Aqueous nanofiltration

Zhou M, Kidd TJ, Noble RD, Gin DL. Supported lyotropic liquid-crystal polymer membranes promising materials for molecular-size-selective aqueous nanofiltration. Adv Mater 2005 17 1850-3. 

An excellent review of composite RO and nanofiltration (NE) membranes is available (8). These thin-fHm, composite membranes consist of a thin polymer barrier layer formed on one or more porous support layers, which is almost always a different polymer from the surface layer. The surface layer determines the flux and separation characteristics of the membrane. The porous backing serves only as a support for the barrier layer and so has almost no effect on membrane transport properties. The barrier layer is extremely thin, thus allowing high water fluxes. The most important thin-fHm composite membranes are made by interfacial polymerization, a process in which a highly porous membrane, usually polysulfone, is coated with an aqueous solution of a polymer or monomer and then reacts with a cross-linking agent in a water-kniniscible solvent. 

Concentration polarization can dominate the transmembrane flux in UF, and this can be described by boundary-layer models. Because the fluxes through nonporous barriers are lower than in UF, polarization effects are less important in reverse osmosis (RO), nanofiltration (NF), pervaporation (PV), electrodialysis (ED) or carrier-mediated separation. Interactions between substances in the feed and the membrane surface (adsorption, fouling) may also significantly influence the separation performance fouling is especially strong with aqueous feeds. 

For relatively porous nanofiltration membranes, simple pore flow models based on convective flow will be adapted to incorporate the influence of the parameters mentioned above. The Hagen-Poiseuille model and the Jonsson and Boesen model, which are commonly used for aqueous systems permeating through porous media, such as microfiltration and ultrafiltration membranes, take no interaction parameters into account, and the viscosity as the only solvent parameter. It is expected that these equations will be insufficient to describe the performance of solvent resistant nanofiltration membranes. Machado et al. [62] developed a resistance-in-series model based on convective transport of the solvent for the permeation of pure solvents and solvent mixtures ... 

Cuperus, F.P. and Ebert, K. (2005) Non-aqueous applications of NF, Chapter 21 in Nanofiltration, Principles and Applications (eds A.I. Schafer, A.G. Fane and T.D. Waite), Elsevier, Oxford, New York and Tokyo. 

However, it can be assumed for most electrochemical applications of ionic liquids, especially for electroplating, that suitable regeneration procedures can be found. This is first, because transfer of several regeneration options that have been established for aqueous solutions should be possible, allowing regeneration and reuse of ionic liquid based electrolytes. Secondly, for purification of fiesh ionic liquids on the laboratory scale a number of methods, such as distillation, recrystallization, extraction, membrane filtration, batch adsorption and semi-continuous adsorption in a chromatography column, have already been tested. The recovery of ionic liquids from rinse or washing water, e.g. by nanofiltration, can also be an important issue. 

Buekenhoudt A, Dotremont C, Aerts S, Vanckelecom I, and Jacobs PA. Successful applications of ceramic nanofiltration in non-aqueous solvents. Proceedings of the Eight International Conference on Inorganic Membranes, Cincinnati, OH, July 18-22, 2004 278-281. 

Using different membranes and various membrane techniques, isotopes of chlorine [155], carbon [156], lithium in aqueous solutions [157,158], and uranium in CH4 [158,159] were separated. Isotopes of gadolinium and neodymium were separated in hybrid system of nanofiltration/complexation [50]. 

Gaubert, E. et al.. Selective strontium removal from a sodium nitrate aqueous medium by nanofiltration-complexation, Sep. Sci. TechnoL, 32, 1, 585, 1997. 

Chitry, F. et al., Cesium/sodium separation by nanofiltration-complexation in aqueous medium, Sep. Sci. TechnoL, 36, 5-6, 1053,... 

Nanofiltration is a pressure-driven process where the solvent is forced through the membrane by pressure, and other feed constituents randomly pass through the membrane by diffusion. The relative rates of solvent and solute passage determine the quality of the product. Nanofiltration membranes are adept at the separation of small, neutral, and charged solutes in aqueous solutions because they allow the passage of monovalent ions and retain multivalent ions due to their charge. Nanofiltration membranes exhibit two important features in their actual apphcations. They provide... 

The aqueous process solution containing sodium thiocyanate with impurities was provided by a local industry. All the membrane modules, pressure vessels, and accessories for the nanofiltration pUot plant used in this study were purchased from Permionics Membranes Pvt. Ltd. (Baroda, India) and assembled in the laboratory. Citric acid, tetra sodium salt of ethylene diamine tetraacetic acid (EDTA), trisodium phosphate (TSP), and sodium metabisulfite for cleaning and maintenance of the membranes were procured from Lob a Chemie (Mumbai, India). 

B. Van der Bruggen, A. Koninchx, and C. Vandecasteele, Separation of monovalent and divalent ions from aqueous solution by electrodialysis and nanofiltration. Water Research (in press) (2004). 

The second section refers to polyelectrolyte membranes prepared by alternating electrostatic layer-by-layer assembly of cationic and anionic polyelectrolytes on porous supports. Mass transport across ultrathin polyelectrolyte multilayer membranes is described. The permeation of gas molecules, liquid mixtures, and ions in aqueous solution has been investigated. The studies indicate that the membranes are excellently suited for separation of alcohol/water mixtures under pervaporation conditions and for ion separation, e.g. under nanofiltration conditions. 

Reverse osmosis-extraction In certain applications, reverse osmosis (RO) or nanofiltration membranes may be used to reduce the volume of an aqueous stream and increase the solute concentration, in... 

Schirg and Widmer [52] published mathematical models for the calculation of retention and selectivity for nanofiltration of aqueous dye-salt solutions. A modification of Eqs. (12.11) and (12.12) has been proposed in which the integral salt permeability co could be described by the introduction of an exponential function... 

P. Schirg and F. Widmer, Characterisation of nanofiltration membranes for the separation of aqueous dye-salt solutions. Desalination, 89 (1992) 89. 

A third general approach to encapsulation involves the use of permselective membrane devices of the types employed in ultrafiltration and nanofiltration of aqueous solutions, especially those devices that employ the membrane in the form of hollow fibres. In effect, the enzymes are retained within a macrocapsule. An aqueous solution of the soluble enzyme or whole cells is contained on the retentate side of the membrane, while a solution containing the substrates is supplied... 

The key part of the reactor is a nanofiltration membrane unit (a), which allows the permeation of small molecules but not macromolecules such as enzymes. In operation, the reactor is initially charged with d-LDH and FDH before the start of the reaction. An aqueous mixture, which consists of 6, ammonium formate, and a catalytic amount of NAD, is then continuously fed into the reactor by a peristaltic pump (b). After passing a check valve (c), the substrate solution is mixed with enzymes inside the reactor by a circulation pump (d). The product is collected continuously as an effluent from the filtration membrane unit. In this fashion, both enzymes are retained inside the reactor by the membrane leading to high turnover. 

Ionic Aqueous salts Nanofiltration, reverse osmosis, chromatogr hy... 

For membrane processes involving liquids the mass transport mechanisms can be more involved. This is because the nature of liquid mixtures currently separated by membranes is also significantly more complex they include emulsions, suspensions of solid particles, proteins, and microorganisms, and multi-component solutions of polymers, salts, acids or bases. The interactions between the species present in such liquid mixtures and the membrane materials could include not only adsorption phenomena but also electric, electrostatic, polarization, and Donnan effects. When an aqueous solution/suspension phase is treated by a MF or UF process it is generally accepted, for example, that convection and particle sieving phenomena are coupled with one or more of the phenomena noted previously. In nanofiltration processes, which typically utilize microporous membranes, the interactions with the membrane surfaces are more prevalent, and the importance of electrostatic and other effects is more significant. The conventional models utilized until now to describe liquid phase filtration are based on irreversible thermodynamics good reviews about such models have been reported in the technical literature [1.1, 1.3, 1.4]. 

A transmission electron microscope (TEM) observation of the Z1O2 nanocrystallites obtained at 500°C is given in Fig. 13. Homogenous, quite spherical, consolidated particles of about 5-6 nm in size can be observed. This clearly shows that, to maintain nanoporosity inside the membrane, an upper limit for membrane sintering exists. Because these results, a zirconia nanofil-ter has been obtained by coating a 1 pm thick layer on a microfiltration zirconia layer. The separation performance of this membrane, characterized with model solutes in aqueous media, is in the nanofiltration range [26]. 

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