Polyamide Active Layer Research Articles

Dual-layered nanocomposite substrate membrane based on polysulfone/graphene oxide for mitigating internal concentration polarization in forward osmosis

A novel thin-film composite (TFC) forward osmosis (FO) membrane with dual-layered substrate membrane was fabricated by a double-blade casting technique using different polysulfone (PSf) concentrations for top (15 wt%) and bottom (7 wt%) substrate layers. Graphene oxide (GO) was incorporated in the substrate layer, and the dual casting approach resulted in a membrane support with a highly porous bottom structure and a dense top skin layer on which the polyamide active layer was effectively formed. The dual-layered TFC PSf/GO membrane (TFC-PSfdGO) exhibited high water permeability, and ion selectivity was enhanced by the presence of well dispersed hydrophilic GO in the PSf substrate. The TFC-PSfdGO also exhibited the lowest specific reverse salt flux (Js/Jv = 0.19 g L-1) and a more favorable structural parameter (S = 130 μm) compared to GO-free membranes. Using deionized water as feed solution and 1 M NaCl as draw solution (DS), TFC-PSfdGO had Jv = 33.8 L m−2 h−1 and Js = 6.9 g−2 h−1 under AL-FS mode, and Jv = 61.5 L m−2h−1 and Js = 14.0 g−2 h−1 under AL-DS mode. The potential of TFC-PSfdGO for commercial application was further evaluated by fabricating it with a fabric backing support (denoted as TFC-PSfdGOf). Compared to TFC-PSfdGO, TFC-PSfdGOf exhibited only 14% decline in its water flux. The overall results reveal that, fabrication of TFC substrate membrane via dual-blade casting approach along with GO incorporation produced high-performance TFC FO membranes which likely reduced the internal concentration polarization effects.

Synergetic effect of graphene oxide nanosheets embedded in the active and support layers on the performance of thin-film composite membranes

We herein report the first instance of the enhancement in membrane performance achieved by embedding graphene oxide (GO) nanosheets in both the polyamide (PA) active and polysulfone (PSf) support layers. The GO nanosheets were prepared via chemical exfoliation and separately dispersed in a PSf solution to fabricate the support layer and in an m-phenylenediamine (MPD) aqueous solution for interfacial polymerization of the PA layer. After precise characterization of the prepared membranes, membrane performances were evaluated using a lab-scale RO device. The water permeability and anti-biofouling property of the TFC membranes with embedded GO nanosheets in both the active and support layers were increased by 19% and 77%, respectively, compared with those of the TFC membranes with the GO nanosheets embedded only in the active layer. These increases were attributed to the changes in hydrophilicity, surface charge and surface roughness due to embedding of GO nanosheets in both layers.

Micro-to nano-scale characterisation of polyamide structures of the SW30HR RO membrane using advanced electron microscopy and stain tracers

The development of new reverse osmosis (RO) membranes with enhanced performance would benefit from a detailed knowledge of the membrane structures which participate in the filtration process. Here, we examined the hierarchical structures of the polyamide (PA) active layer of the SW30HR RO membrane. Scanning electron microscopy combined with focused ion beam milling (FIB-SEM) was used to obtain the 3-D reconstructions of membrane morphology with 5nm cross-sectional resolution (comparable with the resolution of low magnification TEM imaging in 2D) and 30nm slice thickness. The complex folding of the PA layer was examined in 3 dimensions, enabling the quantification of key structural properties of the PA layer, including the local thickness, volume, surface area and their derivatives. The PA layer was found to exhibit a much higher and convoluted surface area than that estimated via atomic force microscopy (AFM). Cross-sectional scanning transmission electron microscopy (STEM) was used to observe the distribution of a tracer stain under various conditions. The behaviour of stain in dry and wet PA indicated that the permeation pathways have a dynamic nature and are activated by water. High resolution STEM imaging of the stained PA nano-films revealed the presence of <1nm pore-like structures with a size compatible with free volume estimations by positron annihilation lifetime spectroscopy (PALS). This study presents a comprehensive map of the active PA layer across different length scales (from micro- to sub-nanometre) and mechanistic insight into their role in the permeation process.

Thin-film composite membrane on a compacted woven backing fabric for pressure assisted osmosis

The water flux in forward osmosis (FO) process declines substantially when the draw solution (DS) concentration reaches closer to the point of osmotic equilibrium with the feed solution (FS). Using external hydraulic pressure alongside the osmotic driving force in the pressure assisted osmosis (PAO) has been found effective in terms of enhancing water flux and even potentially diluting the DS beyond osmotic equilibrium. The net gain in water flux due to the applied pressure in the PAO process closely depends on the permeability of the FO membrane. The commercial flat sheet cellulose triacetate (CTA) FO membrane has low water permeability and hence the effective gain in water flux in the PAO process is low. In this study, a high performance thin film composite membrane was developed especially for the PAO process through casting polyethersulfone (PES) polymer solution on a compacted woven fabric mesh support followed by interfacial polymerisation for polyamide active layer. This PAO membrane possesses a water flux of 37Lm2h−1 using 0.5M NaCl as DS and deionised water as the feed at an applied hydraulic pressure of 10bar. Besides, the membrane was able to endure the external hydraulic pressure required for the PAO process owing to the embedded backing fabric support. While the membranes with low structural parameters are essential for higher water flux, this study shows that for PAO process, polymeric membranes with larger structural parameters may not be suitable for PAO. They generally resulted in compaction and poor mechanical strength to withstand hydraulic pressure.

Effect of MCM-48 nanoparticles on the performance of thin film nanocomposite membranes for reverse osmosis application

Monodispersed spherical MCM-48 nanoparticles with three-dimensional cubic mesostructure were synthesized and used as nanofillers dispersed in the aqueous or organic phase to prepare thin film nanocomposite (TFN) membranes through the interfacial polymerization of m-phenylenediamine and trimesoyl chloride. Scanning and transmission electron microscope images of the isolated polyamide (PA) active layer confirm that MCM-48 nanoparticles were embedded throughout the PA layer with dispersion in the organic phase as they were clipped between the PA layer and the polysulfone support with dispersion in the aqueous phase. Water flux was measured at 16bar with 2000ppm NaCl solution. Furthermore, water flux increased gradually from 24L/m2·h to 40L/m2·h with the increase in MCM-48 content in the organic phase, without significantly affecting salt rejection (>95%). Water flux increased from 24L/m2·h to 68L/m2·h, whereas salt rejection decreased from 97% to 80% with the increase in MCM-48 content in the aqueous phase. Less MCM-48 can be used in the aqueous phase than in the organic phase with similar performances. The two kinds of TFN membranes exhibit enhanced long-term durability, which confirms that MCM-48 can stably reside in the TFN membranes by dispersing whether in the organic or aqueous phase.

Nanocomposite Membrane with Different Carbon Nanotubes Location for Nanofiltration and Forward Osmosis Applications

We investigated the effect of carbon nanotubes (CNTs) location on the property and performance of prepared membranes. Four different types of membranes were prepared, including (1) thin film composite (TFC, polyamide active layer on polysulfone substrate), (2) nanocomposite-supported thin film composite (nTFC, polyamide active layer on CNT-embedded polysulfone substrate), (3) thin film nanocomposite (TFN, CNT-incorporated polyamide active layer on polysulfone substrate), and (4) nanocomposite-supported thin film nanocomposite (nTFN, CNT-incorporated polyamide active layer on CNT-embedded polysulfone substrate). The water permeability followed the sequence of nTFN > TFN > nTFC > TFC. However, the rejection and salt permeability exhibited the opposite trend. The incompatibility between CNTs and polymers provided nanocorridors, through which both water and solutes could pass. The nTFN membrane exhibited the highest porosity and lowest structural parameter. Moreover, the nTFN membrane possessed the best antif...

Improving the water permeability and antifouling property of thin-film composite polyamide nanofiltration membrane by modifying the active layer with triethanolamine

Interfacially synthesized polyamide (PA) thin-film composite (TFC) membranes, as the state-of-the-art polymeric membranes for nanofiltration (NF), are prone to fouling in processing wastewaters and industrial fluids due to the inherent surface physicochemical properties of polyamide active layer. Here, we report the facile and effective modification of PA-TFC nanofiltration membranes for improved water permeability and antifouling property by surface grafting. Ttriethanolamine (TEOA) molecules were bonded onto the surface of PA-TFC NF membrane through esterification reaction between the hydroxyl groups of TEOA and the residual acyl chloride groups on the nascent PA-TFC membrane by pouring the TEOA aqueous solution on membrane surface. The covalent attachment of TEOA molecules was shown to tailor the surface chemistry of the composite membrane without altering the morphology of active layer. The PA-TFC membrane became more hydrophilic after modification and thus more permeable to water and antifouling against hydrophobic foulants, while maintaining its molecular weight cut-off. The results of tertiary treatment of industrial effluent also demonstrated that the modification performed in our study could effectively improve the water permeability and antifouling property of the PA-TFC NF membrane in processing wastewater.

Interconnection of electrospun nanofibers via a post co-solvent treatment and its open pore size effect on pressure-retarded osmosis performance

Design of support layer structures for asymmetric thin film composite membranes has drawn keen attention to improve the power density for salinity gradient power based on pressure-retarded osmosis. This study has interests on electrospun nanofiber-based support layers, and the effects of its open pore sizes are attractively stated. To control the open pore size, a counter charge deposition method was introduced. To retain the open pore size, all the nanofibers were interconnected by a post co-solvent treatment technology. For a thin film composite membrane, an interfacial polymerization was used to fabricate a polyamide active layer on the electrospun nanofiber-based support layers. It was found that although the maximum power density achieved with an open pore size of 2.4 μm2 was 0.14 W/m2, it increased significantly up to 9.5 W/m2 when the pore size was reduced to 0.65 μm2. The cause is the salt flux which increases with increasing the open pore sizes under applied pressures.

Biodegradation of cellulose triacetate and polyamide forward osmosis membranes in an activated sludge bioreactor: Observations and implications

This study investigated long-term stability of forward osmosis (FO) membranes against biodegradation during prolonged exposure to activated sludge. Results show that cellulose triacetate (CTA) membranes were more resistant against biodegradation than polyamide thin film composite (TFC) ones. Nevertheless, CTA membrane biodegradation was discernible after seven months of exposure to activated sludge as manifested by an increase in the membrane average pore size, water and salt permeability, and membrane structural parameter. As a result, of prolonged exposure to activated sludge, the water and reverse salt fluxes of the CTA membrane increased; and concomitantly the rejection of a range of trace organic contaminants decreased significantly. The impact of prolonged exposure to activated sludge on the polyamide active layer of TFC FO membranes was even more severe. Our results indicate that currently available commercial CTA and polyamide TFC FO membranes may not be readily compatible for practical osmotic membrane bioreactor (OMBR) operation. Thus, the development of new and robust FO membrane materials specifically designed for membrane bioreactor operation is essential for commercial OMBR applications.

A Novel Approach Toward Fabrication of High Performance Thin Film Composite Polyamide Membranes.

A practical method is reported to enhance water permeability of thin film composite (TFC) polyamide (PA) membranes by decreasing the thickness of the selective PA layer. The composite membranes were prepared by interfacial polymerization (IP) reaction between meta-phenylene diamine (MPD)-aqueous and trimesoyl chloride (TMC)-organic solvents at the surface of polyethersulfone (PES) microporous support. Several PA TFC membranes were prepared at different temperatures of the organic solution ranging from −20 °C to 50 °C. The physico-chemical and morphological properties of the synthesized membranes were carefully characterized using serval analytical techniques. The results confirmed that the TFC membranes, synthesized at sub-zero temperatures of organic solution, had thinner and smoother PA layer with a greater degree of cross-linking and wettability compared to the PA films prepared at 50 °C. We demonstrated that reducing the temperature of organic solution effectively decreased the thickness of the PA active layer and thus enhanced water permeation through the membranes. The most water permeable membrane was prepared at −20 °C and exhibited nine times higher water flux compared to the membrane synthesized at room temperature. The method proposed in this report can be effectively applied for energy- and cost-efficient development of high performance nanofiltration and reverse osmosis membranes.

Improving the performance of TFC membranes via chelation and surface reaction: applications in water desalination

We modified TFC membranesviaa series of chemical and surface reactions. A nascent polyamide (PA) active layer was prepared by interfacial polymerization and composited with GO nanosheets then reacted with CuCl2·2H2O. Finally, the composite membrane was treated with NH4OH solutions.

Amide Link Scission in the Polyamide Active Layers of Thin-Film Composite Membranes upon Exposure to Free Chlorine: Kinetics and Mechanisms.

The volume-averaged amide link scission in the aromatic polyamide active layer of a reverse osmosis membrane upon exposure to free chlorine was quantified at a variety of free chlorine exposure times, concentrations, and pH and rinsing conditions. The results showed that (i) hydroxyl ions are needed for scission to occur, (ii) hydroxide-induced amide link scission is a strong function of exposure to hypochlorous acid, (iii) the ratio between amide links broken and chlorine atoms taken up increased with the chlorination pH and reached a maximum of ∼25%, (iv) polyamide disintegration occurs when high free chlorine concentrations, alkaline conditions, and high exposure times are combined, (v) amide link scission promotes further chlorine uptake, and (vi) scission at the membrane surface is unrepresentative of volume-averaged scission in the active layer. Our observations are consistent with previously proposed mechanisms describing amide link scission as a result of the hydrolysis of the N-chlorinated amidic N-C bond due to nucleophilic attack by hydroxyl ions. This study increases the understanding of the physicochemical changes that could occur for membranes in treatment plants using chlorine as an upstream disinfectant and the extent and rate at which those changes would occur.

Second interfacial polymerization on polyamide surface using aliphatic diamine with improved performance of TFC FO membranes

High performance thin film composite forward osmosis (TFC-FO) membranes were prepared by in situ modification of nascent polyamide (PA) active layer with ethane diamine (EDA) and 2-[(2-aminoethyl)amino]- ethane sulfonic acid monosodium salt (SEA) via second interfacial polymerization (SIP). The membranes were characterized with respect to membrane chemistry (by ATR–FTIR and XPS spectroscopy), surface properties comprising wettability (by static contact angle), roughness (by AFM), zeta potential, surface free energy, osmosis performance, and antifouling ability in FO process. The results indicates that SIP with aliphatic diamine is an effective strategy towards improving the FO membrane performance, such as high permeability, selectivity and antifouling tendency. EDA would shadow the valleys of pristine PA layer, and gain the highest fouling resistance; while additional hydrophilic group (−SO3Na) in SEA can enhance the wettability of membrane, consequently resulting in higher water flux without scarifying the selectivity. Meanwhile, we found that the initial osmotic water flux plays a more important role in the cake layer formation than the surface properties of membrane. When the initial flux was set as 19 LMH, the final normalized flux of SEA-M declined to about 0.45 at the end of fouling test with BSA as the foulant model, while that of three membranes can keep more than 0.8 at the initial flux 9 LMH.

Identifying facile and accurate methods to measure the thickness of the active layers of thin-film composite membranes – A comparison of seven characterization techniques

Despite the important role played by active layer thickness in the performance of thin-film composite (TFC) membranes, and in the understanding of the intrinsic transport properties (i.e., permeability, and water and solute partition and diffusion coefficients) of active layers, there is no study in the peer-reviewed literature evaluating whether existing measurement techniques provide consistent results among each other. Thus, we compared active layer thickness results obtained for each of six commercial TFC membranes with seven measurement techniques, including four techniques commonly used in the literature (scanning electron microscopy – SEM, transmission electron microscopy – TEM, atomic force microscopy – AFM, Rutherford backscattering spectrometry – RBS) and three non-commonly used techniques (quartz crystal microbalance – QCM, profilometry and ellipsometry). The six membranes tested covered performance levels ranging from nanofiltration to seawater reverse osmosis. Our results showed that AFM, RBS, QCM, profilometry and ellipsometry produced consistent results among each other, and thus likely provide the most accurate thickness results. SEM and TEM produced thickness results that were greater than those obtained with the five non-electron microscopy techniques, thus suggesting that SEM and TEM should only be used for rough estimates of active layer thickness. On the basis of nine different factors used to evaluate the advantages and disadvantages of the measurement techniques, ellipsometry was found to be the most advantageous technique for measuring active layer thickness. Results for active layer thickness and mass were used to obtain experimental estimates of the density of polyamide active layers, which for uncoated polyamide layers was found to be 1.26±0.21gcm−3.

Investigating the void structure of the polyamide active layers of thin-film composite membranes

The potential presence of voids in the fully-aromatic polyamide active layers of thin-film composite (TFC) membranes for water purification was studied in a selection of commercial membranes with a broad range of performance levels. The membranes were characterized for their potential void fractions using three independent methods: (i) analysis of transmission electron microscopy (TEM) images of membrane cross-sections, (ii) water uptake measurements by quartz crystal microbalance (QCM), and (iii) estimates of the effective refractive indices of active layers by spectroscopic ellipsometry. Results revealed that voids having tens of nanometers in diameter exist in the fully-aromatic polyamide active layers of TFC membranes, the voids fill up with water when immersed in it, and the voids account for a significant volume fraction of the active layers (i.e., 15–32% for the membranes studied). It was concluded that the voids in polyamide active layers do not form passageways connecting the feed and permeate sides, but rather are cavities disconnected from the feed side. In addition, it was also concluded that the globular features observable in TEM images of membrane cross sections that had been previously identified as voids or nodules are indeed voids, and not nodules. The finding that a significant volume fraction of fully-aromatic polyamide active layers corresponds to water-filled voids has deep implications on various aspects of TFC membrane science and technology. For example, we illustrate how the presence of voids can potentially increase the effective water permeability of the active layer by as much as a factor of ≈5 compared with the case of an equivalent active layer without any voids. The methods developed in this study to measure void volume fraction represent useful tools for future membrane characterization studies, and the void fractions measured can be used as input or calibration parameters in future modeling studies of active layer formation or water and solute transport.

Thin-film nanocomposite membrane with CNT positioning in support layer for energy harvesting from saline water

The pressure retarded osmosis (PRO) process has been considered as an alternative and renewable technology to generate electricity from mixing two solutions of different salinities. However, improving the osmotic performance of semi-permeable membrane is still a major challenge in the PRO system. Therefore, thin-film nanocomposite (TFN) membrane was synthesized by using carbon nanotubes (CNT)-embedded-polyethersulfone (PES) supporting layer and polyamide active layer in this study. The prepared membranes were further employed in the PRO process to harvest energy from saline water. The water flux increase of the TFN membrane was promoted by CNT-induced porosity and the hydrophilicity of the support layer as well as by the chemical etching of the active layer. The water flux and maximum power density of the developed TFN membrane was found to be 87% (averaged from 2bar to 10bar) and 110% greater than for bare thin-film composite (TFC) membranes, respectively. Furthermore, the TFN membrane preparation could easily be scaled up using conventional fabrication methods with less than 2% additional material cost. Therefore, this finding could contribute to the commercialization of sustainable energy generation by utilizing the tremendous potential of fresh- and salt-water mixing.

Comparison of the performance of two reverse osmosis membranes for the final purification of olive mill wastewater

Two quite different reverse osmosis (RO) polymeric membranes were examined for the final purification of olive mill wastewater from two-phase olive mills (OMW2): the first one is a thin-film composite (TFC) membrane consisting of polyamide active layer on polysulfone ultrafiltration support, whereas the other one is a low-pressure membrane made of asymmetric polyamide. A net operating pressure (PTM) of 25 bar was found as the target for the TFC membrane, whereas for the asymmetric one a PTM of 8 bar was chosen, given that similar flux decay but still significant productivity was observed by increasing the PTM for this membrane. These results are confirmed by the fouling index (b) values calculated for each membrane. Complete removal of suspended solids, phenolic compounds and iron was achieved by both membranes. Otherwise, the asymmetric membrane ensured slightly higher organic matter (COD) and electroconductivity (EC) reduction, leading to a COD concentration in the permeate stream equal to 3.7 mg L-1 and 1.4 mg L-1 (TFC vs. asymmetric), whereas the EC values were 97.0 and 31.0 μs cm-1, respectively. This would permit reusing the purified effluent provided by both membranes in the production process and close the loop at industrial scale. Moreover, the asymmetric membrane provides a steady-state flux value of the same order of that yielded by the TFC membrane upon more than three times less PTM (14.9 L h-1m-2 at PTM = 8 bar vs. 15.2 L h-1m-2 at PTM = 25 bar), implying a reduction of the specific energy consumption above 50 %, from 0.30 € m-3 for the TFC membrane to 0.14€ m-3 for the asymmetric one.

Graphene oxide incorporated polysulfone substrate for the fabrication of flat-sheet thin-film composite forward osmosis membranes

The preparation and performances of the newly synthesized thin film composite (TFC) forward osmosis (FO) membranes with graphene oxide (GO)-modified support layer are presented in this study. GO nanosheets were incorporated in the polysulfone (PSf) to obtain PSf/GO composite membrane support layer. Polyamide (PA) active layer was subsequently formed on the PSf/GO by interfacial polymerization to obtain the TFC-FO membranes. Results reveal that at an optimal amount of GO addition (0.25wt%), a PSf/GO composite support layer with favorable structural property measured in terms of thickness, porosity and pore size can be achieved. The optimum incorporation of GO in the PSF support layer not only significantly improved water permeability but also allowed effective PA layer formation, in comparison to that of pure PSf support layer which had much lower water permeability. Thus, a TFC-FO membrane with high water flux (19.77Lm−2h−1 against 6.08Lm−2h−1 for pure PSf) and reverse flux selectivity (5.75Lg−1 against 3.36Lg−1 for pure PSf) was obtained under the active layer facing the feed solution or AL-FS membrane orientation. Besides the improved structural properties (reduced structural parameter, S) of the support layer, enhanced support hydrophilicity also contributed to the improved water permeability of the membrane. Beyond a certain point of GO addition (≥0.5wt%), the poor dispersion of GO in dope solution and significant structure change resulted in lower water permeation and weaker mechanical properties in support as well as FO flux/selectivity of consequent TFC membrane. Overall, this study suggests that GO modification of membrane supports could be a promising technique to improve the performances of TFC-FO membranes.

Preparation of graphene oxide modified polyamide thin film composite membranes with improved hydrophilicity for natural organic matter removal

The incorporation of nano-materials in thin film composite (TFC) membrane has opened an alternative opportunity to enhance the membrane permeation and anti-fouling properties in many water treatment processes. In this study, graphene oxide (GO) was incorporated into the polyamide active layer of a TFC membrane via interfacial polymerization (IP) reaction. The structure, composition and hydrophilicity of the membranes were characterized by FT-IR, FE-SEM and water contact angle measurements. The separation performance for natural organic matter (NOM) and the anti-fouling properties of the GO modified TFC membranes were investigated with real natural water sampled from a local river. The permeate water samples were analyzed with ultraviolet absorbance (UV), dissolved organic carbon (DOC), fluorescence excitation–emission matrices (FEEMs) and molecular weight distribution (MWD). Within the range of GO loadings assessed in this study, it was found that the TFC membrane incorporated with GO offered not only higher water flux and NOM removal rate, but also better anti-fouling properties, which was ascribed to the favorable change in membrane hydrophilicity, surface morphology and surface charge by the addition of appropriate amount of GO, indicating great potential of such membranes in water industry.

Influence of chitosan coating on the separation performance, morphology and anti-fouling properties of the polyamide nanofiltration membranes

Chitosan was coated into polyamide membranes by in situ reaction of amine and hydroxyl groups of chitosan and un-reacted of acylchloride of polyamide active layer. Effect of chitosan concentration and coagulation time on the membrane chemical structure, morphology, separation performances and anti-fouling properties were characterized by FT-IR, SEM, AFM, cross-flow membrane module device and cetyltrimethylammonium bromide experiment. Water flux of the polyamide membranes enhanced from 32.9 to 59.6L/m2h and anti-fouling results proved that excellent fouling resistance. Thus, chitosan as a secondary modifier has special potential to improve the performance of polyamide membranes.