Membrane Pore Size Research Articles

Shedding light on the transfer of tetracycline in forward osmosis through experimental investigation and machine learning modeling

Tetracycline in wastewater can pose adverse impacts on the environment and human health. Forward osmosis (FO) is a promising method to reject antibiotics due to its low energy demand and high rejection rate. Tetracycline rejection during FO is a complicated process. Mechanistic models have been developed to describe antibiotic rejection by the FO membrane under ideal conditions but cannot be applied to real wastewater. Herein, the effects of draw concentration, pH, and solute type on the fate of tetracycline during FO were investigated by combining experimentation, factor analysis, and artificial neural network (ANN) modeling. High draw concentrations led to high convection that favored tetracycline diffusion. Low draw pH helped reject antibiotics potentially due to the decreased tortuosity and pore size of the FO membrane. When different draw solutes were tested, both convection and electrostatic interaction exerted effects on tetracycline retention on the FO membrane surface, and steric hindrance could further affect the amount of tetracycline in the draw solution. Exploratory factor analysis (EFA) showed that tetracycline rejection was a combined result of convection, steric hindrance, and electrostatic interactions. Path analysis revealed the significant roles of initial conductivity and draw pH in tetracycline rejection. Eight representative input variables were selected from 13 observed explanatory variables using redundancy analysis (RDA), based on which an ANN was trained and successfully predicted tetracycline diffusion and transfer through the FO membrane. These results have provided practical and predictive insights in the development of FO processes for efficient treatment of pharmaceutical wastewater.

Superior separation of industrial oil-in-water emulsions utilizing surface patterned isotropic PES membranes

Developing polymeric membranes for oily wastewater treatment has attracted great interest over the last few years. However, the low permeance and poor fouling resistance are considered the most challenging issues. Introducing topographic patterns on the membrane surface to increase active area and surface roughness is one practical strategy for reducing membrane fouling. Here, isotropic polyethersulfone (PES) membranes with line patterns and continuous sponge-like porous structure were prepared using phase separation micro-molding processes. Various techniques were used to characterize the morphologies and physicochemical properties of the as-prepared membranes. The surface wettability of the membranes was evaluated by measuring contact angle of a water drop in air and of industrial crude oil under water. The as-prepared membranes separation and antifouling performance were assessed using water permeance, oil rejection, and flux recovery ratio after emulsion filtration. The results showed that the patterned PES membrane has superior hydrophilic and antifouling capabilities, with a lower static water contact angle (CA) of 32° as compared to a flat PES membrane (CA = 39°). Furthermore, the patterned membrane had pure water permeance (PWP) of 8036 ± 65 L·h−1·m−2·bar−1, which was ∼ 68 % greater than that of the flat PES membrane. This is mainly attributable to the discernible increase in the membrane pore size distribution and the enhancement in membrane surface hydrophilicity due to the increase in surface roughness as a result of micro-patterning. The patterned membranes showed superior separation efficiency (>99.0 %) and permeance (7031 ± 65 L·h−1·m−2·bar−1) for the oil-in-water emulsion of 0.1 g·L−1. At a higher concentration of crude oil-in-water emulsion (50 g·L−1), the patterned PES membranes achieved promising permeance of 90 L/h.m2.bar and 99.9 % rejection. Furthermore, the flux recovery ratio of the patterned membrane exceeded 91 % after three consecutive cycles of dead-end filtration and backwashing with water, demonstrating the benefit of using micro-patterned membranes for treating high concentration and industrial oil-in-water emulsions.

Preparation and characterization of fiber braided tube reinforced polyethylene hollow fiber membranes via thermally induced phase separation

In wastewater treatment applications, mechanical strength of the membranes often cannot meet practical requirements due to the disturbance of aeration flow or the damage of high-pressure cleaning. In this work, fiber braided tube reinforced polyethylene (BR PE) hollow fiber membranes with desirable filtration performance and high tensile strength were successfully prepared via fiber braided tube reinforcement (BR) and thermally induced phase separation (TIPS) methods. Especially, the effect of heat treatment temperature on structure and properties of the membranes was discussed. The results exhibited that tensile strength and interfacial bonding state between the surface separation layer and the fiber braided tube reinforcement were improved with the increase of heat treatment temperature. While the heat treatment temperature was 130 °C, BR PE hollow fiber membranes exhibited favorable interfacial bonding state, and tensile strength was up to 204.4 MPa, which was significantly higher than that of previously reported PE membranes. Moreover, the increase of heat treatment temperature could decrease the membrane pore size and improve filtration performance. Comprehensive consideration, the optimal heat treatment temperature of BR PE hollow fiber membranes was 110 °C. This work can provide scientific and practical basis for large-scale development of BR hollow fiber membranes with good comprehensive performance.

Modification of Cellulose Acetate Microfiltration Membranes Using Graphene Oxide-Polyethyleneimine for Enhanced Dye Rejection.

Pressure-based membrane processes represent excellent water resource recovery prospects from industrial waste streams. In contrast with conventional pretreatment technologies, studies have shown that membrane pretreatment applications, such as microfiltration (MF), are more cost-effective and improve the results of the overall treatment processes. Hence, enhancing rejection efficiency of MF will enhance the performance of any downstream treatment processes. In this study, 0.45 µm cellulose acetate (CA) microfiltration membranes were modified by vacuum filtration-assisted layer-by-layer deposition of bilayers composed of negatively charged graphene oxide (GO) and positively charged polyethyleneimine (PEI). The performance of 1-, 2-, and 4-bilayer GO-PEI-modified membranes were investigated for their dye-rejection of anionic eriochrome black T (EBT) dye and cationic methylene blue (MB) dye in a cross-flow membrane module. As the number of bilayers on the membrane increased, the membrane thicknesses increased, and the deionized (DI) water flux through the membranes decreased from 4877 LMH/bar for the control (no bilayer) membrane to 2890 LMH/bar for the 4-bilayer membrane. Conversely, the dye-rejection performance of the modified membranes increased as increasing bilayers of GO-PEI deposited on the membranes. The anionic EBT dye saw superior rejection (~90% rejection) compared to the cationic MB dye (~80% rejection), which can be attributable to the electrostatic repulsion between the negatively charged GO surface and anionic EBT dye. After 50% recovery of the saline and dye-laden feed water, there was an observed drop in DI water fluxes of ~40-41% and 36%, respectively. There was also a slight increase in EBT dye-rejection during the composite feed-water experiments, attributed to the precipitation of salts on the membrane feed side or pore spaces, which subsequently reduce the membrane pore sizes.

Poly(vinylidene fluoride) membrane with immobilized TiO2 for degradation of steroid hormone micropollutants in a photocatalytic membrane reactor

The lack of effective technologies to remove steroid hormones (SHs) from aquatic systems is a critical issue for both environment and public health. The performance of a flow-through photocatalytic membrane reactor (PMR) with TiO2 immobilized on a photostable poly(vinylidene fluoride) membrane (PVDF-TiO2) was evaluated in the context of SHs degradation at concentrations from 0.05 to 1000 µg/L under UV exposure (365 nm). A comprehensive investigation into the membrane preparation approach, including varying the surface Ti content and distribution, and membrane pore size, was conducted to gain insights on the rate-limiting steps for the SHs degradation. Increasing surface Ti content from 4 % to 6.5 % enhanced the 17β-estradiol (E2) degradation from 46 ± 12–81 ± 6 %. Apparent degradation kinetics were independent of both TiO2 homogeneity and membrane pore size (0.1–0.45 µm). With optimized conditions, E2 removal was higher than 96 % at environmentally relevant feed concentration (100 ng/L), a flux of 60 L/m2h, 25 mW/cm2, and 6.5 % Ti. These results indicated that the E2 degradation on the PVDF-TiO2 membrane was limited by the catalyst content and light penetration depth. Further exploration of novel TiO2 immobilization approach that can offer a larger catalyst content and light penetration is required to improve the micropollutant removal efficiency in PMR.

Electrocatalytic ultrafiltration membrane reactors designed from dry-spun self-standing carbon nanotube sheets

The development of electrochemically active ultrafiltration membrane reactors offers promising perspectives to achieve simultaneous separation and degradation of persistent organic pollutants and support triggered self-cleaning of membrane materials upon surface fouling. Here, electro-responsive ultrafiltration membranes were synthesised from drawable carbon nanotubes (CNT) dry-spun as ultra-thin sheets onto preformed carbon nanofibre (CNF) supports to generate a unique class of electrically conductive and flexible ultrafiltration membranes. The pore size of the CNT-based membranes, on the order of ∼ 28 nm, was fine-tuned by controlling the dry layering and orientation of the CNT sheets to manage the membrane selectivity. The CNT-based membranes were used as effective conductive platforms to promote charge transfer during electrocatalytic degradation of acetaminophen, as a model contaminant. The CNT-based membranes, besides offering water permeance up to 2.77 × 103 L.m−2.h−1.bar−1, yielded electrocatalytic kinetic constant up to 46.5 × 10−3 min−1 during combined electrochemical reaction and ultrafiltration process, which is 1.4 to 39 times larger than previously reported values. Such high performance was maintained quite stable even after 8 reuse cycles. These results demonstrate the potential of CNT dry spinning technology for the scalable fabrication of highly permeable, but selective CNT-based membranes with remarkable electrochemical properties towards cost-effective water treatment at an exceptional reaction rate.

Direct Membrane Filtration of Municipal Wastewater: Studying the Most Suitable Conditions for Minimizing Fouling Rate in Commercial Porous Membranes at Demonstration Scale.

This study aimed to evaluate the feasibility of applying a commercial porous membrane to direct filtration of municipal wastewater. The effects of membrane pore size (MF and UF), treated influent (raw wastewater and the primary settler effluent of a municipal wastewater treatment plant) and operating solids concentration (about 1 and 2.6 g L-1) were evaluated on a demonstration plant. Filtration periods of 2-8 h were achieved when using the MF membrane, while these increased to 34-69 days with the UF membrane. This wide difference was due to severe fouling when operating the MF membrane, which was dramatically reduced by the UF membrane. Use of raw wastewater and higher solids concentration showed a significant benefit in the filtration performance when using the UF module. The physical fouling control strategies tested (air sparging and backwashing) proved to be ineffective in controlling UF membrane fouling, although these strategies had a significant impact on MF membrane fouling, extending the operating period from some hours to 5-6 days. The fouling evaluation showed that a cake layer seemed to be the predominant reversible fouling mechanism during each independent filtration cycle. However, as continuous filtration advanced, a large accumulation of irreversible fouling appeared, which could have been related to intermediate/complete pore blocking in the case of the MF membrane, while it could have been produced by standard pore blocking in the case of the UF membrane. Organic matter represented more than 70% of this irreversible fouling in all the experimental conditions evaluated.

Preparation of Lateral Flow PVDF Membrane via Combined Vapor- and Non-Solvent-Induced Phase Separation (V-NIPS).

A large pore size Poly(vinylidene fluoride) (PVDF) membrane was prepared by the V-NIPS method using PVDF/N, N-dimethylacetamide (DMAc)/Polyvinyl pyrrolidone (PVP)/Polyethylene glycol (PEG) system. Firstly, the effect of different additive ratios on the membrane morphology and pore size was studied, and it was found that when the PVP:PEG ratio was 8:2, PVDF membranes with a relatively large pore size tend to be formed; the pore size is about 7.5 µm. Then, the effects of different exposure time on the membrane morphology and pore size were investigated, and it was found that as the vapor temperature increased, the pores on the surface of the membrane first became slightly smaller and then increased. Finally, the effects of different vapor temperatures on the membrane properties were discussed. The results showed that the as-prepared membrane exhibited suitable capillary flow rate and similar performance compared with a commercially available membrane in colloidal gold tests. The likely cause is that the amount of negative charge is less and the capillary migration rate is too fast. This paper provides a reference for the preparation of PVDF colloidal gold detection membrane.

Gravity-Driven Membrane as seawater desalination pretreatment: Understanding the role of membrane biofilm on water production and AOC removal

Gravity-Driven Membrane (GDM) is considered an emerging seawater reverse osmosis desalination pretreatment. The objective of this study was to assess the GDM process performance under different operating conditions and to evaluate the impact of membrane biofilm on the produced water quantity and quality. Three lab-scale submerged GDM systems (UF12h, UF24h and MF24h) were tested under different conditions of Hydraulic Retention Time (HRT) (12 h/24 h) and membrane pore size (Microfiltration/Ultrafiltration). The best water quality in terms of Assimilable Organic Carbon (AOC) was achieved with the longer HRT (24 h), where significant AOC was removed by the biofilm developed on the membrane. The next generation sequencing indicated that a longer HRT increased the abundance of Proteobacteria and Planctomycetes in the biofilm, which due to their biodegradation capability yielded to the highest AOC removal (~50–55 %). Furthermore, the use of microfiltration led to the highest water production (9.1 L/m2/h) due to larger membrane pore size, and a less dense biofilm. These findings indicate that employing a microfiltration membrane and 24 h HRT provides the best performance in terms of water quantity and AOC-removal. This study demonstrates that controlling GDM design and operation enabled manipulation of the biofilm proprieties yielding to high water production and high AOC-removal.

Synthesis, characterization, and application of gravity-driven ceramic microfiltration membranes for surface water treatment

Gravity-driven ceramic microfiltration (MF) disk-shaped membranes were synthesized using kaolin and different alumina contents (0 % wt, 25 % wt, and 50 % wt). The pure water flux, mean pore size, porosity and contact angle of membranes were measured. Their structure and composition were characterized by scanning electron microscopy (SEM) and X-ray diffraction (XRD). The effect of alumina content was evaluated for long-term river water filtration in terms of permeate flux, turbidity, dissolved organic carbon (DOC), UV254, pH, and alkalinity removal. The physical characteristics of the biofouling layer, such as thickness and roughness, were studied using optical coherence tomography (OCT) imaging and the concentration of active microorganisms in the biofilm. The results showed acceptable turbidity removal after the flux stabilization period and relatively high performance for DOC, UV, and alkalinity removal during the first three days of filtration. Flux stabilized at 2.5–3 LMH on day 24 of filtration, indicating that the alumina content does not considerably affect the stable flux. As the flux modeling data showed, prior to the flux stabilization time, the fouling was controlled by the pore blocking mechanism. This was confirmed by OCT imaging that showed a very outspread biofilm layer with a low relative roughness; the layer became more compact with a higher relative roughness over time, showing that the cake layer is dominant after the flux stabilization period. Increasing the alumina content of the membranes increased the number of active microorganisms in the biofilm layer; possibly because of an increased adsorption of nutrients in the biofouling layer.

Improved negative charge of tight ceramic ultrafiltration membranes for protein-resistant and easy-cleaning performance

Fouling caused by adsorption is a challenge in protein separation and purification using membrane technology. The interaction between the membrane surface and proteins is significant for developing antifouling modified membranes. In this study, a negative ZrO2 tubular ceramic membrane was constructed by grafting polyacrylic acid (PAA) to mitigate protein fouling. PAA of 2000 Da had little effect on the membrane pore and surface wettability, as characterized by XPS, dextran retention, contact angle, and water permeance. The protein resistance of the modified membrane was investigated by protein filtration. The modified membranes had a higher stable normalized flux (0.6) and flux recovery ratio (90 %) than that of the original membrane. In addition, the performance of the modified membranes was sensitive to the pore sizes of the original membranes. When the original membrane was unable to completely reject bovine serum albumin (BSA), the weak interaction between BSA and the membrane pores would reduce rejection after PAA modification. And the flux recovery ratio of modified membrane increased by 55 %. Overall, PAA modification reduces irreversible fouling by electrostatic repulsion, and improves the protein-resistant and easy-cleaning performance of ceramic membranes.

Effect of different pore membrane on ultrafiltration treatment of wastewater contained heavy metals complexed by palygorskite

The treating effects of four hollow fiber membranes on heavy metals complexed by palygorskite in wastewater by complexation–ultrafiltration were investigated. The membranes contained three asymmetric polysulfone membranes and a polyethersulfone membrane with nominal molecular weight cut-off (MWCO) between 6,000 and 100,000 Da. The results through the comprehensive analysis of the rejection rate and the flux of normalized membranes showed that 6,000; 10,000 and 6,000 Da ultrafiltration membranes were suitable for the ultrafiltration of Cu2+, Zn2+, Cd2+, respectively. The pseudo-first-order reaction rate equation can better represent the reaction behavior of the complexes of favorite-ultraviolet at four membrane pore sizes. Four kinds of ultrafiltration membranes with molecular weight cut-offs of 6, 10, 50, and 100 kDa were fouled with wastewater containing Cu2+, Zn2+, Cd2+ complexed by palygorskite in complexation–ultrafiltration process at a transmembrane pressure of 1 bar and 20°C. The influence of molecular weight cutoff was investigated. The permeation flux decreased with time. Permeate flux declined with time gradually and finally stabilized after about 140 min of operation for all the membranes tested. 50,000 Da membrane showed the highest flux decline. The concentration of heavy metals and types of materials affect the distribution of resistance. The highest proportion of resistance for 6,000 and 10,000 Da membranes is the resistance of the membrane. For 50,000 and 100,000 Da membranes, the highest proportion of its resistance is the filter cake layer resistance. The results provide a kind of inorganic clay complexing agent in large reserves for the complexation–ultrafiltration of heavy metal wastewater.

Separation Mechanisms and Anti-Fouling Properties of a Microporous Polyvinylidene Fluoride-Polyacrylic Acid-Graphene Oxide (PVDF-PAA-GO) Composite Membrane with Salt and Protein Solutions.

This research demonstrates the preparation of composite membranes containing graphene oxide (GO) and investigates the separation mechanisms of various salts and bovine serum albumin (BSA) solutions. A microporous polyvinylidene fluoride-polyacrylic acid-GO (PVDF-PAA-GO) separation layer was fabricated on non-woven support. The GO-incorporating composite resulted in enlarged pore size (0.16 μm) compared with the control membrane (0.12 μm). The zeta potential of the GO composite was reduced to -31 from -19 mV. The resulting membranes with and without GO were examined for water permeability and rejection efficiency with single salt and BSA solutions. Using the non-woven/PVDF-PAA composite, the permeance values were 88-190 kg/m2hMPa, and the salt rejection coefficients were 9-28% for Na2SO4, MgCl2, MgSO4, and NaCl solutions. These salt removals were based on the Donnan exclusion mechanism considering the ion radii and membrane pore size. Incorporating GO into the separation layer exhibited limited impacts on the filtration of salt solutions, but significantly reduced BSA membrane adhesion and increased permeance. The negatively charged protein reached almost complete removal (98.4%) from the highly negatively charged GO-containing membrane. The GO additive improved the anti-fouling property of the composite membrane and enhanced BSA separation from the salt solution.

Gold nanoparticle conjugate-based lateral flow immunoassay (LFIA) for rapid detection of RBD antigen of SARS-CoV-2 in clinical samples using a smartphone-based application.

The coronavirus disease 2019 (COVID-19) pandemic has emphasized the need for development of a rapid diagnostic device for the effective treatment and its mitigation. Lateral flow immunoassay (LFIA) belongs to a class of diagnostic devices, which has the benefit of providing quick results, easy to handle, low cost, and on-site applicable. So far, several LFIA has been developed for the detection of infectious severe acute respiratory syndrome-coronavirus-2(SARS-CoV-2), however, only a few of them are antigen (Ag)-based. Here, we describe an antibody (Ab)-labeled gold nanoparticles (AuNPs)-based LFIA(AuNPs-LFIA) for the detection of Receptor-Binding Domain (RBD) of SARS-CoV-2. For this, RBD Abof SARS-CoV-2 was conjugated with the AuNPs, which served as a detecting probe. The fabricated LFIA strip was optimized for different parameters such as membrane pore size, blocking conditions, Ab coating concentration, and conjugate incubation. The optimized LFIA strips were validated in spiked buffer samples and the optimal limit of detection was found to be 1 ng/ml, which was confirmed by a smartphone-based application. Moreover, the developed AuNPs-LFIA strips effectively detected RBD Agin 100 clinical samples with 94.3% sensitivity and 90.9% specificityin clinical samples when compared with the gold standard (RT-PCR). The fabricated LFIAs are reported to have storage stability of up to 21 days at 4°C and room temperature (RT).Hence, the developed LFIA can be used as a portable, cost-effective diagnostic device for rapid detection of SARS-CoV-2.

Effect of spraying polyvinyl alcohol solution on the surface of liquid film on the structure and antifouling properties of polyvinylidene membrane

Abstract Polyvinylidene fluoride (PVDF) membrane was prepared by a two-step method of surface gelation-immersion precipitation. The surface of the scraping solution film was sprayed with polyvinyl alcohol (PVA) aqueous solution to gel the film surface first, and then the liquid film was immersed in a coagulation bath for phase transformation to obtain the surface modified PVDF membrane. The effects of PVA solution with different mass fraction on the structure and properties of PVDF membrane were studied. The results showed that with the increase of spraying PVA mass fraction, the contact angle of the upper surface of PVDF membranes gradually decreased, the porosity and mean pore size of PVDF membranes gradually increased, and the crystallinity of PVDF membranes gradually decreased. The pores on upper surface of the membranes first increased and then decreased, and the finger-like macropore structure appeared inside the membranes, the water flux first increased and then decreased, and the rejection rate increased. Dynamic cycle filtration experiment of bovine serum protein (BSA) solution showed an increase in membrane flux recovery from 68.31 to 95.08% and a decrease in the irreversible fouling rate from 31.69 to 4.92%, indicating an improvement in the hydrophilicity and antifouling properties of the modified PVDF membrane.

Cellulose-based carbon membranes for gas separations - Unraveling structural parameters and surface chemistry for superior separation performance

Carbon molecular sieve membranes were prepared from the carbonization of a cellulose-based polymeric precursor doped with urea. The addition of urea to the cellulose precursor induces an increase in structural disorder and an increase in pore volume inside the structure of prepared membranes. This unique preparation procedure proved to be an extremely effective method for tuning the pore size of carbon membranes to the desired separations. Urea acts as a pore-forming agent that allows the fabrication of carbon membranes with high porosity. The addition of 2.8 wt% of urea doubled the permeability of the prepared carbon membrane to hydrogen. In addition, a permeability to oxygen of 333 barrer was obtained, without impairing the selectivity. The proposed preparation procedure is compatible with industrial production and scaling, hopefully making carbon membranes a viable solution to produce oxygen-enriched air, recovering of hydrogen from hydrocarbon streams and carbon dioxide removal from natural gas/biogas.

Electro-nanofiltration membranes with high Li+/Mg2+ selectivity prepared via sequential interfacial polymerization

With the rapid development of batteries and other industries, the demand for lithium has exploded. As about 70 wt% of the lithium resources exist in brines, the extraction of lithium by selective electrodialysis (S-ED) has attracted extensive attention. However, the similar hydrated ionic radius of Li+ and Mg2+ caused many challenges. Herein, a simple sequential interfacial polymerization (SIP) method was developed to adjust the surface charge and pore size of the electro-nanofiltration membranes (ENFMs), and the ENFMs were endowed with excellent selectivity due to the electrostatic repulsion and pore-size sieving effect. Polyethyleneimine (PEI) with different molecular weights was used for the SIP process based on the nascent polyamide separation layer prepared by interfacial polymerization. With the increase of the PEI molecular weight, the zeta potential of the membrane surface changed from negative to positive, while the pore size of the membranes also decreased. The optimal ENFMs achieved outstanding selectivity for Li+/Mg2+ (16.55) and high Li+ flux (3.08 × 10−8 mol·cm−2·s−1) at a current density of 10 mA·cm−2. Moreover, the optimal ENFMs exhibited high limiting current density (50.6 mA·cm−2) and low membrane electrical resistance (2.42 Ω·cm2), which has a great potential to be used for practical applications.

Antifouling modification of PVDF membranes via incorporating positive-charge tuned quaternized chitosan magnetic particles

Membrane separation technology often suffers from fouling during operation and undergoes a trade-off effect between permeability and selectivity. We report a new strategy for fabricating antifouling poly(vinylidene fluoride) (PVDF) ultrafiltration membrane by incorporating hydrophilic, positive-charge tuned quaternized chitosan magnetic particles (QCMP). Fe3O4-chitosan particles were synthesized by co-precipitation and cross-linked by glutaraldehyde, and further quaternization modification was applied to obtain a series of QCMP with different charge densities. Then, the QCMP were incorporated into the matrix of PVDF to prepare hybrid membrane via the non-solvent-induced phase separation method. The effect of incorporated QCMP on the membrane morphology and properties was determined using scanning electron microscopy, water contact angle, surface free energy, membrane pore size and porosity. The incorporation of QCMP into the PVDF membrane effectively increased the surface hydrophilicity. Results of bovine serum albumin (BSA) solution filtration showed a higher flux recovery ratio (FRR) and a lower total fouling ratio(Rt) of the QCMP/PVDF hybrid membranes than those of the PVDF pristine membrane, indicating the addition of QCMP was helpful to enhance antifouing performance for BSA via improving hydrophilicity and neutralizing the surfaces of negatively charged PVDF membranes. Furthermore, the permeability flux of BSA was increased by 53 % and the rejection rate was also increased by 26 %, indicating the hybrid membranes have the potential to break the permeability-selectivity trade-off. Therefore, this study provided a green, simple and effective method for the rational design and construction membranes with enhanced the permeability and selectivity as well as antifouling performance.

Rigorous determination of pore size non-uniformity for nanofiltration membranes by incorporating the effects on mass transport

The pore size uniformity of a nanofiltration (NF) membrane has an important impact on its rejection capacity and separation selectivity, which remains yet difficult to be scientifically and accurately characterized. In this study, a method was proposed for characterizing the pore size distribution of NF membranes, by assuming its obeyance of the log-normal distribution and incorporating the Donnan and steric pore model to describe water permeation and solute rejection. The distribution parameters are derived through data-fitting the experimental rejections of reference solutes in the model framework. The non-uniformity index is newly defined as the degree of deviation of a special pore size from the median pore size. The index makes that possible to quantitatively compare the non-uniformity among different NF membranes. The fast convergence of the method was verified. Furthermore, we put forward some suggestions on the use of the proposed method for pore size distribution characterization. The determination for the rejections of highly-rejected (∼90 %) solutes and the radii of the lowly-rejected (∼30 %) solutes must be sufficiently accurate. The proposed method is more advantageous over the conventionally adopted method due to the higher accuracy and more convenience in the assessment of membrane pore size distribution as well as membrane performance.

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.