Physicochemical basis of the sol-gel process giving selective layers of ultrafiltration ceramic membranes from zirconia hydrosol

Selective layers of ceramic ultrafiltration membranes were prepared by the sol-gel method using zirconia hydrosol as the initial material. On the basis of turbidimetric investigations, a compatibility diagram was constructed in the zirconia (sol) — polymer thickener — water system. The results of x-ray phase analysis and thermogravimetric analysis were used to choose the regime for heat treatment of the sol — polymer thickener compositions in order to prepare selective layers. The characteristics of the pore structure of the selective layers meet the requirements imposed on ultrafiltration membranes.     

Spray coating of polysulfone/poly(ethylene glycol) block polymer on macroporous substrates followed by selective swelling for composite ultrafiltration membranes

Polysulfone(PSF) is extensively used for the production of ultrafiltration(UF) membranes thanks to its high strength,chemical stability,and good processibility.However,PSF is intrinsically hydrophobic,and hydrophilic modification is always required to PSF-based membranes if they are intended to be used in aqueous systems.Facile strategies to prepare hydrophilic PSF membranes are thus highly demanded.Herein we spray coat a PSF-based amphiphilic block polymer onto macroporous substrates followed by selective swelling to prepare flat-sheet PSF UF membranes.The polymer is a triblock polymer containing PSF as the majority middle block tethered with shorter block of polyethylene glycol(PEG) on both ends,that is,PEG-b-PSF-b-PEG.We use the technique of spray coa ting to homogeneously dispense diluted triblock polymer solutions on the top of macroporous supports,instantly resulting in uniform,defect-free polymer coating layers with the thickness down to ～1.2 μm.The bi-layered composite structures are then immerged in ethanol/acetone mixture to generate mesoscale pores in the coating layers following the mechanism of selective swelling-induced pore generation,thus producing composite membranes with the mesoporous triblock polymer coating as the selective layers.This facile strategy is free from additional hydrophilic modification and much smaller dosages of polymers are used compared to conventional casting methods.The pore sizes,porositie s,hydrophilicity,and consequently the separation properties of the membranes can be flexibly tuned by changing the swelling duration and the composition of the swelling bath.This strategy combining spray coating and selective swelling is upscalable for the production of highperformance PSF UF membranes.     

Preparation and Properties of Ultrafiltration Ceramic Membranes

A technology for preparing ceramic ultrafiltration membranes with a ZrO₂-based selective layer and their pore and separating characteristics are determined. The potential use of these membranes for separation or fractionation of polyvinylpyrrolidone and purification of apple pectin is demonstrated.     

陶瓷多层选择膜工艺

综述了制取陶瓷微滤和超滤多层选择膜的6种工艺方法，并指出了膜层的结构特点。     

Selective swelling of polysulfone/poly(ethylene glycol) block copolymer towards fouling-resistant ultrafiltration membranes

Fouling resistance of ultrafiltration (UF) membranes is critical for their long-term usages in terms of stable performance, so convenient approaches to prepare fouling-resistant membranes are always anticipated. Herein, we demonstrate the facile fabrication of antifouling polysulfone-block-poly(ethylene glycol) (PSF-b-PEG, SFEG) composite membranes. SFEG layer was coated onto macroporous supports and cavitated by immerging them in acetone/n-propanol following the mechanism of selective swelling induced pore generation. Thus-produced SFEG membranes possessed high permeance and excellent mechanical strength. Meanwhile, the structures and separation performances of the SFEG layers can be continuously tuned through simply changing swelling durations. More importantly, the hydrophilic PEG chains were spontaneously enriched onto the pore walls through swelling treatment, endowing intrinsic antifouling property to the SFEG membranes. Bovine serum albumin (BSA)/humic acid (HA) fouling tests proved the prominent fouling resistance of SFEG membranes, and the fouling resistance is expected to be long-standing because of the firm connection between PEG chains and PSF matrix by covalent bonding.     

多孔纳米材料改性水处理超滤膜的研究进展

基于压力驱动的超滤膜面临渗透性和选择性的制衡及膜污染问题。多孔纳米材料是高性能超滤膜改性制备中一类重要的添加剂,是新型水处理功能膜的研究热点之一。多孔纳米材料的添加为膜提供了额外水通道,其可调的孔尺寸又为在膜内构建出具有高度选择性的纳米通道提供了潜在有利条件,进而突破膜渗透性和选择性的Trade-off效应。同时,亲水性多孔纳米材料的添加有利于提升膜的抗污染性能。本文综述了近年来多孔纳米材料对超滤膜的改性方法,总结了微孔沸石分子筛、介孔炭、介孔二氧化硅、金属有机骨架材料和共价有机骨架材料对超滤膜的改性研究进展,着重评价了不同改性材料对超滤膜在亲水性、渗透性、污染物截留和抗污染能力等方面的影响。最后对未来多孔纳米材料改性超滤膜的研究及应用的发展趋势进行了展望。     

Preparation of polysulfone-based block copolymer ultrafiltration membranes by selective swelling and sacrificing nanofillers

Selective swelling of block copolymers of polysulfone-b-poly(ethylene glycol) is an emerging strategy to prepare new types of polysulfone ultrafiltration membranes. Herein, we prepared nanoporous polysulfone-b-poly(ethylene glycol) ultrafiltration membranes by selective swelling and further promoted their porosity and ultrafiltration performances by using CaCO₃ nanoparticles as the sacrificial nanofillers. Different contents of CaCO₃ nanoparticles were doped into the solution of polysulfone-b-poly(ethylene glycol), and thus obtained suspensions were used to prepare both self-supported and bi-layered composite structures. Selective swelling was performed on the obtained block copolymer structures in the solvent pair of ethanol/acetone, producing nanoporous membranes with poly(ethylene glycol) lined along pore walls. The CaCO₃ nanoparticles dispersed in polysulfone-b-poly(ethylene glycol) were subsequently etched away by hydrochloric acid and the spaces initially occupied by CaCO₃ provided extra pores to the block copolymer layers. The porosity of the membranes was increased with increasing CaCO₃ content up to 41%, but further increase in the CaCO₃ content led to partial collapse of the membrane. The sacrificial CaCO₃ particles provided extra pores and enhanced the connectivity between adjacent pores. Consequently, the membranes prepared under optimized conditions exhibited up to 80% increase in water permeance with slight decrease in rejection compared to neat membranes without the use of sacrificial CaCO₃ particles.     

Ceramic Membranes with Selective Layers Based on SiO2, TiO2, and ZrO2

A technology for producing ultrafiltration ceramic membranes using the sol-gel method has been developed. Experimental lots of membranes with selective layers based on SiO₂, TiO₂, and ZrO₂ are obtained, and their characteristics are investigated.     

工业化陶瓷超滤膜的表征

以液-液排除法测定工业化陶瓷超滤膜的孔径及孔径分布，比较了两种渗透体系对测定结果的影响；用已知分子量标准物质考察了不同膜的截留行为，获得了各膜的切割分子量值，并分析了截留率与孔径的对应关系，为工业化陶瓷膜的开发提供依据。     

湿式催化氧化/膜过滤组合工艺膜过滤机理

采用湿式催化氧化/膜过滤组合工艺,以Mo-Zn-Al-O粉末作为催化剂降解阳离子红GTL模拟染料废水,探讨在膜过滤过程中平均孔径为0.1 μm的微滤和0.022 μm的超滤聚偏氟乙烯(PVDF)中空纤维膜的过滤机理。实验结果表明,两种膜过滤组合工艺对染料的降解效果均优于单独湿式催化氧化,0.01 MPa恒压条件下运行120 min后微滤和超滤的膜通量分别衰减了26.63%和16.48%,其原因是粉末催化剂可在微滤膜孔内部沉积形成中间阻塞过滤,后在表面形成滤饼层;而在超滤膜表面仅形成滤饼层。通过实验结果对膜阻力进行计算,可知在相同反应过程后微滤膜产生的不可逆阻力大于超滤膜。在不同反应条件下,催化剂的沉积量与膜阻力呈现线性相关。     

Influence of covalent attachment of low molecular weight substances on the rejection and adsorption properties of crystalline proteinaceous ultrafiltration membranes

Many bacteria possess crystalline cell surface layers (S-layers) as their outermost cell envelope component. High-resolution electron microscopical and permeability studies on S-layers revealed pores with a size from 2–8 nm. Each type of S-layer lattice showed pores of identical size and morphology. By depositing S-layer fragments on appropriate microfiltration membranes and crosslinking the S-layer protein with glutaraldehyde, it was possible to produce ultrafiltration membranes with a crystalline and isoporous active filtration layer. Due to a surplus of free carboxyl groups on the surface of the S-layer lattice and in the pore areas, these composite S-layer ultrafiltration membranes (SUMs) showed a net negative charge. Upon converting the free carboxyl groups into neutral or positively charged groups by amidation, both the adsorption and rejection properties of SUMs could be altered. As expected, neutral SUMs showed the lowest unspecific protein adsorption. Membrane modification studies further demonstrated that in addition to the molecular size, the physicochemical properties of the immobilized compounds and the resulting interactions with the test proteins in solution determine the rejection characteristics of SUMs.     

Deposition of thin ultrafiltration membranes on commercial SiC microfiltration tubes

Porous SiC based materials present high mechanical, chemical and thermal robustness, and thus have been largely applied to water-filtration technologies. In this study, commercial SiC microfiltration tubes with nominal pore size of 0.04 μm were used as carrier for depositing thin aluminum oxide (Al₂O₃) ultrafiltration membranes. These ultrafiltration membranes were obtained by coating, drying and calcination of a colloidal suspension of boehmite particles. After calcination, the membrane material consisted of nanosized γ-Al₂O₃ crystallites and had a narrow pore size distribution with average pore size of 5.5 nm. Membrane thickness was tuned by repeating the coating of boehmite sol. By doing so, we were able to reduce the defect density on the membrane surface, as evidenced by SEM analysis and by the significant reduction of water permeance after depositing the second γ-Al₂O₃ layer. After five times coating, a 5.6 µm thick γ-Al₂O₃ layer was obtained. This membrane shows retention of ~75% for polyethylene glycol molecules with M_n of 8 and 35 kDa, indicating that, despite their intrinsic surface roughness, commercial SiC microfiltration tubes can be applied as carrier for thin ultrafiltration membranes. This work also indicates that an improvement of the commercial SiC support surface smoothness may greatly enhance permeance and selectivity of γ-Al₂O₃ ultrafiltration membranes by allowing the deposition of thinner defect-free layers.     

多通道陶瓷超滤膜孔径分布及截留率测定

本文对工业化多通道陶瓷超滤膜进行了研究,以异丁醇－蒸馏水体系用液－液排除法测定了超滤膜的孔径及孔径分布,对同一膜管及体系,进行了重复性实验,测定了膜的截留率,比较了截留率与孔径及孔径分布的关系,讨论了操作条件对截留率的影响,为工业化膜的制备及选取提供指导。     

Pore size control technique in the spinning of polysulfone hollow fiber ultrafiltration membranes

Conditions for finger-like void formation in polysulfone (PS) ultrafiltration membranes were studied empirically. It was found that both the “mixability parameter” which was obtained in separate titration experiments and the viscosity of coagulants could be used to predict membrane morphology. It was also found that finger-like void layers appeared under the conditions which dope had concentrations quite apart from the phase separation area and which coagulant had both a low mixability parameter and low viscosity.

These factors proved to play a very important role in controlling the pore size of membranes. The results of this study were applied to obtaining pore size variations of PS hollow fiber membranes.     

Development of antimicrobial membranes via the surface tethering of chitosan

To render the surface of ultrafiltration membranes biocidal, cellulose membranes were modified with chitosan, a naturally occurring polycationic biocide. Through the use of chitosans of different molecular weights and membranes with different pore sizes, the alteration of the morphological structure of tethered chitosan layers was achieved. The importance of such structural differences in the antimicrobial activity of the prepared membranes against gram‐positive Staphylococcus aureus and gram‐negative Escherichia coli was studied. The antimicrobial efficiency improved with the use of chitosans with higher molecular weights and membranes with smaller pore sizes. This suggested that the surface location of the grafted chitosan chains was more preferential for a higher antimicrobial activity of the surface. Membranes modified with chitosan showed higher antimicrobial efficiency against gram‐positive S. aureus than against gram‐negative E. coli. © 2008 Wiley Periodicals, Inc. J Appl Polym Sci, 2009     

Multilayer ultrathin-film composite membranes for oxygen enrichment

Multilayer composite membranes were made of poly(4-methylpentene-1) (PMP), an ethyl cellulose (EC) + heptyl cellulose (HC) blend, polycarbonate (PC), polysulfone, poly(2,6-dimethylphenylene oxide), cellulose triacetate ultrathin films as selective layers, and polysulfone, poly(ether sulfone), and poly(sulfone amide) ultrafiltration membranes with a 10–45 nm pore size and 100–120 μm thickness as porous support layers. The effects of the ultrathin-film type and its casting solution concentration, operating pressure, temperature, as well as time on the oxygen-enriched air (OEA) flux and oxygen concentration in the OEA permeated in a single step through the composite membranes were investigated using a constant pressure—variable volume method. The OEA flux increases significantly with an increasing transmembrane pressure difference and operating temperature. The oxygen concentration in the OEA also increases with an increasing pressure difference but decreases slightly with an increasing operating temperature. In long-term tests, the oxygen-enrichment properties were maintained almost constant for as long as 170 h. The composite membranes consisting of the bilayer ultrathin film cast from a more dilute solution (0.11–0.26 wt %) on the porous support with a smaller pore size combine a higher oxygen-enriching ability and a higher stability than do those of monolayer and tetralayer ultrathin films. The maximum OEA flux and oxygen concentration produced at 20–75°C and a 500 kPa transmembrane pressure difference in a single pass across the PMP/98EC + 2HC bilayer and PC bilayer ultrathin-film composite membranes are 3.1 × 10⁻³ cm³(STP)/s cm² and 50%, respectively. © 1997 John Wiley & Sons, Inc. J Appl Polym Sci 66: 2139–2147, 1997     

The structure and morphology of the skin of polyethersulfone ultrafiltration membranes: A comparative atomic force microscope and scanning electron microscope study

Atomic Force Microscopy (AFM) and Scanning Electron Microscopy (SEM) were used to investigate the surface structure and morphology of 10,000, 30,000, and 100,000 dalton molecular weight cutoff (MWCO) polyethersulfone (PES) ultrafiltration membranes, and the results are compared. Although both approaches reveal the pore structure in the 30,000 and 100,000 MWCO membranes, the pore diameters derived from SEM are smaller than those measured by AFM. This discrepancy is a result of the diminution in pore dimensions during the sample preparation for SEM, that is, the solvent exchange procedure needed to remove the water from the membrane prior to the high vacuum gold coating deposition step. In contrast to SEM, which requires a high vacuum both during heavy metal coating and during examination, AFM can be performed on wet ultrafiltration membranes. Consequently, the potential of altering the membranes' pore structures during sample preparation is eliminated. Therefore, the pore diameters obtained from AFM are more accurate than those derived from SEM.     

Gas transport in coextruded multilayered membranes with alternating dense and porous polymeric layers

Multilayered gas separation membranes with alternating layers of β crystalline polypropylene and PEBAX copolymers were produced via coextrusion and biaxial stretching for potential applications in modified atmosphere packaging. The number of layers and PEBAX compositions were varied to study the effect of various membrane configurations on pore formation in the polypropylene layers, which ideally serve as a mechanical support for the selective PEBAX layers. Gas permeabilities of multilayered films were compared to control PEBAX films using a simple model, which allowed inferences about the completeness of pore formation to be drawn. Preliminary results show that increasing the layer count degrades membrane throughput and separation capability, while varying PEBAX composition results in a tradeoff between these properties.     

Modification of porous polyacrylonitrile membrane

The effects of the modification of polyacrylonitrile ultrafiltration membranes with sodium hydroxide are discussed. The NaOH-induced hydrolysis of nitrile groups on the membrane surface results in membranes with decreasing pore diameter. The average pore diameter changes from 2.6 to 0.6 nm in the progress of hydrolysis. The modified membranes are less prone for protein deposition. Fouling causes a pore diameter reduction of 80% for the untreated and 20% for the modified membranes. Modification results also in the creation of the membranes working in the nanofiltration mode. The unmodified membrane does not reject salt while a membrane immersed in the modification bath is capable of rejecting about 50% of calcium carbonate.     

Substrate matters:The influences of substrate layers on the performances of thin-film composite reverse osmosis membranes

Thin-film composite (TFC) reverse osmosis (RO) membranes are playing the dominating role in desalination.Tremendous efforts have been put in the studies on the polyamide selective layers. However, the effect of the substrate layers is far less concerned. In this review, we summarize the works that consider the impacts of the substrates, including pore sizes, surface hydrophilicity, on the processes of interfacial polymerization and consequently on the morphologies of the active layers and on final RO performances of the compositemembranes. All the works indicate that the pore sizes and surface hydrophilicity of the substrate evidently influence the RO performances of the composite membranes. Unfortunately, we find that the observations and understandings on the substrate effect are frequently varied from case to case because of the lack of substrates with uniform pores and surface chemistries.We suggest using track-etchedmembranes or anodized aluminamembranes having relatively uniformpores and functionalizable porewalls asmodel substrates to elucidate the substrate effect.Moreover, we argue that homoporous membranes derived from block copolymers have the potential to be used as substrates for the large-scale production of high-performances TFC RO membranes.