Preparation of reverse osmosis membranes by plasma polymerization of organic compounds

The plasma polymerization of organic compounds was used to prepare a composite reverse osmosis membrane which consists of an ultrathin semipermeable membrane formed by plasma polymerization of an organic compound or compounds and a porous substrate. Many nitrogen-containing compounds (aromatic amines, heteroaromatic compounds, aliphatic amines, and nitriles) were found to yield excellent reverse osmosis membranes by plasma polymerization directly onto porous substrates such as Millipore filters, porous polysulfone filters, and porous glass tubes. Factors involved in the preparation of reverse osmosis membranes by plasma polymerization were investigated and discussed. The plasma polymerized membranes have the following unique features: (1) very stable performance independent of salt concentration and applied pressure (practically no water flux decline was observed with many membranes): (2) salt rejection and water flux both increase with time in the initial stage of reverse osmosis (consequently, the performance of the membrane improves with time of operation); (3) very high salt rejection (over 99%) with high water flux (up to 38 gfd) can be obtained with 3.5% NaCl at 1500 psi (membranes perform equally well under conditions of sea water conversion and brakish water treatment).     

The use of direct osmosis tests as complementary experiments to determine the water and salt permeabilities of reinforced cellulose acetate membranes

In this article the use of the direct osmosis test as a complementary experiment to the reverse osmosis test is described. The membranes investigated are cellulose acetate membranes reinforced with mineral fillers. In both tests the same influence of the type of fillers on the water permeability coefficients is found. The salt permeability coefficients indicate the presence of pressure sensitive defects in the reinforced membranes. The direct osmosis test is found to be a suitable test to confirm and predict the membrane properties under reverse osmosis conditions.     

Crosslinked poly(hydroxyethyl methacrylate) membranes for desalination by reverse osmosis

A series of crosslinked hydroxyethyl methacrylate (HEMA) membranes for reverse osmosis desalination has been prepared. The crosslinkers used were trimethylol propane trimethacrylate (TPT) or ethylene glycol dimethacrylate (EGD). Membranes were synthesized by polymerizing the monomers as a thin homogeneous film. In addition to reverse osmosis tests, the membranes were also characterized by osmosis experiments and sorption measurements. The reverse osmosis water flux (1500 psi applied pressure, 4% NaCl brine, pH = 5) for these membranes decreases from 0.6 gallonmil/ft²-day (GMFD) to 0.055 GMFD and salt rejection increase from 78% to a maximum of 94% as the amount of TPT is increased from 0 to 11 mole-%. Water contents decrease from 42% to 15% over the same range of crosslinker, but the preferential sorption of water to salt does not vary. Thus, rises in reverse-osmosis semipermeability were found to result from changes in water–salt diffusivity ratios. The mechanism of permselectivity has been interpreted in terms of parallel diffusive fluxes across the membrane of primary H-bonded water and secondary water plus salt ions.     

Concentration and pressure dependence of rate of membrane permeation

The permeation of water and sodium chloride in cellulose acetate membranes has been examined over a wide range of concentration and pressure. The results obtained from reverse osmosis experiments have been used to evaluate relations derived on the assumption that permeation of both solution components takes place by diffusion down a concentration gradient in the membrane. With the aid of equilibrium and raidoactive tracer measurements, most of the deviations could be attributed to nonconstancy of the diffusion, and, especially, the distribution coefficients of water. A comparison of the net flux in reverse osmosis with the rate of tracer permeation in the same membrane provided positive evidence to show that hydrodynamic flow under pressure cannot account for the water flux through the membrane. Differences in the shape of the distribution isotherms for salt and water between solution and membrane provide an explanation for the high selectivity of cellulose acetate membranes in favor of water.     

反渗透膜的研究进展与展望

反渗透具有低能耗、高效率等突出优点,是目前应用最为广泛的分离技术之一。反渗透膜的性能是影响反渗透过程效率的决定因素,反渗透膜的研制一直是国内外膜领域的研究热点。特别是近年来,石墨烯、碳纳米管等新型材料展现出优异的水传递行为,成为新型反渗透膜材料的研究热点。本论文回顾了反渗透膜的研制发展历程,介绍了不同单体通过界面聚合反应成膜的研究进展,综述了国内外新型混合基质膜和无机分子筛反渗透膜材料及其成膜研究,最后提出了新型反渗透膜的研究方向。     

循环水排污水回收处理中过滤工艺的选择

循环水由于含盐量较高,在对其排污水进行回收处理时一般采用反渗透除盐工艺。反渗透膜对于进水的洁净度要求较为严格,为此采用压力式过滤器加超滤的预处理工艺处理循环水排污水,对三种压力式过滤器和两种超滤膜的出水特性比较试验,试验表明,多介质过滤器加外压式中空纤维超滤膜的过滤工艺,其出水水质和运行稳定性均满足后续反渗透设备的要求。     

具有三维聚酰胺脱盐网络结构的无纺布复合正渗透膜

利用间苯二胺（MPD）和均苯三甲酰氯（TMC）,直接在聚酯无纺布（NV）织物的多孔空间中进行界面聚合,形成大通量无纺布复合正渗透（NVC-FO）膜。NVC-FO膜在无纺布内部形成的多层次三维（3-D）聚酰胺结构,分布在30~50μm深的聚对苯二甲酸乙二醇酯支撑材料的内部。这种相对松散的有深度的3-D聚酰胺网络,不仅透水表面积大,而且可以避免薄层聚酰胺缺陷导致的高漏盐性,有较低的反向盐通量。进一步研究发现,在一定范围内降低单体质量分数（MPD 1%~0.01%,TMC 0.5%~0.005%）,可以形成更宽广的3-D聚酰胺网络结构,在保持较低的反向盐通量的同时得到更高的水通量。使用1mol/L NaCl作为汲取溶液,优化的NVC-FO膜水通量最高可以达到193.54L/(m²·h),反向盐通量为0.047mol/(m²·h)。采用加压正渗透实验,发现这些高通量NVC-FO膜的盐穿透破裂压力在200~1400Pa之间,而且证实了降低单体质量分数会导致膜的耐压性能显著降低。尽管NVC-FO膜的耐压性能有待提高,但是该研究有可能为构建高脱盐性能的FO膜提供一条新的思路。     

Removal of methyl orange dye and Na2SO4 salt from synthetic waste water using reverse osmosis

The efficiency of reverse osmosis (RO) membranes used for treatment of colored water effluents can be affected by the presence of both salt and dyes. Concentration polarization of each of the dye and the salt and the possibility of a dynamic membrane formed by the concentrated dye can affect the performance of the RO membrane. The objective of the current work was to study the effect of varying the Na₂SO₄ salt and methyl orange (MO) dye concentrations on the performance of a spiral wound polyamide membrane. The work also involved the development of a theoretical model based on the solution diffusion (SD) mass transport theory that takes into consideration a pressure dependent dynamic membrane resistance as well as both salt and dye concentration polarizations. Control tests were performed using distilled water, dye/water and salt/water feeds to determine the parameters for the model. The experimental results showed that increasing the dye concentration from 500 to 1000 ppm resulted in a decrease in the salt rejection at all of the operating pressures and for both feed salt concentrations of 5000 and 10,000 ppm. Increasing the salt concentration from 5000 to 10,000 ppm resulted in a slight decrease in the percent dye removal. The model’s results agreed well with these general trends.     

用于反渗透复合膜的海绵状结构支撑膜制备研究

通过调节铸膜液中聚砜浓度和非溶剂含量,浸没沉淀法制备海绵状结构的支撑膜,并在支撑膜上界面聚合制备聚酰胺反渗透复合膜。分别对支撑膜及反渗透复合膜的结构和性能进行表征,考察聚砜浓度对支撑膜结构和性能的影响,以及不同结构支撑膜对反渗透复合膜结构和性能的影响。结果显示,随着聚砜浓度的增加,支撑膜表面孔径和孔隙率下降,断面结构变致密,耐压性增强。在不同支撑膜上制备的反渗透复合膜具有不同的通量和脱盐率。综合考虑支撑膜及反渗透复合膜的性能,以聚砜浓度为15%制备的海绵状结构支撑膜更适于作为制备反渗透复合膜的支撑层。     

Precise measurement of membrane constants of cellulose acetate membranes by direct osmosis tests

A method for the determination of membrane constants by direct osmosis tests was studied using cellulose acetate membranes. A countercurrent type osmosis cell was designed and made for this study, and a method for precise measurements of permeated water and solutes through the membrane was established. Based on the membrane constants derived from direct osmosis tests, membrane performances of cellulose acetate membranes under pressure of 40 atm were predicted. The predicted values were in good agreement with the observed values in reverse osmosis experiments and it was confirmed that membrane performances under pressure could be predicted by the direct osmosis with considerably good accuracy.     

Spinning of cellulose acetate hollow fiber by dry-wet technique of 3C-shaped spinneret

Cellulose acetate hollow fibers were spun by a new method—a dry–wet spinning technique of a 3C-shaped spinneret. The spinning technique parameters effecting the form and the reverse osmosis performances of the hollow fiber were investigated in detail, such as polymer concentration, the kind of solvent and additive, spinneret temperature, extrasion rate, evaporation distance, and take-up rate. Heat treating for different times in several treating baths was tested. The results showed that cellulose acetate hollow fiber spun by this method is feasible and is a kind of “loose” reverse osmosis membrane and suitable to operate at ultralow pressure, 0.8 MPa, and exhibits a higher flux rate at a salt rejection of 60–85% for tap water. Cellulose acetate hollow fiber for ultralow pressure reverse osmosis should find wide application in industrial processes. © 1996 John Wiley & Sons, Inc.     

Membranes composites preparees a partir d'alcool polyvinylique et de diisocyanate de toluylene destinees a l'osmose inverse

Composite reverse osmosis membranes have been prepared by coating a Millipore filter with an ultrathin layer of polyvinyl alcohol crosslinked by toluene diisocyanate. The membranes present a good selectivity towards aqueous salt solutions (for instance water flux of 500 liters/m²-day at 28°C, salt rejection of 98% with a 3.5% NaCl solution under a pressure of 100 bars) and have better temper-ature and pressure stabilities than cellulose acetate membranes. Their resistance to acid and alkaline hydrolysis is satisfactory. The influence of different preparation factors on the osmotic properties of the membranes was examined.     

Fractionation of aqueous organic mixtures by reverse osmosis

Modern synthetic polymer membranes for reverse osmosis, although primarily developed for seawater desalination, are remarkably selective and chemically resistant in the case of organic solvents. Accordingly, reverse osmosis is increasingly employed for the treatment of industrial effluents such as solvent-contaminated waste water. In this case flux and selectivity can no longer be calculated with the simple relationships valid for seawater desalination. Semi-empirical relationships are presented, capable of quantitatively describing the local partial fluxes of organic-aqueous solutions. The relationships are based on the solution diffusion model but contain less stringent simplifying assumptions than in the case of dilute salt solutions.

Compared to empirical relationships, very few experiments are required to determine the model parameters. In combination with the differential mass and material balances, these equations are sufficient for process design. This is illustrated by a comparison of calculated results and experiments on a pilot plant scale and is even valid for quasi-binary systems.     

Fundamental water and salt transport properties of polymeric materials

Fundamental water and salt transport properties of polymers are critical for applications such as reverse osmosis (RO), nanofiltration (NF), forward osmosis (FO), pressure-retarded osmosis (PRO), and membrane capacitive deionization (MCDI) that require controlled water and salt transport. Key developments in the field of water and salt transport in polymer membranes are reviewed, and a survey of polymers considered for such applications is provided. Many polymers considered for such applications contain charged functional groups, such as sulfonate groups, that can dissociate in the presence of water. Water and ion transport data from the literature are reviewed to highlight the similarities and differences between charged and uncharged polymers. Additionally, the influence of other polymer structure characteristics, such as cross-linking and morphology in phase separated systems, on water and salt transport properties is discussed. The role of free volume on water and salt transport properties is discussed. The solution–diffusion model, which describes the transport of water and ions in nonporous polymers, is used as a framework for discussing structure/property relations in polymers related to water and salt transport properties. Areas where current knowledge is limited and opportunities for further research are also noted.     

Theoretical modification of the finely porous model for reverse osmosis transport

One of the most significant models used to describe and predict the performance of reverse osmosis type membranes is the finely porous model (FPM). In this paper, the basic assumptions of the model are examined and modified. The two most serious problems with FPM are that an incorrect form of material balance on the solute is used and that the osmotic pressure effects are not completely taken into account for electrolytes. A modified model (called MD-FPM), which is based on the same physical precepts is derived. Equations describing the concentration profile for both models have been derived and compared. It has been shown that the FPM can predict physically unacceptable results. Difficulties in using the parameters from the model for prediction or for membrane development work are discussed. Simulation results for the MD-FPM model are consistent with what is expected for reverse osmosis type membranes.     

Theory of reverse osmosis and some other membrane permeation operations

The general permeation equations for various transport operations using membranes were correlated according to the solution-diffusion theory. It was shown that for some important conditions, the permeation properties for reverse osmosis can be generated from those of pervaporation. The use of reverse osmosis with pressure smaller than 2000 psi is calculated to be of limited use for the purification of water with small amounts of organic compounds.     

Nanofiltration membranes broaden the use of membrane separation technology

Most reverse osmosis (RO) research has concentrated on the development of single-pass seawater membranes. The success of these high rejection membranes has created interest in other applications requiring less demanding salt rejection, or desiring the elimination of salt from a feed stream (diafiltration), or having severe chemical resistance requirements. All would prefer to operate at lower net driving pressures than demanded by the high rejection membranes. This paper reports on the characteristics of three such tailored membranes. These membranes have been designated as “quo;nanofiltration”quo; membranes to distinguish them from the “quo;hyperfiltration”quo; seawater membranes. The first is XP45, a polyamide membrane with a low sodium chloride rejection that makes it an excellent candidate for applications such as the processing of salty cheese wheys and pharmaceutical preparations. The second is NF70, another polyamide, a low pressure membrane with rejections suited for converting mildly brackish water and organic-laden raw water to potable water that meets WHO standards. The third is XP20, a new developmental membrane for the maintenance of electroless copper plating baths.     

Synthesis and crosslinking of partially disulfonated poly(arylene ether sulfone) random copolymers as candidates for chlorine resistant reverse osmosis membranes

Biphenol-based, partially disulfonated poly(arylene ether sulfone)s synthesized by direct copolymerization show promise as potential reverse osmosis membranes. They have excellent chlorine resistance over a wide range of pHs and good anti-protein and anti-oily water fouling behavior. Crosslinking of these copolymers that have high degrees of disulfonation may improve salt rejection of the membranes for reverse osmosis performance. A series of controlled molecular weight, phenoxide-endcapped, 50% disulfonated poly(arylene ether sulfone)s were synthesized. The copolymers were reacted with a multifunctional epoxy resin and crosslinked thermally. The effects on network properties of various factors such as crosslinking time, copolymer molecular weight and epoxy concentration were investigated. The crosslinked membranes were characterized in terms of gel fraction, water uptake, swelling and self-diffusion coefficients of water. The salt rejection of the cured membranes was significantly higher than that for the uncrosslinked copolymer precursors.     

Highly permeable reverse osmosis membranes incorporated with hydrophilic polymers of intrinsic microporosity via interfacial polymerization

Enhancing the water permeation while maintaining high salt rejection of existing reverse osmosis (RO) membranes remains a considerable challenge. Herein, we proposed to introduce polymer of intrinsic microporosity, PIM-1, into the selective layer of reverse osmosis membranes to break the trade-off effect between permeability and selectivity. A water-soluble a-LPIM-1 of low-molecular-weight and hydroxyl terminals was synthesized. These designed characteristics endowed it with high solubility and reactivity. Then it was mixed with m-phenylenediamine and together served as aqueous monomer to react with organic monomer of trimesoyl chloride via interfacial polymerization. The characterization results exhibited that more “nodule” rather than “leaf” structure formed on RO membrane surface, which indicated that the introduction of the high free-volume of a-LPIM-1 with three dimensional twisted and folded structure into the selective layer effectively caused the frustrated packing between polymer chains. In virtue of this effect, even with reduced surface roughness and unchanged layer thickness, the water permeability of prepared reverse osmosis membranes increased 2.1 times to 62.8 L·m^-2·h^-1 with acceptable NaCl rejection of 97.6%. This attempt developed a new strategy to break the trade-off effect faced by traditional polyamide reverse osmosis membranes.     

Preparation and properties of poly(acrylic acid) composite membranes

A new type of membrane has been prepared for hyperfiltration (reverse osmosis) desalination that is essentially a very thin polyelectrolyte membrane. It is prepared by casting an aqueous solution of a polyelectrolyte, specifically poly(acrylic acid) (PAA), directly on one surface of a finely porous support membrane. In hyperfiltration tests, these composite membranes exhibit desalination performance comparable in dilute solutions to that observed with cellulose acetate membranes of the Loeb-Sourirajan type. The water flux through these membranes is linear in the pressure up to 100 atm. Salt rejection is a function of pressure; it is also a function of the concentration of the feed solution and the charge of the counterion, in qualitative agreement with the Donnan ion-exclusion mechanism. Typical long-term results range from water fluxes of 2 × 10^?3 g/cm²-sec (50 gal/ft²-day) and 80% salt rejection to 0.2 × 10^?3 g/cm²-sec (5 gal/ft²-day) and >99.5% salt rejection at 1500 psi with 0.3 wt-% NaCl. These membranes appear to be useful for brackish water desalination.