Novel stand-alone PVA mixed matrix membranes conjugated with graphene oxide for highly improved reverse osmosis performance

In this research, an innovative Poly (vinyl alcohol) (PVA) reverse osmosis (RO) membrane with exceptional attributes was fabricated. Graphene Oxide (GO) nanosheets and Pluronic F-127 were infused within crosslinked PVA to fabricate thin film mixed matrix membranes. The newly synthesized membranes were evaluated in terms of several parameters like surface roughness, hydrophilicity, salt rejection, water permeability, Chlorine tolerance and anti-biofouling property, utilizing a dead-end RO filtration unit. Typical characterization techniques were used to assess the characteristics of the membranes. These include SEM, AFM, contact angle measurements and mechanical strength analysis. The conjugation of Pluronic F-127 and GO enhanced the overall performance of the membranes. The modified membranes surfaces had less roughness and higher hydrophilicity in comparison with the unmodified ones. This research showed that membranes that contained 0.08 wt% and 0.1 wt% GO exhibited superior selectivity, mechanical strength, Chlorine tolerance and anti-biofouling property. The truly significant outcome to evolve from this investigation is that improvements have been accomplished while PVA was used as a stand-alone RO layer without the use of any substrate. This study showed that crosslinking of PVA and modifying it with proper fillers overcame the common PVA downsides, primarily swelling and rupture under exceptionally high pressure.     

A novel method of surface modification on thin-film-composite reverse osmosis membrane by grafting hydantoin derivative

Membrane degradations by biofouling and free chlorine oxidation are the major obstacles for aromatic polyamide thin-film-composite (TFC) reverse osmosis (RO) membranes to realize high performance over a long period of operation. In this work, a hydantoin derivative, 3-monomethylol-5,5-dimethylhydantoin (MDMH), was grafted onto the nascent aromatic polyamide membrane surfaces by the reactions with active groups (e.g., acyl chloride groups) in the surfaces. The grafted MDMH moieties with high reaction activity and free chlorine could play as sacrificial pendant groups when membranes suffer from chlorine attacks, and the chlorination products N-halamines with strong antimicrobial function could sterilize microorganisms on membrane surfaces and then regenerate to MDMH. This was designed as a novel means to improve both chlorine resistances and anti-biofouling properties of the aromatic polyamide TFC RO membranes.Attenuated total reflectance mode Fourier transform infrared spectroscopy (ATR-FTIR) revealed that the MDMH-modified membranes had two characteristic bands at 1772 and 1709 cm⁻¹ corresponding to two carbonyl groups in hydantoin ring. This suggested the successful grafting of MDMH onto the membrane surfaces, which was further confirmed and quantified by X-ray photoelectron spectroscopy (XPS) analysis. After modification with MDMH, the membrane surface hydrophilicity increased obviously as contact angles decreased from 57.7° to 50.4–31.5°. But, there was no obvious change in membrane surface roughness after modification. The MDMH-modified membranes were shown to possess high chlorine resistances with small changes in water fluxes and salt rejections after chlorination with 100–2000 ppm h chlorine at pH 4. The chlorinated MDMH-modified membranes demonstrated obvious sterilization effects on Escherchia coli and substantial preventions against microbial fouling. Therefore, the MDMH-modified membranes offer a potential use as a new type of chlorine resistance and anti-biofouling TFC RO membranes.     

Effects of bisphenol monomer structure on the surface morphology and reverse osmosis (RO) performance of thin-film-composite membranes composed of polyphenyl esters

Four reverse osmosis (RO) composite membranes, in which thin-film active layers were polyphenyl esters, were prepared by interfacial polymerization of a series of bisphenol monomers and trimesoyl chloride (TMC). An atomic force microscope (AFM) was used to investigate the surface morphology and RO experiments were carried out to measure the rejection and flux characteristics of the membranes. Correlations between the inherent chemical nature of bisphenols possessing structural variations in the middle of phenyl rings and the surface morphology/RO performance of the membranes were studied. Polarity of the connectors between two phenyl rings of bisphenols played an important role in determining the surface morphology and RO performance. Nonpolar bisphenol gave a morphology of uniform, distinct nodular corrugation and a superior RO rejection but a relatively low flux, while the polar one resulted in an irregular, ambiguous nodule structure and a high flux. The size of the bisphenol connectors was also found to be important; the smaller one was more favorable for the formation of membrane with better salt rejection, while the larger one contributed to higher flux. © 1996 John Wiley & Sons, Inc.     

Correlations of chemical structure,atomic force microscopy (AFM) morphology,and reverse osmosis (RO) characteristics in aromatic polyester high-flux RO membranes

A homologous series of thin-film composite membranes was prepared by interfacial polymerization of various bisphenols possessing structural variations and trimesoyl chloride (TMC). Correlations between the inherent chemical nature of bisphenols with methyl or halogen substitutions on the biphenyl rings, reverse osmosis (RO) characteristics, and surface features characterized by atomic force microscopy (AFM) were studied. The methyl substitutions in bisphenol phenyl rings resulted in membranes with higher RO water flux but lower RO rejection, tending to give membrane surface morphology of irregular ambiguous nodule structure with reduced size and a smoother surface. The halogen substitutions were found to play an important role in enhancing the RO rejection of the resulting membranes; the rough surface appearance of uniform distinct nodule structure may also have contributed to obtaining higher rejections.     

Hydrophobic modification of poly(phthalazinone ether sulfone ketone) hollow fiber membrane for vacuum membrane distillation

New hydrophobic poly(phthalazinone ether sulfone ketone) (PPESK) hollow fiber composite membranes coated with silicone rubber and with sol–gel polytrifluoropropylsiloxane were obtained by surface-coated modification method. The effects of coating time, coating temperature and the concentration of silicone rubber solution on the vacuum membrane distillation (VMD) properties of silicone rubber coated membranes were investigated. It was found that high water permeate flux could be gotten in low temperature and low concentration of silicone rubber solution. When the coating temperature is 60 °C, the coating time is 9 h and the concentration of silicone rubber solution is 5 g L⁻¹ the water permeate flux of the silicone rubber coated membrane is 3.5 L m⁻² h⁻¹. The prepolymerization time influence the performance of polytrifluoropropylsiloxane coated membranes, and higher prepolymerization time decrease the water permeate flux of the membrane. The water permeate flux and the salt rejection was 3.7 L m⁻² h⁻¹ and 94.6%, respectively in 30 min prepolymerization period. The VMD performances of two composite membranes during long-term operation were studied, and the results indicated that the VMD performances of two composite membranes are quite stable. The salt rejection of silicone rubber coated membrane decreased from 99 to 95% and the water permeate flux fluctuated between 2.0 and 2.5 L m⁻² h⁻¹. The salt rejection of polytrifluoropropylsiloxane coated membrane decreased from 98 to 94% and the water permeate flux fluctuated in 1 L m⁻² h⁻¹ range.     

The effect of gel layer thickness on the salt rejection performance of polyelectrolyte gel-filled nanofiltration membranes

The effect of gel layer thickness on salt separation of positively charged pore-filled nanofiltration membranes has been examined both theoretically and experimentally. The extended Nernst-Planck (ENP) equation coupled with the Teorell-Meyer-Sievers (TMS) model were used to calculate the pressure-driven sodium chloride rejections for membranes having gel densities in the range typically used in nanofiltration applications. It was found that salt rejection was dependent on membrane (gel-layer) thickness with salt rejections increasing rapidly with thickness up to 50–75 μm. Further increases in thickness beyond this point had a much smaller effect on salt rejection. The theoretical predictions were examined experimentally by preparing a series of membranes with cross-linked poly(3-acrylamidopropyl)-trimethylammonium chloride (PAPTAC) gels with varying densities within the pores of a thin microporous polyethylene (PE) support. The membranes were characterized by their polymer volume fractions (gel concentration), thicknesses and effective charge densities. The effect of membrane thickness was examined by using single and stacks of two membranes. The pure water fluxes and salt rejections of the membranes and membrane stacks were determined in the pressure range 50–550 kPa. The single salt rejections of the membranes which were very dependent on the thickness of the membrane or membrane stack, were fully in accord with the calculated salt rejections of the membranes.     

Synthesis and Characterization of the Organic Surface Modifications of Monodisperse Colloidal Silica

Monodisperse colloidal silica spheres were prepared from tetraethylorthosilicate in mixtures of water, ammonia and ethanol. The surfaces of the spheres were successfully modified by chemical reaction with silane coupling agents. Several qualitative and quantitative methods were employed to analyse the organic surface modifications. As a result, the surface coverage of silica spheres with silane coupling agents could be calculated using different characterization methods.     

Preparation of thin-film-composite polyamide membranes for desalination using novel hydrophilic surface modifying macromolecules

A new concept for the preparation of thin-film-composite (TFC) reverse osmosis (RO) membrane by interfacial polymerization on porous polysulfone (PS) support using novel additives is reported. Hydrophilic surface modifying macromolecules (LSMM) were synthesized both ex situ by conventional method (cLSMM), and in situ within the organic solvent of the TFC system (iLSMM). The effects of these LSMMs on the fouling of the TFC RO membranes used in the desalination processes were studied. FTIR results indicated that both cLSMM and iLSMM were present in the active layer of the TFC membranes. SEM micrographs depicted that heterogeneity of the surface increases for TFC membranes compared to the control PS membrane, and that higher concentrations of LSMM provided smoother surface. AFM characteristic data presented that the surface roughness of the skin surface increases for TFC membranes compared to the control. The RO performance results showed that the addition of the cLSMM significantly decreased the salt rejection of the membrane and slightly reduced the flux, while in the case of the iLSMM, salt rejection was improved but the flux declined at different rates for different iLSMM concentrations. The membrane prepared by the iLSMM exhibited less flux decay over an extended operational period.     

Influence of Substitutional Defects in ZIF-8 Membranes on Reverse Osmosis Desalination: A Molecular Dynamics Study

In this study, molecular dynamics simulation is used to investigate the effects of water-based substitutional defects in zeolitic imidazolate frameworks (ZIF)-8 membranes on their reverse osmosis (RO) desalination performance. ZIF-8 unit cells containing up to three defect sites are used to construct the membranes. These substitutional defects can either be Zn defects or linker defects. The RO desalination performance of the membranes is assessed in terms of the water flux and ion rejection rate. The effects of defects on the interactions between the ZIF-8 membranes and NaCl are investigated and explained with respect to the radial distribution function (RDF) and ion density distribution. The results show that ion adsorption on the membranes occurs at either the nitrogen atoms or the defect sites. Complete NaCl rejection can be achieved by introducing defects to change the size of the pores. It has also been discovered that the presence of linker defects increases membrane hydrophilicity. Overall, molecular dynamics simulations have been used in this study to show that water-based substitutional defects in a ZIF-8 structure reduce the water flux and influence its hydrophilicity and ion adsorption performance, which is useful in predicting the type and number of defect sites per unit cell required for RO applications. Of the seven ZIF-8 structures tested, pristine ZIF-8 exhibits the best RO desalination performance.     

An efficient nanofiltration membrane based on blending of polyethersulfone with modified (styrene/maleic anhydride) copolymer

In this study, styrene–maleic anhydride (SMA) copolymer was modified by ring opening reaction of its anhydride groups with diethanolamine (DEA). The modified SMA copolymer was blended in different concentrations (2.5, 4 and 5.5 %) with Polyethersulfone (PES) to improve the hydrophilicity of PES membranes and the corresponding blend membrane was prepared through phase inversion. The influence of SMA copolymer on morphology, mechanical properties, water flux, rejection and anti-fouling properties of blend membrane were investigated. The modified SMA and their composition were confirmed by FT-IR and ¹HNMR techniques. The asymmetric structure of membrane was revealed by SEM. The water flux and contact angle results show that the hydrophilicity of membrane surface was increased by addition of SMA copolymer. The better anti-fouling properties of the PES/modified SMA blend membranes in comparison with the PES membrane also confirmed that the hydrophilicity of blend membrane enhances.     

Exploring the characteristics,performance, and modification of Matrimid for development of thin‐film composite and thin‐film nanocomposite reverse osmosis membranes

Reverse osmosis (RO) membrane technology is widely employed to address the demands for freshwater. In this study, fabrication and performance evaluation of customized RO membranes comprised of Matrimid and polyacrylonitrile (PAN) is carried out. While exploring adoption of slip coating procedure, the effects of various modification techniques including incorporation of TiO₂ nanoparticles and polyethylene glycol (PEG) into the skin layer as well as cross‐linking were investigated. The individual and combined effects of parameters on membrane morphology, surface characteristics and performance were also examined. Despite the distinctive characteristics of involved materials, delamination‐free composite membranes were successfully formed with an intimate contact at the interface of two layers. The results also indicated that increasing concentration of Matrimid in dope solution led to increase in membrane thickness and consequently decline in water flux. In the best case, membrane prepared using 1 wt.% Matrimid in dope exhibited water flux of 0.98 LMH and NaCl rejection of 95.7%. Also, incorporation of 3 wt.% TiO₂ nanoparticles offered membranes with improved water flux of 1.37 LMH and salt rejection of 95.8%. On the other hand, water flux and salt rejection in membranes containing 5 wt.% PEG were 1.18 LMH and 96.2%, respectively. The co‐presence of both nanoparticles and PEG provided more insights about the contributing factors in tuned membranes. Modification of skin layer by cross‐linking significantly improved salt rejection at the expense of water flux. The results are scientifically interpreted and compared to the values reported in literature.     

PEG-coated reverse osmosis membranes: Desalination properties and fouling resistance

This study focuses on the use of surface-coated reverse osmosis (RO) membranes to reduce membrane fouling in produced water purification. A series of crosslinked PEG-based hydrogels were synthesized using poly(ethylene glycol) diacrylate as the crosslinker and poly(ethylene glycol) acrylate, 2-hydroxyethyl acrylate, or acrylic acid as comonomers. The hydrogels were highly water permeable, with water permeabilities ranging from 10.0 to 17.8 (L μm)/(m² h bar). The hydrogels were applied to a commercial RO membrane (AG brackish water RO membrane from GE Water and Process Technologies). The water flux of coated membranes and a series-resistance model were used to estimate coating thickness; the coatings were approximately 2 μm thick. NaCl rejection for both uncoated and coated membranes was 99.0% or greater, and coating the membranes appeared to increase salt rejection, in contrast to predictions from the series-resistance model. Zeta potential measurements showed a small reduction in the negative charge of coated membranes relative to uncoated RO membranes. Model oil/water emulsions were used to probe membrane fouling. Emulsions were prepared with either a cationic or an anionic surfactant. Surfactant charge played a significant role in membrane fouling even in the absence of oil. A cationic surfactant, dodecyltrimethyl ammonium bromide (DTAB), caused a strong decline in water flux while an anionic surfactant, sodium dodecyl sulfate (SDS), resulted in little or no flux decline. In the presence of DTAB, the AG RO membrane water flux immediately dropped to 30% of its initial value, but in the presence of SDS, its water flux gradually decreased to 74% of its initial value after 24 h. DTAB-fouled membranes had lower salt rejection than membranes not exposed to DTAB. In contrast, SDS-fouled membranes had higher salt rejection than membranes not exposed to SDS, with rejection values increasing, in some cases, from 99.0 to 99.8% or higher. In both surfactant tests, coated membranes exhibited less flux decline than uncoated AG RO membranes. Additionally, coated membranes experienced little fouling in the presence of an oil/water emulsion prepared from DTAB and n-decane. For example, after 24 h the water flux of the AG RO membrane fell to 26% of its initial value, while the water flux of a PEGDA-coated AG RO membrane was 73% of its initial value.     

Fouling of reverse osmosis and nanofiltration membranes by humic acid—Effects of solution composition and hydrodynamic conditions

Fouling of reverse osmosis (RO) and nanofiltration (NF) membranes by humic acid, a recalcitrant natural organic matter (NOM), was systematically investigated. The membrane flux performance depended on both hydrodynamic conditions (flux and cross-flow velocity) and solution composition (humic acid concentration, pH, ionic strength, and calcium concentration), and was largely independent of virgin membrane properties. While increasing humic acid concentration and ionic strength, and lowering cross-flow velocity affected flux performance moderately, severe flux reduction occurred at high initial flux, low pH, and high calcium concentration. At a calcium concentration of 1 mM, all the membranes exhibited an identical stable flux, independent of their respective intrinsic membrane permeabilities. The effect of solution composition was more significant at higher fluxes. Improved salt rejection was observed as a result of humic acid fouling, which was likely due to Donnan exclusion by humic material close to membrane surfaces. Greater rejection improvement was observed for membranes with rougher surfaces.     

Fabrication,characterization, and performance evaluation of polyethersulfone/TiO2 nanocomposite ultrafiltration membranes for produced water treatment

In the present work, development of neat and nanocomposite polyethersulfone membranes composed of TiO₂ nanoparticles is presented. Membranes are fabricated using nonsolvent phase inversion process with the objective of improving antifouling, hydrophilicity, and mechanical properties for real and synthetic produced water treatment. Membranes are characterized using scanning electron microscopy, Fourier‐transform infrared, contact angle, porosity measurement, compaction factor, nanoparticles stability, and mechanical strength. The performance of prepared membranes was also characterized using flux measurement and oil rejection. Fourier‐transform infrared spectra indicated that noncovalence bond formed between Ti and polyethersulfone chains. The contact angle results confirmed the improved hydrophilicity of nanocomposite membranes upon addition of TiO₂ nanoparticles owing to the strong interactions between fillers and water molecules. The increased water flux for nanocomposite membranes in comparison with neat ones can be due to coupling effects of improved surface hydrophilicity, higher porosity, and formation of macrovoids in the membrane structure. The membrane containing 7 wt% of TiO₂ nanoparticles was the best nanocomposite membrane because of its high oil rejection, water flux, antifouling properties, and mechanical stability. The pure water flux for this membrane was twice greater than that of neat membrane without any loss in oil rejection. The hydrophilicity and antifouling resistance against oil nominates developed nanocomposite membranes for real and synthetic produced water treatment applications with high performance and extended life span.     

水性纳米硅溶胶的表面改性及其应用研究

将几种功能性有机硅烷偶联剂与不同pH值的无机纳米硅溶胶水溶液通过表面改性反应和溶胶-凝胶技术制备出水分散纳米杂化改性硅溶胶,并用透射电镜TEM和红外光谱FT-IR对其进行表征和分析,研究了硅烷偶联剂碳链长度及偶联剂用量、溶液pH值对改性硅溶胶稳定性的影响,初步探讨了其在水性涂料、水性纳米罩光防污涂料及砂浆防水中的性能和应用。     

Fouling of reverse osmosis membranes by biopolymers in wastewater secondary effluent: Role of membrane surface properties and initial permeate flux

Reverse osmosis (RO) is being increasingly used in treatment of domestic wastewater secondary effluent for potable and non-potable reuse. Among other solutes, dissolved biopolymers, i.e., proteins and polysaccharides, can lead to severe fouling of RO membranes. In this study, the roles of RO membrane surface properties in membrane fouling by two model biopolymers, bovine serum albumin (BSA) and sodium alginate, were investigated. Three commercial RO membranes with different surface properties were tested in a laboratory-scale cross-flow RO system. Membrane surface properties considered include surface roughness, zeta potential, and hydrophobicity. Experimental results revealed that membrane surface roughness had the greatest effect on fouling by the biopolymers tested. Accordingly, modified membranes with smoother surfaces showed significantly lower fouling rates. When Ca²⁺ was present, alginate fouled RO membranes much faster than BSA. Considerable synergistic effect was observed when both BSA and alginate were present. The larger foulant particle sizes measured in the co-existence of BSA and alginate indicate formation of BSA-alginate aggregates, which resulted in greater fouling rates. Faster initial flux decline was observed at higher initial permeate flux even when the flux was measured against accumulative permeate volume, indicating a negative impact of higher operating pressure.     

Qualitative spectroscopic characterization of the matrix–silane coupling agent interface across metal fibre reinforced ion exchange resin composite membranes

The characterization of novel metal reinforced electro-dialysis ion exchange membranes, for water desalination, by attenuated total reflectance Fourier transform infrared spectroscopy mapping is presented in this paper. The surface of the porous stainless steel fibre meshes was treated in order to enhance the amount of surface oxide groups and increase the material hydrophilicity. Then, the metal membranes were functionalized through a sol–gel reaction with silane coupling agents to enhance the affinity with the ion exchange resins and avoid premature metal oxidation due to redox reactions at the metal–polymer interface. Polished cross sections of the composite membranes embedded into an epoxy resin revealed interfaces between metallic frameworks and the silane layer at the interface with the ion exchange material. The morphology of the metal–polymer interface was investigated with scanning electron microscopy and Fourier transform infrared micro-spectroscopy. Fourier transform infrared mapping of the interfaces was performed using the attenuated total reflectance mode on the polished cross-sections at the Australian Synchrotron. The nature of the interface between the metal framework and the ion exchange resin was shown to be homogeneous and the coating thickness was found to be around 1 μm determined by Fourier transform infrared micro-spectroscopy mapping. The impact of the coating on the properties of the membranes and their potential for water desalination by electro-dialysis are also discussed.     

Electrochemical behavior of methyl viologen at graphite electrodes modified with Nafion sol–gel composite

Electrodes were prepared by spin-coating spectroscopic graphite rods with a Nafion doped sol. Coating solutions consisting of Nafion:TEOS (tetraethoxysilane) ratios of 3:1 and 4:1 gave smooth films on the electrode surface. These modified electrodes were evaluated and compared with Nafion modified and bare spectroscopic graphite electrodes using methyl viologen (MV²⁺) as a representative cationic electroactive probe. Substantial partitioning of MV²⁺ into the Nafion:sol–gel matrix to the electrode surface was observed by cyclic voltammetry and square wave voltammetry. Cyclic voltammograms of MV²⁺ in 0.1 M NaCl at Nafion:sol–gel 4:1 modified electrodes showed a reversible reduction to MV⁺ with E^0′=−0.695 V vs. Ag/AgCl. Results of scan rate variation showed the wave to be characterized by semi-infinite diffusion for scan rates in the range 50–500 mV/s. Slowing the scan rate below 50 mV/s resulted in a transition to thin-layer behavior. MV²⁺ partitioned much more quickly into the sol–gel-Nafion modified electrodes compared to pure Nafion modified electrodes. Reversibility of the MV²⁺-loaded modified Nafion-doped sol–gel coatings on electrodes was obtained by soaking in 1 M NaCl solution. Concentration calibration plots for MV²⁺ at the sol–gel-Nafion modified electrodes were nonlinear. Substantial enhancement of current signal at low concentrations was observed by square wave voltammetry.     

Surface modification of γ-alumina membranes by silane coupling for CO2 separation

Top layers of γ-Al₂O₃ composite membranes have been modified by the silane coupling technique using phenyltriethoxysilane for improving the separation factor of CO₂ to N₂. The separation efficiency of the modified membranes was strongly dependent upon the hydroxylation tendency of the support materials and the amount of the special functional group (i.e. phenyl radical) which was coupled onto a top layer. The separation factor through the TiO₂ supported γ-Al₂O₃ membrane was found to be fairly enhanced by silane coupling, but in case of the -Al₂O₃ supported membrane was not. The CO₂/N₂ separation factor through the modified γ-Al₂O₃/TiO₂ composite membrane is 1.7 at 90°C and ΔP = 2 × 10⁵ Pa for the binary mixture containing 50 vol% CO₂. The separation factor is proportional to the CO₂ concentration in the gas mixture, and the modified membrane is stable up to 100°C. The main mechanism of the CO₂ transport through the modified γ-Al₂O₃ layer is known to be a surface diffusion.     

Application of amperometric sol-gel biosensor to flow injection determination of glucose

A glucose biosensor with enzyme immobilised by sol–gel technology was constructed and evaluated. The glucose biosensor reported is based on encapsulated GOX within a sol–gel glass, prepared with 3-aminopropyltriethoxy silane, 2-(3,4-epoxycyclohexyl)-ethyltrimetoxy silane and HCl. A flow system incorporating the amperometric biosensor constructed was developed for the determination of glucose in the 1×10⁻⁴–5×10⁻³ mol l⁻¹ range with a precision of 1.5%. The results obtained for the analysis of electrolytic solution for iv administration and human serum samples showed good agreement between the proposed method and the reference procedure, with relative error <5%.