Asymmetric ion exchange mosaic membranes with unique selectivity

In this paper, we describe the investigation of membranes to concentrate aqueous low molecular weight (<500Da) organics streams, while removing electrolytes including divalent salts such as sodium sulfate. Such membranes would be useful in many industrial applications as currently used pressure driven process such as nanofiltration (NF) or electrical processes such as electric dialysis (ED) cannot achieve such separations and concentrations. An analysis of ion/water transport in different membranes and, selectivity and flux requirements indicated that ion exchange mosaics in the form of integrally skinned asymmetric structures could achieve the required performance. The relationship between the internal structure of the mosaic membrane elements and the required separation properties was further analyzed as a development guide. It was found that such membranes could be made by casting a homogenous solution of two mutually incompatible polymers in a common solvent, containing non-solvents and additives, followed by a chemical modification. The process of forming such membranes involves phase separation between the two polymers and the phase inversion of each polymer. In this study the membrane consists of a cation exchange asymmetric membrane with a uniform distribution of anion exchange particles in the dense integrally skin layer. The choice of polymeric materials, their molecular weights, solvent combinations and surfactants determined the membranes' surface morphology, mosaic dimensions and particle density. In this way membranes were formed with ∼1μm sized anion exchange particles uniformly dispersed in a thin (∼1.0μm) cation exchange selective layer of an asymmetric membrane. The best performance to date: Fluxes of 500+LMD, 10% rejection to sodium sulfate, 90% to sucrose and >98% rejection to 400 molecular weight organic ions. The membranes also show a mosaic effect of decreasing sulfate rejection with decreasing sulfate concentration. The membranes also show a musaic effect of decreasing sulfate rejection with decreasing sulfate concentration, which is desired to perform effectively the removal of mono and bivalent ions during diafiltration.