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Self-Assembled Nanofiltration Membranes with Thermo- and pH-Responsive Behavior
Polymerization within nanoconfinement offers a versatile approach to creating nanostructured materials with unique properties and a wide range of applications. Therefore, it is important to understand the nature of polymerization...
Bicontinuous cubic (Q) lyotropic liquid crystal (LLC)‐derived polymer networks possess periodic, uniform‐size, 3D‐interconnected nanopores that make them highly desirable organic materials for selective molecular separation and uptake applications. To date, there are no reported examples of a Q‐phase network with additional functional properties, such as catalytic reactivity, response to external stimuli, or gated transport, that would expand the usefulness of this class of porous materials. Here, a functionalized gyroid‐type Q polymer network material that can be used for potential gated transport or adaptive uptake applications is presented. This material contains a novel spiropyran‐containing dopant monomer that upon blending and cross‐linking with a gyroid‐forming LLC monomer, yields a gyroid polymer material that retains its phase architecture while reversibly responding to changes in external solution and vapor pH. Studies also demonstrate that this system is capable of strongly binding to aq. Pb2+ ions when activated by UV light, allowing it to function as a potential colorimetric sorbent or gated‐response material. A spiropyran‐based monomer is synthesized and blended with a bicontinuous cubic (Q) phase‐forming lyotropic liquid crystal monomer. Self‐assembly with added water and polymerization yields stimuli‐responsive, gyroid‐type Q‐phase polymer networks. This novel nanoporous gyroid polymer material retains its phase structure while responding to external stimuli such as acid (aqueous solution or vapor phase) or UV light.
Mono-/multivalent ion-selective separation has become a common requirement at the water-energy nexus, including energy storage and conversion, water purification, and sustainable industrial processes. In this review, we summarize the theory of ion transport through membrane and mechanisms of selective ion separation in nanofiltration (NF) briefly. Recent advancing in improving the mono-/multivalent ion selectivity of thin-film composite (TFC) NF membrane via size sieving enhancement, electric charge property regulation and co-enhancement of size sieving and electric charge properties are concluded. What's more, three material classes—surface assembly materials, nanomaterials and biomimetic ion channels are highlighted as candidates for the preparation of ion-selective NF membranes. Lastly, design directions and critical challenges for developing high-selectivity nanofiltration membranes based on the ion-selective mechanisms are provided.
Organic solvent-stable membranes exhibiting strong selectivity and high permeance have the potential to transform energy utilization in chemical separation processes. A key goal is developing materials with uniform, well-defined pores at the 1-nm scale, with sizes that can be tuned in small increments with high fidelity. Here, we demonstrate a class of organic solvent-stable nanoporous membranes derived from self-assembled liquid crystal mesophases that display such characteristics and elucidate their transport properties. The transport-regulating dimensions are defined by the mesophase geometry and can be controlled in increments of ~0.1 nm by modifying the chemical structure of the mesogen or the composition of the mesophase. The highly ordered nanostructure affords previously unidentified opportunities for the systematic design of organic solvent nanofiltration membranes with tailored selectivity and permeability and for understanding and modeling rejection in nanoscale flows. Hence, these membranes represent progress toward the goal of enabling precise organic solvent nanofiltration.
This study describes the development of a renewable and biodegradable biopolymer-based hydrogel for application in agriculture and horticulture as a soil conditioning agent and for release of a nutrient or fertilizer. The novel product is based on a combination of cellulose derivatives (carboxymethylcellulose and hydroxyethylcellulose) cross-linked with citric acid, as tested at various concentrations, with acid whey as a medium for hydrogel synthesis in order to utilize the almost unusable by-product of the dairy industry. The water uptake of the hydrogel was evaluated by swelling tests under variations in pH, temperature and ion concentration. Its swelling capacity, water retention and biodegradability were investigated in soil to simulate real-world conditions, the latter being monitored by the production of carbon dioxide during the biodegradation process by gas chromatography. Changes in the chemical structure and morphology of the hydrogels during biodegradation were assessed using Fourier transform infrared spectroscopy and scanning electron microscopy. The ability of the hydrogel to hold and release fertilizers was studied with urea and KNO3 as model substances. The results not only demonstrate the potential of the hydrogel to enhance the quality of soil, but also how acid whey can be employed in the development of a soil conditioning agent and nutrient release products.
Nanofiltration (NF) with advantages of high efficiency and low-cost has attracted increasing attentions in bio-separation. However, the large-scale application is limited by the inferior molecular selectivity, low chemical stability and serious membrane fouling. Many efforts, thus, have been devoted in NF materials design for specific applications to enhance the separation efficiency of bio-products and increase membrane lifetime , as well as reduce the operating cost. This review summarized the recent progress of NF applications in bio-separation, discussed various demands for NF membrane in the bio-products purification and corresponding material innovations, finally proposed several practical suggestions for future research, which provided directions and guidance toward further product development and process industrialization.
In this work, graphene oxide (GO) and titanium dioxide (TiO2) were used as an additive to fabricate the nanofiltration (NF) membrane. GO was synthesised via electrochemical exfoliation method utilising sodium bis(3,5,5-trimethyl-1-hexyl) sulphosuccinate (AOT4) surfactant. The synthesised GO was then used to fabricate PVDF-based NF membrane namely PVDF/GO_TiO2 via non-solvent induced phase separation (NIPS) method. The effects of embedded GO and TiO2 on the morphology and structural properties of PVDF/GO_TiO2 were investigated by using field emission scanning electron microscopy (FESEM) and micro-Raman spectroscopy. The finding shows that PVDF/GO_TiO2 present thin and dense top layer supported by macro-voids sub-layer with sponge-like layer at the bottom. Based on its morphology, the fabricated PVDF/GO_TiO2 membrane is potential to be applied as membrane filtration for water treatment application.
The structure of the electrical double layer at the interface of planar electrodes and aqueous solutions is investigated. Electrical impedance spectroscopy is used to measure the impedance of aqueous solutions of sodium chloride and two different surfactants over a wide range of concentrations. The electrode capacitance is directly inferred from the admittance spectra, as well as by regression of the impedance spectra to an equivalent circuit. It is found that the electrode capacitance remains on the same order of magnitude over the entire range of investigated concentrations. This is contradictive to the predictions of the Gouy-Chapman-Stern theory which predicts that at low concentrations, the electrode capacitance should be determined by the diffuse layer. It is concluded that the Stern layer capacitance always dominates the electrode capacitance, even at very low concentrations, and the establishment of a diffuse layer capacitance requires an ionic strength of around 1 mM.
Injectable biomaterials have gained significant attention in recent years since they can be delivered to the defect sites through minimally-invasive approaches. In this work, thermo-responsive injectable hydrogels containing borate-based bioactive glass particles and carbon-based nanopowders were designed for bone tissue engineering applications. For this purpose, mixtures of Pluronic F127 block-copolymer and 13-93B3 bioactive glass particles with different sizes (2.3 µm, 14 µm, 150 µm) were prepared in aqueous medium and their in situ gelation were investigated through rheological measurements as a function of temperature. Effects of graphene nanopowders and multi-walled carbon nanotubes on the flow behavior of the designed hydrogel system were also investigated. Results revealed that viscosity of the prepared hydrogel system was strongly dependent on the temperature and the bioactive glass particle size. Inclusion of graphene and multi-walled carbon nanotubes in this system caused a further increase in viscosity. All of the hydrogel compositions designed in the study showed shear thinning flow behavior which is a crucial parameter for injectability. Drug release studies showed that the addition of bioactive glass and carbon-based nanoparticles improved the drug release behavior of the prepared hydrogels.
A dual-responsive hydrogel (pH/temperature) was developed from a thermos-responsive polymer, pluronic F-127 (PF127), and pH-responsive polymers, N,N,N-trimethyl chitosan (TMC) and polyethylene glycolated hyaluronic acid (PEG-HA). Gallic acid, the principal component of the traditional Chinese drug Cortex Moutan was loaded into the hydrogel (PF127/TMC/PEG-HA) for possible application in textile-based transdermal therapy as Cortex Moutan has been proven to be an effective drug for the treatment of atopic dermatitis (AD). TMC and PEG-HA were synthesized, characterized (1H-NMR and FTIR), and added to the formulations to enhance drug release from the hydrogels, and increase the drug targeting of the carriers. The thermo-responsive properties of the hydrogel were assessed by dynamic viscosity analysis and the tube inversion method, and the pH-responsiveness of the formulation was determined by changing the pH of the external media. Rheology study of the hydrogels showed that complex viscosity and storage/loss moduli for PF127/TMC/PEG-HA hydrogel formulation are higher than PF127 hydrogel. The microstructure analysis by reflection SAXS indicated similar type of frozen inhomogeneity of hydrogel formulations. Various characterizations such as FTIR, SEM, TEM, zeta potential, and degradation of the hydrogel formulation indicated that the PF127/TMC/PEG-HA hydrogel showed better physico-chemical properties and morphology than did the PF127 hydrogel, and drug release was also higher for the PF127/TMC/PEG-HA hydrogel than for PF127. The drug release from hydrogels followed more closely first-order rate model than other rate models.
Self-assembled materials are attractive for next-generation membranes. However, the need to align self-assembled nanostructures (e.g. cylinders, lamellae) and the narrow stability windows for ordered bicontinuous systems present serious challenges. We propose and demonstrate a novel approach that circumvents these challenges by exploiting size-selective transport in the water-continuous medium of a nanostructured polymer templated from a self-assembled lyotropic H 1 mesophase. Optimization of the mesophase composition enables high-fidelity retention of the H 1 structure on photoinduced cross-linking. The resulting material is a mechanically robust nanostructured polymer possessing internally and externally cross-linked nanofibrils surrounded by a continuous aqueous medium. Fabricated membranes show size selectivity at the 1- to 2-nm length scale and water permeabilities of ~10 liters m ⁻² hour ⁻¹ bar ⁻¹ μm. Moreover, the membranes display excellent antimicrobial properties due to the quaternary ammonium groups on the nanofibril surfaces. These results represent a breakthrough for the potential use of polymerized lyotropic mesophase membranes in practical water purification applications.
Self-assembly of block copolymers in the presence of solvents forms ordered mesophase structures, also known as lyotropic liquid crystals (LLCs). The aim of this work is to investigate the rheological properties of Pluronic block copolymer/water/oil mesophases with lamellar and hexagonal structures. The flow behavior of lamellar and hexagonal mesophases indicates that they have yield stress. Oscillatory shear experiments show that mesophases have solid-like behavior and exhibit type III non-linear behavior. The elastic modulus of mesophases is probably controlled by the van der Waals interaction between micelles. We suggest that at relatively low frequencies, defects control the rheological behavior of LLCs, while at high frequencies, the contributions in micellar scale are dominant. Applying high strains on the LLCs induces two relaxation times after cessation of flow, decreases the storage modulus in the whole frequency range, and decreases the loss modulus in small-frequency regime with negligible effect at high frequencies. The decrease in moduli is reversible and the system relaxes back to its original elastic modulus at rest. The observed behavior can be attributed to the elimination of defects under high strains and their re-formation during long enough rest times.
The preferential removal of sodium sulfate over sodium chloride (fractionation) by nanofiltration (NF) is studied. A fractionation metric, M, is defined, and a simple set of equations are derived to describe its variation due to operating parameters, such as temperature and pressure. The use of these equations to explain known behavior of NF systems and suggest improvements in system or membrane design is demonstrated. Furthermore, the derived framework is applicable to all membranes used in pressure-driven technologies, without detailed characterization of specific membrane properties, such as structural parameters (pore radius, active layer thickness, tortuosity, porosity) or charge-based parameters. Given that the principal applications of NF involve the selective removal of multivalent ions over monovalent ions, the introduced metric provides a direct measure of its efficacy in such applications, and is shown to be more accurate in some cases than comparing salt rejection ratios, as usually done. Finally, the concept of ‘breakthrough,’ commonly observed as the point at which rejection ratio begins to decrease with increase in pressure, is mathematically described, and its implication on selectivity discussed.
Organic solvents, such as N-methyl-2-pyrrolidone (NMP) and dimethylacetamide (DMAc), have been traditionally used to fabricate polymeric membranes. These solvents may have a negative impact on the environment and human health; therefore, using renewable solvents derived from biomass is of great interest to make membrane fabrication sustainable. Methyl-5-(dimethylamino)-2-methyl-5-oxopentanoate (Rhodiasolv PolarClean) is a bio-derived, biodegradable, nonflammable and nonvolatile solvent. Polysulfone is a commonly used polymer to fabricate membranes due to its thermal stability, strong mechanical strength and good chemical resistance. From cloud point curves, PolarClean showed potential to be a solvent for polysulfone. Membranes prepared with PolarClean were investigated in terms of their morphology, porosity, water permeability and protein rejection, and were compared to membranes prepared with traditional solvents. The pores of polysulfone/PolarClean membranes were sponge-like, and the membranes displayed higher water flux values (176.0 ± 8.8 LMH) along with slightly higher solute rejection (99.0 ± 0.51%). On the other hand, PSf/DMAc membrane pores were finger-like with lower water flux (63.1 ± 12.4 LMH) and slightly lower solute rejection (96 ± 2.00%) when compared to PSf/PolarClean membranes.
Mixed Pluronic micelles from very hydrophobic and very hydrophilic copolymers were selected to scrutinize the synergistic effect on the self-assembly process as well as the solubilization capacity of ibuprofen. The tendency of mixing behavior between parent copolymers was systematically examined from two perspectives: different block chain lengths at same hydrophilicity (L92 + F108, +F98, +F88, and +F68), as well as various hydrophobicities at the same PPO moiety (L92 + F88, +F87, and +P84). Temperature-dependent micellization in these binary systems was clearly inspected by the combined use of high sensitivity differential scanning calorimeter (HSDSC) and dynamic light scattering (DLS). Changes in heat capacity and size of aggregates at different temperatures during the whole micellization process were simultaneously observed and examined. While distinction of block chain length between parent copolymers increases, the monodispersity of the binary Pluronic systems decreases. However, parent copolymers with distinct PPO moieties do not affirmatively lead to non-cooperative binding, such as the L92 + P84 system. The addition of ibuprofen promotes micellization as well as stabilizes aggregates in the solution. The partial replacement of the hydrophilic Pluronic by a more hydrophobic Pluronic L92 would increase the total hydrophobicity of mixed Pluronics used in the system to substantially enhance the solubility of ibuprofen. The solubility of ibuprofen in the 0.5 wt % L92 + 0.368 wt % P84 system is as high as 4.29 mg/mL, which is 1.4 times more than that of the 0.868 wt % P84 system and 147 times more than that in pure water at 37 °C.
New and advanced opportunities are arising for the synthesis and functionalization of membranes with selective separation, reactivity, and stimuli-responsive behavior. One such advancement is the integration of bio-based channels in membrane technologies. By a layer-by-layer (LbL) assembly of polyelectrolytes, outer membrane protein F trimers (OmpF) or “porins” from Escherichia coli with central pores ∼2 nm in diameter at their opening and ∼0.7 × 1.1 nm at their constricted region are immobilized within the pores of poly(vinylidene fluoride) microfiltration membranes, in contrast to traditional ruptured lipid bilayer or vesicle processes. These OmpF-membranes demonstrate selective rejection of non-charged organics over ionic solutes, allowing the passage of up to 2 times more salts than traditional nanofiltration membranes starting with rejections of 84% for 0.4 to 1.0 kDa organics. The presence of charged groups in OmpF-membranes also leads to pH-dependent salt rejection through Donnan exclusion. These OmpF-membranes also show exceptional durability and stability, delivering consistent and constant permeability and recovery for over 160 h of operation. Characterization of the solutions containing OmpF and the membranes was conducted during each stage of the process, including detection by fluorescence labelling (FITC), zeta potential, pH responsiveness, flux changes, and rejection of organic–inorganic solutions.
A novel pH-responsive nanofiltration membrane was fabricated by means of layer-by-layer (LbL) technique based on porphyrin supramolecular self-assembly. The multilayer membrane was prepared on a hydrolyzed poly(acrylonitrile) (PAN) support membrane, and was composed of poly(allylamine hydrochloride) (PAH) as a polycation and poly(styrene sulfonate) (PSS) as a polyanion and 5,10,15,20-tetrakis-(4-sulfonatophenyl)-porphyrin (TPPS) as the pH-responsive functional supramolecular. Aggregation of TPPS and the shielding effect of salt in solution affected the adsorption of TPPS onto the membrane, while the higher ionization degree of the oppositely charged membrane with PAH favored the adsorption of TPPS. A coil structure of polyelectrolytes caused by the lower ionization degree of PAH or by the shielding effect of salt led to higher adsorption of polyelectrolytes on the membrane. The LbL assembly membrane showed higher and pH-responsive water flux and salt rejection compared with that without TPPS. At pH 1.0, TPPS assembled into J-aggregates on the membrane surface, and the membrane showed relatively lower water flux and higher rejection. When the pH value was increased above 2.0, TPPS transformed into H-aggregates and monomers, and the membrane showed relatively higher water flux and lower salt rejection.
Membrane-based separations for water purification and desalination have been increasingly applied to address the global challenges of water scarcity and the pollution of aquatic environments. However, progress in water purification membranes has been constrained by the inherent limitations of conventional membrane materials. Recent advances in methods for controlling the structure and chemical functionality in polymer films can potentially lead to new classes of membranes for water purification. In this Review, we first discuss the state of the art of existing membrane technologies for water purification and desalination, highlight their inherent limitations and establish the urgent requirements for next-generation membranes. We then describe molecular-level design approaches towards fabricating highly selective membranes, focusing on novel materials such as aquaporin, synthetic nanochannels, graphene and self-assembled block copolymers and small molecules. Finally, we highlight promising membrane surface modification approaches that minimize interfacial interactions and enhance fouling resistance.
An amphiphilic thermo responsive cross linked polyvinylcaprolactam-co-polysulfone (PVCL-co-PSF) copolymer was synthesized via solution polymerization of vinylcaprolactam (VCL) in PSF solution by use of three different initial ratios of PSF to VCL monomer. After the synthesis of the copolymer, the required amount of PSF was dissolved in PVCL-co-PSF copolymer solution. The presence of copolymer in the blended membrane was confirmed by Fourier transform infrared-attenuated total reflectance (FTIR-ATR) spectroscopy. Blended membranes showed enhanced pure water flux, hydrophilicity and evident thermo sensitivity. The hydration capacity for the modified membrane decreased from 279 to 161 mg cm−3 when the temperature changed from 25 to 40 °C. The hydration capacity of the modified PSF membrane compared to the plain PSF membrane increased from 127 to 279 mg cm−3, and the adsorbed protein amount decreased from 0.14 mg cm−2 to 0.03 mg cm−2 at 25 °C. The reversible volume phase transition of PVCL around the lower critical solution temperature (LCST) was used as an environmentally-friendly approach for membrane cleaning. A temperature change water elution hydraulic cleaning for the modified membranes around the LCST of the PVCL-co-PSF copolymer brushes was proposed (as shown in Fig. 7). Following the alternating temperature-change (40 °C/25 °C) cleaning, flux recoveries of about 92.5% (in the case of BSA) and 95% (in the case of HA) were obtained for the modified PSF membrane (the flux recoveries of the plain membrane were only about 39% and 36% after BSA and HA ultrafiltration, respectively).
Polybenzimidazole (PBI), a polymer known for its superior chemical stability, thermal properties, and mechanical resistance has been regarded as a suitable membrane material for a wide range of applications. Herein, we took advantage of these properties and its amphoteric nature that could endow the formed membranes with capabilities to have different states and responses in different pH conditions for enhanced nanofiltration (NF) performance. Integrally skinned asymmetric flat-sheet PBI membranes were fabricated using the nonsolvent-induced phase separation (NIPS) method under various pH in a coagulant bath. For the first time, we have thoroughly investigated the effects of nonsolvent acidity and alkalinity on PBI membrane formation and their performances. It was revealed that the membrane formed under pH 12 showed a doubled MgCl2 rejection than the one formed under pH 7. Moreover, a green modification method was employed to enhance the membranes’ sieving capabilities by using hyperbranched polyethyleneimine polymers (HPEIs) with different molecular weights (MWs). The PBI membrane formed under pH 12 and then surface modified with a HPEI of MW = 25,000 g/mol (i.e., PBI 12–25K membrane) has a molecular weight cut-off (MWCO) as low as 288 Da and an MgCl2 rejection more than 97%. The membrane also displays pH-responsive characteristics for salt separation with unequal valence ratios of co-ions and counter-ions. It is able to show a competitive SLi,Mg value for Li⁺/Mg²⁺ separation surpassing the state-of-the-art performance of commercial membranes.
Biological sodium channels have selectivity filters with extraordinary Na⁺ selectivity. Realizing this function in ion exchange membranes is highly desirable for technologies related to water, energy, and the environment, but it remains a challenge. Here we report a sodium selective isoporous membrane (NaSIM) derived from lyotropic liquid crystals. This membrane consists of uniform ion conductive channels lined with carboxylate groups. These negatively charged ion channels demonstrate charge-governed ion transport, pH responsiveness, and Na⁺ selectivity. The Na⁺ selectivity was 1.88 against K⁺ as revealed from single ion permeation experiment and approached 2.10 in binary salt solutions. The prominent Na⁺ selectivity may arise from specific interactions between Na⁺ ions and the carboxylate groups inside the channels, which regulate the energy barriers for monovalent cation transfer. The NaSIM we developed may promote high-precision separation and provides a cornerstone for designing a new generation of ion selective membranes.
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 (NF) membrane has been applied for the treatment of wastewater owing to its unique features such as higher selectivity towards divalent/polyvalent ions while allowing permeation for monovalent ions and small molecules of less than 100 Da. Thus, the use of NF in wastewater treatment is promising for water recycling, reuse, and recovery of other valuable products in industrial wastewater treatment. This review highlights the current application of NF for water recycling, reuse, and product recovery within multiple industries such as textile, food, oil and gas, mining, tannery, pharmaceutical as well as pulp and paper industry. The performance of NF either as stand-alone or integrated with other processes for improving the overall treatment efficiency and minimizing membrane issues is discussed. Finally, future perspectives for NF applications in industrial wastewater treatment for water recycling, reuse, and product recovery are discussed.
Lyotropic liquid crystals (LLCs) have drawn attention in numerous technical fields as they feature a variety of nanometer-scale structures, processability, and diverse chemical functionality. However, they suffer from poor mechanical...
Regulating the monomer diffusion during interfacial polymerization (IP) process is the key for tailoring the pore structure and desalination performance of thin-film composite polyamide nanofiltration (NF) membrane. Recently a surfactant-assembly strategy to regulate IP process and NF membranes with sub-Å separation precision is proposed. However, little is known for the role of the molecular structure of surfactant on IP process and membrane performance. In this work, five sulfate surfactants with different length of alkyl chains are used to construct surfactant monolayer assembly at the water/hexane interface to regulate IP process. The results show that for the sulfate surfactants, the longer the alkyl chain, the more uniform the pore size distribution of polyamide active layer is and the higher the ion separation selectivity of NF membrane is. Among them, the NF membrane prepared from sodium n-hexadecyl sulfate (SHS) exhibits the highest separation performance with the rejection of Na2SO4, MgSO4, MgCl2, and CaCl2 up to 99.39%, 99.12%, 98.09%, and 97.38%, respectively. Overall, this study further confirms the regulatory role of surfactant-assembled monolayer during IP process and provides important insights into how the polyamide structure and the resulting NF membranes are influenced by the alkyl chain length of the surfactants.
Fabricating membranes with homogenous pore size at sub-nanometer scale is highly demanded for precise separation but remains a challenge. Herein, we propose a new strategy for preparing polyamide (PA) nanofiltration membranes with high solute-solute selectivity via a pre-diffusion interfacial polymerization (PDIP) process. The enrichment of amine monomers on the organic phase side of the oil-water interface facilitates a stoichiometric interfacial polymerization reaction with acyl chloride and endows the PA active layer more uniform pores with sub-Angstrom separation precision. The nanofiltration membrane prepared by PDIP strategy has lower surface roughness and higher degree of polymerization reaction inside the PA active layer compared with the membrane prepared by the traditional method. The pre-diffusion process enables PA nanofiltration membranes to have a 99.7% rejection rate of Na2SO4 and 98.7% of MgCl2 with no loss of flux while the rejection of that is 96.7% and 62.4% for traditional IP. Most importantly, the membrane exhibits fairly high mono/divalent salt selectivity of both cations and anions (e.g. 246 for NaCl/Na2SO4 and 51 for NaCl/MgCl2), which is one of the highest mono/divalent salt selectivity among all the reported polymer membranes. The PDIP process with improved ion selectivity is expected to change the traditional manufacturing mode of PA thin film composite (TFC) membranes, including nanofiltration and reverse osmosis membranes those are the most important members in the family of filtration membranes for water purification.
Nanostructured materials with precisely defined and water-bicontinuous 1-nm-scale pores are highly sought after as advanced materials for next-generation nanofiltration membranes. While several self-assembled systems appear to satisfy this need, straightforward fabrication of such materials as submicron films with high-fidelity retention of their ordered nanostructure represents a nontrivial challenge. We report the development of a lyotropic liquid crystal mesophase that addresses the aforementioned issue. Films as thin as ∼200 nm are prepared on conventional support membranes using solution-based methods. Within these films, the system is composed of a hexagonally ordered array of ∼3 nm diameter cylinders of cross-linked polymer, embedded in an aqueous medium. The cylinders are uniformly oriented in the plane of the film, providing a transport-limiting dimension of ∼1 nm, associated with the space between the outer surfaces of nearest-neighbor cylinders. These membranes exhibit molecular weight cutoffs of ∼300 Da for organic solutes and are effective in rejecting dissolved salts, and in particular, divalent species, while exhibiting water permeabilities that rival or exceed current state-of-the-art commercial nanofiltration membranes. These materials have the ability to address a broad range of nanofiltration applications, while structure-property considerations suggest several avenues for potential performance improvements.
Thermoresponsive amphiphilic Pluronic F127 triblock copolymer solutions have been widely investigated in smart biomaterial applications due to the proximity of its critical gel temperature to human body temperature. Meanwhile, cellulose nanocrystals (CNCs) have quickly become the focus of many drug delivery and tissue engineering applications due to their biocompatibility, abundance, ability to conjugate with drug molecules, and superior rheological properties. Herein, we investigate the phase behavior and thermo-rheological properties of the composite hydrogels containing cellulose nanocrystals (up to 5% by weight) and the temperature responsive Pluronic F127. Our results revealed an unprecedented role of CNC network formation on micellization and gelation behavior of the triblock copolymer. Linear and nonlinear rheological analysis suggest that at low and moderate nanocrystal loadings (1–3% by weight), the composite gel remarkably becomes softer and deformable compared to the neat Pluronic F127 gels. The softening effect results from the disruption of the close packed micelles by the rodlike CNCs. At high concentrations, however, the nanocrystals form their own network and the micelles are trapped within the CNC meshes. As a result, the original (neat F127) hard-gel modulus is recovered at 4 to 5% nanocrystal loading, yet the composite gel is much more deformable (and tougher) in the presence of the CNC network. Our temperature sweep experiments show that the CNC addition up to 3% does not change the rapid thermal gelation of the F127 solutions; therefore, these composites are suitable for smart drug delivery systems. On the other hand, at higher CNC concentrations, abrupt viscosity transition is not observed, rather the composite gels smoothly thicken with temperature in contrast to thermal thinning of the aqueous neat CNC. Thus, they can be used as smartly adaptive biolubricants and bioviscostatic materials.
Charged functional groups are often incorporated onto the surface of nanofiltration (NF) membranes to facilitate the selective rejection of multivalent ions over monovalent ions. However, since fouling-resistant surfaces tend to be electrically neutral, the incorporation of charged functionality exacerbates membrane fouling. Multifunctional Janus membrane architectures, which incorporate chemically-distinct domains over their cross section, provide a strategy for balancing the competing demands associated with making fouling-resistant, ion rejecting NF membranes. Here, through the controlled exposure of poly(trifluoroethyl methacrylate-co-oligo-(ethylene glycol) methyl ether methacrylate-co-(3-azido-2-hydroxypropyl methacrylate)) copolymer substrates to a series of reactive solutions containing alkyne-terminated molecules, the process for creating dual-functional membranes using the copper(I)-catalyzed azide-alkyne cycloaddition (CuAAC) reaction was analyzed. Under the appropriate conditions, the CuAAC reaction propagates into the copolymer substrate as a front. This phenomenon results in a process for creating layered domains of distinct functionality whereby the distribution of anti-fouling zwitterionic moieties and ion rejecting sulfonate moieties can be modified by manipulating the exposure time. The ion rejection and fouling propensity for a family of dual-functional membranes was examined. For short initial reaction times, which introduced a thin anti-fouling layer on top of an ion rejection layer, the rejection of K2SO4, 87%, was comparable to the value for full charge control membranes, 90%. Moreover, when exposed to a fouling solution containing bovine serum albumin (BSA), these dual-functional membranes exhibited an 18% decline in normalized flux and recovered 99% of their flux upon rinsing with water. In comparison, the full charge membranes exhibited a 44% decline in normalized flux and recovered 65% of their flux upon washing. As such, the results demonstrate that the controlled functionalization process reported here is capable of balancing anti-fouling and ion rejection capabilities. Furthermore, the versatile nature of the click chemistry mechanism at the center of this process offers a means by which to design and fabricate multifunctional membranes for numerous future applications.
With the rapid growth of world population and water contamination, nanofiltration membranes with extraordinary permeance and high salt rejection are eminently desirable for addressing global water scarcity. Here, an innovative approach to design thin film composite (TFC) membranes with ultrahigh permeance via hydrogel assisted interfacial polymerization was reported. The hydrogel containing piperazine (PIP) monomers serving an aqueous phase in interfacial polymerization enables a homogeneous interfacial polymerization, reduces the diffusion rate of PIP monomers, and provides mechanical strength to the thin polyamide (PA) selective layer. As a result, the synthesized advanced TFC membranes have ultrahigh high permeance of 52.8 L m-2 h-1 bar-1 and 62.9 L m-2 h-1 bar-1 while maintaining a satisfactorily high rejection of 96.4 % and 93.5% for Na2SO4, demonstrating a desalination performance superior to PA TFC nanofiltration membranes reported so far. Moreover, the mechanism behind the exceptional high permeance is delineated by exploring theoretically hydrogel assisted interfacial polymerization through simulation. Most importantly, the fabrication process of Hydrogel-TFC membranes is simple, enabling a cost-efficient and scalable manufacturing.
Non-solvent induced phase separation (NIPS) is a frequently used technique for the production of polymeric membranes. It enables the production of membranes with a broad range of different characteristics. Current solvents used in membrane preparation are often toxic, environmentally unfriendly and prepared from non-sustainable resources. This is why a replacement of solvents like N-methyl-2-pyrrolidone (NMP) and dimethylacetamide (DMAc) is highly desirable. In order to substitute a solvent whilst achieving the same desired membrane properties, it is necessary to understand the formation mechanisms and its influencing factors. One important set of parameters for controlling the membrane features is the polymer solution composition. This is why the aim of this study was to improve the understanding of membrane formation by gaining a holistic picture of the influences of systematic additive variations, focusing on the comparison between conventional and alternative sustainable solvent systems. Thus, 72 different polyethersulfone (PES) membrane prototypes were produced by immersion precipitation from polymer solutions prepared in NMP and DMAc, as well as in the sustainable alternatives 2-pyrrolidone (2P) and dimethyllactamide (DML). In all four solvent systems varying concentrations and molecular weights of the polymeric additives polyvinylpyrrolidone (PVP) and polyethylene glycol (PEG) were applied. The viscosity of the polymer solutions was determined, and thereof formed membranes were analyzed in terms of permeability, protein retention, surface properties, mechanical stability and morphology. The results indicate that both, solvents and additives, significantly impact the membrane properties. It was shown that the influences of the additives on all investigated membrane features were strongly dependent on the applied solvent. The observed effects were similar for the conventional solvents NMP and DMAc, but differed from those found for the alternative solvents 2P and DML, which among themselves also showed comparable outcomes. In conclusion, this study proves that it is possible to obtain desired membrane properties with 2P or DML as long as the solution composition is chosen appropriately.
This work describes the kinetics of thermal polymerization in nanoconfined domains of lyotropic liquid crystal (LLC) templates by using chemorheological studies at different temperatures. We investigate lamellar and reverse hexagonal LLC phases with the same concentration of monomeric phase. Results show that the mesophase structures remain intact during thermal polymerization with very slight changes in the domain size. The polymerization rate decreases in the nanoconfined structure compared to the bulk state due to the segregation effect that increases the local monomer concentration and enhances the termination rate. Additionally, the polymerization rate is faster in the studied reverse hexagonal systems compared to the lamellar ones due to their lower degree of confinement. A higher degree of confinement also induces a lower monomer conversion. Differential scanning calorimetry confirms the obtained results from chemorheology.
Industrially, ultrafiltration (UF) membranes are produced through non-solvent induced phase separation (NIPS), but due to environmental hazards inherent to the NIPS process as well as the low surface porosity and fouling resistance of membranes produced via this method, an alternative route to UF membranes is desirable. This work presents the self-assembly of Pluronic block copolymers in the presence of water and a monomeric phase as a new technique for the preparation of UF membranes without the need for organic solvent or post-modification. Different compositions of block copolymer, water, and monomer were polymerized to obtain both hexagonal and lamellar mesostructures, as indicated by small angle X-ray scattering (SAXS) and cross-polarized light microscopy. As-synthesized membranes were found to have pore sizes in the range of 3–4 nm with a molecular weight cutoff of 1500 g/mol and displayed both excellent fouling resistance and high permeance of water, vastly outperforming a conventional NIPS UF membrane. Further, in contrast to NIPS, the proposed method provides flexibility in terms of both the final membrane chemistry and pore size. As such, it is a versatile approach that can be easily tailored to produce membranes for a wide range of applications including wastewater treatment and food processing.
We demonstrate the fabrication of a loose, negatively charged nanofiltration (NF) membrane with tailored selectivity for the removal of perfluoroalkyl substances with reduced scaling potential. A selective polyamide layer was fabricated on top of a polyethersulfone support via interfacial polymerization of trimesoyl chloride and a mixture of piperazine and bipiperidine. Incorporating high molecular weight bipiperidine during the interfacial polymerization enables the formation of a loose, nanoporous selective layer structure. The fabricated NF membrane possessed a negative surface charge and had a pore diameter of ~1.2 nm, much larger than a widely used commercial NF membrane (i.e., NF270 with pore diameter of ~0.8 nm). We evaluated the performance of the fabricated NF membrane for the rejection of different salts (i.e., NaCl, CaCl2, and Na2SO4) and perfluorooctanoic acid (PFOA). The fabricated NF membrane exhibited a high retention of PFOA (~90%) while allowing high passage of scale-forming cations (i.e., calcium). We further performed gypsum scaling experiments to demonstrate lower scaling potential of the fabricated loose porous NF membrane compared to NF membranes having a dense selective layer under solution conditions simulating high water recovery. Our results demonstrate that properly designed NF membranes are a critical component of a high recovery NF system, which provide an efficient and sustainable solution for remediation of groundwater contaminated with perfluoroalkyl substances.
Stimuli-responsive polymeric (SRP) materials undergo changes in their physical morphologies and chemical properties in response to small changes in their external environment, such as temperature, pH or light. When immobilised, SRP materials, fabricated in various formats and compositions, provide new opportunities for the separation of products generated by the biotechnology industry. This Current Opinion highlights the potential of these functional materials for the capture, purification and analysis of these products via batch capture methods, column chromatography or electrophoresis, drawing on break-through developments achieved particularly over the last five years.
In this work, stimuli-responsive, superabsorbent materials are created by copolymerization of stimuli-responsive poly(n-isopropyl acrylamide) (PNIPAM) in combination with superabsorbent sodium acrylate (SA) via photopolymerization in lyotropic liquid crystal (LLC) templates. Templating PNIPAM in LLC mesophases imparts nanostructure to the polymer that significantly increases transport and swelling when compared to isotropic hydrogels. These materials exhibit twice the equilibrium swelling of analogous non-templated materials and show a dynamic range between the swollen and deswollen state that is 5 times greater. To further augment this stimuli-responsive range, LLC-templated PNIPAM was combined with the superabsorbent monomer sodium acrylate (SA). As SA concentration is increased, significant structure changes are observed during polymerization that leads to less-defined nanostructure and lower stimuli response. Maximum swelling and temperature response are observed at low concentrations of SA (∼2 wt%). These LLC-templated copolymers exhibit stimuli-responsive volume transitions up to 40 times and equilibrium swelling ratios of 60 times their dry mass. This 600% increase in thermal response is due to the combined high swelling capabilities of SA with the enhanced thermal-response behavior induced by the LLC-templated nanostructure. Additionally, the nanostructure induces fast deswelling rates at temperatures above the lowest critical solution temperature of PNIPAM. The high dynamic range and quick response of this composition could allow for the development of superabsorbent, stimuli-responsive hydrogels in a variety of biomaterial, microfluidic, and water remediation applications.
We developed two-step thermoresponsive membranes that have two different lower critical solution temperatures. To achieve this, poly(2-dimethylaminoethyl methacrylate)-block-poly(N-isopropylacrylamide) (PDMAEMA-b-PNIPAM, PDN) was introduced into a polyethersulfone (PES) ultrafiltration membrane. The combined Fourier-transform infrared spectroscopy, nuclear magnetic resonance spectroscopy, attenuated total reflection Fourier-transform infrared spectroscopy, and field-emission scanning electron microscopy results showed that PDN was successfully synthesized, and introduced into the PES membrane. The hydration capacity changes showed that the PES membrane with introduced PDN (PES/PDN) was temperature responsive, and that its pore structure changed. Water flux and protein separation experiments showed that the permeation properties of the PES/PDN membrane effectively changed, and protein separation was achieved based on pore structure changes in response to temperature changes. In particular, PES/PDN containing 5% PDN compared to PES separated water and three types of protein with high efficiency via three clear separation steps, at 30, 50, and 70 °C. In addition, the thermoresponsive polymer block in the PES/PDN membrane improved the flux recovery ratio from 29% to 69%.
Smart materials with controllable responses to various stimulations offer considerable potential in many applications. Herein we report a simple and efficient method to fabricate ionic strength- and thermo-responsive polyethersulfone composite membranes using zwitterionic copolymers of sulfobetaine methacrylamide (SBMI) and sulfobetaine methacrylate (SBMA). The permeability of the modified membranes could be adjusted by changing the ionic strength varying from 0 mol/kg and 1.0 mol/kg and/or changing the surrounding temperature varying from 15 oC to 90 oC, and both the responses were reversible. The thermo-response transition temperature could be regulated by changing the chemical compositions of zwitterionic copolymers. Furthermore, the zwitterionic copolymer modified membranes exhibited excellent antifouling resistances to protein solutions and surfactant-stabilized oil-in-water emulsions. Ionic strength- and thermo-responsive flux, separation ability, together with excellent antifouling property endowed the membranes with wide applications in the field of separation.
Interrelationship between physicochemical properties and separation performance of nine commercial nanofiltration (NF) membranes was systematically investigated. Seven NF membranes, NT103 and NT102 from Microdyn-Nadir, DF90 and DF30 from OriginWater, NF70, NF40-I and NF40-II from Hangzhou Development Center of Water Treatment Technology, were reported for the first time. FTIR spectra demonstrated that the skin layer of NF90, NT103 and DF90 was made from fully aromatic polyamide, while other membranes were the semi-aromatic ones. The fully aromatic membranes had rougher, thicker and less hydrophilic polyamide layer as well as lower permeability than the semi-aromatic ones. When applying these membranes to concentrate the pretreated dairy wastewater, NF270, DF30 and NF40-I with high lactose rejection, low salt rentention low transmembrane pressure (TMP) and negligible irreversible fouling (IF) are preferable. Furthermore, it was found that the pore size and skin thickness dominated the membrane permeability. Meanwhile, the pore size and TMP produced a negligible effect on the IF, while the higher roughness and contact angle resulted in the higher IF, implying that the main fouling mechanism in this case is the foulants adsorption at membrane surface rather than pore blocking (affected by pore size) or cake formation (its compressibility was influenced by TMP). Moreover, the retentions of both lactose and salts by the tight NF membranes appeared to be governed by steric hindrance. Thus, this work provides a new method to study membrane separation and fouling mechanisms by correlating the physiochemical properties and separation performance of different membranes.
A temperature (T)-dependent coarse-grained (CG) Hamiltonian of polyethylene glycol/oxide (PEG/PEO) in aqueous solution is reported to be used in implicit-solvent material models in a wide temperature (i.e., solvent quality) range. The T-dependent nonbonded CG interactions are derived from a combined “bottom-up” and “top-down” approach. The pair potentials calculated from atomistic replica-exchange molecular dynamics simulations in combination with the iterative Boltzmann inversion are post-refined by benchmarking to experimental data of the radius of gyration. For better handling and a fully continuous transferability in T-space, the pair potentials are conveniently truncated and mapped to an analytic formula with three structural parameters expressed as explicit continuous functions of T. It is then demonstrated that this model without further adjustments successfully reproduces other experimentally known key thermodynamic properties of semi-dilute PEG solutions such as the full equation of state (i.e., T-dependent osmotic pressure) for various chain lengths as well as their cloud point (or collapse) temperature.
pH dependent polyethylene glycol/acrylic acid(AA/PEG) hydrogels of an anti-depressant, venlafaxine hydrochloride were prepared by free-radical co-polymerization method, using benzyl peroxide (BPO) as initiator and methylene bisacrylamide (MBA) as cross-linker. Different formulations were prepared with varying concentration of polymer, monomer and cross-linker to study their effect on gel swelling, diffusion characteristics and subsequent drug release. Structural parameters of AA/PEG hydrogels were investigated by measuring diffusion co-efficient, volume fraction of polymer in the swollen state, effective molecular weight of polymer chain between cross-linking points, the number of links between two crosslink and solvent interaction parameters. Swelling coefficients were found to be increased with increase in pH i.e. minimum at pH 1.2 and maximum at pH 7.5. Increase in acrylic acid concentration increases swelling while increase in cross-linker concentration has an opposite effect due to denser structure. AA/PEG hydrogels were characterized by fourier-transform infrared spectroscopy (FT-IR), scanning electron microscope (SEM), X-Ray diffraction (XRD), thermo gravimetric analysis (TGA) to determine the polymer structure, its morphology and strength. Drug release studies performed at pH 1.2, 5.5 and 7.5 shows higher release at higher pH. Drug release was increased by increasing acrylic acid concentration and decreasing cross-linker concentration.
A thin-film composite, bicontinuous cubic lyotropic liquid crystal polymer (TFC QI) membrane with uniform-size, ionic nanopores was studied for the treatment of hydraulic fracturing flowback water. The TFC QI membrane performance was compared to those of a commercial nanofiltration (NF) membrane (NF270) and a commercial reverse osmosis (RO) membrane (SW30HR) for the filtration of flowback water from the Denver-Julesburg Basin. The permeability, salt rejection, and organic solute rejection for each membrane was evaluated. The results illustrate that the TFC QI membrane maintained its performance to a similar degree as the commercial NF and RO membranes while demonstrating a unique selectivity not observed in the commercial membranes. Specifically, the TFC QI membrane rejected 75% of the salt while recovering 9.6% of the dissolved organic carbon (DOC) and 50% of the water. Of particular interest was the recovery of labile DOC, which was assessed through biodegradation experiments. Analysis following biodegradation of the TFC QI membrane permeate demonstrates the membrane's ability to recover labile DOC in a reduced-saline permeate. Improved recovery of labile DOC (increased to 22%) was demonstrated by reducing the pH of the flowback water. Therefore, the selectivity of the TFC QI membrane provides an opportunity to recover resources from hydraulic fracturing flowback.
A new method for determining the molecular weight cut-off (MWCO) of an organic solvent nanofiltration (OSN) membrane has been developed utilising poly(propylene) glycol (PPG) oligomers. This new MWCO method overcomes the limitations of the currently popular methods: namely the high molecule cost in the popular polystyrene method, the Donnan Exclusion effects when using dye molecules and the solvent compatibility and HPLC separation resolution limitations of the lesser used poly(ethylene) glycol (PEG) method. A new reverse phase high-performance liquid chromatography separation with evaporative light scattering detection (ELSD) allows the concentration of each oligomer of PPG to be accurately determined and from this the MWCO curves are constructed. The method has a high resolution (size increment of 58 g mol⁻¹ corresponding to the OCH(CH3)CH2 structural unit) and can be used in polar, polar aprotic, and non-polar solvents. The accuracy of the method has been demonstrated in three different solvents (methanol, acetone, and toluene) and 5 different OSN membranes (DuraMem® 150, 200, 500, PuraMem® 280 and StarMemTM 240). Other advantages include; oligomers of PPG are cheap and widely available, can probe a wide range of MWCO and provide high resolution MWCO curves. Consequently, it is proposed that that this method be adopted as a new standard MWCO test for OSN membranes.
The self-assembly of an end-functionalized PEO106-PPO70-PEO106 triblock copolymer (BCP) with acrylic end groups (Pluronic F127 diacrylate or FDA) in a protic ionic liquid (deuterated ethylammonium nitrate, dEAN) is studied using small-angle neutron scattering, shear rheology, and dielectric spectroscopy. This functionalization constitutes less than 1 wt % of the copolymer and has no effect on micelle formation in dEAN. The sol-gel transition and the supramolecular crystal structure of the concentrated FDA/dEAN solutions are also found to be unaffected by the acrylation of the end groups. Photo-cross-linking results in viscoelasticity enhancement of the FDA/dEAN solutions with FDA contents below the gel point. Cross-linking gels produces soft elastomers with exceptional elasticity and conductivity comparable to the un-cross-linked solutions. Tuning BCP microstructure via self-assembly in ionic liquids followed by chemical cross-linking is shown to be a promising route for the design of materials with specific mechanical properties and ionic conductivities.
Although multiple methods and materials have been developed to fabricate pH-responsive membranes, separating organics/inorganic salts mixtures and divalent/monovalent ions systems with these membranes still remains a challenging task. In this study, pH-responsive nanofiltration membranes (PCHMs) with tunable ion selectivity were first prepared with poly(carboxybetaine methacrylamide (CBMA) -co-N-(Hydroxymethyl) acrylamide) (PCHs) through surface coating and glutaraldehyde cross-linking method. The membrane ion selectivity towards divalent ions and monovalent ions can be well tuned by adjusting the pH of feeding solution. At pH 3.0, the retention of PCHM3 (CBMA content in PCHs is 48.8 mol%) to MgCl2 (92.9%) is much higher than that to NaCl (3.4%). And by adjusting the feed pH value to pH 10.0, the retention to Na2SO4 (91.8%) is much higher than that to NaCl (3.8%). While at neutral environment, PCHM3 allows the transport of the inorganic salts but blocks the transport of PEG800. It indicates that PCHMs could be a kind of high performance membranes for divalent/monovalent ions separation and organics/inorganic salts separation. Furthermore, the PCHMs possess a reversible and stable pH-responsive property, which is essential for practical charge-based separations. The responsiveness and adaptiveness of the PCHMs to the feeding solution make it being a versatile material for custom-designed membrane separations.
Nanofiltration (NF) membranes have been used previously for the recovery of dyes, salts, and water from textile wastewaters with high salinity. However, commercially available NF membranes have a high rejection for divalent salts (i.e., Na2SO4), substantially reducing the salt recovery and membrane flux when treating textile wastewater containing Na2SO4. In this study, a tight ultrafiltration membrane (UH004, Microdyn-Nadir) was proposed to fractionate the dye and Na2SO4 in the textile wastewater. The UH004 membrane with a molecular weight cutoff of 4700 Da provided complete passage of monovalent salts, with little rejection of Na2SO4. This significantly increases the filtrate flux that can be achieved with high-salinity wastewater since osmotic pressure and concentration polarization effects are minimized. Furthermore, the retention behavior of four different dyes was evaluated to determine the efficiency of this membrane process. This tight ultrafiltration membrane offered the high retention for direct dyes (i.e., direct red 80, direct red 23, and Congo red) and reactive blue 2. For instance, the UH004 membrane yielded >98.9% rejection for all of the dyes at a pressure of 4 bar even in the presence of 60 g·L−1 Na2SO4. Subsequently, an ultrafiltration-diafiltration process was designed to separate a dye/Na2SO4 aqueous mixture with 98% desalination efficiency and greater than 97% dye recovery after 5 diavolumes. These results clearly demonstrate that tight ultrafiltration membranes can be a stand-alone alternative to NF membranes for the effective fractionation of dye and Na2SO4 in the direct treatment of high-salinity textile wastewater.
Although protein fouling is one of the critical factors governing the effectiveness of many microfiltration processes, the underlying chemical and physical mechanisms that influence the initiation and growth of the fouling layer have not yet been clearly established. We have obtained data for the flux decline during the stirred cell microfiltration of bovine serum albumin (BSA) preparations with different physical and/or chemical characteristics through isotropic polyvinylidene fluoride microfiltration membranes. The initial fouling in this system was caused by the convective deposition of protein aggregates onto the membrane surface. Native (non-aggregated) BSA only fouled the membrane by chemical attachment to an existing protein deposit via the formation of intermolecular disulfide linkages. A mathematical model was developed to describe this dual-mode fouling process, with the model calculations being in very good agreement with the experimental data. These results provide important new insights into the physical and chemical interactions governing protein fouling during microfiltration.