运用醋酸纤维素丁酯和芳香聚酰胺膜的二元有机液体混合物的反渗透法分离

本文运用实验室制备的醋酸纤维素丁酯膜和芳香聚酰胺膜系统研究并证实了反渗透技术用于分离液体有机混合物的适用性,并用液相色谱法则量了上述二种膜材料的优先吸附性能。研究发现:反渗透枝术能够应用于液体有机混合物的分离。通过考虑进料液中每一个组分的优先吸附特性和斯托克斯半径,能够预测出二元混合物中哪一组分会在膜透过液中得到浓缩。     

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.     

Ionically crosslinked poly(acrylic acid) membranes. II. Dry technique

The ionic crosslinking of poly(acrylic acid) for dialysis and reverse osmosis applications has been studied. A new dry technique has been developed that is faster than the wet technique reported in the previous paper to synthesize aluminum/poly(acrylic acid) membranes. It is based on casting a solution containing both the aluminum salt and the unneutralized poly(acrylic acid) (PAA) and subjecting the cast film to a heat treatment (curing step) to promote ionic crosslinking. Postcuring treatment in nonsolvents for PAA such as acetone induced structural modifications in the membrane. Membranes with optical clarity ranging from colorless and transparent to white and opaque were obtained and were characterized by their water and aluminum contents. Transport properties of these membranes were investigated and indicated a selective rejection of ionic solutes such as NaCl compared to organic solutes such as ethylene glycols. Typical results for reverse osmosis under static conditions at fluxes of 1250 g/hm² (0.75 U.S. gallons/ft²-day) gave rejections of 85% for a feed of 0.1% (by wt.) NaCl at 600 psi. Much lower rejection (28%) for ethylene glycol suggests the possible use of these membranes in the fractionation of mixtures of ionic and nonionic solutes by reverse osmosis.     

Pervaporation of Water/Ethanol Mixtures by an Aromatic Polyetherimide Membrane

Abstract

The pervaporation of water/ethanol mixtures through an aromatic polyetherimide membrane was attempted. The membrane was laboratory prepared using the solution casting technique. The sorption characteristics in relation to pervaporation were also studied. It was found that the preferential sorption was altered when the liquid composition was changed, whereas the water component permeated through the membrane preferentially over the whole range of feed mixture compositions. The experimental results were analyzed in terms of sorption ratio and permeation ratio to characterize nonideality of sorption and pervaporation. The effects of some operating parameters, including temperature, feed concentration, and permeate pressure, on the pervaporation performance were also investigated.     

Use of Liquid Chromatography for Studying Reverse Osmosis and Ultrafiltration

Abstract

A method of observing the preferential adsorption of solute or solvent onto a polymer surface by liquid chromatography is illustrated, and the results are discussed in relation to the solute separation by reverse osmosis and membrane fouling. This method shows the existence of a preferentially adsorbed water layer at the polymer-solution interface, where its amount and thickness depend on the polymer material. The adsorption data of undissociated organic solutes indicate that cellulosic materials are intrinsically less susceptible to fouling by such solutes than synthetic polymer materials. It is also shown that the steric repulsion involved in the solute's distribution between the interfacial water phase and the bulk water phase is not sufficient to describe the solute transport through the membrane, and an additional steric effect associated with the frictional force working on the solute molecule in the membrane pore has to be considered.     

Synthesis of permselective membranes by radiation induced grafting of hydrophilic monomers into poly(tetrafluoroethylene) films

A large variety of permselective membranes were prepared by radiation grafting techniques. Grafting of styrene into poly-(tetrafluoroethylene) films followed by sulfonation led to strong anionic membranes showing good performance in reverse osmosis. Carboxylic membranes were prepared by grafting acrylic and methacrylic acids. Cationic membranes were obtained via grafting 4-vinylpyridine. These membranes were tested in various fractionation processes including reverse osmosis. N-vinylpyrrolidone grafts exhibited high selectivity for separating water from organic solvents by pervaporation. Bifunctional membranes containing both carboxylic and pyridine groups were obtained by two successive grafting steps either in bulk giving rise to “homogeneous” membranes with unusual swelling properties, or into localized areas generating mosaic membranes which exhibit negative osmosis and increased rates of ion-pair diffusion.     

Prediction of reverse osmosis performance of cellulose acetate membrane

This study illustrates the analytical techniques involved in specifying the membrane and outlines the procedure for predicting the reverse osmosis (RO) performance of these membranes using feed solutions, containing either single solutes or mixed electrolytes having a common ion. The scientific basis for such specification and prediction techniques has been extensively discussed in the literature. In the present work, the governing transport equations for RO systems, involving preferential sorption of water at the membrane–solution interface, are utilized.     

The composition and structure of reverse osmosis foulant deposits formed from rand water board water: A preliminary investigation

The elemental composition, mineral composition and physical structure of reverse osmosis foulant deposits formed from Rand Water Board (RWB) water were investigated. Most deposits comprised complex mixtures which included organics, colloidal clays, quartz, and iron zinc compounds. The finely divided clays were layered, orientated semi-parallel to the membrane surface and interwoven with organic matter. They were easily scraped off the membrane and were substantially soluble in strong alkali. It is suggested that RWB fouling may be ameliorated by turbulent flushing, flow reversal, sponge ball cleaning and alkali cleaning solutions. A simplified procedure for analysing foulant deposits is also proposed.     

Removal of organic substances from aqueous solutions by reagent enhanced reverse osmosis

The reagent enhanced reverse osmosis treatment of aqueous solutions containing phenol and its derivatives has been studied. A combination of the reverse osmosis with the oxidation of organic substances by hydrogen peroxide in the presence of FeCl₂ salt as a catalyst was shown to lead to the effective removal of phenol and its derivatives from aqueous solutions. The phenol catalytic decomposition was believed to occur not only in the bulk solution but also on the membrane surface. This phenomenon is of particular interest, but additional studies are necessary. In the case of solutions containing lignosulphonates (LS), a combination of the reverse osmosis treatment with the LS oxidation by hydrogen peroxide was found to provide the high degree of solution purification from both organic substances and inorganic salts. Furthermore, it shows a possibility for the recirculation of water.     

Reverse osmosis characteristics of composite membranes prepared by plasma polymerization of allylamine. Effects of deposition conditions

A study has been conducted to determine the effects of flow configuration and reaction conditions on the performance of composite reverse osmosis membranes prepared by plasma polymerization of allylamine over a porous polymer substrate. It was established that superior membranes were obtained by using a gas-flow configuration avoiding direct monomer flow over the substrate. High rejections of NaCl could be attained when the plasma-deposited film was sufficiently thick to bridge all of the pores in the substrate. It was observed that in addition to influencing the rate of polymerization, the conditions used to sustain the plasma also affected the reverse osmosis characteristics of the deposited film. The effects of these conditions and other preparation procedures are discussed. Attempts to use infrared spectroscopy and ESCA to identify the relationship between polymer structure and reverse osmosis performance were not successful. ESCA did prove useful, though, in confirming an earlier postulated hypothesis that degradation of reverse osmosis performance is associated with the hydrolysis of nitrogen-containing structures in the plasma-deposited film.     

Reverse osmosis for the separation of organics from aqueous solutions

The properties of modern polymer membranes with respect to organic aqueous solutions are discussed to some extent. The potential of reverse osmosis for the treatment of organics-contaminated waste water is demonstrated by a discussion of two examples—landfill-drainage water and ‘desorbate’. It is shown that in the case of partial immiscibility, a combination of reverse osmosis and phase separation allows operation at very high recovery rates without exceeding tolerable concentrations of organics within the reverse osmosis modules. Furthermore, it is shown that a purification of the organic phase by pervaporation is economically feasible.     

Reverse osmosis separation of organic acids in aqueous solutions using porous cellulose acetate membranes

Reverse osmosis data obtained using porous cellulose acetate membranes and aqueous feed solutions involving one of 22 monocarboxylic acids, seven dicarboxylic acids, and four hydroxycarboxylic acids have been analyzed. The operating pressure used was 250 psig in all cases, and the solute concentration used was ～100 ppm in most cases. The results yield the following physicochemical criteria for preferential sorption at the membrane–solution interface with respect to the un-ionized acid. At pK_a = 4.2 (for monocarboxylic acids), or Taft number (σ^*) = 0.6 or Hammett number (σ) = 0, neither the un-ionized acid nor water is preferentially sorbed at the membrane–solution interface; at pK_a < 4.2 (for monocarboxylic acids) or σ^* > 0.6 or σ > 0, the un-ionized acid is preferentially sorbed at the membrane–solution interface. For practical purposes, preferential sorption of water at the membrane–solution interface may be considered negligible in the σ^* region of 0 to 0.6. The results also show that the criterion of acidity of the molecule governing the extent of its repulsion or attraction at the membrane–solution interface is valid for both ionized and un-ionized acid. Further, when the acid molecule contains three or more straight-chain carbon atoms not associated with a ? COOH group, the nonpolar character of the molecule also affects its separation in reverse osmosis.     

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).     

反渗透法处理浓水存在的问题及解决方案

反渗透浓水中有机物含量较高,因此常发生生物污染,如果处理不当,会使反渗透系统发生严重的污染堵塞。不仅可以使膜的性能下降,还会缩短其使用寿命。通过离线清洗,并选择了合适的化学药剂进行处理,反渗透系统已经恢复正常运行。     

天然有机物提取及表征技术近期发展动态

在阐述天然有机物(NOM)来源和危害的基础上,对NOM的提取、分离和表征技术的近期发展动态进行了全面综述。介绍了NOM的各种提取和分离方法,对各种提取方法的优缺点和产率进行了分析比较,指出以反渗透、大孔树脂和膜分离为核心的工艺为目前最主要的提取分离手段。此外,详细列举了包括光谱、色谱、核磁共振、原子力显微成像等在内的NOM表征方法,对各分析结果所对应的NOM的结构特征和物化性质进行了阐述。     

Evaluation of Ozone Pretreatment on Flux Parameters of Reverse Osmosis for Surface Water Treatment

This research details the effects of ozone pretreatment on flux of a reverse osmosis membrane. Initial tests were conducted to determine the effects of ozonation on solids removal, turbidity, and chemical oxygen demand concentrations using various doses on a simulated surface water. These initial tests showed that the best reduction of solids in the 2–5 microns range was at 0.30 mg/L of ozone. Next, a series of bench scale tests was run for 62.5 hours using a standard reverse osmosis system under constant pressure with three pre-ozonation doses and a no ozone baseline dose. Temporal models were developed using the flux data from these tests to determine the effects of operating hours and ozone dosage on flux. It was found that the laboratory data were not linear and followed power law models. Statistical analysis was used to determine the significance of each ozone dose on the four developed models. The change in flux over the 62.5 operating hours with an ozone dosage of 0.30 mg/L showed the lowest flux change. Last, the models were tested using Hermia's filtration models and resistance versus time data to determine the type of membrane fouling that existed. It was concluded that the major fouling was pore blockage. This work demonstrated that ozone pretreatment is effective prior to use with reverse osmosis since it shows a better solid and organic removal rate as well as decreased flux and resistance changes over time.     

Polymer-water interaction and its relation to reverse osmosis desalination efficiency

Water and salt sorption and the homogeneity of water distribution have been determined in various polymers and compared with their reverse osmosis desalination properties. A general relationship between the polymer water sorption, the water distribution in the polymer matrix, and the membrane water- and salt-permeabilities has been determined, which indicates that the uniformity of water distribution in the polymer is an important parameter controlling reverse osmosis desalination efficiency.     

The mechanism of liquid transport through swollen polymer membranes

The pressure-driven transport of liquids employed in reverse osmosis has been shown to occur by a solution-diffusion mechanism in highly swollen polymer membranes. A theory based on this mechanism was successfully used earlier to correlate permeation fluxes for such membranes. Positive confirmation of this theory is provided here by direct measurement of the proposed concentration gradient. A study of the temperature dependence of the liquid diffusion coefficient in the polymer membrane has provided additional evidence of a hydrodynamic regime of diffusion in highly swollen membranes. It is also shown that the proposed ceiling flux in reverse osmosis is equal to the pervaporation flux.     

醋酸丁酯酯化反应体系组分在PEBA膜中的吸附性能

采用实验和模型计算两种方法研究了醋酸丁酯酯化反应体系组分在涂布法制备的用于渗透汽化分离的PEBA膜材料中的吸附行为.分别用Flory-Huggins和ONIQUAC模型预测了全浓度范围内的水/有机物二元混合液在PEBA膜中的吸附量.用溶解度参数法得到Flory-Huggins模型中的相互作用参数;用纯溶剂与膜的平衡吸附数据拟合得到UNIOUAC模型中溶剂与膜的二元相互作用参数,而溶剂之间的二元相互作用参数可用文献中汽液平衡数据关联拟合.结果表明PEBA膜材料的亲有机物性能良好,对于四种组分的选择吸附性顺序为:醋酸正丁酯>正丁醇>醋酸>水,并且两种模型的计算结果基本一致.     

Effect of Residual Solvent on Physicochemical Properties of Poly(Phenylene Isophtalamide) Membrane

The influence of a plasticizer in the form of the residual solvent dimethylacetamide on pervaporation and sorption properties of membranes based on poly(phenylene isophtalamide) (PA) was investigated. To analyze the influence of the plasticizer on membrane transport properties, pervaporation of binary water–ethyl acetate mixtures was studied. The method of sorption calorimetry was used to investigate water sorption properties of the studied membranes and PA powder. Moreover, to characterize the PA membranes, contact angle measurements and thermogravimetric analysis (TGA) were applied. It is shown that the presence of a residual organic solvent significantly changes the sorption and transport characteristics of the membranes. The residual organic solvent increases the amount of water absorbed by the polymer membrane. In pervaporation of water/ethyl acetate mixtures, the presence of the residual solvent dimethylacetamide makes the membrane more permeable but less selective for water separation.