A highly selective surface coating for enhanced membrane rejection of endocrine disrupting compounds: Mechanistic insights and implications

Incorporating functionalized graphene oxide into diethylene triamine-based nanofiltration membranes can improve the removal of emerging organic micropollutants

The presence of emerging organic micropollutants (OMPs) in drinking and potable waters is a matter of great concern due to the health hazards associated with these. In this work, we present the preparation and application of a thin-film nanocomposite (TFN) membrane containing functionalized graphene oxide to effectively remove low-molecular-weight OMPs from water. Graphene oxide was functionalized with amino silane to enhance its cross-linking capability during the formation of the polyamide active layer via interfacial polymerization of diethylene triamine and trimesoyl chloride. The TEM analysis showed that amino silane functionalized GO had 2-3 layered sheets, while non-functionalized graphene oxide appeared multilayered or stacked. XPS analysis confirmed the successful functionalization of GO. Characterization of the membranes with advanced techniques confirmed the successful incorporation of the GO and its functionalization: spectra from Fourier Transform Infra Red spectroscopy had the characteristic peaks of GO and NH groups; scanning Electron Microscopy (SEM) images showed a continuous presence of GO nanosheets. Contact angle measurements showed the TFN membranes to be more hydrophilic than their thin film composite (TFC) counterparts. Incorporating functionalized oxide nanosheets in the active polyamide layer produced additional water permeation channels, resulting in an improvement of ∼25 % in permeate flux compared to the pristine TFC and the TFN membrane with non-functionalized GO. The removal efficiencies of four OMPs commonly found in natural waters: Amitriptylene HCl (ATT HCl) and Bisphenol-A (BPA), Acetaminophen (ACT), and Caffeine (CFN) were determined for the synthesized membranes. The TFN membrane with functionalized GO outperformed its TFC counterpart with ∼100 % removal for BPA, ∼ 90 % for CFN and ATT HCl, and ∼80 % removal for the low molecular weight ACT. The high-efficiency rejection of OMPs was attributed to the synergistic effects of size exclusion as well as the reduced specific interactions between the functional groups.

Biofouling behaviors of reverse osmosis membrane in the presence of trace plasticizer for circulating cooling water treatment: Characteristics and mechanisms

Reverse osmosis (RO) system has been increasingly applied for circulating cooling water (CCW) reclamation. Plasticizers, which may be dissolved into CCW system in plastic manufacturing industry, cannot be completely removed by the pretreatment prior to RO system, possibly leading to severe membrane biofouling. Deciphering the characteristics and mechanisms of RO membrane biofouling in the presence of trace plasticizers are of paramount importance to the development of effective fouling control strategies. Herein, we demonstrate that exposure to a low concentration (1 - 10 μg/L) of three typical plasticizers (Dibutyl phthalate (DBP), Tributyl phosphate (TBP) and 2,2,4-Trimethylpentane-1,3-diol (TMPD)) detected in pretreated real CCW promoted Escherichia coli biofilm formation. DBP, TBP and TMPD showed the highest stimulation at 5 or 10 μg/L with biomass increasing by 55.7 ± 8.2 %, 35.9 ± 9.5 % and 32.2 ± 14.7 % respectively, relative to the unexposed control. Accordingly, the bacteria upon exposure to trace plasticizers showed enhanced adenosine triphosphate (ATP) activity, stimulated extracellular polymeric substances (EPS) excretion and suppressed intracellular reactive oxygen species (ROS) induction, causing by upregulation of related genes. Long-term study further showed that the RO membranes flowing by the pretreated real CCW in a polypropylene plant exhibited a severer biofouling behavior than exposed control, and DBP and TBP parts played a key role in stimulation effects on bacterial proliferation. Overall, we demonstrate that RO membrane exposure to trace plasticizers in pretreated CCW can upregulate molecular processes and physiologic responses that accelerate membrane biofouling, which provides important implications for biofouling control strategies in membrane-based CCW treatment systems.

Trade-off between Endocrine-Disrupting Compound Removal and Water Permeance of the Polyamide Nanofiltration Membrane: Phenomenon and Molecular Insights

The polyamide (PA) nanofiltration (NF) membrane has the potential to remove endocrine-disrupting compounds (EDCs) from water and wastewater to prevent risks to both the aquatic ecosystem and human health. However, our understanding of the EDC removal-water permeance trade-off by the PA NF membrane is still limited, although the salt selectivity-water permeance trade-off has been well illustrated. This constrains the precise design of a high-performance membrane for removing EDCs. In this study, we manipulated the PA nanostructures of NF membranes by altering piperazine (PIP) monomer concentrations during the interfacial polymerization (IP) process. The upper bound coefficient for EDC selectivity-water permeance was demonstrated to be more than two magnitudes lower than that for salt selectivity-water permeance. Such variations were derived from the different membrane-solute interactions, in which the water/EDC selectivity was determined by the combined effects of steric exclusion and the hydrophobic interaction, while the electrostatic interaction and steric exclusion played crucial roles in water/salt selectivity. We further highlighted the role of the pore number and residual groups during the transport of EDC molecules across the PA membrane via molecular dynamics (MD) simulations. Fewer pores decreased the transport channels, and the existence of residual groups might cause steric hindrance and dynamic disturbance to EDC transport inside the membrane. This study elucidated the trade-off phenomenon and mechanisms between EDC selectivity and water permeance, providing a theoretical reference for the precise design of PA NF membranes for effective removal of EDCs in water reuse.

Empowering ultrathin polyamide membranes at the water-energy nexus: strategies, limitations, and future perspectives

Membrane-based separation is one of the most energy-efficient methods to meet the growing need for a significant amount of fresh water. It is also well-known for its applications in water treatment, desalination, solvent recycling, and environmental remediation. Most typical membranes used for separation-based applications are thin-film composite membranes created using polymers, featuring a top selective layer generated by employing the interfacial polymerization technique at an aqueous-organic interface. In the last decade, various manufacturing techniques have been developed in order to create high-specification membranes. Among them, the creation of ultrathin polyamide membranes has shown enormous potential for achieving a significant increase in the water permeation rate, translating into major energy savings in various applications. However, this great potential of ultrathin membranes is greatly hindered by undesired transport phenomena such as the geometry-induced "funnel effect" arising from the substrate membrane, severely limiting the actual permeation rate. As a result, the separation capability of ultrathin membranes is still not fully unleashed or understood, and a critical assessment of their limitations and potential solutions for future studies is still lacking. Here, we provide a summary of the latest developments in the design of ultrathin polyamide membranes, which have been achieved by controlling the interfacial polymerization process and utilizing a number of novel manufacturing processes for ionic and molecular separations. Next, an overview of the in-depth assessment of their limitations resulting from the substrate membrane, along with potential solutions and future perspectives will be covered in this review.

New directions on membranes for removal and degradation of emerging pollutants in aqueous systems

Emerging pollutants (EPs) refer to a group of non-regulated chemical or biological substances that have been recently introduced or detected in the environment. These pollutants tend to exhibit resistance to conventional treatment methods and can persist in the environment for prolonged periods, posing potential adverse effects on ecosystems and human health. As we enter a new era of managing these pollutants, membrane-based technologies hold significant promise in mitigating impact of EPs on the environment and safeguarding human health due to their high selectivity, efficiency, cost-effectiveness and capability for simultaneous separation and degradation. Moreover, these technologies continue to evolve rapidly with the development of new membrane materials and functionalities, advanced treatment strategies, and analyses for effectively treating EPs of more recent concerns. The objective of this review is to present the latest directions and advancements in membrane-based technologies for addressing EPs. By highlighting the progress in this field, we aim to share valuable perspectives with researchers and contribute to the development of future directions in sustainable treatments for EPs.

A review on artificial water channels incorporated polyamide membranes for water purification: Transport mechanisms and performance

While thin-film composite (TFC) polyamide (PA) membranes are advanced for removing salts and trace organic contaminants (TrOCs) from water, TFC PA membranes encounter a water permeance-selectivity trade-off due to PA layer structural characteristics. Drawing inspiration from the excellent water permeance and solute rejection of natural biological channels, the development of analogous artificial water channels (AWCs) in TFC PA membranes (abbreviated as AWCM) promises to achieve superior mass transfer efficiency, enabling breaking the upper bound of water permeance and selectivity. Herein, we first discussed the types and structural characteristics of AWCs, followed by summarizing the methods for constructing AWCM. We discussed whether the AWCs acted as the primary mass transfer channels in AWCM and emphasized the important role of the AWCs in water transport and ion/TrOCs rejection. We thoroughly summarized the molecular-level mechanisms and structure-performance relationship of water molecules, ions, and TrOCs transport in the confined nanospace of AWCs, which laid the foundation for illustrating the enhanced water permeance and salt/TrOCs selectivity of AWCM. Finally, we discussed the challenges encountered in the field of AWCM and proposed future perspectives for practical applications. This review is expected to offer guidance for understanding the transport mechanisms of AWCM and developing next-generation membrane for effective water treatment.

Enhanced EDC removal from water through electron beam-mediated adsorber particle integration in microfiltration membranes

The existence of endocrine disrupting chemicals (EDCs) in water and wastewater gives rise to significant environmental concerns. Conventional treatment approaches demonstrate limited capacity for EDC removal. Thus, incorporation of advanced separation procedures becomes essential to enhance the efficiency of EDC removal. In this work, adsorber composite microfiltration polyethersulfone membranes embedded with divinyl benzene polymer particles were created. These membranes were designed for effectively removing a variety of EDCs from water. The adsorber particles were synthesized using precipitation polymerization. Subsequently, they were integrated into the membrane scaffold through a phase inversion process. The technique of electron beam irradiation was applied for the covalent immobilization of particles within the membrane scaffold. Standard characterization procedures were carried out (i.e., water permeance, contact angle, X-ray photoelectron spectroscopy and scanning electron microscopy) to gain a deep understanding of the synthesized membrane properties. Dynamic adsorption experiments demonstrated the excellent capability of the synthesized composite membranes to effectively remove EDCs from water. Particularly, among the various target molecules examined, testosterone stands out with the most remarkable enhancement, presenting an adsorption loading of 220 mg m-2. This is an impressive 26-fold increase in the adsorption when compared to the performance of the pristine membrane. Similarly, androst-4-ene-3,17-dione exhibited an 18-fold improvement in adsorption capacity in comparison to the pristine membrane. The composite membranes also exhibited significant adsorption capacities for other key compounds, including 17β-estradiol, equilin, and bisphenol-A. With the implementation of an effective regeneration procedure, the composite membranes were put to use for adsorption over three consecutive cycles without any decline in their adsorption capacity.

Enhancing the removal of organic micropollutants by nanofiltration membrane with Fe (III)-tannic acid interlayer: Mechanisms and environmental implications

Nanofiltration technology has been applied in a variety of water treatment scenarios. However, conventional thin-film composite (TFC) membranes fail to remove emerging organic micropollutants (OMPs) efficiently. Here we applied thin-film nanocomposite membrane with an interlayer (TFNi) of Fe (III)-tannic acid to remove various types of OMPs, such as endocrine disrupting chemicals (EDCs), pharmaceutically active compounds (PhACs), and perfluoroalkyl substances (PFASs). Compared to the pristine TFC membrane, TFNi membrane exhibited crumpled morphology and its rejection layer was denser, better cross-linked and possessed smaller average pore size with narrower distribution. Significant enhancement in water-OMPs selectivity of PhACs and PFASs was observed. The mechanism lies in the effects of interlayer in improving the membrane permeance to water and meanwhile reducing the permeance to some OMPs by enhancing size exclusion effects. This work confirms the effectiveness of using TFNi membrane to simultaneously enhance the OMPs rejection and water permeance. The unraveled mechanism might inspire the future development of high-performance nanofiltration membranes targeting OMPs removal.

NaHCO3 addition enhances water permeance and Ca/haloacetic acids selectivity of nanofiltration membranes for drinking water treatment

The existence of disinfection by-products such as haloacetic acids (HAAs) in drinking water severely threatens water safety and public health. Nanofiltration (NF) is a promising strategy to remove HAAs for clean water production. However, NF often possesses overhigh rejection of essential minerals such as calcium. Herein, we developed highly selective NF membranes with tailored surface charge and pore size for efficient rejection of HAAs and high passage of minerals. The NF membranes were fabricated through interfacial polymerization (IP) with NaHCO3 as an additive. The NaHCO3-tailored NF membranes exhibited high water permeance up to ∼24.0 L m - 2 h - 1 bar-1 (more than doubled compared with the control membrane) thanks to the formation of stripe-like features and enlarged pore size. Meanwhile, the tailored membranes showed enhanced negative charge, which benefitted their rejection of HAAs and passage of Ca and Mg. The higher rejection of HAAs (e.g., > 90%) with the lower rejection of minerals (e.g., < 30% for Ca) allowed the NF membranes to achieve higher minerals/HAAs selectivity, which was significantly higher than those of commercially available NF membranes. The simultaneously enhanced membrane performance and higher minerals/HAAs selectivity would greatly boost water production efficiency and water quality. Our findings provide a novel insight to tailor the minerals/micropollutants selectivity of NF membranes for highly selective separation in membrane-based water treatment.

Effect of Interlayer Construction on TFC Nanofiltration Membrane Performance: A Review from Materials Perspective

Polyamide (PA) thin-film composite (TFC) nanofiltration (NF) membranes, which are extensively utilized in seawater desalination and water purification, are limited by the upper bounds of permeability-selectivity. Recently, constructing an interlayer between the porous substrate and the PA layer has been considered a promising approach, as it may resolve the trade-off between permeability and selectivity, which is ubiquitous in NF membranes. The progress in interlayer technology has enabled the precise control of the interfacial polymerization (IP) process, which regulates the structure and performance of TFC NF membranes, resulting in a thin, dense, and defect-free PA selective layer. This review presents a summary of the latest developments in TFC NF membranes based on various interlayer materials. By drawing from existing literature, the structure and performance of new TFC NF membranes using different interlayer materials, such as organic interlayers (polyphenols, ion polymers, polymer organic acids, and other organic materials) and nanomaterial interlayers (nanoparticles, one-dimensional nanomaterials, and two-dimensional nanomaterials), are systematically reviewed and compared. Additionally, this paper proposes the perspectives of interlayer-based TFC NF membranes and the efforts required in the future. This review provides a comprehensive understanding and valuable guidance for the rational design of advanced NF membranes mediated by interlayers for seawater desalination and water purification.

Preparation and characterization of superior hydrophilic PVDF/DA membranes by the self-polymerization approach of dopamine

Polyvinylidene fluoride (PVDF) membranes are favored for their excellent properties. However, the inherent strong hydrophobicity of PVDF membranes limits their development in the field of water treatment. The objective of this study was to improve the performance of PVDF membranes using the self-polymerization, strong adhesion properties, and biocompatible effects of dopamine (DA). The PVDF/DA membrane modification conditions were simulated and optimized using response surface methodology (RSM), and the experimental design was used to investigate three main parameters. The results showed that the DA solution concentration of 1.65 g/L, the coating time of 4.5 h, the post-treatment temperature of 25°C, the contact angle decreased from 69° to 33.9°, and the pure water flux on the PVDF/DA membrane was higher than that on the original membrane. The absolute value of the relative error between the actual and predicted values is only 3.36 %. In the MBR parallel comparison test, compared with the PVDF/DA membrane, the total amount of extracellular polymers (EPS) of the PVDF membrane increased by 1.46 times and the polysaccharide increased by 1.56 times, which further showed that the PVDF/DA modified membrane had the excellent anti-pollution ability. Through Alpha diversity analysis, the biodiversity detected on PVDF/DA membranes was higher than that of PVDF membranes, which further proved its good bio-adhesion ability. These findings could offer a reference for the hydrophilicity, antifouling, and stability of PVDF/DA membranes, which would establish the foundation for the comprehensive applications in MBR.

Molecular Design of the Polyamide Layer Structure of Nanofiltration Membranes by Sacrificing Hydrolyzable Groups toward Enhanced Separation Performance

Nanofiltration (NF) is an effective technology for removing trace organic contaminants (TrOCs), while the inherent trade-off effect between water permeance and solute rejections hinders its widespread application in water treatment. Herein, we propose a novel scheme of "monomers with sacrificial groups" to regulate the microstructure of the polyamide active layer via introducing a hydrolyzable ester group onto piperazine to control the diffusion and interfacial polymerization process. The achieved benefits include narrowing the pore size, improving the interpore connectivity, enhancing the microporosity, and reducing the active layer thickness, which collectively realized the simultaneous improvement of water permeance and enhancement of TrOCs rejection performance. The resulting membranes were superior to both the control and commercial membranes, especially in water-TrOCs selectivity. The effects of using the new monomers on the membrane physicochemical properties were systematically studied, and underlying mechanisms for the enhanced separation performance were further revealed by simulating the polymerization process through density functional theory calculation and measuring the trans-interface diffusion rate of monomers. This study demonstrates a novel promising NF membrane synthesis strategy by designing the structure of reaction monomers for achieving excellent rejection of TrOCs with a low energy input in water treatment.

Efficient capture of endocrine-disrupting compounds by a high-performance nanofiltration membrane for wastewater treatment

Conventional polyamide (PA) nanofiltration (NF) membranes can readily adsorb aromatic compounds, such as endocrine disrupting compounds (EDCs). Therefore, these substances can easily be transported across the membrane by solution-diffusion, resulting in a poor EDC-rejection. In this work, a novel thin film nanocomposite (TFN) membrane was fabricated by incorporating covalent organic frameworks (COFs) into the PA layer via an interfacial polymerization reaction. COFs with functional groups can provide abundant active binding sites for highly efficient EDC-capture. The rejection of the optimal TFN-COF membrane for bisphenol A, bisphenol AF, and sodium 2-biphenylate was 98.3%, 99.1%, and 99.3%, respectively, which was much higher than of the rejection of the pristine NF-membrane (82.4%, 95.5%, and 96.4%, respectively). Additionally, the TFN-COF membrane could be regenerated fast and efficiently by washing with ethanol for some minutes. COF nanofillers with porous structures provide additional water channels, making it possible to overcome the permeability-selectivity trade-off of NF membranes. The water permeance (17.1 L m^-2 h^-1 bar^-1) of the optimal membrane was about two times higher than for the pristine NF-membrane (8.7 L m^-2 h^-1 bar^-1). In addition, the TFN-COF membrane with a COF-loading of 0.05% w/v had an excellent Na₂SO₄ rejection (95.2%) due to size exclusion and strong Donnan effect. This work combines traditional NF membranes and adsorption materials to achieve efficient capture and rapid release of EDCs without sacrificing salt rejections, which opens the door to develop fit-for-purpose adsorptive NF membranes.

Boosting the Performance of Nanofiltration Membranes in Removing Organic Micropollutants: Trade-Off Effect, Strategy Evaluation, and Prospective Development

In view of the high risks brought about by organic micropollutants (OMPs), nanofiltration (NF) processes have been playing a vital role in advanced water and wastewater treatment, owing to the high membrane performance in rejection of OMPs, permeation of water, and passage of mineral salts. Though numerous studies have been devoted to evaluating and technically enhancing membrane performance in removing various OMPs, the trade-off effect between water permeance and water/OMP selectivity for state-of-the-art membranes remains far from being understood. Knowledge of this effect is significant for comparing and guiding membrane development works toward cost-efficient OMP removal. In this work, we comprehensively assessed the performance of 88 NF membranes, commercialized or newly developed, based on their water permeance and OMP rejection data published in the literature. The effectiveness and underlying mechanisms of various modification methods in tailoring properties and in turn performance of the mainstream polyamide (PA) thin-film composite (TFC) membranes were quantitatively analyzed. The trade-off effect was demonstrated by the abundant data from both experimental measurements and machine learning-based prediction. On this basis, the advancement of novel membranes was benchmarked by the performance upper-bound revealed by commercial membranes and lab-made PA membranes. We also assessed the potentials of current NF membranes in selectively separating OMPs from inorganic salts and identified the future research perspectives to achieve further enhancement in OMP removal and salt/OMP selectivity of NF membranes.

Removal of emerging organic micropollutants via modified-reverse osmosis/nanofiltration membranes: A review

Hazardous micropollutants (MPs) such as pharmaceutically active compounds (PhACs), pesticides and personal care products (PCPs) have emerged as a critical concern nowadays for acquiring clean and safe water resources. In the last few decades, innumerable water treatment methods involving biodegradation, adsorption and advanced oxidation process have been utilized for the removal of MPs. Of these methods, membrane technology has proven to be a promising technique for the removal of MPs due to its sustainability, high efficiency and cost-effectiveness. Herein, the aim of this article is to provide a comprehensive review regarding the MPs rejection mechanisms of reverse osmosis (RO) and nanofiltration (NF) membranes after incorporation of nanomaterials and also surface modification atop the PA layer. Size exclusion, adsorption and electrostatic charge interaction mechanisms play important roles in governing the MP removal rate. In addition, this review also discusses the state-of-the-art research on the surface modification of thin film composite (TFC) membrane and nanomaterials-incorporated thin film nanocomposite (TFN) membrane in enhancing MPs removal performance. It is hoped that this review can provide insights in modifying the physicochemical properties of NF and RO membranes to achieve better performance in water treatment process, particularly for the removal of emerging hazardous substances.

Enhancing the Permselectivity of Thin-Film Composite Membranes Interlayered with MoS2 Nanosheets via Precise Thickness Control

The demand for highly permeable and selective thin-film composite (TFC) nanofiltration membranes, which are essential for seawater and brackish water softening and resource recovery, is growing rapidly. However, improving and tuning membrane permeability and selectivity simultaneously remain highly challenging owing to the lack of thickness control in polyamide films. In this study, we fabricated a high-performance interlayered TFC membrane through classical interfacial polymerization on a MoS₂-coated polyethersulfone substrate. Due to the enhanced confinement effect on the interface degassing and the improved adsorption of the amine monomer by the MoS₂ interlayer, the MoS₂-interlayered TFC membrane exhibited enhanced roughness and crosslinking. Compared to the control TFC membrane, MoS₂-interlayered TFC membranes have a thinner polyamide layer, with thickness ranging from 60 to 85 nm, which can be tuned by altering the MoS₂ interlayer thickness. A multilayer permeation model was developed to delineate and analyze the transport resistance and permeability of the MoS₂ interlayer and polyamide film through the regression of experimental data. The optimized MoS₂-interlayered TFC membrane (0.3-inter) had a 96.8% Na₂SO₄ rejection combined with an excellent permeability of 15.9 L m^-2 h^-1 bar^-1 (LMH/bar), approximately 2.4 times that of the control membrane (6.6 LMH/bar). This research provides a feasible strategy for the rational design of tunable, high-performance NF membranes for environmental applications.

Tailored design of nanofiltration membranes for water treatment based on synthesis-property-performance relationships

Tailored design of high-performance nanofiltration (NF) membranes is desirable because the requirements for membrane performance, particularly ion/salt rejection and selectivity, differ among the various applications of NF technology ranging from drinking water production to resource mining. However, this customization greatly relies on a comprehensive understanding of the influence of membrane fabrication methods and conditions on membrane properties and the relationships between the membrane structural and physicochemical properties and membrane performance. Since the inception of NF, much progress has been made in forming the foundation of tailored design of NF membranes and the underlying governing principles. This progress includes theories regarding NF mass transfer and solute rejection, further exploitation of the classical interfacial polymerization technique, and development of novel materials and membrane fabrication methods. In this critical review, we first summarize the progress made in controllable design of NF membrane properties in recent years from the perspective of optimizing interfacial polymerization techniques and adopting new manufacturing processes and materials. We then discuss the property-performance relationships based on solvent/solute mass transfer theories and mathematical models, and draw conclusions on membrane structural and physicochemical parameter regulation by modifying the fabrication process to improve membrane separation performance. Next, existing and potential applications of these NF membranes in water treatment processes are systematically discussed according to the different separation requirements. Finally, we point out the prospects and challenges of tailored design of NF membranes for water treatment applications. This review bridges the long-existing gaps between the pressing demand for suitable NF membranes from the industrial community and the surge of publications by the scientific community in recent years.

Nanofiltration for drinking water treatment: a review

In recent decades, nanofiltration (NF) is considered as a promising separation technique to produce drinking water from different types of water source. In this paper, we comprehensively reviewed the progress of NF-based drinking water treatment, through summarizing the development of materials/fabrication and applications of NF membranes in various scenarios including surface water treatment, groundwater treatment, water reuse, brackish water treatment, and point of use applications. We not only summarized the removal of target major pollutants (e.g., hardness, pathogen, and natural organic matter), but also paid attention to the removal of micropollutants of major concern (e.g., disinfection byproducts, per- and polyfluoroalkyl substances, and arsenic). We highlighted that, for different applications, fit-for-purpose design is needed to improve the separation capability for target compounds of NF membranes in addition to their removal of salts. Outlook and perspectives on membrane fouling control, chlorine resistance, integrity, and selectivity are also discussed to provide potential insights for future development of high-efficiency NF membranes for stable and reliable drinking water treatment.

Surface modification of nanofiltration membranes to improve the removal of organic micropollutants: Linking membrane characteristics to solute transmission

Surface modification of nanofiltration (NF) membranes has great potential to improve the removal of organic micropollutants (OMs) by NF membranes. This study used polydopamine (PDA) as a model coating to comprehensively link the changes in membrane properties with the changes in transmission of 34 OMs. The membrane characterization demonstrated that a thicker, denser, and more hydrophilic PDA coating can be achieved by increasing the PDA deposition time from 0.5 to 4 hours. Overall, the transmissions of target OMs were reduced by PDA-coated NF membranes compared to unmodified NF membranes. The neutral hydrophobic compounds showed lower transmissions for longer PDA coating (PDA4), while the neutral hydrophilic compounds tended to show lower transmissions for shorter PDA coating (PDA0.5). To explain this, competing effects provided by the PDA coatings are proposed including sealing defects, inducing cake-enhanced concentration polarization in the coating layer for neutral hydrophilic compounds, and weakened hydrophobic adsorption for neutral hydrophobic compounds. For charged compounds, PDA4 with the greatest negative charge among the PDA-coated membranes showed the lowest transmission. Depending on the molecular size and hydrophilicity of the compounds, the transmission of OMs by the PDA4 coating could be reduced by 70% with only a 26.4% decline in water permeance. The correlations and mechanistic insights provided by this work are highly useful for designing membranes with specific surface properties via surface modification to improve the removal of OMs without compromising water production.

Predicting Micropollutant Removal by Reverse Osmosis and Nanofiltration Membranes: Is Machine Learning Viable?

Predictive models for micropollutant removal by membrane separation are highly desirable for the design and selection of appropriate membranes. While machine learning (ML) models have been applied for such purposes, their reliability might be compromised by data leakage due to inappropriate data splitting. More importantly, whether ML models can truly understand the mechanisms of membrane separation has not been revealed. In this study, we evaluate the capability of the XGBoost model to predict micropollutant removal efficiencies of reverse osmosis and nanofiltration membranes. Our results demonstrate that data leakage leads to falsely high prediction accuracy. By utilizing a model interpretation method based on the cooperative game theory, we test the knowledge of XGBoost on the mechanisms of membrane separation via quantifying the contributions of input variables to the model predictions. We reveal that XGBoost possesses an adequate understanding of size exclusion, but its knowledge of electrostatic interactions and adsorption is limited. Our findings suggest that future work should focus more on avoiding data leakage and evaluating the mechanistic knowledge of ML models. In addition, high-quality data from more diverse experimental conditions, as well as more informative variables, are needed to improve the accuracy of ML models for predicting membrane performance.

Incorporation of Cellulose Nanocrystals into Graphene Oxide Membranes for Efficient Antibiotic Removal at High Nutrient Recovery

Two-dimensional (2D) material-based membranes hold great promise in wastewater treatment. However, it remains challenging to achieve highly efficient and precise small molecule/ion separation with pure 2D material-fabricated lamellar membranes. In this work, laminated graphene oxide (GO)-cellulose nanocrystal (CNC) hybrid membranes (GO/CNC) were fabricated by taking advantages of the unique structures and synergistic effects generated from these two materials. The characterization results in physiochemical properties, and the structure of the as-synthesized hybrid membranes displayed enhanced membrane surface hydrophilicity, enhanced crumpling surface structure, and slightly enlarged interlayer-spacing with the incorporation of CNCs. Water permeability increases by two to four times with the addition of different CNC weight ratios in comparison to a pristine GO membrane. The optimal GO/CNC membrane achieved efficient rejection toward three typical antibiotics at 74.8, 90.9, and 97.2% for sulfamethoxazole (SMX), levofloxacin (Levo), and norfloxacin (Nor), respectively, while allowing a high passage of desirable nutrients such as NO₃^- and H₂PO₄^-. It was found that SMX removal is primarily governed by electrostatic repulsion, while adsorption plays a crucial role in removing Levo and Nor. Moreover, the density functional theory calculations confirmed the increased antibiotic removal in the presence of an organic foulant, humic acid. Such a 2D material-based hybrid membrane offers a new strategy to develop fit-for purpose membranes for resource recovery and water separation.

Fit-for-Purpose Design of Nanofiltration Membranes for Simultaneous Nutrient Recovery and Micropollutant Removal

Domestic wastewater is a valuable reservoir of nutrients such as nitrogen and phosphorus. However, the presence of emerging micropollutants (EMPs) hinders its applications in resource recovery. In this study, we designed and fabricated a novel thin-film composite polyamide membrane, which enables highly selective nanofiltration (NF) that removes EMPs effectively while preserving valuable nutrients in the permeate. By incorporating polyethylenimine as an additional monomer to piperazine and surfactant sodium dodecyl sulfate in interfacial polymerization, we precisely tuned membrane pore size, pore size distribution, and surface charge. The resultant NF membrane achieved desirable solute-solute selectivity between EMPs (rejection rate > 75%) and nutrient N and P ions (rejection rate < 25%). By applying a modified Donnan steric pore model with dielectric exclusion, which takes membrane pore size distribution into consideration, we demonstrate the synergistic effect of membrane pore size, pore size distribution, and surface charge in regulating membrane solute-solute selectivity. Designing solute-solute selective NF membranes for fit-for-purpose wastewater treatment has great potential to improve the flexibility of membrane technologies that can convert wastewater streams to valuable water and nutrient resources.

Comparative studies on the separation of endocrine disrupting compounds from aquatic environment by emulsion liquid membrane and hollow fiber supported liquid membrane

Endocrine disrupting compounds have been found to limit the natural working of the endocrine system like synthesis, secretion, transference and binding. Endocrine disrupting compounds are released from humans, animals and from production industries to soil, surface water and sediments mostly through the sewage treatment system. Studies have revealed the impact of these compounds on the nervous system, lungs, liver, thyroid, prostate, metabolism, obesity and reproductive system. So removal of these compounds from sewage water/wastewater by appropriate processes is essential. Conventional techniques like coagulation, precipitation, flocculation, microfiltration and ultrafiltration are effective for the removal of these compounds but limitations like low molecular weight of these compounds and pore size of membrane restricts the complete removal. Liquid membrane is a promising technology which combines the steps like extraction and stripping in a single step thereby providing the instantaneous removal and recovery of solutes and also results in high selectivity and savings of chemicals. This paper mainly focuses on the use of liquid membrane techniques like emulsion liquid membrane and hollow fiber supported liquid membrane which are the promising techniques for the removal of endocrine disrupting compounds from aqueous streams. The working principle, mechanism and implementation of these two techniques in the removal of several endocrine disrupting compounds from aquatic streams are also discussed.

Differentiating Solutes with Precise Nanofiltration for Next Generation Environmental Separations: A Review

Selective removal or enrichment of targeted solutes including micropollutants, valuable elements, and mineral scalants from complex aqueous matrices is both challenging and pivotal to the success of water purification and resource recovery from unconventional water resources. Membrane separation with precision at the subnanometer or even subangstrom scale is of paramount importance to address those challenges via enabling "fit-for-purpose" water and wastewater treatment. So far, researchers have attempted to develop novel membrane materials with precise and tailored selectivity by tuning membrane structure and chemistry. In this critical review, we first present the environmental challenges and opportunities that necessitate improved solute-solute selectivity in membrane separation. We then discuss the mechanisms and desired membrane properties required for better membrane selectivity. On the basis of the most recent progress reported in the literature, we examine the key principles of material design and fabrication, which create membranes with enhanced and more targeted selectivity. We highlight the important roles of surface engineering, nanotechnology, and molecular-level design in improving membrane selectivity. Finally, we discuss the challenges and prospects of highly selective NF membranes for practical environmental applications, identifying knowledge gaps that will guide future research to promote environmental sustainability through more precise and tunable membrane separation.

A Critical Review on Thin-Film Nanocomposite Membranes with Interlayered Structure: Mechanisms, Recent Developments, and Environmental Applications

The separation properties of polyamide reverse osmosis and nanofiltration membranes, widely applied for desalination and water reuse, are constrained by the permeability-selectivity upper bound. Although thin-film nanocomposite (TFN) membranes incorporating nanomaterials exhibit enhanced water permeance, their rejection is only moderately improved or even impaired due to agglomeration of nanomaterials and formation of defects. A novel type of TFN membranes featuring an interlayer of nanomaterials (TFNi) has emerged in recent years. These novel TFNi membranes show extraordinary improvement in water flux (e.g., up to an order of magnitude enhancement) along with better selectivity. Such enhancements can be achieved by a wide selection of nanomaterials, ranging from nanoparticles, one-/two-dimensional materials, to interfacial coatings. The use of nanostructured interlayers not only improves the formation of polyamide rejection layers but also provides an optimized water transport path, which enables TFNi membranes to potentially overcome the longstanding trade-off between membrane permeability and selectivity. Furthermore, TFNi membranes can potentially enhance the removal of heavy metals and micropollutants, which is critical for many environmental applications. This review critically examines the recent developments of TFNi membranes and discusses the underlying mechanisms and design criteria. Their potential environmental applications are also highlighted.

Mechanistic Insights into the Role of Polydopamine Interlayer toward Improved Separation Performance of Polyamide Nanofiltration Membranes

Interlayered thin-film nanocomposite membranes (TFNi) are an emerging type of membranes with great potential to overcome the permeability-selectivity upper bound of conventional thin-film composite (TFC) nanofiltration and reverse osmosis membranes. However, the exact roles of the interlayer and the corresponding mechanisms leading to enhanced separation performance of TFNi membranes remain poorly understood. This study reports a polydopamine (PDA)-intercalated TFNi nanofiltration membrane (PA-PSF2, PDA coating time of 2 h) that possessed nearly an order of magnitude higher water permeance (14.8 ± 0.4 Lm^-2 h^-1 bar^-1) than the control TFC membrane (PA-PFS0, 2.4 ± 0.5 Lm^-2 h^-1 bar^-1). The TFNi membrane further showed enhanced rejection toward a wide range of inorganic salts and small organic molecules (including antibiotics and endocrine disruptors). Detailed mechanistic investigation reveals that the membrane separation performance was enhanced due to both the direct "gutter" effect of the PDA interlayer and its indirect effects resulting from enhanced polyamide formation on the PDA-coated substrate, with the "gutter" effect playing a more dominant role. This study provides a mechanistic and comprehensive framework for the future development of TFNi membranes.

Hydrophilic Selective Nanochannels Created by Metal Organic Frameworks in Nanofiltration Membranes Enhance Rejection of Hydrophobic Endocrine-Disrupting Compounds

Rejection of endocrine-disrupting compounds (EDCs) by thin-film composite (TFC) polyamide membranes remains a challenging issue in wastewater reclamation applications because of the unfavorable hydrophobic interaction between EDCs and membranes. Herein, we investigated the incorporation of hydrophilic metal organic frameworks (MOFs) into the polyamide layer to create water/EDC selective nanochannels for enhancing EDC rejection. Using MIL-101(Cr) MOF as a nanofiller, the water flux of the MOF0.20 TFC membrane (0.20 wt/v % MOF in n-hexane) was 2.3 times that of the control. The rejection rates against EDCs involving methylparaben, propylparaben, benzylparaben, and bisphenol A (BPA) by MOF0.20 were also significantly higher than the respective values of the control membrane, with the water/EDC selectivity (e.g., A/B_BPA) of MOF0.20 approximately doubled compared to that of the control. Further single salt rejection and gold nanoparticle filtration tests confirmed that the hydrophilic nanochannels created by MOFs played a critical role in membrane transport, accounting for the significant enhancement of EDC rejection of the modified TFC membrane. This study demonstrates a promising membrane modification protocol using hydrophilic MOFs for achieving selective removal of EDCs and high-efficient wastewater reclamation using TFC membranes.

Hydrophilic Silver Nanoparticles Induce Selective Nanochannels in Thin Film Nanocomposite Polyamide Membranes

Thin-film nanocomposite (TFN) membranes have been widely studied over the past decade for their desalination applications. For some cases, the incorporation of nonporous hydrophilic nanofillers has been reported to greatly enhance membrane separation performance, yet the underlying mechanism is poorly understood. The current study systematically investigates TFN membranes incorporated with silver nanoparticles (AgNPs). For the first time, we reveal the formation of nanochannels of approximately 2.5 nm in size around the AgNPs, which can be attributed to the hydrolysis of trimesoyl chloride monomers and thus the termination of interfacial polymerization by the water layer around each hydrophilic nanoparticle. These nanochannels nearly tripled the membrane water permeability for the optimal membrane. In addition, this membrane showed increased rejection against NaCl, boron, and a set of small-molecular organic compounds (e.g., propylparaben, norfloxacin, and ofloxacin), thanks to its combined effects of improved size exclusion, enhanced Donnan exclusion, and suppressed hydrophobic interaction. Our work provides fundamental insights into the formation and transport mechanisms involved in solid-filler incorporated TFN membranes. Future studies should take advantage of this spontaneous nanochannel formation in the design of TFN to overcome the classical membrane permeability-selectivity trade-off.

Calcium-Carboxyl Intrabridging during Interfacial Polymerization: A Novel Strategy to Improve Antifouling Performance of Thin Film Composite Membranes

This study reports a novel intrabridging strategy to improve the antifouling performance of a thin-film composite (TFC) membrane. We demonstrate that the addition of Ca²⁺ during the interfacial polymerization reaction led to the formation of stable Ca²⁺-carboxyl complexes within the polyamide rejection layer. This intrabridging of carboxyl groups by Ca²⁺ effectively sequestrated them, reducing their availability for binding divalent metal ions in the aqueous solution and for forming foulant-metal-membrane interbridges. Membrane fouling and cleaning experiments confirmed improved flux stability and fouling reversibility for the Ca²⁺ modified membranes. The greatly enhanced antifouling performance of these membranes, together with their better surface hydrophilicity and greater water permeability, makes the intrabridging approach highly attractive in overcoming the classical permeability-selectivity-antifouling trade-off. Our findings pave a new direction for synthesizing high-performance TFC membranes.

Non-Polyamide Based Nanofiltration Membranes Using Green Metal-Organic Coordination Complexes: Implications for the Removal of Trace Organic Contaminants

Polyamide-based thin film composite (TFC) membranes are generally optimized for salt rejection but not for the removal of trace organic contaminants (TrOCs). The insufficient rejection of TrOCs such as endocrine disrupting compounds (EDCs) by polyamide membranes can jeopardize product water safety in wastewater reclamation. In this study, we report a novel nonpolyamide membrane chemistry using green tannic acid-iron (TA-Fe) complexes to remove TrOCs. The nanofiltration membrane formed at a TA-Fe molar ratio of 1:3 (TA-Fe3) had a continuous thin rejection layer of 10-30 nm in thickness, together with a water permeability of 5.1 Lm^2-h^-1bar^-1 and a Na₂SO₄ rejection of 89.7%. Meanwhile, this membrane presented significantly higher rejection of EDCs (up to 99.7%) than that of polyamide membranes (up to 81.8%). Quartz crystal microbalance results revealed that the sorption amount of a model EDC, benzylparbaen, by TA-Fe3 layer was nearly 2 orders of magnitude less than that by polyamide, leading to reduced transmission and higher rejection. Further analysis of membrane revealed a much greater water/EDC selectivity of the TA-Fe3 membrane compared to the polyamide membranes.