A parametric study on the synergistic impacts of chemical additives on permeation properties of thin film composite polyamide membrane

"Like Dissolves Like" Strategy Facilitates Interfacial Polymerization for Facile Synthesis of Highly Permeable Reverse Osmosis Membranes

Existing reverse osmosis (RO) membranes often feature a polyamide rejection layer fabricated by interfacial polymerization (IP) between m-phenylenediamine (MPD) and trimesoyl chloride. However, polyamide RO membrane formation is limited by the poorly soluble polar MPD in the nonpolar organic solvent (e.g., hexane). Herein, we developed a dual organic solvent system to increase MPD solubility via introducing a polar solvent of dioxane into the hexane as inspired by the classical "like dissolves like" theory and thus promoting the IP reaction efficiency. Consequently, the optimal RO membrane exhibited a superior desalination performance with a rejection of 99.2% for 35,000 ppm of NaCl, simultaneous with a high water permeance of 3.1 L m-2 h-1 bar-1. Meanwhile, it had a boron rejection of 90.3% that far exceeds commercial RO membranes. These findings demonstrate that a dual organic solvent IP system can offer a facile yet effective strategy for scalable fabrication of high-performance RO membranes.

Analysis of Metal-Organic Framework and Polyamide Interfaces in Membranes for Water Treatment and Antibacterial Applications

Integrating biocidal nanoparticles (NPs) into polyamide (PA) membranes shows promise for enhancing resistance to biofouling. Incorporating techniques can tailor thin-film nanocomposite (TFN) membranes for specific water purification applications. In this study, silver-based metal-organic framework Ag-MOFs (using silver nitrate and 1,3,5-benzentricarboxylic acid as precursors) are incorporated into PA membranes via three different methods: i) incorporation, ii) dip-coating, and iii) in situ ultrasonic techniques. The characterizations, such as top-surface and cross-section scanning and transmission microscopy, reveal that the incorporation methods for the modified TFN membranes substantially control morphology and surface characteristics. For example, the in situ ultrasonically interlayered Ag-MOFs showed the largest pores (average pore diameter of 14 Å ± 0.1), resulting in the highest water permeance (water flux of 10.9 LMH/bar for Na2SO4). It also show superior antifouling and anti-biofouling performance, with a flux recovery ratio (FRR) of 94.1% in both fouling tests due to its improved surface hydrophilicity and the antibacterial properties of incorporated Ag-MOFs. Conversely, the surface-grafted dip-coated Ag-MOFs offered the highest salt rejection, attributed to its highly negatively charged surface and a dense PA network with narrow pores (average pore diameter of 10 Å ± 0.06).

Diffusion-Regulated Interfacial Polymerization of Hierarchically Microporous Polyamide Membranes for Permselective Gas Separations

Interfacial polymerization has emerged as a robust method for fabricating task-specific polyamide (PA) membranes. However, the limited microporosity of highly cross-linked PA membranes constrains their effectiveness in gas separation applications. Herein, we introduce an ionic liquid (IL)-regulated interfacial polymerization process to fabricate polyamide nanofilms incorporating kinked tetrakis (4-aminophenyl) methane monomers. In situ ultraviolet-visible spectroscopy demonstrates that the diffusion of 1,3,5-benzenetricarbonyl trichloride (TMC) toward the interface increases with the IL/H2O ratio, leading to the formation of a more compact membrane with a higher cross-linking degree. The PA-TAM7/3-60 min membrane exhibits a CO2 permeance of 29.8 GPU and a CO2/CH4 selectivity of 109, exceeding the 2008 Robeson upper bond. Additionally, the highly cross-linked structure imparts the membranes with notable plasticization resistance. Mixed-gas tests (CO2/CH4 = 50/50, v/v) reveal that the PA-TAM7/3-60 min membrane experiences only a 2% reduction in CO2 permeance and a 10% decrease in CO2/CH4 selectivity at a CO2 partial pressure of 300 PSIG, compared to its performance at 30 PSIG. The ease of tuning membrane structure and gas separation performance, along with its excellent plasticization resistance, underscores the potential of these PA membranes for task-specific gas separations.

Micelles regulated thin film nanocomposite membrane with enhanced nanofiltration performance

The desalination performance of thin film nanocomposite (TFN) membranes is significantly influenced by the nature of nanofillers and the structure of the polyamide (PA) layer. Herein, a micelles regulated interfacial polymerization (MRIP) strategy is reported for the preparation of TFN membranes with enhanced nanofiltration (NF) performance. Specially, stable and ultrafine micelles, synthesized from the poly(ethylene oxide)-b-poly(4-vinyl pyridine)-b-polystyrene (PEO-PVP-PS) triblock copolymers, were utilized as regulators in the aqueous phase during the interfacial polymerization (IP) process. TFN membranes were fabricated with varying concentrations of micelles to improve their properties and performances. The structure of the PA layer was further regulated by modulating the content of trimesoyl chloride (TMC), which significantly enhances the performance of the TFN membrane with micelles. Attributable to the homogeneously dispersed micelles and the modified PA layer, the optimized membrane denoted as TFN-2-0.3 exhibits an improved separation performance of 20.7 L m-2h-1 bar-1 and 99.3 % Na2SO4 rejection, demonstrating nearly twice the permeance and 2.7 % higher rejection than that of the original control membrane, respectively. The mechanism of this MRIP strategy was investigated through the diffusion experiments of piperazine (PIP) and interfacial tension tests. The incorporated micelles effectively lower the interfacial tension, promote the diffusion of PIP and accelerate the IP reaction, resulting in a denser and thinner PA layer. Collectively, these findings demonstrate that TFN membranes with micelles exhibit increased roughness, enhanced hydrophilicity, superior rejection to divalent salts, and better acid-base resistance, highlighting their potential applications in the design of TFN membranes.

Comprehensive Characterization of Commercial Reverse Osmosis Membranes through High-Temperature Cross-Flow Filtration

Developing thermally stable reverse osmosis membranes is a potential game-changer in high-temperature water treatment. In this work, the performance of three commercial reverse osmosis membranes was evaluated with a series of high-temperature filtrations. The membranes were tested with different filtration methodologies: long-term operation, cyclic tests, controlled stepwise temperature increment, and permeability tests. The morphological and physiochemical characterizations were performed to study the impact of high-temperature filtration on the membranes' chemical composition and morphological characteristics. An increase in the temperature deteriorated the membrane performance in terms of water flux and salt rejection. Flux decline at high temperatures was recognized as the primary concern for high-temperature filtrations, restricting the applications of commercial membranes for long-term operations. This research provides valuable insights for researchers aiming to thoroughly characterize reverse osmosis membranes at high temperatures.

Recent Advances in the Support Layer, Interlayer and Active Layer of TFC and TFN Organic Solvent Nanofiltration (OSN) Membranes: A Review

Although separation of solutes from organic solutions is considered a challenging process, it is inevitable in various chemical, petrochemical and pharmaceutical industries. OSN membranes are the heart of OSN technology that are widely utilized to separate various solutes and contaminants from organic solvents, which is now considered an emerging field. Hence, numerous studies have been attracted to this field to manufacture novel membranes with outstanding properties. Thin-film composite (TFC) and nanocomposite (TFN) membranes are two different classes of membranes that have been recently utilized for this purpose. TFC and TFN membranes are made up of similar layers, and the difference is the use of various nanoparticles in TFN membranes, which are classified into two types of porous and nonporous ones, for enhancing the permeate flux. This study aims to review recent advances in TFC and TFN membranes fabricated for organic solvent nanofiltration (OSN) applications. Here, we will first study the materials used to fabricate the support layer, not only the membranes which are not stable in organic solvents and require to be cross-linked, but also those which are inherently stable in harsh media and do not need any cross-linking step, and all of their advantages and disadvantages. Then, we will study the effects of fabricating different interlayers on the performance of the membranes, and the mechanisms of introducing an interlayer in the regulation of the PA structure. At the final step, we will study the type of monomers utilized for the fabrication of the active layer, the effect of surfactants in reducing the tension between the monomers and the membrane surface, and the type of nanoparticles used in the active layer of TFN membranes and their effects in enhancing the membrane separation performance.

When self-assembly meets interfacial polymerization

Interfacial polymerization (IP) and self-assembly are two thermodynamically different processes involving an interface in their systems. When the two systems are incorporated, the interface will exhibit extraordinary characteristics and generate structural and morphological transformation. In this work, an ultrapermeable polyamide (PA) reverse osmosis (RO) membrane with crumpled surface morphology and enlarged free volume was fabricated via IP reaction with the introduction of self-assembled surfactant micellar system. The mechanisms of the formation of crumpled nanostructures were elucidated via multiscale simulations. The electrostatic interactions among m-phenylenediamine (MPD) molecules, surfactant monolayer and micelles, lead to disruption of the monolayer at the interface, which in turn shapes the initial pattern formation of the PA layer. The interfacial instability brought about by these molecular interactions promotes the formation of crumpled PA layer with larger effective surface area, facilitating the enhanced water transport. This work provides valuable insights into the mechanisms of the IP process and is fundamental for exploring high-performance desalination membranes.

Tailoring properties and performance of thin-film composite membranes by salt additives for water treatment: A critical review

During the fabrication of thin film composite (TFC) membranes by interfacial polymerization (IP), the utilization of salt additives is one of the effective methods to regulate membrane properties and performance. Despite gradually receiving widespread attention for membrane preparation, the strategies, effects and underlying mechanisms of using salt additives have not yet been systematically summarized. This review for the first time provides an overview of various salt additives used to tailor properties and performance of TFC membranes for water treatment. By classifying salt additives into organic and inorganic salts, the roles of added salt additives in the IP process and the induced changes in membrane structure and properties are discussed in detail, and the different mechanisms of salt additives affecting membrane formation are summarized. Based on these mechanisms, the salt-based regulation strategies have shown great potential for improving the performance and application competitiveness of TFC membranes, including overcoming the trade-off relationship between water permeability and salt selectivity, tailoring membrane pore size distribution for precise solute-solute separation, and enhancing membrane antifouling performance. Finally, future research directions are suggested to focus on the long-term stability assessment of salt-modified membranes, the combined use of different salt additives, and the integration of salt regulation with other membrane design or modification strategies.

Forward Osmosis Membranes: The Significant Roles of Selective Layer

Forward osmosis (FO) is a promising separation technology to overcome the challenges of pressure-driven membrane processes. The FO process has demonstrated profound advantages in treating feeds with high salinity and viscosity in applications such as brine treatment and food processing. This review discusses the advancement of FO membranes and the key membrane properties that are important in real applications. The membrane substrates have been the focus of the majority of FO membrane studies to reduce internal concentration polarization. However, the separation layer is critical in selecting the suitable FO membranes as the feed solute rejection and draw solute back diffusion are important considerations in designing large-scale FO processes. In this review, emphasis is placed on developing FO membrane selective layers with a high selectivity. The effects of porous FO substrates in synthesizing high-performance polyamide selective layer and strategies to overcome the substrate constraints are discussed. The role of interlayer in selective layer synthesis and the benefits of nanomaterial incorporation will also be reviewed.

Thin Film Polyamide Nanocomposite Membrane Decorated by Polyphenol-Assisted Ti3C2Tx MXene Nanosheets for Reverse Osmosis

Transition-metal carbides (MXenes), multifunctional 2D materials, have caught the interest of researchers in the fabrication of high-performance nanocomposite membranes. However, several issues regarding MXenes still remain unresolved, including low ambient stability; facile restacking and agglomeration; and poor compatibility and processability. To address the aforementioned challenges, we proposed a facile, green, and cost-efficient approach for coating a stable layer of plant-derived polyphenol tannic acid (TA) on the surface of MXene (Ti₃C₂T_x) nanosheets. Then, high-performance reverse osmosis polyamide thin film nanocomposite (RO-PA-TFN) membranes were fabricated by the incorporation of modified MXene (Ti₃C₂T_x-TA) nanosheets in the polyamide selective layer through interfacial polymerization. The strong negative charge and hydrophilic multifunctional properties of TA not only boosted the chemical compatibility between Ti₃C₂T_x MXene nanosheets and the polyamide matrix to overcome the formation of nonselective voids but also generated a tight network with selective interfacial pathways for efficient monovalent salt rejection and water permeation. In comparison to the neat thin film composite membrane, the optimum TFN (Ti₃C₂T_x-TA) membrane with a loading of 0.008 wt % nanofiller revealed a 1.4-fold enhancement in water permeability, a well-maintained high NaCl rejection rate of 96% in a dead-end process, and enhanced anti-fouling tendency. This research offers a facile way for the development of modified MXene nanosheets to be successfully integrated into the polyamide-selective layer to improve the performance and fouling resistance of TFN membranes.

Nanodiamond-Enabled Thin-Film Nanocomposite Polyamide Membranes for High-Temperature Water Treatment

Despite growing demands for high-temperature wastewater treatment, most available polymeric membranes are limited to mild operating temperatures (<50 °C) and become less efficient at high temperatures. Herein we show how to make thermally stable reverse osmosis thin-film nanocomposite (TFN) membranes by embedding nanodiamond (ND) particles. Polyamide composite layers containing different loadings of surface-modified ND particles were synthesized through interfacial polymerization. The reactive functional groups and the hydrophilic surface of the NDs intensified the interactions of the nanoparticles with the polymer matrix and increased the surface wettability of the TFN membranes. Contact angle measurement showed a maximum decrease from 88.4° for the pristine membrane to 58.3° for the TFN membrane fabricated with 400 ppm ND particles. The addition of ND particles and ethyl acetate created larger surface features on the polyamide surface of TFN membranes. The average roughness of the membranes increased from 108.4 nm for the pristine membrane to 177.5 nm for the TFN membrane prepared with highest ND concentration. The ND-modified TFN membranes showed a higher pure water flux (up to 76.5 LMH) than the pristine membrane (17 LMH) at ambient temperature at 220 psi and room temperature. The TFN membrane with the highest loading of ND particles overcame the trade-off relation between the water flux and NaCl rejection with 76.5 LMH and 97.3% when 2000 ppm of NaCl solution was filtered at 220 psi. Furthermore, with increasing ND concentration, the TFN membrane showed a lower flux decline at high temperatures over time. The TFN400 prepared with 400 ppm of m-phenylene diamine functionalized ND particles had a 13% flux decline over a 9 h filtration test at 75 °C. This research provides a promising path to the development of high-performance TFN membranes with enhanced thermal stability for the treatment of wastewaters at high temperatures.

Modifying thin film composite membrane with zeolitic imidazolate framework-8@polydopamine for enhanced antifouling property

Biofouling and organic fouling are major obstacles for polymeric membranes during application. In this work, zeolitic imidazolate framework-8@polydopamine (ZIF-8@PDA) nanoparticles were prepared by an aqueous synthesis strategy and incorporated into the polyamide (PA) selective layer to synthesize thin film nanocomposite membrane (TFN) during interfacial polymerization. The permeability and selectivity of the composite membrane were simultaneously improved with the introduction of ZIF-8@PDA. The water permeability of the TFN membrane increased to 3.74 ± 0.19 L/(m2·h·bar), which is 43.8% higher than that of the control membrane. Besides, the rejection of TFN membrane to sodium chloride is 98.68 ± 0.13%, which shows 0.99% increment than the unmodified membrane. Moreover, organic fouling and biofouling of the TFN membrane were also alleviated thanks to the introduction of the hydrophilic ZIF-8@PDA. The short-term filtration results indicate the performance of the TFN membrane is stable during operation.

Ultrathin Thin-Film Composite Polyamide Membranes Constructed on Hydrophilic Poly(vinyl alcohol) Decorated Support Toward Enhanced Nanofiltration Performance

Traditional polyamide-based interfacial polymerized nanofiltration (NF) membranes exhibit upper bound features between water permeance and salt selectivity. Breaking the limits of the permeability and rejections of these composite NF membranes are highly desirable for water desalination. Herein, a high-performance NF membrane (TFC-P) was fabricated via interfacial polymerization on the poly(vinyl alcohol) (PVA) interlayered poly(ether sulfone) (PES) ultrafiltration support. Owing to the large surface area, great hydrophilicity, and high porosity of the PES-PVA support, a highly cross-linked polyamide separating layer was formed with a thickness of 9.6 nm, which was almost 90% thinner than that of the control membrane (TFC-C). In addition, the TFC-P possessed lower ζ-potential, smaller pore size, and greater surface area compared to that of the TFC-C, achieving an ultrahigh water permeance of 31.4 L m^-2 h^-1 bar^-1 and a 99.4% Na₂SO₄ rejection. Importantly, the PVA interlayer strategy was further applied to a pilot NF production line and the fabricated membranes presented stable water flux and salt rejections as comparable to the lab-scaled membranes. The outstanding properties of the PVA-interlayered NF membranes highlight the feasibility of the fabrication method for practical applications, which provides a new avenue to develop robust polyamide-based NF desalination membranes for environmental water treatment.

Fabrication of Highly Permeable and Thermally Stable Reverse Osmosis Thin Film Composite Polyamide Membranes

Developing thermally stable polymer membranes for high-temperature water treatment is in high demand, as the recommended usage temperatures of most commercial membranes are lower than 50 °C. In this study, we synthesized novel thin film composite polyamide membranes by modifying the chemical structure of their selective layers. Triaminopyrimidine was used to synthesize a polyamide selective layer with high cross-linking density over a microporous poly(ether sulfone) support. The addition of triamiopyrimidine to the classic m-phenylenediamine/trimesoyl chloride combination remarkably improved the permeation of the membranes. All synthesized thin film composite membranes showed consistent permeate flux for 9 h of operation at 75 °C with only a slight reduction in salt rejection. This study provides a promising and reproducible methodology to develop thermally stable high-flux thin film composite membranes, opening up a new paradigm for high-temperature water treatment processes.

Removal of trace organic contaminants by melamine-tuned highly cross-linked polyamide TFC membranes

In recent years, forward osmosis (FO) has represented numerous potential applications in safe water production. In this study, we improved the performance of FO thin film composite (TFC) membranes for the removal of trace organic compounds (TOrCs) by tuning the chemistry of its top active layer. The TFC membranes were synthesized by interfacial polymerization (IP) reaction between amine-containing monomers, e.g., meta-phenylene diamine (MPD) or para-phenylenediamine (PPD), and an acid chloride monomer, e.g., trimesoyl chloride (TMC). Owing to three free amine functionals over main core, melamine was used in the amine monomers solution to increase cross-linking among polyamide chains. Chemical and morphological characterization of the prepared membranes confirmed that melamine was successfully incorporated into the chemical structure of the top PA layer. Two agricultural toxic materials (atrazine and diazinon) were used to investigate the capability of the newly fabricated membranes in the removal of TOrCs. The obtained results showed that melamine-improved FO membranes provided higher atrazine and diazinon rejections in two different FO membrane configurations, including active layer facing feed solution (ALF) and active layer facing draw solution (ALD). The highest rejections of both diazinon (99.4%) and atrazine (97.3%) were achieved when the melamine modified MPD-based membrane served in ALF mode with 2 M NaCl as a draw solution.

Sulfaguanidine nanofiltration active layer towards anti-adhesive and antimicrobial attributes for desalination and dye removal

A novel sulfaguanidine (SG)-modified polyamide thin-film composite (TFC) nanofiltration (NF) membrane was constructed by the strategy referred to as co-solvent assisted interfacial polymerization (CASIP), which involves the respective interfacial polymerization (IP) of piperazine (PIP) and SG with trimesoyl chloride (TMC) on porous polysulfone (PSf) supports. CASIP enables the formation of a defect-free thin dense active layer and favors higher water permeance up to 79.0 L m⁻² h⁻¹ with rejection above 98.3% for Na₂SO₄. The resulting PA membrane also demonstrates a high flux recovery ratio of nearly 98.9% to bovine serum albumin protein after being cleaned. More importantly, the current membrane shows excellent anti-adhesive and antimicrobial performances against Gram-negative Escherichia coli, Gram-positive Bacillus pumilus LDS.33 and Aspergillus parasiticus JFS. This promises great potential application of the PA membrane for practical water/wastewater treatment. The prospect of using the co-solvent mediated SG-modified layer as a next-generation anti-fouling/antimicrobial membrane is very encouraging.

The resulting sulfaguanidine nanofiltration membrane demonstrates higher water permeance and better antifouling property. The membrane shows excellent anti-adhesive and antimicrobial performances against E. coli, B. pumilus LDS.33 and A. parasiticus JFS.

Treatment of oil sands produced water using combined electrocoagulation and chemical coagulation techniques

Hybrid electrocoagulation-chemical coagulation (EC-CC) process has attracted a growing attention for the removal of various types of wastewaters contaminants. In this paper, the feasibility of EC-CC technique as an alternative to conventional chemical processes for the treatment of steam assisted gravity drainage (SAGD) produced water has been systematically studied. Eight parameters, namely electrode material, cell configuration, pH and temperature of the solution, chemical coagulant dosage, intensity of the electrical current, mixing rate, and treatment time were studied. To explore the synergistic effect of the design parameters, the experimental trials were arranged using Taguchi method. Analysis of variance (ANOVA) was performed to evaluate the effect of each design parameter on the organic matter removal from the SAGD produced water. It was found that all parameters except the electrode arrangement had a significant effect on the removal efficiency of the EC-CC process. Among these parameters, the chemical coagulant and the treatment time had the most significant contribution to the efficiency by 40% and 26%, respectively. The optimum condition for the highest TOC removal efficiency (39.8%) was obtained by applying 0.34 A to Al electrode in a bipolar (BP) configuration when the pH, temperature, coagulant concentration, mixing rate, and reaction time were set to 8, 60 °C, 200 mg/L, 700 rpm, and 90 min, respectively. Moreover, a second-order polynomial regression model was proposed to predict the removal efficiency in terms of design parameters. An excellent agreement between the model predictions and experimental data was obtained with the adjusted R² of about 99%.