Influence of the surface charge on the permeate flux in the dead-end filtration with ceramic membranes

The filtering properties of membranes in the ultra and nano filtration rang e cannot only be derived from a pure sieving effect. With decreasing membra ne pore size the surface charge of the membranes and the charge density bec ome more and more important with regard to its retention and selectivity. T ypical parameters like permeate flux and retention of salt ions, particles or molecules are governed by a so-called effective pore diameter depending on the surface charge and the thickness of the electrochemical double layer on the pore walls. Fouling mechanisms, i.e. cake forming and pore blocking , which cause a permeate flux decline in the filtration process are also st rongly influenced by the surface charge of the membrane. In the case of liq uid saturated oxide ceramic membranes a surface charge results from dissoci ated surface groups and adsorbed charge determining ions. Ceramic ultra fil tration membranes made of TiO2, ZrO2, alpha- and gamma -Al2O3 were investig ated by streaming potential measurements. For these measurements an Electro kinetic Analyser EKA from Anton Paar, Austria, was used. The membranes were characterized by a tangential flow method. This means, an electrolyte solu tion is forced through a narrow slit formed by the surfaces of two similar membranes which are positioned facing one another. In this way the dependen ce of the surface charge on the pH was shown. Using the Helmholtz-Smoluchow ski equation the surface charge density can be expressed by the C-potential . The permeate fluxes of polyethylenimine solutions and dextrane solutions through the membranes in a dead-end filtration process at different pH-valu es were investigated by means of a filtration testing device. The pH values chosen for these experiments result either in a high positive, a high nega tive or in a net zero charge of the appropriate membrane material. The abso lute permeate flux and its drop with increasing filtering time strongly dep end on the charge conditions of the membrane surface and the charge of the particles, molecules or ions in the feed solution. Improved filtering condi tions can be achieved if the membrane and the content of the feed solution show a similar charge, since the electrostatic repulsion decelerates the fo rmation of a flux reducing fouling layer on the membrane surface. (C) 2001 Elsevier Science B.V. All rights reserved.