Organo-mineral ultrafiltration membranes

Abstract

A new kind of membrane material, resulting from the addition of minerals (inorganics) to a polymer solution, is presented. This innovative concept allows us to combine the interesting properties of organic membranes (such as flexibility) with those of mineral membranes (such as pressure resistance, and surface properties). These organo-material membranes are prepared by immersion precipitation in a water bath, as is frequently applied for polymeric membranes. The resulting membrane structure consists of a porous polymer network with incorporated mineral particles. It is shown that the presence of these particles in the casting solution modifies the resulting membrane structure and surface properties in a very favourable manner. This study focuses first on the organomineral combination, polysulfone/zirconia. It was found that increasing the weight percentage of ZrO₂ in the casting solution causes a dramatic increase in membrane permeability. Above a certain minimum amount of mineral filler, field emission scanning electron images of the membrane's top surface revealed the presence of a large number of equally sized pores with diameters of about 10 nm. Within a certain filler content range, the mean pore size and the surface porosity did not change noticeably, although large differences in permeability were measured. The surface porosity for these membranes of about 11% (deduced from image analysis) is much higher than the reported values of 2–3% for pure polymeric ultrafiltration membranes. The cutoff values of these high surface porosity membranes confirm no significant changes in skin pore size. It is shown that the concept is also applicable for preparing high flux membranes from other organo-mineral combinations, for example with polyacrylonitrile as the binding polymer. The addition of suitable amounts of a mineral filler increases the membrane surface porosity with little effect on the retention. Moreover, it is demonstrated that the surface properties (surface charge) of the mineral filler can be used for the development of membranes with nanofiltration properties.