Membrane synthesis

RO membranes are generally prepared by the classical phase inversion method, in which the polymer solution is formed in two phases from which the selective layer and porous support are fabricated by selective solvent evaporation. Integrally skinned phase inversion membranes are prepared in this method. Interfacial composite membranes (IFC) are prepared in general by generating the selective layer by chemical reaction on the already prepared porous support [1]. The advantage of the IFC membrane is that the chemistry of the critical selective layer can be chosen independently from the underlying porous support as thin-film chemistry and morphology determines the membranes transport properties.

Syntheses of IFC membranes involve impregnating the porous polymer support, preferably polysulfone, with aqueous solutions containing a multifunctional amine [1,2]. The impregnated membrane is then contacted with a hexane solution containing a multifunctional acid chloride. As the two solutions are immiscible, polycondensation reaction occurs at the interface, leading to the formation of a thin selective layer at the surface of porous support. Various parameters such as diffusion rates of amines into organic solvent, polymer film permeability, interfacial tension, concentration of the reactants, and polymer film growth rate play a crucial role in the synthesis of IFC membranes. Acid produced during the condensation reaction is neutralized by using suitable bases, such as sodium hydroxide or excess diamines in the reaction. Porous polymeric supports are prepared by phase inversion method in the IFC membranes.

Integrally skinned phase inversion membranes are casted from various organic polymers such as cellulose acetate [3], nylon 4, and polyvinyl alcohol. Once the desired polymer material is prepared through melt phase polymerization, membranes are casted by phase inversion process. The polymers are dissolved in suitable polar organic solvents to form appropriate weight percent solutions. The membranes are prepared by casting these solutions on dry Pyrex glass plates to a uniform thickness with the aid of a specially designed glass rod. The plates are then introduced into an oven at a desired temperature for a selected period to facilitate the partial evaporation of the solvent. These two parameters, solvent evaporation temperature and solvent evaporation period, play a vital role in determining the flux and salt rejection properties of the resulting RO membranes [4]. The casted film side facing the air has higher solvent evaporation than the side facing the glass, which results in smaller pore size on the former side. The glass plates are then immersed in deionized water overnight to complete the exchange of residual solvent with water. The appearance of the membrane at this stage is an indication of the residual solvent present in the membrane. A transparent membrane indicates low solvent content in the membrane. This can be achieved by optimizing solvent evaporation temperature and period. The solvent-free membranes are optimized after annealing in deionized water at preselected temperatures.

Apart from the above mentioned parameters, the chosen polymer and its structure play a very important role in determining the properties of RO membranes. Uniformity and symmetry in the polymer chain structure provide tighter and uniform pore structures, resulting in improved salt rejections [4,5]. Lack of uniformity in the molecular structure can result in reduced rejection due to openings in the polymer structure [4].

Pore size distribution (PSD) of a membrane predominantly determines its filtration characteristics. The thin selective layer is characterized by the presence of two distinct kinds of pore sizes on the membrane surface [3,6]. These are tight "polymer network pores'' or wider "polymer aggregate pores,'' which facilitate the transfer of matter across the membrane. In most cases, the membrane is designed to only allow water to pass through. The water goes into solution in the polymer of which the membrane is manufactured, and crosses it mostly by diffusion. However, convection may aid filtration when membranes with wider pores are used. Water transport across membranes requires that a high pressure be exerted on the feed side of the membrane, usually 5-20 MPa (50-200 bar).

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