MembrFactory: A Force Field and composition Double Independent Universal Tool for Constructing Polyamide Reverse Osmosis Membranes

The topmost polyamide (PA) layer of the thin-film-composite reverse osmosis (RO) membrane is the most important part in the membrane-based RO technology. With the aid of molecular dynamics simulations, many PA layer-related features in the RO process can be revealed. With many novel types of PA RO membranes out of trimesoyl chloride/m-phenylenediamine monomers developed in the laboratory, a convenient model building tool for these PA layer systems is urgently needed to conduct the theoretical analysis. Here, we develop a new universal toolkit for constructing PA RO membranes, named as MembrFactory, which combines flexibility of force fields and membrane compositions. A key characteristic of our approach is the use of monomers as the starting state, and the final membrane model was obtained automatically by stepwise reaction between the functional groups on the monomers. The reliability of MembrFactory has been validated by constructing several common PA RO membranes. © 2019 Wiley Periodicals, Inc.     

Effect of skin layer surface structures on the flux behaviour of RO membranes

The relationship between the surface structures of skin layers of crosslinked aromatic polyamide composite reverse osmosis (RO) membranes and their RO performances have been studied using two surface analytical techniques: SEM and AFM. As a result, it was found that RO membranes whose skin layer surface structures were rough produced high fluxes, and an approximately linear relationship existed between this surface roughness and RO membrane flux. Accordingly, skin layer surface unevenness of crosslinked aromatic polyamide composite RO membranes is regarded as an enlargement of the effective RO membrane area.     

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.     

The relationship between polymer molecular structure of RO membrane skin layers and their RO performances

The relationship between polymer molecular structure of the skin layers of crosslinked polyamide composite reverse osmosis (RO) membranes and their RO performances was studied. The molecular structures of these RO membrne skin layers was designed using a computer calculation method. It was found that the molecular structures of crosslinked polyamides exhibiting good RO performances possessed relatively flat and uniform network structures.     

Membrane potential across reverse osmosis membranes under pressure gradient

Membrane potential measurement has been widely used for the characterization of ionic membranes such as ion-exchange membranes without solvent permeability. However, there have been few studies on membrane potentials across pressure-driven processes such as reverse osmosis (RO) membranes with solvent permeability. In the present study, the membrane potential across RO membranes in NaCl and MgCl2 under the pressure gradient, DeltaP=0-0.3 MPa, was measured. The experimental results were analyzed by the theoretical model based on the Donnan equilibrium and the extended Nernst-Planck flux equation considering the pressure effect. The theoretical values agreed well with the experimental ones. This indicates that membrane potential is useful for characterizing the effective charge density of the active layer of RO membranes under pressure gradient.     

Scalable Antifouling Reverse Osmosis Membranes Utilizing Perfluorophenyl Azide Photochemistry

We present a method to produce anti‐fouling reverse osmosis (RO) membranes that maintains the process and scalability of current RO membrane manufacturing. Utilizing perfluorophenyl azide (PFPA) photochemistry, commercial reverse osmosis membranes were dipped into an aqueous solution containing PFPA‐terminated poly(ethyleneglycol) species and then exposed to ultraviolet light under ambient conditions, a process that can easily be adapted to a roll‐to‐roll process. Successful covalent modification of commercial reverse osmosis membranes was confirmed with attenuated total reflectance infrared spectroscopy and contact angle measurements. By employing X‐ray photoelectron spectroscopy, it was determined that PFPAs undergo UV‐generated nitrene addition and bind to the membrane through an aziridine linkage. After modification with the PFPA‐PEG derivatives, the reverse osmosis membranes exhibit high fouling‐resistance.

     

羟丙基醋酸纤维素反渗透膜

研究了羟丙基醋酸纤维素反渗透膜的制膜工艺和膜性能。实验表明,以甘油—正丙醇或磷酸为添加剂,蒸发时间30—60s,预热处理温度70℃,时间3—5min,可得到在2MPa操作压力下,氯化钠脱除率95—98％,水通量1.0～2.1ml/cm~2·h的反渗透膜。羟丙基醋酸纤维素膜具有一定的耐热性,其使用温度上限比醋酸纤维素膜至少提高了10℃以上。羟丙基醋酸纤维素的溶解性能与醋酸纤维素相似。X射线衍射和热重分析显示羟丙基醋酸纤维素具有与醋酸纤维素相似的聚集态结构。     

Nuclear magnetic resonance microscopy studies of membrane biofouling

There is a substantial need for novel measurement techniques that enable non-invasive spatially resolved observation of biofouling in nanofiltration (NF) and reverse osmosis (RO) membrane modules. Such measurements will enhance our understanding of the key design and operational parameters influencing biofilm fouling. In this study we demonstrate the first application of nuclear magnetic resonance microscopy (NMR) to a spiral wound reverse osmosis (RO) membrane module. The presented NMR protocols allow the extraction of the evolution with biofouling of (i) the spatial biofilm distribution in the membrane module, (ii) the spatially resolved velocity field and (iii) displacement propagators, which are distributions of molecular displacement of a passive tracer (in our case, water) in the membrane. From these measurements, the effective membrane surface area is quantified. Despite the opaque nature of membrane design, NMR microscopy is shown to be able to provide a non-invasive quantitative measurement of RO membrane biofouling and its impact on hydrodynamics and mass transport. Minimal biofilm growth is observed to have a substantial impact on flow field homogeneity.     

Factors influencing reverse osmosis rejection of inorganic solutes from aqueous solution

Reverse osmosis (RO) rejection is strongly influenced by the distribution of solute between the membrane and solvent phases. For this reason, we examined the partition coefficients of inorganic compounds between water and cellulose acetate (CA) membranes. Cation and anion partition coefficients were determined by independent analyses. Effects of fixed (negative) membrane charges on CA are clearly apparent at low solute concentrations. The mean cation/anion partition coefficients decrease with the product of the cation and anion valence, and increase with increasing ionic size. Un-ionized inorganic compounds, HgC1₂ and HAuC1₄, are strongly sorbed by CA membranes. All of these observations are consistent with electrostatic theory.Experimental membrane/water partition coefficients are influenced by temperature, pH, and ion-pairing. CA membranes exhibit swelling and shrinkage when exposed to certain aqueous solutions. Swelling and shrinkage influence solute partition and diffusion coefficients, the water content of the membranes, and their RO rejection.The present results provide a comprehensive experimental basis for understanding the mechanism of RO rejection by CA membranes. Moreover, these results can be used to predict RO behavior under a wide variety of experimental conditions. The potential use of reverse osmosis in a variety of wastewater applications is considered in some detail.     

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.     

Role of membrane surface morphology in colloidal fouling of cellulose acetate and composite aromatic polyamide reverse osmosis membranes

Laboratory-scale colloidal fouling tests, comparing the fouling behavior of cellulose acetate and aromatic polyamide thin-film composite reverse osmosis (RO) membranes, are reported. Fouling of both membranes was studied at identical initial permeation rates so that the effect of the transverse hydrodynamic force (permeation drag) on the fouling of both membranes is comparable. Results showed a significantly higher fouling rate for the thin-film composite membranes compared to that for the cellulose acetate membranes. Addition of an anionic surfactant (sodium dodecyl sulfate, SDS) to mask variations in chemical and electrokinetic surface characteristics of the cellulose acetate and aromatic polyamide membranes resulted in only a small change in the fouling behavior. The higher fouling rate for the thin-film composite membranes is attributed to surface roughness which is inherent in interfacially polymerized aromatic polyamide composite membranes. AFM and SEM images of the two membrane surfaces strongly support this conclusion. These surface images reveal that the thin-film composite membrane exhibits large-scale surface roughness of ridge-and-valley structure, while the cellulose acetate membrane surface is relatively smooth.     

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.     

Osmotic backwash mechanism of reverse osmosis membranes

A new osmotic backwash (BW) model for reverse osmosis (RO) membranes was developed for conditions of no applied pressures across the membrane. This analytical model has one adjustable parameter representing the coefficient of a linearized convection term in the general convection–diffusion equation. An experimental RO/BW system was used for 12 data sets to verify the proposed BW model and illustrate its predictability. Results show deviations of the model from the data within a range of 5–15%. The described dilution mechanism of the feed concentration polarization (CP) layer is based on RO originated concentrated layer detachment from the membrane surface followed by its gradual dilution.The understanding gained in this research may be applied to automatic RO/BW cleaning cycles. A dominant RO parameter of the BW process is the RO initial driving force—the concentration difference across the membrane. Other RO process parameters – applied pressure and feed flow rate – have lesser effects. Both theoretical and experimental methods provide quantitative relationships between RO and BW variables that enable an understanding and control of the BW process.     

Influence of membrane surface properties on initial rate of colloidal fouling of reverse osmosis and nanofiltration membranes

Recent studies have shown that membrane surface morphology and structure influence permeability, rejection, and colloidal fouling behavior of reverse osmosis (RO) and nanofiltration (NF) membranes. This investigation attempts to identify the most influential membrane properties governing colloidal fouling rate of RO/NF membranes. Four aromatic polyamide thin-film composite membranes were characterized for physical surface morphology, surface chemical properties, surface zeta potential, and specific surface chemical structure. Membrane fouling data obtained in a laboratory-scale crossflow filtration unit were correlated to the measured membrane surface properties. Results show that colloidal fouling of RO and NF membranes is nearly perfectly correlated with membrane surface roughness, regardless of physical and chemical operating conditions. It is further demonstrated that atomic force microscope (AFM) images of fouled membranes yield valuable insights into the mechanisms governing colloidal fouling. At the initial stages of fouling, AFM images clearly show that more particles are deposited on rough membranes than on smooth membranes. Particles preferentially accumulate in the “valleys” of rough membranes, resulting in “valley clogging” which causes more severe flux decline than in smooth membranes.     

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.     

Reverse osmosis filtration for space mission wastewater: membrane properties and operating conditions

Reverse osmosis (RO) is a compact process that has potential for the removal of ionic and organic pollutants for recycling space mission wastewater. Seven candidate RO membranes were compared using a batch stirred cell to determine the membrane flux and the solute rejection for synthetic space mission wastewaters. Even though the urea molecule is larger than ions such as Na+, Cl-, and NH4+, the rejection of urea is lower. This indicates that the chemical interaction between solutes and the membrane is more important than the size exclusion effect. Low pressure reverse osmosis (LPRO) membranes appear to be most desirable because of their high permeate flux and rejection. Solute rejection is dependent on the shear rate, indicating the importance of concentration polarization. A simple transport model based on the solution-diffusion model incorporating concentration polarization is used to interpret the experimental results and predict rejection over a range of operating conditions. Grant numbers: NAG 9-1053.     

PDMS coating of used TFC-RO membranes for O2/N2 and CO2/N2 gas separation applications

A new application for used reverse osmosis (RO) membranes as gas separation membranes was studied. In this regard, firstly, three pretreatment procedures were used to remove the foulants from the surface of used membrane and then they were coated with polydimethylsiloxane (PDMS). The results indicated that PDMS-coated used RO membranes were capable of separating O₂/N₂ and CO₂/N₂. The maximum O₂/N₂ and CO₂/N₂ selectivities of coated membranes were 5.9 and 32.5, respectively. The O₂/N₂ and CO₂/N₂ selectivities of PDMS membrane were reported in the range of 2.1–2.2 and 11–12, respectively. Finally, an economic assessment was carried out to compare prepared PDMS coated RO membranes with commercial PPO membrane. This showed that coated membranes are less expensive than PPO membrane for CO₂/N₂ gas separation. The outcome of the research was a simple method for converting used RO membranes to cost effective gas separation membranes.     

Experimental study of desalination using direct contact membrane distillation: a new approach to flux enhancement

New membrane distillation configurations and a new membrane module were investigated to improve water desalination. The performances of three hydrophobic microporous membranes were evaluated under vacuum enhanced direct contact membrane distillation (DCMD) with a turbulent flow regime and with a feed water temperature of only 40 °C. The new configurations provide reduced temperature polarization effects due to better mixing and increased mass transport of water due to higher permeability through the membrane and due to a total pressure gradient across the membrane. Comparison with previously reported results in the literature reveals that mass transport of water vapors is substantially improved with the new approach. The performance of the new configuration was investigated with both NaCl and synthetic sea salt feed solutions. Salt rejection was greater than 99.9% in almost all cases. Salt concentrations in the feed stream had only a minor effect on water flux. The economic aspects of the enhanced DCMD process are briefly discussed and comparisons are made with the reverse osmosis (RO) process for desalination.     

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.     

Grafting of polyaniline by a dynamic inverse emulsion polymerization technique onto reverse osmosis membranes as an antibiofouling agent

This study reports on the benefits of an in situ interfacial dynamic inverse emulsion polymerization process under sonication of aniline in the presence of a commercial reverse osmosis (RO) membrane. This polymerization method is simple and much faster (5‐15 min) than systems reported in the literature. During polymerization, the membranes are coated with polyaniline (PANI) as verified by high‐resolution scanning electron microscopy (HRSEM) images and Fourier‐transform‐infrared (FTIR) measurements. A colony‐counting antimicrobial activity test showed that whereas the reference RO membrane developed a large bacterial colony, the polyaniline‐coated RO membrane had no colonies at all. Surface resistivity was the lowest when the pH levels were below 6, which corresponded to the polyaniline‐grafted conductive layer. The membrane flow properties were only modified slightly as a result of the polyaniline grafting, compared with a pristine reference membrane.