microfiltration Potential from a Bibliometric Analysis

A bibliometric study to analyze the scientific documents released until 2024 in the database Scopus related to the use of pressure-driven membrane technologies (microfiltration, ultrafiltration, nanofiltration and reverse osmosis) for heavy metal removal was conducted. The work aimed to assess the primary quantitative attributes of the research in this field during the specified period. A total of 2205 documents were identified, and the corresponding analysis indicated an exponential growth in the number of publications over time. The contribution of the three most productive countries (China, India and USA) accounts for more than 47.1% of the total number of publications, with Chinese institutions appearing as the most productive ones. Environmental Science was the most frequent knowledge category (51.9% contribution), followed by Chemistry and Chemical Engineering. The relative frequency of the keywords and a complete bibliometric network analysis allowed the conclusion that the low-pressure technologies (microfiltration and ultrafiltration) have been more deeply investigated than the high-pressure technologies (nanofiltration and reverse osmosis). Although porous low-pressure membranes are not adequate for the removal of dissolved heavy metals in ionic forms, the incorporation of embedded adsorbents within the membrane structure and the use of auxiliary chemicals to form metallic complexes or micelles that can be retained by this type of membrane are promising approaches. High-pressure membranes can achieve rejection percentages above 90% (99% in the case of reverse osmosis), but they imply lower permeate productivity and higher costs due to the required pressure gradients.

Removal of per- and polyfluoroalkyl substances by nanofiltration: Effect of molecular structure and coexisting natural organic matter

This study investigates the removal efficiency of anionic, cationic, and zwitterionic per- and polyfluoroalkyl substances (PFAS) by nanofiltration (NF) in the presence of three representative natural organic matter (NOM) types: bovine serum albumin (BSA), humic acid (HA), and sodium alginate (SA). In particular, effects of PFAS molecular structure and coexisting NOM on the transmission and adsorption efficiency of PFAS during NF treatment were analyzed. The results indicate that NOM types dominate membrane fouling behavior despite the coexistence of PFAS. SA exhibits the most significant fouling propensity, resulting in maximum water flux decline. NF effectively removed both ether and precursor PFAS. The effects of the three typical NOM on the membrane-passing behavior of PFAS were consistent for all PFAS investigated. Generally, PFAS transmission decreased in the order of SA-fouled > pristine > HA-fouled > BSA-fouled, indicating that the presence of HA and BSA enhanced PFAS removal while SA declined. Furthermore, reduced PFAS transmission was observed with increased perfluorocarbon chain length or molecular weight (MW), regardless of the presence or type of the NOM. The impacts of NOM on PFAS filtration diminished when the PFAS van der Waals radius was > 4.0 Å, MW > 500 Da, polarization > 20 Å, or LogKow > 3. These findings suggest that both steric repulsion and hydrophobic interactions, especially the former, play important roles in PFAS rejection by NF. This study provides insights into the specific applicability and performance of membrane-based processes for eliminating PFAS during drinking and wastewater treatments, and highlighting the importance of coexisting NOM.

A review on the green synthesis of nanoparticles, their biological applications, and photocatalytic efficiency against environmental toxins

Green synthesis of nanoparticles (NPs) using plant materials and microorganisms has evolved as a sustainable alternative to conventional techniques that rely on toxic chemicals. Recently, green-synthesized eco-friendly NPs have attracted interest for their potential use in various biological applications. Several studies have demonstrated that green-synthesized NPs are beneficial in multiple medicinal applications, including cancer treatment, targeted drug delivery, and wound healing. Additionally, due to their photodegradation activity, green-synthesized NPs are a promising tool in environmental remediation. Photodegradation is a process that uses light and a photocatalyst to turn a pollutant into a harmless product. Green NPs have been found efficient in degrading pollutants such as dyes, herbicides, and heavy metals. The use of microbes and flora in green synthesis technology for nanoparticle synthesis is biologically safe, cost-effective, and eco-friendly. Plants and microbes can now use and accumulate inorganic metallic ions in the environment. Various NPs have been synthesized via the bio-reduction of biological entities or their extracts. There are several biological and environmental uses for biologically synthesized metallic NPs, such as photocatalysis, adsorption, and water purification. Since the last decade, the green synthesis of NPs has gained significant interest in the scientific community. Therefore, there is a need for a review that serves as a one-stop resource that points to relevant and recent studies on the green synthesis of NPs and their biological and photocatalytic efficiency. This review focuses on the green fabrication of NPs utilizing diverse biological systems and their applications in biological and photodegradation processes.

Relating critical and limiting fluxes to metastable and long-term stable fluxes in colloidal membrane filtration through collision-attachment theory

In membrane technology for water/wastewater treatment, the concepts of critical flux (J_C) and limiting flux (J_L) suggest the existence of a threshold flux below which no fouling occurs. However, their important roles on stable flux duration have not been sufficiently understood. This work adopts a collision-attachment approach to clarify the relationship of J_C, J_L to metastable (i.e., short-term stable) and long-term stable fluxes based on their dependence on initial flux (J₀), foulant-clean-membrane energy barrier (E_f-m), and foulant-fouled-membrane energy barrier (E_f-f). When J₀ is below J_L, water flux remains stable over a long time even for the case of J₀ over J_C, thanks to the strongly repulsive E_f-f. At J₀ > J_L and J₀ > J_C, the water flux is unstable at the beginning of filtration, and the flux ultimately decreases to J_L as the long-term stable flux. Under the condition of J_L < J₀ ≤ J_C, an initial metastable flux appears owing to the high E_f-m, with longer metastable period observed at lower J₀ and for more hydrophilic/charged membrane or colloids. Nevertheless, rapid flux decline occurs subsequently due to the energy barrier shifting to weak E_f-f, and the water flux eventually degenerates to J_L in long-term fouling duration. Our results provide significant guidelines for fouling control strategies with respect to membrane design, feedwater pretreatment, and operational optimization.

Loosely Sandwich-Structured Membranes Decorated with UiO-66-NH2 for Efficient Antibiotic Separation and Organic Solvent Resistance

Thin-film nanocomposite (TFN) membranes with efficient molecular separation and organic solvent resistance are active in demand in wastewater treatment and resource reclamation, meeting the goal of emission peaks and carbon neutrality. In this work, a simple and rational design strategy has been employed to construct a sandwich-structured membrane for removing fluoroquinolone antibiotics and recycling organic solvents. The sandwich-structured membrane is composed of a porous substrate, a hydrophilic tannic acid-polyethyleneimine (TA-PEI) interlayer, and a polyamide (PA) selective layer decorated with metal-organic framework (PA-MOF). Results manifest that the hydrophilic TA-PEI interlayer played a bridging and gutter effect to achieve effective control in amide storage, amine diffusion, and nanomaterial downward leakage at the immiscible interface. The PA-MOF selective layer has been changed to a loosely crumpled surface, endowing functionalities on the sandwich-structured membrane that included limited pores, strengthened electronegativity, and stronger hydrophilicity. Thus, an enhanced water flux of 87.23 ± 7.43 LMH was achieved by the TFN-2 membrane (0.04 mg·mL^-1 UiO-66-NH₂), which is more than five times that of the thin-film composite membrane (17.46 ± 3.88 LMH). The rejection against norfloxacin, ciprofloxacin, and levofloxacin is 92.94 ± 1.60%, 94.62 ± 1.29%, and 96.92 ± 1.05%, respectively, effectively breaking through the "trade-off" effect between membrane permeability and rejection efficiency. Further antifouling results showed that the sandwich-structured membrane had lower flux decay ratios (3.36∼7.07%) and higher flux recovery ratios (93.40∼98.40%), as well as superior long-term stability after 30 days of filtration. Moreover, organic solvent resistance testing confirms that the sandwich-structured membrane maintained stable solvent flux and better recovery rates in ethanol, acetone, isopropanol, and N,N-dimethylformamide. Detailed nanofiltration mechanism studies revealed that these outstanding performances are based on the joint effect of the TA-PEI interlayer and PA-MOF selective layer, proposing a new perspective to break through the bottleneck of nanofiltration application in a complex environment.

Characteristics of dissolved ozone flotation for the enhanced treatment of bio-treated drilling wastewater from a gas field

Working fluid preparation using treated drilling wastewater is of great potential for drilling wastewater reuse, especially in water-deficient and ecologically fragile areas, which require low levels of organic matter and suspended solids (SS). This study established the dissolved ozone flotation (DOF) process as the advanced treatment process to replace the original electrocatalytic system (ECS) which exhibited low organic and suspended solids removal efficiency. Higher and more stable organic matter, suspended solids and turbidity removal efficiency were obtained for the DOF process. Consequently, the lower fouling potential and higher water production rate of treated water from DOF process was observed for the following reverse osmosis (RO) system. In addition, brine drilling fluids can be successfully prepared using DOF effluent directly, which exhibited promising practical implications in the advanced drilling wastewater treatment. Based on organic matter fractionation and redundancy analysis (RDA), the hydrophobic acid (HOA), hydrophobic neutral (HON) and hydrophilic fraction (HI) contents significantly impacted brine drilling fluid preparation. Based on X-ray photoelectron spectroscopy (XPS) analysis, the aromatic carbon species in the HOA, HON and HI fractions were found to be the critical factors deteriorating the brine drilling fluid preparation. However, oxygen-containing groups played a positive role. The favorable brine drilling fluid preparation performance using DOF effluent directly can be ascribed to the removal of HOA, HON and HI fractions and enhanced generation of oxygen-containing groups in ozone flotation zone.

Identifying active concentrations of biopolymers for enhancing membrane nanofiltration performance: From bench-scale tests to real production considerations

In the last decades, membrane-based nanofiltration (NF) technique has been widely applied for safe and high-quality drinking water production worldwide. NF membrane fouling has become one of the main obstacles in its application due to high operation cost, and thus numerous efforts have been made. However, there is still a large disconnect between academic findings and their applications. Hence, novel approaches for further exploitation and application are required based on feasibility of implementation. In this work, an optimized design of membrane-based NF plants was proposed, inspired by natural biopolymers present in feed water of NF unit. Specifically, we found beneficial functions of biopolymers, including NF membrane fouling alleviation and effluent quality improvement; these advantages could only be "activated" under a certain concentration range of biopolymers (0-1 mg C/L here), and less or more is not acceptable. This indicated that a NF unit is better to follow a microfiltration (MF) (instead of ultrafiltration (UF) which removes biopolymers) process during which natural biopolymers could be remained; also, this approach is suggested to be valid across different seasons when biopolymers' concentrations could be controlled within an "activated" range by mixing MF and UF permeates. Furthermore, three representative reference biopolymers with different, confirmed spatial structures and molecular weight (MW) were used to elucidate the micro-level functions of natural biopolymers on NF membranes, suggesting that cake layer structures shaped by various biopolymers determine the resulting NF performance. Overall, this innovative proposal is expected to be considered and adopted towards more energy-efficient NF technology for drinking water supply.

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.

Combined Nanofiltration and Thermocatalysis for the Simultaneous Degradation of Micropollutants, Fouling Mitigation and Water Purification

Due to progressive limitation of access to clean drinkable water, it is nowadays a priority to find an effective method of water purification from those emerging organic contaminants, which might have potentially harmful and irreversible effects on living organisms and environment. This manuscript reports the development of a new strategy for water purification, which combines a novel and recently developed Al2O3-doped silica nanofiltration membrane with a thermocatalytic perovskite, namely cerium-doped strontium ferrate (CSF). The thermocatalytic activity of CSF offers the opportunity to degrade organic pollutants with no light and without input of chemical oxidants, providing simplicity of operation. Moreover, our studies on real samples of secondary effluent from wastewater treatment showed that the thermocatalyst has the ability to degrade also part of the non-toxic organic matter, which allows for reducing the chemical oxygen demand of the retentate and mitigating membrane fouling during filtration. Therefore, the new technology is effective in the production of clean feed and permeate and has a potential to be used in degradation of micropollutants in water treatment.

Accelerated catalytic oxidation of dissolved manganese(II) by chlorine in the presence of in situ-growing 3D manganese(III)/(IV) oxide nanosheet assembly in zeolite filter

Manganese contamination is ubiquitous in ground water. Water eutrophication also exaggerates manganese release and contamination in surface water. However, conventional manganese(II) removal process through sand filter is low-efficiency and long-term ripening. Manganese exceeding standard is still a bottleneck issue for drinking water plants. To provide a quick-setup and low-cost means, we invented an accelerated catalytic oxidation filtration process through porous zeolite filter with dynamically coating of manganese oxide nanocatalysts. In dynamic filtration process, the addition of chlorine less than redox stoichiometric consumption can efficiently remove dissolved manganese(II) from contaminated tap water, ground water and Songhua river water. Characterization results showed that a continuous manganese(III)/(IV) oxide nanosheet catalyst was dynamically in situ-growing and assembled into 3D porous superstructure in the reactive Zeolite@MnO_x(s) filter. Active Mn(III) species on the edges of MnO_x(s) nanosheets were dynamically generated and transferred into stable Mn(IV) species on the layer-structured surface. The cycling transformation of manganese(III)/(IV) species was responsible for the accelerated catalytic oxidation of dissolved manganese(II) by chlorine. Without process changes in drinking water plant, the porous Zeolite@MnO_x(s) media could be feasibly integrated onto the existing sand filtration tanks for emergence handling of manganese(II) contamination. This novel reactive Zeolite@MnO_x(s) filter with higher hydraulic conductivity provides a high-efficiency, scalable and low-cost technique for the manganese(II) removal from various of water environments.

Green Synthesis of Metallic Nanoparticles: Applications and Limitations

The past decade has witnessed a phenomenal rise in nanotechnology research due to its broad range of applications in diverse fields including food safety, transportation, sustainable energy, environmental science, catalysis, and medicine. The distinctive properties of nanomaterials (nano-sized particles in the range of 1 to 100 nm) make them uniquely suitable for such wide range of functions. The nanoparticles when manufactured using green synthesis methods are especially desirable being devoid of harsh operating conditions (high temperature and pressure), hazardous chemicals, or addition of external stabilizing or capping agents. Numerous plants and microorganisms are being experimented upon for an eco–friendly, cost–effective, and biologically safe process optimization. This review provides a comprehensive overview on the green synthesis of metallic NPs using plants and microorganisms, factors affecting the synthesis, and characterization of synthesized NPs. The potential applications of metal NPs in various sectors have also been highlighted along with the major challenges involved with respect to toxicity and translational research.

Enhanced Water Permeability and Antifouling Property of Coffee-Ring-Textured Polyamide Membranes by In Situ Incorporation of a Zwitterionic Metal-Organic Framework

Modulation of the polyamide structure is critically important for the reverse-osmosis performance of thin-film composite (TFC) membranes in the field of water reuse and desalination. Herein, zwitterionic nanoparticles of zeolitic imidazolate framework-8 (PZ@ZIF-8) were fabricated and incorporated into the polyamide active layer through the interfacial polymerization method. A hydrophilic, zwitterionic coffee-ring structure was formed on the surface of polyamide thin-film nanocomposite (TFN) membranes due to the adjusted diffusion rate of m-phenylenediamine (MPD) from the aqueous phase into the organic phase during the interfacial polymerization process. Surface characterization demonstrated that the coffee-ring structure increased the amounts of water transport channels on the membrane surface and the intrinsic pores of PZ@ZIF-8 maintained the salt rejection. Antifouling and bactericidal activities of TFN membranes were enhanced remarkably owing to the bacterial-"defending" and bacterial-"attacking" behaviors of hydrophilic and zwitterionic groups from PZ@ZIF-8 nanoparticles. This work would provide a promising method for the application of MOFs to enhance the bio-/organic-fouling resistance of TFN membranes with high water permeation and salt rejection.