Pore Size Distribution Of Membranes Research Articles

Chemically enhanced pretreatment (CEPT) to reduce irreversible fouling during the clean-in-place process for membranes operating under constant flux and constant pressure filtration

The goal of this study was to evaluate membrane degradation after 10 clean-in-place (CIP) cycles for polyvinylidene fluoride (PVDF) membranes exposed to chemically enhanced pretreated (CEPT) feed water under constant flux (CF) and constant pressure (CP) filtration modes. Cycles of filtration (20 min) and backwashing (3 min) were employed for both filtration modes for a period of 10 h, followed by the CIP process. The changes in the membrane pore size distribution, membrane symmetry, and bond structure were analyzed using the Brunauer-Emmett-Teller method, scanning electron microscopy, and Fourier-transform infrared radiation, respectively. Severe damage to the membrane pores and significant structural variation were observed with the use of untreated feed water, particularly under CF mode, due to the defluorination and oxidation of the membrane arising from the action of the residual inorganic foulants and cleaning chemicals. Membranes exposed to CEPT feed water exhibited a recovery of 79 % and 83 % of the initial membrane flux under CF and CP modes, respectively at end of study. Interestingly, a decrease in irreversible fouling of 16.7 % was observed for CEPT feed water, with fewer inorganic foulants (Al and Mn) on the membrane surface. The fouling propensity was 14.54 % and 14.78 % lower when using CEPT feed water under CP and CF modes.

A nanofiltration membrane with outstanding antifouling ability: Exploring the structure-property-performance relationship

Membrane fouling is inevitable during nanofiltration of industrial liquids. Understanding the influence of membrane physicochemical properties and structures on its antifouling performance can guide the membrane selection and preparation. In this study, the antifouling ability of three commercial nanofiltration membranes (NF270, NFA4 and DK) was assessed using organic foulants with different charge patterns. Besides, the pristine and fouled membranes were systematically characterized. The results showed that although the negatively charged polyamide membrane had a strong antifouling ability to negatively charged substances, the fouling extent was still affected by the membrane pore size distribution. For the small foulants with positive charge, driven by electrostatic adsorption effect, they were easier to be anchored on the membrane surface with higher roughness. Due to the synergistic effect of membrane charge and roughness, the flux decay ratio increased from 5.1% for the NFA4 to 63.1% for the DK with more negative charge and highest roughness. Moreover, according to FTIR, XPS deep profiling and SEM cross-section analysis, it was found that the structure of the separation layer (e.g., single/double layer or cross-linking density in horizontal/longitudinal) closely related to its antifouling performance. Therefore, a nanofiltration membrane with narrow pore size distribution, moderate charge, smooth and hydrophilic surface is more prone to resist organic fouling formation. The outcomes of the work also offer several strategies to prepare antifouling nanofiltration membranes.

Narrowing the pore size distribution of polyamide nanofiltration membranes via dragging piperazines to enhance ion selectivity

Traditional polyamide nanofiltration membranes often can't obtain ideal ion selectivity due to their wide pore size distribution. In this work, a β-cyclodextrin modified sodium hyaluronate (HA-CD) was introduced to achieve the dragging effect of piperazine by non-specific hydrogen bond interaction between HA-CD and substrate and piperazine, thus realizing the purpose of sustained release of piperazine. The MD simulation results demonstrated that the diffusion rate of piperazine was reduced by approximately two orders of magnitude due to the interaction effect. In addition to making the polyamide layer thinner and smoother, HA-CD also narrows the pore size distribution. The salt filtration test showed that the prepared membrane has excellent anion selectivity (αmax:114) and permeability (10.66 L m−2 h−1 bar−1). Long-term filtration and nano-scratch test results demonstrated that the prepared membranes have good operational stability (the permeability only decreased about 30%) and mechanical strength (the interface bonding strength increased about 270%). By exploring the effect of the interaction between HA-CD and substrate and piperazine on the interfacial polymerization process, which proved the feasibility of realizing high permeability selectivity and stability of the membrane by constructing the functional bridge between substrates and aqueous monomers.

Study on the performance of CO2 capture from flue gas with ceramic and PTFE membrane contactors

Excessive CO2 emissions contribute to global warming, which may cause potential threat to survival of humans and other creatures, and it is necessary to take effective measures to curb CO2 emissions. Gas-liquid membrane contact CO2 separation technology is an emerging CO2 capture technology that combines advantages of membrane separation and chemical absorption. In this paper, differences of CO2 capture performance between ceramic and polytetrafluoroethylene (PTFE) membranes with pore size of 10 nm and 100 nm are investigated comprehensively. The morphological structure, pore size distribution, wettability and membrane flux of ceramic and PTFE membranes are analyzed by membrane characterization. Moreover, with monoethanolamine (MEA) as absorbent, CO2 capture performance under different operating parameters (e.g., absorber flow rate and temperature, gas flow rate and pressure) is investigated through experiments based on ceramic and PTFE membrane modules. The CO2 capture efficiency of ceramic and PTFE membrane is 94.7% and 99.3%, respectively. However, the mass transfer rate of ceramic membrane is much higher than that of PTFE membrane. The results show that ceramic membrane exhibits favorable CO2 capture performance under appropriate operation parameters. This paper will provide theoretical and technical references for subsequent industrial CO2 capture applications.

Designing of amino silica covalently functionalized carboxylic multi-wall carbon nanotubes-based polyethersulfone membranes for enhancing oily wastewater treatment

The accumulation of oily wastewater is a critical issue that can cause environmental pollution. Membrane technology can play a vital role in the treatment of oily wastewater due to its productivity and economical cost. In this work, the amino silica covalently functionalized carboxylated multi-walled carbon nanotubes (MWCNTs-COOH) nanohybrid was synthesized and blended with polyethersulfone (PES) membranes for oil/water separation to enhance the antifouling property. Membranes were fabricated with various loadings of SiO2-f-MWCNTs nanohybrid (0–2 wt%) using phase inversion technique. The incorporation of nanohybrid resulted in the enhancement of wettability and pore size distribution of nanocomposite membranes. The morphological structure, surface and chemical features of the synthesized membranes were examined using scanning electron microscopy, Fourier transform infrared spectroscopy, water contact angle and X-ray diffraction analyses. Furthermore, the impact of nanohybrid blending with PES membrane on water permeability and oil rejection was investigated. The water permeability enhanced from 188 ± 8 L/m2.h.bar, for the pristine PES membrane, to 293 ± 9 L/m2.h.bar for nanocomposite membrane blended with 2 % SiO2-f-MWCNTs nanohybrid. Similarly, the oil rejection was improved up to 97 ± 0.1 % using the same nanohybrid loading. The results revealed that using 2 % nanohybrid loading has increased the humic acid (HA) flux by 16 %, while the irreversible HA fouling was significantly reduced to 0.9 % compared to 9.9 % for the pristine PES.

Facile plasma grafting of zwitterions onto nanofibrous membrane surface for improved antifouling properties and filtration performance

To improve the permeability and antifouling properties of filtration membrane, zwitterionic sulfobetaine methacrylate (SBMA) was introduced onto the surface of ethylene vinyl alcohol copolymer (EVOH) nanofibrous membrane (NFM) via facile spray-coating followed by plasma treatment method. Surface chemical compositions, morphology, pore size distribution, roughness, zeta potential, hydrophilicity, permeability, separation performance, antifouling and mechanical properties of the pristine EVOH nanofibrous membrane, plasma treated membrane (PNFM) and the SBMA modified membrane (PZNFM) were systematically characterized and compared. Due to the improved hydrophilicity and antifouling properties, PZNFM showed higher water permeability (i.e. 9426.7 L h−1 m−2, 0.2 MPa), lower bovine serum albumin (BSA) adsorption (i.e. 0.15%), and higher flux recovery ratio (i.e. 95.65%) compared with the original NFM. Meanwhile, the newly prepared zwitterionic membrane can separate and intercept of E. coli, S. aureus and Brevundimonas dimimuta absolutely. The mutiphysical-chemical properties of the membrane allow it to function with synergy between permeability and sterile filtration efficiency for further bacterial separation and interception application. Our work provides a simple and scalable method to fabricate sterile membrane with outstanding performance, highlighting its great potential in sterile filtration.

Use of the Dispersion Coefficient as the Sole Structural Parameter to Model Membrane Chromatography.

The characterization and modelling of membrane chromatography processes require the axial dispersion coefficient as a relevant and effective intrinsic property of porous media, instead of arbitrary assumptions on pore size distribution. The dispersion coefficient can be easily measured by experiments completely independent of chromatographic tests. The paper presents the prediction of experimentally obtained breakthrough curves using B14-TRZ-Epoxy2 membranes as a test case; the mathematical model implemented is based on the use of the experimentally measured axial dispersion coefficient as an input parameter. Application of the model and its comparison with the data demonstrate that alternative ways of explaining the shape of breakthrough curves, based on unverified assumptions about the membrane pore size distribution, are not feasible and not effectively supported by experimental evidence. In contrast, the axial dispersion coefficient is the only measurable parameter that accounts for all the different contributions to the dispersion phenomenon that occurs in the membrane chromatography process, including the effects due to porous structure and pore size distribution. Therefore, mathematical models that rely on the mere assumption of pore size distribution, regardless of the role of the axial dispersion coefficient, are in fact arbitrary and ultimately misleading.

Effect of synthesis conditions on the non-uniformity of nanofiltration membrane pore size distribution

The uniformity of membrane pore sizes, which is essentially determined by the membrane synthesis conditions, significantly affects the rejection performance of nanofiltration (NF) membranes. In this study, we applied two modeling methods, i.e., the DSPM (Donnan Steric Pore Model) and the log-normal distribution methods, for the determination of the average membrane pore size and pore size uniformity of lab-made NF membranes. The synthesis conditions included concentration of monomers (e.g., piperazine and trimesoyl chloride), (thermal) curing temperature and time, and activation solvent type and duration. Results showed that both high piperazine (PIP) concentration (≥0.5 wt%) and curing temperature (≥40 °C) could enhance the membrane pore size uniformity. Although the average membrane pore size calculated by the DSPM method was higher than that by the log-normal distribution method, they significantly correlated. It appears that the log-normal distribution method could more directly characterize membrane pore size uniformity. Obviously, the pore uniformity of NF membranes affected the rejection of small molecules, such as trace organic compounds. These insights provided a theoretical foundation for the characterization of membrane pore size distribution with more accuracy and the fabrication of membranes with higher pore size uniformity. • Two different modeling methods were used to characterize membrane pore size uniformity. • Thermal curing could improve membrane pore size uniformity while solvent activation generally not. • The average pore radius determined by the two methods significantly correlated, but pore size uniformity not. • Membrane pore size non-uniformity substantially decreased the TrOCs rejection performance.

Examination of the bubble gas transport method to estimate the membrane pore size distribution

The pore size distribution (PSD) of the membrane is often measured by the bubble gas transport method. However, the method is not free from some concerns over its accuracy. In the present work, two questions are raised; 1) the validity of the computational procedure commonly used to calculate PSD 2) the validity of the assumption that all pores are open to gas transport in the linear part of the flow rate versus pressure curve. To answer these questions, the flow rate versus pressure lines were generated rigorously based on a given PSD by the model that we have recently developed [1], and using the lines so produced, the PSD was back-calculated. It was found that the agreement of original PSD and the back-calculated PSD is reasonable when the PSD is narrow, but when the PSD is broad, the back-calculated PSD shifts toward the larger pore sizes. It was also found that the calculated PSD is slightly affected when some small pores are not included in the linear part of flow rate versus pressure wet-line.

Electrospun nanofibrous membranes as promising materials for developing high-performance desalination technologies

Rising population, together with rapid industrial development, are making freshwater shortage an ever-increasing crisis globally. In addition to the conventional strategy such as water reuse, desalination has proven to be an effective method to meet this requirement, especially membrane-based desalination technology. Besides the conventional reverse osmosis (RO) and nanofiltration (NF) desalination, other membrane-based methods such as membrane distillation (MD) and forward osmosis (FO) have also been exploited as promising alternatives. For all the membrane-based desalination, performance of the membrane is the key issue. However, the currently used membranes for the main desalination technologies are all faced with some challenges, e.g., low permeation for RO, low selectivity for NF, wetting for MD, and internal concentrative polarization for FO. Thus, researchers are all pursuing more advanced membranes to meet these challenges. Particularly, developing novel membranes is a promising alternative. Electrospun nanofibrous membranes (ENMs) provide an alternative for various membrane-based desalination thanks to their versatility in tuning membrane morphology, structure, composition, fiber diameter, pore size and pore size distribution, and fiber/membrane functionalization. In this review paper, we discussed the application of ENMs in typical membrane desalination process presented by RO, NF, MD, and FO. We emphasize on the usage of superiorities of ENMs in tackling the problems of the currently used membranes such as low membrane permeability, membrane wetting, and internal concentrative polarization. We also proposed the future directions of ENMs development in developing more advanced membranes for desalination.

Three-dimensional printing of high-flux ceramic membranes with an asymmetric structure via digital light processing

In this study, a novel method was proposed for preparing high-flux ceramic membranes via digital light processing (DLP) three-dimensional (3D) printing technology. Two different alumina powders were well dispersed in a photosensitive resin to form a UV-curable slurry for DLP 3D printing. The effects of the grading ratio on the viscosity of the slurry and the porosity, pore size distribution, mechanical strength, roughness, and permeability of the ceramic membranes were systematically investigated. The thermal treatment conditions were also studied and optimized. The obtained ceramic membranes exhibited a uniform pore size distribution, a high porosity, a low tortuosity factor, and an asymmetric structure. The combination of these factors led to a high flux for the 3D-printed ceramic membranes. DLP 3D printing exhibited a good potential to be a strong candidate for the next generation of ceramic membrane fabrication technology.

Enhancement of Physical Characteristics of Styrene-Acrylonitrile Nanofiber Membranes Using Various Post-Treatments for Membrane Distillation.

Insufficient mechanical strength and wide pore size distribution of nanofibrous membranes are the key hindrances for their concrete applications in membrane distillation. In this work, various post-treatment methods such as dilute solvent welding, vapor welding, and cold-/hot-pressing processes were used to enhance the physical properties of styrene–acrylonitrile (SAN) nanofiber membranes fabricated by the modified electrospinning process. The effects of injection rate of welding solution and a working distance during the welding process with air-assisted spraying on characteristics of SAN nanofiber membranes were investigated. The welding process was made less time-consuming by optimizing system parameters of the electroblowing process to simultaneously exploit residual solvents of fibers and hot solvent vapor to reduce exposure time. As a result, the welded SAN membranes showed considerable enhancement in mechanical robustness and membrane integrity with a negligible reduction in surface hydrophobicity. The hot-pressed SAN membranes obtained the highest mechanical strength and smallest mean pore size. The modified SAN membranes were used for the desalination of synthetic seawater in a direct contact membrane distillation (DCMD). As a result, it was found that the modified SAN membranes performed well (>99.9% removal of salts) for desalination of synthetic seawater (35 g/L NaCl) during 30 h operation without membrane wetting. The cold-/hot-pressing processes were able to improve mechanical strength and boost liquid entry pressure (LEP) of water. In contrast, the welding processes were preferred to increase membrane flexibility and permeation.

Study on pore size distributions of microporous polymer membranes having different physical architecture using capillary flow porometry

Capillary flow porometry (CFP) is an emerging technique to measure the most constricted part of the through pores in membranes. In our previous work, the technique was successfully tested for track-etch membranes, which are model membranes with straight cylindrical pores. In this work, this technique has been used to measure the mean pore size, smallest pore size, and bubble point pore size for commercially available microporous membranes such as poly(propylene), poly(ethersulfone), poly(tetrafluoroethylene), and poly(vinylidene fluoride). The scanning electron microscopy images revealed that these membranes have different microporous architecture with no well-defined pores in these membranes. The study is important because the pore architecture in these membranes is not straight and cylindrical, as assumed by CFP. Also, the influence of different parameters such as the shape factor, wetting liquid, and tortuosity on the performance of capillary flow porometer has been studied. It is seen that it is important to incorporate shape factor for CFP measurements of membranes with such pore architecture for obtaining the reliable results. The studies with different wetting liquids (Porefil, dodecane, and 1-decanol) show that for the membranes studied, dodecane can serve as a wetting liquid to obtain information about membrane pore sizes, without much effect on the results. The technique has been applied to measure the pore size distribution of highly cross-linked poly(vinylbenzylchloride) pore-filled membranes synthesized in our laboratory.

Analysis of tradeoffs between purification factor and yield for high-performance countercurrent membrane purification for protein separations.

High-performance countercurrent membrane purification (HPCMP) has recently been presented as a new approach for protein separations, exploiting differences in diffusive transport across a semipermeable membrane to achieve high selectivity for protein separations. This study presents a set of design equations and diagrams that describe the tradeoff between the yield and purification factor in HPCMP processes in terms of two parametric variables: the diffusive membrane selectivity and the ratio of the draw to bulk solution flow rates. Conditions are identified that provide the high yields and purification factors of interest in bioprocessing. In addition, hydrodynamic models for solute transport were used to evaluate the selectivity as a function of the membrane pore size distribution for purely size-based separations. Model calculations demonstrate that diffusive transport provides significantly greater selectivity than traditional pressure-driven membrane separations for the same pore size distribution due to differences in hindered transport rates for diffusion and convection. These results provide a framework that can be used for the development of HPCMP processes for highly selective protein separations.

Improved separation efficiency of polyamide-based composite nanofiltration membrane by surface modification using 3-aminopropyltriethoxysilane

• Separation efficiency of PA NF membrane was improved by modification with APTES. • Modification was fulfilled via surface cross-linking and in-situ condensation. • Homogenized pore size and reduced mean pore diameter were achieved by modification. • Removal efficiency to both cationic and anionic dye aqueous solutions was improved. • Anti-fouling resistances to model foulants CTAB, HA and BSA were enhanced. Surface modification has been adopted to tune surface property of nanofiltration membrane for improved separation performance. However, it has rarely been explored to modulate membrane pore size distribution. Here, we reported a novel modification strategy combined with surface cross-linking and in-situ condensation for fabricating composite nanofiltration membrane with reduced and homogenized pore size. The strategy was fulfilled by soaking the surface of nascent polyamide-based membrane with an aqueous solution of 3-aminopropyltriethoxysilane (APTES). Membrane physico-chemical property analyses demonstrated that the hydrolyzed APTES molecules bonded onto membrane surface through amidation and esterification reactions with the residual acyl chloride groups, and chemically combined with each other through condensation reaction. Modification was found to hydrophilize membrane surface, homogenize pore size and reduce mean pore diameter. The modification under desired conditions led to an improvement in pure water permeance from 12.2 to 14.0 l/m 2 h bar, a decrease in geometric mean pore diameter from 0.71 nm to 0.63 nm, and a decline in geometric standard deviation from 1.32 to 1.23. Permeation test results showed that the APTES-modified membrane also exhibited improved removal performance to methylene blue and cresol red removal and enhanced antifouling resistances to model foulants cetyltrimethylammonium bromide, humic acid and bovine serum albumin.

Ionic liquid regulated interfacial polymerization process to improve acid-resistant nanofiltration membrane permeance

In order to enhance the permeance of the acid resistance membrane, an ionic liquid (IL) regulating strategy was proposed to rearrange the interfacial polymerization process. Herein, we revisited polyethylenimine (PEI) and cyanuric chloride (CC) as the pristine acid-resistant membrane. 1-aminopropyl-3-methylimidazolium chloride ([AEMIm][Cl]) and 1-aminopropyl-3-methylimidazolium bis((trifluoromethyl)sulfonyl)imide ([AEMIm][Tf2N]) ILs were used to regulate the interfacial polymerization process. Molecular dynamic (MD) simulation was used to reveal IL and PEI diffusion behavior in aqueous solution, membrane pore size distribution, and porosity. The effects of ILs on membrane surface morphology, surface zeta potential, chemical composition, and separation property were analyzed. The IL regulating strategy endow the membrane with uniform smaller pore and higher porosity, thus improved membrane permeance and selectivity simultaneously. For the [AEMIm][Cl] IL, the corresponding AEMIC-PEI-CC membrane showed high permeance of 79.1 L m−2h−1bar−1, which is 1.36 times the pristine PEI-CC membrane, combined with high Y3+ rejection of 97.5%, low H+ rejection of 1.35%. In addition, this membrane showed good acid stability in 30 days long-term test.

A reverse approach to evaluate membrane pore size distribution by the bubble gas transport method using fewer experimental data points

Membrane technologies have been successfully developed as the separation and purification processes, such as desalination, wastewater treatment, gas separation and so forth. It is known that membrane pore size and pore size distribution (PSD) are the factors by which membrane performance is significantly affected. In this study, a novel mathematical model is presented to evaluate the PSD. Although the new model is based on the bubble gas transport method, it is different from the conventional approach since this model starts from the assumption of a distribution function, typically Gaussian normal distribution, for the PSD, and a flow rate versus pressure line is drawn theoretically based on the assumed PDS. Then, the mean pore size and standard deviation of the PSD are optimized by the best fit of the theoretical line to the experimental one using the nonlinear regression analysis. A unique feature of the proposed model is that fewer experimental data points are necessary to find the optimal PSD. In addition, experiments with the dry membrane are no longer required. The PSDs estimated by this model for three commercial membranes were compared with those obtained by the conventional method, and an excellent agreement was observed.

Calcium alginate and barium alginate hydrogel filtration membrane coated on fibers for molecule/ion separation

Calcium alginate (CaAlg) hydrogel has good anti-pollution performance. However, the self-supporting CaAlg hydrogel filtration membrane is difficult to be batch prepared due to its poor mechanical properties. In this paper, polyester (PET) non-woven fabric, polypropylene (PP) non-woven fabric and polyhydroxybutyrate (PHB) nanofibers were used as support layer to prepare CaAlg composite membranes. A series of studies proved that CaAlg-PET membrane showed higher stability and flux, which revealed that PET non-woven fabric was the best carrier among the three fiber carriers to enhance the various properties of hydrogel. Methyl blue (MB) and sodium chloride (NaCl) were used as the representatives of molecules and ions, respectively. The effects of concentration, operating pressure and temperature on the separation performance of MB/NaCl by CaAlg-PET membrane were investigated. The flux of CaAlg-PET membrane was 19.3 L·m−2·h−1·bar−1, and the membrane showed a high selectivity of 183.4 to MB/NaCl mixture (99.5% rejection of MB and 8.3% rejection of NaCl). Moreover, Barium ion was used to cross-link sodium alginate to prepare barium alginate (BaAlg) hydrogel coated composite membrane (BaAlg-PET) using PET fiber as the carrier. The BaAlg-PET membrane exhibited higher strength, stability, flux and better separation performance than CaAlg-PET membrane. BaAlg-PET membrane exhibited a high flux of above 35.5 L·m−2·h−1·bar−1, and the molecular ion selectivity was 306.3 (99.7% rejection of MB and 8.1% rejection of NaCl). Membrane pore structure and pore size distribution of BaAlg hydrogel membrane were generated by molecular dynamic (MD) simulations. In sum, this work provided a green, economical and expandable new idea and membrane material for the separation of molecules and ions.

Tailoring pore structure and surface chemistry of microporous Alumina-Carbon Molecular Sieve Membranes (Al-CMSMs) by altering carbonization temperature for optimal gas separation performance: An investigation using low-field NMR relaxation measurements

In this work, we applied low-field, NMR spin–lattice measurements to evaluate for the first time the effect of carbonization temperature (range 600–1000 ℃) on the preparation of Alumina-Carbon Molecular Sieve Membranes (Al-CMSMs), providing new insights into intra-pore fluid interactions. The results show that the average Al-CMSM pore size generally increases with carbonization temperature whilst the hydrophilicity of the pore surface, and the amount of strongly adsorbed H2O, decreases with an increasing carbonization temperature. As such, lower carbonization temperatures produce more hydrophilic membranes, with further evidence provided by FTIR measurements demonstrating the presence of polar functional groups on the surface, with water interacting more strongly with the membrane surface, as evidenced by NMR.It was found that the Al-CMSM carbonization temperature significantly affected permeance and H2O/CH4 permselectivity by altering the membrane pore size distribution and pore hydrophilicity. H2O permeance values are seen to be up to 100 times larger than respective CH4 permeance values. The greater permeance of H2O is attributed to the larger kinetic diameter of CH4 relative to H2O and the adsorption of water in the hydrophilic pores enhancing the adsorption-diffusion transport mechanism. Optimal water permeation temperatures are thus higher for the more hydrophilic membranes, obtained at lower carbonization temperatures, as more energy is required to remove strongly adsorbed water blocking the pores. At higher carbonization temperatures, the Knudsen diffusion mechanism of permeance dominates over the adsorption-diffusion mechanism thereby reducing permeance as diffusion slows due to collisions between gas molecules and the pore walls. CH4 permeation always occurs via Knudsen diffusion with CH4 permeance increasing with permeation temperature due to the increased rate of CH4 diffusion.

Targeted modification of polyamide nanofiltration membrane for efficient separation of monosaccharides and monovalent salt

Separation of monosaccharides and monovalent salts is important in the biorefinery and food industry. Nanofiltration (NF) technology is promising for this purpose but its poor selectivity still needs to be addressed. In this work, the separation of monosaccharides and monovalent salts by NF is improved by regulating the surface charge and pore size distribution of polyamide NF membranes. The carboxyl groups on the polyamide membrane are selectively activated by N-(3-Dimethylaminopropyl)-N′-ethyl carbodiimide (EDC) and N-hydroxy succinimide (NHS), and subsequently molecules with amino groups are grafted on the membrane to reduce the effective pore size and electronegativity. The conditions and grafted molecules are optimized, and polyethyleneimine (600 Da) is selected as the best for enhancing the separation of glucose/fructose and KCl. Such targeted modification is found to reduce the effective mean pore size while maintaining the porosity of the NF270 membrane, due to pore segmentation. This results in a remarkable improvement in glucose/fructose rejection (from 67.96% to 84.14%) and separation factor (from 2.20 to 6.78), with only a 4.70% permeability loss. The modified membrane also maintains separation performance in crossflow filtration and after alkaline cleaning (pH 12), which outperforms the pristine NF270 and those modified by mussel-inspired coating and simple physical adsorption.