Polyamide Active Layer Research Articles

Nanoparticle-templated nanofiltration membranes for ultrahigh performance desalination

Nanofiltration (NF) membranes with ultrahigh permeance and high rejection are highly beneficial for efficient desalination and wastewater treatment. Improving water permeance while maintaining the high rejection of state-of-the-art thin film composite (TFC) NF membranes remains a great challenge. Herein, we report the fabrication of a TFC NF membrane with a crumpled polyamide (PA) layer via interfacial polymerization on a single-walled carbon nanotubes/polyether sulfone composite support loaded with nanoparticles as a sacrificial templating material, using metal-organic framework nanoparticles (ZIF-8) as an example. The nanoparticles, which can be removed by water dissolution after interfacial polymerization, facilitate the formation of a rough PA active layer with crumpled nanostructure. The NF membrane obtained thereby exhibits high permeance up to 53.5 l m−2h−1 bar−1 with a rejection above 95% for Na2SO4, yielding an overall desalination performance superior to state-of-the-art NF membranes reported so far. Our work provides a simple avenue to fabricate advanced PA NF membranes with outstanding performance.

Synthesis and characterization of novel Cellulose Nanocrystals-based Thin Film Nanocomposite membranes for reverse osmosis applications

A novel TFN membrane was fabricated by embedding Cellulose Nanocrystals (CNCs) into the polyamide active layer. Membranes were synthesized by in-situ interfacial polymerization of m-phenylenediamine (MPD) and trimesoyl chloride (TMC) containing different amounts of CNCs. Successful incorporation of CNCs into the polyamide layer was confirmed by XRD analyses. Surface morphology of the membranes was characterized by SEM and AFM. Water contact angle measurements showed improved hydrophilicity of the TFN membranes. Desalination experiments with synthetic brackish water at 20 bar(g) revealed doubling of the water flux from 30 to 63 ± 10 L/m2·h, without significantly compromising the salt rejection (97.8%) for 0.1% (w/v) CNCs loading. In the fouling/filtration experiments with 300 ppm of Bovine Serum Albumin (BSA) in the feed solution, the TFN membrane had 11% smaller water flux reduction compared to the control TFC membrane. The promising performance of the CNCs-based TFN membranes and the fact that CNCs are inexpensive and abundant nanoparticles indicates their potential for a large-scale use in water desalination. Moreover, since CNCs are safe and environmentally friendly nanoparticles their possible leaching out during membrane operation would not impose health and environmental concerns.

Simultaneous Improvement of Antimicrobial, Antifouling, and Transport Properties of Forward Osmosis Membranes with Immobilized Highly-Compatible Polyrhodanine Nanoparticles.

This work shows that incorporating highly compatible polyrhodanine nanoparticles (PRh-NPs) into a polyamide (PA) active layer allows for fabricating forward osmosis (FO) thin-film composite (TFC)-PRh membranes that have simultaneously improved antimicrobial, antifouling, and transport properties. To the best of our knowledge, this is the first reported study of its kind to this date. The presence of the PRh-NPs on the surface of the TFC-PRh membranes active layers is evaluated using FT-IR spectroscopy, SEM, and XPS. The microscopic interactions and their impact on the compatibility of the PRh-NPs with the PA chains were studied using molecular dynamics simulations. When tested in forward osmosis, the TFC-PRh-0.01 membrane (with 0.01 wt % PRh) shows significantly improved permeability and selectivity because of the small size and the high compatibility of the PRh-NPs with PA chains. For example, the TFC-PRh-0.01 membrane exhibits a FO water flux of 41 l/(m2·h), higher than a water flux of 34 l/(m2·h) for the pristine TFC membrane, when 1.5 molar NaCl was used as draw solution in the active-layer feed-solution mode. Moreover, the reverse solute flux of the TFC-PRh-0.01 membrane decreases to about 115 mmol/(m2·h) representing a 52% improvement in the reverse solute flux of this membrane in comparison to the pristine TFC membrane. The surfaces of the TFC-PRh membranes were found to be smoother and more hydrophilic than those of the pristine TFC membrane, providing improved antifouling properties confirmed by a flux decline of about 38% for the TFC-PRh-0.01 membranes against a flux decline of about 50% for the pristine TFC membrane when evaluated with a sodium alginate solution. The antimicrobial traits of the TFC-PRh-0.01 membrane evaluated using colony-forming units and fluorescence imaging indicate that the PRh-NPs hinder cell deposition on the TFC-PRh-0.01 membrane surface effectively, limiting biofilm formation.

Grafting of bioactive 2-aminoimidazole into active layer makes commercial RO/NF membranes anti-biofouling

Innovative anti-biofouling reverse osmosis (RO) and nanofiltration (NF) membranes were developed. Membranes were produced via grafting of a bioactive, non-biocidal 2-aminoimidazole (2-AI) to the polyamide active layer of commercial RO/NF membranes, by coupling 2-AI to free carboxylate groups in active layers using 1-ethyl-3-(3-dimethylaminopropyl)carbodiimide (EDC). 2-AI was grafted in the 0.44–1.08 M range, which is several orders of magnitude higher than required for biofilm inhibition (IC50 = 162–420 µM). Results showed that, compared to control (unmodified) membranes, 2-AI-modified membranes inhibited the growth of Pseudomonas aeruginosa biofilms by 61–96% (p = 0.01–0.12), and that the measured changes in surface charge, hydrophobicity, and roughness due to 2-AI grafting were not the main contributors to biofilm inhibition. Some loss of 2-AI from water filtration and membrane cleaning was observed, but 2-AI concentrations remained orders of magnitude higher than required for biofilm inhibition. Short-term performance results showed that compared to commercial control membranes, 2-AI-grafting resulted in statistically insignificant changes in salt rejection, and decreases in initial water permeability < 25%, being statistically insignificant for two out of the four membrane types tested. Overall, results support that 2-AI grafting to polyamide RO/NF membranes via activation of carboxylate groups with EDC is a promising approach to enhance current RO/NF water purification membranes.

Adsorption of pharmaceuticals onto isolated polyamide active layer of NF/RO membranes

Adsorption of trace organic compounds (TrOCs) onto the membrane materials has a great impact on their rejection by nanofiltration (NF) and reverse osmosis (RO) membranes. This study aimed to investigate the difference in adsorption of various pharmaceuticals (PhACs) onto different NF/RO membranes and to demonstrate the necessity of isolating the polyamide (PA) active layer from the polysulfone (PS) support layer for adsorption characterization and quantification. Both the isolated PA layers and the PA+PS layers of NF90 and ESPA1 membranes were used to conduct static adsorption tests. Results showed that apparent differences existed between the PA layer and the PA+PS layer in the adsorption capacity of PhACs as well as the time necessary to reach the adsorption equilibrium. PhACs with different physicochemical properties could be adsorbed to different extents by the isolated PA layer, which was mainly attributed to electrostatic attraction/repulsion and hydrophobic interactions. The PA layer of ESPA1 exhibited apparently higher adsorption capacities for the positively charged PhACs and similar adsorption capacities for the neutral PhACs although it had significantly less total interfacial area (per unit membrane surface area) for adsorption compared to the PA layer of NF90. The higher affinity of the PA layer of ESPA1 for the PhACs could be due to its higher capacity of forming hydrogen bonds with PhACs resulted from the modified chemistry with more –OH groups. This study provides a novel approach to determining the TrOC adsorption onto the active layer of membranes for the ease of investigating adsorption mechanisms.

Highly-efficient forward osmosis membrane tailored by magnetically responsive graphene oxide/Fe3O4 nanohybrid

Emerging forward osmosis (FO) process as a potentially more energy efficient method has recently gained remarkable attention. Herein, considering the unique features of graphene oxide (GO), a new facile method has been proposed to magnetically modify GO within the polyamide active layer to obtain highly efficient osmotically driven membranes. While exposed to magnetic field, thin film nanocomposite membranes modified by GO/Fe3O4 nanohybrids (TFN-MMGO/Fe3O4) were synthesized by in-situ interfacial polymerization of the prepared monomer solution and organic trimesoyl chloride. Water permeability, salt rejection, and fouling tendency of the modified membranes were then evaluated and compared with both pristine thin film composite (TFC) membrane and the ones modified by GO/Fe3O4 nanohybrides in the absence of magnetic field (TFN-GO/Fe3O4). According to the experimental results, when compared to the TFC and TFN-GO/Fe3O4 membranes, respectively, 117.4% and 63.2% water flux enhancements were achieved in TFN-MMGO/Fe3O4 membrane with optimal GO/Fe3O4 nanohybrid concentration of 100 ppm. In spite of such improvements in water flux, little compromise in reverse salt leakages were observed in the TFN-MMGO/Fe3O4 membranes compared to the TFC one. As well, the TFN-MMGO/Fe3O4 and TFN-GO/Fe3O4 membranes revealed higher fouling resistances than the TFC membrane due to their distinguished manipulated surface characteristics.

Performance evaluation of polyamide TFC membranes: Effects of free volume properties on boron transport

To comprehensively investigate the correlation between free volume properties (size and distribution) in the interior of polyamide (PA) active layer and mass transport mechanism, six polyamide thin film composite (TFC) membranes were characterized using positron annihilation lifetime spectroscopy (PALS). In case of pressurized filtration conditions, the rejection rate of boron across all PA membranes was found to be inversely proportional to the free volume size. The more boron transport occurred at the membrane containing larger free volume. In addition, the transition of the neutral boron transport mechanism from convection to diffusion was found to occur at a membrane free volume radius around 0.275nm within the applied pressure range (2 and 10bar), as verified by PALS and the dimensionless Peclet number. We believe that mass transport mechanism transition from convection to diffusion is caused by the compression of polyamide active layer due to applied hydraulic pressure (10bar), resulting in lowering the convective mass transport pathway inside polyamide active layer. These membrane free volume size criteria and experimental filtration results may subsequently be used as new design guidelines for the development of high boron rejection polyamide TFC membranes having a low energy consumption.

A robust thin film composite membrane incorporating thermally rearranged polymer support for organic solvent nanofiltration and pressure retarded osmosis

Thin film composite (TFC) polymer membranes are ubiquitous in membrane-based liquid separation processes, especially in reverse osmosis (RO). While the ultrathin polyamide separating layer employed in TFC membranes has exhibited excellent performance in many liquid separation processes, the polymer support has been identified as a bottleneck to practical applications. In this work, we report a highly porous, thermally and chemically robust support comprising a thermally rearranged polymer which is combined with a polyamide active layer to form a thermally rearranged, thin film composite (TR-TFC) polymer membrane for general use in liquid separation, and for environmentally-friendly power generation. The precursor polymer has good processability for scale-up of synthesis and membrane fabrication. After thermal rearrangement, the developed TR-TFC membranes containing polybenzoxazole-co-imide can be utilized in separations in any organic liquids, including under harsh environments such as dimethyl formamide even at elevated temperatures, with a remarkable performance. Moreover, the membrane achieves 40Wm−2 of power density through pressure retarded osmosis using a concentrated brine, similar to those obtained from RO plants. These results points to the possibilities for next-generation TFC polymer membranes for general use in liquid separation and power generation.

Carbon Nanotube Incorporated Nanocomposite Membranes for CO2 Removal

Thin-film nanocomposite (TFN) membranes that consist of multi-walled carbon nanotube (MWNT) incorporated polyamide selective layer formed on polysulfone substrate were developed. The resultant TFN membranes were used for CO2 gas removal. For inclusion into these active layers, a grafting procedure for CNT was established in this study to enhance their hydrophobicity. MWNTs grafted with poly (methyl methacrylate) (PMMA) were synthesized via a micro emulsion polymerization of methyl methacrylate (MMA) in the presence of acid-modified multi-walled carbon nanotubes (c-MWNTs). Subsequently, polyamide TFN membranes containing PMMA–MWNTs were prepared via interfacial polymerization reaction between aqueous and organic phases. The resultants TFN were characterized by using FTIR and SEM. Morphology studies demonstrate that MWNTs have been successfully embedded into the active polyamide layer. The gas selectivity increased about 6.6 times compared to the thin-film composite membrane when using 0.35 w/v% amine in the aqueous phase, 0.28 w/v% trimesoyl chloride (TMC) in organic phase and 0.50g/L PMMA–MWNTs in coating layer.

Efficient removal of heavy metal ions based on the optimized dissolution-diffusion-flow forward osmosis process

In order to efficiently remove the heavy metal ions in wastewater, a novel thin film composite (TFC)-type forward osmosis (FO) membrane was prepared specifically with the glass nanofiber supporting layer and the bovine serum albumin (BSA)-embedded polyamide (PA) swellable active layer, and used for the removal of heavy metal ions from water by enhanced dissolution-diffusion-flow FO process. The separation results indicated that the appropriate addition of BSA in the PA active layer improved the dissolution-diffusion process of the traditional PA layer by providing the swellable sites as water channels in aqueous solutions and enhanced the rejection effect on heavy metal ions. In addition, the ultra large porosity and pore size of the glass nanofiber supporting layer provided smooth flow channels, accelerating the flow process and alleviating the concentration polarization problem, with the structural parameter (S) value decreasing to 172 μm. This paper proved that the specific structural construction of the functional membranes was a convenient method to prepare applicable membranes with tunable FO performances.

Hydrophilic polyvinyl alcohol coating on hydrophobic electrospun nanofiber membrane for high performance thin film composite forward osmosis membrane

In this study, the hydrophilic property of polyvinyl alcohol (PVA) was utilized to improve the hydrophilicity and mechanical strength of electrospun polyvinylidene fluoride (PVDF)-supported thin film composite (TFC) forward osmosis (FO) membranes. The PVDF nanofiber support was modified with PVA via dip coating and acid-catalyzed crosslinking with glutaraldehyde prior to formation of polyamide active layer on the support via interfacial polymerization. The influence of PVA modification on the morphology and physical properties of PVDF support was evaluated through several characterization techniques while the flux performance was assessed using lab-scale FO membrane unit. The fabricated PVA-modified TFC FO membranes exhibited high hydrophilicity, porosity, and mechanical strength. FO performance tests reveal excellent flux performance (34.2LMH using 1M NaCl and DI water as draw and feed solution, respectively) and low structural parameters (154μm) of the PVA-modified TFC FO membrane. Dip coating of the nanofiber support in PVA is therefore a simple and effective method for the improvement of PVDF support hydrophilicity to fabricate high performance TFC FO membranes.

Study of polyamide thin film characteristics impact on permeability/selectivity performance and fouling behavior of forward osmosis membrane.

In recent years, forward osmosis (FO) has received considerable attention due to its huge potentials in water desalination. The thin film composite (TFC) membrane used in the FO desalination consists of a bottom support layer covered by an active layer on top. Polyamide (PA) is commonly employed as an active layer forming via interfacial polymerization between m-phenylenediamine (MPD) and trimesoyl chloride (TMC) monomers. In this study, the effects that the MPD and TMC concentrations could have on the performance and anti-fouling behavior of the obtained FO membrane have been investigated. Results showed that there is a trade-off relationship between the water flux and salt rejection, which by increasing MPD concentration, the water flux was reducedو while the salt rejection was enhanced. Also, by increasing the TMC concentration, an opposite trend was observed. Using 0.20wt.% of TMC monomer, the highest water fluxes of 21.6 LMH and 29.3 LMH were achieved in two different membrane configurations. Furthermore, higher TMC concentration caused better anti-fouling property, when PA active layer of the membrane was in a high fouling potential environment.

Reconciling DLVO and non-DLVO Forces and Their Implications for Ion Rejection by a Polyamide Membrane.

Recognizing the significance of surface interactions for ion rejection and membrane fouling in nanofiltration, we revise the theories of DLVO (named after Derjaguin, Landau, Verwey, and Overbeek) and non-DLVO forces in the context of polyamide active layers. Using an atomic force microscope, surface forces between polyamide active layers and a micrometer-large and smooth silica colloid were measured in electrolyte solutions of representative monovalent and divalent ions. While the analysis of DLVO forces, accounting for surface roughness, provides how surface charge of the active layer changes with electrolyte concentration, scrutiny of non-DLVO hydration forces gives molecular insight into the composition of the membrane-solution interface. Importantly, we report an expansion of the diffuse layer at high ionic strength, consistent with the recent development of the electrical double layer theory, but in contrast to the widely accepted phenomenon of aggregation in the secondary minimum. Further, the enhanced repulsion acting on modified membranes via polyelectrolyte adsorption can be quantitatively predicted by DLVO and non-DLVO forces. This work serves to solve past misunderstandings about the interaction forces acting on nanofiltration membranes, and it provides guidance for future work on the relation between surface properties and rejection mechanisms and fouling.

A Novel Fabrication Approach for Multifunctional Graphene-based Thin Film Nano-composite Membranes with Enhanced Desalination and Antibacterial Characteristics

A practical fabrication technique is presented to tackle the trade-off between the water flux and salt rejection of thin film composite (TFC) reverse osmosis (RO) membranes through controlled creation of a thinner active selective polyamide (PA) layer. The new thin film nano-composite (TFNC) RO membranes were synthesized with multifunctional poly tannic acid-functionalized graphene oxide nanosheets (pTA-f-GO) embedded in its PA thin active layer, which is produced through interfacial polymerization. The incorporation of pTA-f-GOL into the fabricated TFNC membranes resulted in a thinner PA layer with lower roughness and higher hydrophilicity compared to pristine membrane. These properties enhanced both the membrane water flux (improved by 40%) and salt rejection (increased by 8%) of the TFNC membrane. Furthermore, the incorporation of biocidal pTA-f-GO nanosheets into the PA active layer contributed to improving the antibacterial properties by 80%, compared to pristine membrane. The fabrication of the pTA-f-GO nanosheets embedded in the PA layer presented in this study is a very practical, scalable and generic process that can potentially be applied in different types of separation membranes resulting in less energy consumption, increased cost-efficiency and improved performance.

Neutron Reflectivity and Performance of Polyamide Nanofilms for Water Desalination

The structure and hydration of polyamide (PA) membranes are investigated with a combination of neutron and X‐ray reflectivity, and their performance is benchmarked in reverse osmosis water desalination. PA membranes are synthesized by the interfacial polymerization of m‐phenylenediamine (MPD) and trimesoyl chloride (TMC), varying systematically reaction time, concentration, and stoichiometry, to yield large‐area exceptionally planar films of ≈10 nm thickness. Reflectivity is employed to precisely determine membrane thickness and roughness, as well as the (TMC/MPD) concentration profile, and response to hydration in the vapor phase. PA film thickness is found to increase linearly with reaction time, albeit with a nonzero intercept, and the composition cross‐sectional profile is found to be uniform, at the conditions investigated. Vapor hydration with H2O and D2O from 0 to 100% relative humidity results in considerable swelling (up to 20%), but also yields uniform cross‐sectional profiles. The resulting film thickness is found to be predominantly set by the MPD concentration, while TMC regulates water uptake. A favorable correlation is found between higher swelling and water uptake with permeance. The data provide quantitative insight into the film formation mechanisms and correlate reaction conditions, cross‐sectional nanostructure, and performance of the PA active layer in RO membranes for desalination.

Anti-fouling and high water permeable forward osmosis membrane fabricated via layer by layer assembly of chitosan/graphene oxide

To date, forward osmosis (FO) has received considerable attention due to its potential application in seawater desalination. FO does not require external hydraulic pressure and consequently is believed to have a low fouling propensity. Despite the numerous privileges of FO process, a major challenge ahead for its development is the lack of high performance membranes. In this study, we fabricated a novel highly-efficient FO membrane using layer-by-layer (LbL) assembly of positive chitosan (CS) and negative graphene oxide (GO) nanosheets via electrostatic interaction on a porous support layer. The support layer was prepared by blending hydrophilic sulfonated polyethersulfone (SPES) into polyethersulfone (PES) matrix using wet phase inversion process. Various characterization techniques were used to confirm successful fabrication of LbL membrane. The number of layers formed on the SPES-PES support layer was easily adjusted by repeating the CS and GO deposition cycles. Thin film composite (TFC) membrane was also prepared by the same SPES-PES support layer and polyamide (PA) active layer to compare membranes performances. The water permeability and salt rejection of the fabricated membranes were obtained by two kinds of draw solutions (including Na2SO4 and sucrose) under two different membrane orientations. The results showed that membrane coated by a CS/GO bilayers had water flux of 2–4 orders of magnitude higher than the TFC one. By increasing the number of CS/GO bilayers, the selectivity of the LbL membrane was improved. The novel fabricated LbL membrane showed better fouling resistance than the TFC one in the feed solution containing 200ppm of sodium alginate as a foulant model.

Middle support layer formation and structure in relation to performance of three-tier thin film composite forward osmosis membrane

In recent years, nanofibers have been directly used as the support layer for developing high-flux forward osmosis (FO) membrane due to their high porosity and low tortuosity. However, the rougher surface and larger pore size weaken the adhesion to the polyamide (PA) active layer. Conventional FO membrane made of phase inversion support layer exhibits favorable mechanical stability, however the water flux is poor due to severe internal concentration polarization (ICP). In order to simultaneously reduce the ICP and enhance the mechanical stability of FO membrane, a thin film composite (TFC) FO membrane composed of three layers was prepared, namely bottom electrospun hydrophobic/hydrophilic interpenetrating network composite nanofibers (HH-IPN-CNF) support layer, a phase separation formed microporous polyvinylidene fluoride (PVDF) middle support layer, and the top PA active layer formed by interfacial polymerization. The factors including polymer concentration, coagulation bath composition and the gate height of the casting blade that affect the mid-layer structure formation were investigated, respectively. With the mid-layer of FO membrane cast under different conditions, water flux ranged from 10.39 to 30.62LMH, by using 0.5M NaCl draw solution and deionized water feed solution. The TFC FO membrane with high structure integrity may be promising candidate for engineered osmosis.

Partitioning of Alkali Metal Salts and Boric Acid from Aqueous Phase into the Polyamide Active Layers of Reverse Osmosis Membranes.

The partition coefficient of solutes into the polyamide active layer of reverse osmosis (RO) membranes is one of the three membrane properties (together with solute diffusion coefficient and active layer thickness) that determine solute permeation. However, no well-established method exists to measure solute partition coefficients into polyamide active layers. Further, the few studies that measured partition coefficients for inorganic salts report values significantly higher than one (∼3-8), which is contrary to expectations from Donnan theory and the observed high rejection of salts. As such, we developed a benchtop method to determine solute partition coefficients into the polyamide active layers of RO membranes. The method uses a quartz crystal microbalance (QCM) to measure the change in the mass of the active layer caused by the uptake of the partitioned solutes. The method was evaluated using several inorganic salts (alkali metal salts of chloride) and a weak acid of common concern in water desalination (boric acid). All partition coefficients were found to be lower than 1, in general agreement with expectations from Donnan theory. Results reported in this study advance the fundamental understanding of contaminant transport through RO membranes, and can be used in future studies to decouple the contributions of contaminant partitioning and diffusion to contaminant permeation.

In situ surface modification of thin film composite forward osmosis membranes with sulfonated poly(arylene ether sulfone) for anti-fouling in emulsified oil/water separation

Forward osmosis (FO) is a promising technology for treating oily wastewater thanks to its high rejection and high water recovery. However, the performance of an FO membrane can be severely affected by the fouling of emulsified oil onto its rejection layer. In this work, we report a novel thin film composite (TFC) FO membrane with enhanced fouling resistance to emulsified oil. Specifically, we performed in situ surface grafting of its nascent polyamide active layer using amine-terminated sulfonated poly(arylene ether sulfone), i.e., NH2-BPSH100, over a range of molecular weights (2, 10 and 18kg/mol). The results of multi-cycle fouling experiments using a feed solution of 40,000ppm soybean oil/water emulsion showed optimum antifouling performance for the membrane modified with 10kg/mol of NH2-BPSH100 (10k-g-TFC). The improved antifouling performance of this membrane can be attributed to its superhydrophilic and underwater superoleophobic properties. In the current study, the permeability of 10k-g-TFC membrane was easily recovered by simple hydraulic flushing. Even without any membrane cleaning, its flux maintained as high as 69.8% of its initial value at 80% water recovery, in direct contrast to 11.0% for the control membrane without surface modification. The current study has important implications for anti-fouling surface modification of FO membranes.

Highly permeable and stable forward osmosis (FO) membrane based on the incorporation of Al2O3 nanoparticles into both substrate and polyamide active layer

In the present study, hydrophilic Al2O3 nanoparticles were used as additives in both substrate and polyamide active (PA) layer to improve forward osmosis (FO) membrane properties.