Dynamically formed hydrous zirconium (IV) oxide/polyacrylic membranes; low pressure formation, high pressure evaluation

Abstract

Dynamic membranes are formed in situ when a dilute solution of one or more specific additives is passed over the surface of a porous support. Zirconium (iv) species are polymerised in aqueous solution and polymerisation increases with decreasing acidity. At concentrations as low as 10 −4 molar zirconium, a colloid phase exists even at low pH values (pH 3). This colloidal condition is important in the formation of a membrane on a porous support by cross-flow techniques. The ability of zirconium to react strongly with oxygen containing species is another important property leading to the chelation of polyelectrolytes such as polyacrylic acid onto a preformed hydrous zirconium (iv) oxide membrane, and so producing a composite membrane. In this investigation porous stainless steel supports have been selected as the main support medium. The pore size in the supports varied from 2.0 to 7.0 microns. Tests were carried out in forming membranes at low pressures, up to 600 kPa and evaluating such membranes at high pressures up to 6 000 kPa. This enabled comparison with similar membranes formed at high pressure (6 000 kPa). A standard method of formation, similar to that developed at Oak Ridge National Laboratory, USA, was used. It was found that the low pressure technique (600 kPa) for dynamic membranes of the zirconium/polyelectrolyte type resulted in satisfactory membranes when compared to those prepared at high pressure (6 000 kPa). There was, however, some variation in flux and rejection of the membranes. This may have been related to the large pore size of the tubes used. Tests were therefore carried out on pretreating the tubes with a suspension of fumed silica. The degree of reproducibility of results at formation pressure (600 kPa) improved markedly yielding membranes with high flux and good rejection. When these tubes were operated at higher pressure (6 000 kPa), the integrity of the membrane system tended to fail. Nevertheless, the indications are that satisfactory high flux membranes could be developed but further investigations and development work are necessary.