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 range cannot only be derived from a pure sieving effect. With decreasing membrane pore size the surface charge of the membranes and the charge density become more and more important with regard to its retention and selectivity. Typical 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 strongly influenced by the surface charge of the membrane. In the case of liquid saturated oxide ceramic membranes a surface charge results from dissociated surface groups and adsorbed charge determining ions. Ceramic ultra filtration membranes made of TiO2, ZrO2, alpha- and gamma -Al2O3 were investigated by streaming potential measurements. For these measurements an Electrokinetic Analyser EKA from Anton Paar, Austria, was used. The membranes were characterized by a tangential flow method. This means, an electrolyte solution is forced through a narrow slit formed by the surfaces of two similar membranes which are positioned facing one another. In this way the dependence of the surface charge on the pH was shown. Using the Helmholtz-Smoluchowski 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-values were investigated by means of a filtration testing device. The pH values chosen for these experiments result either in a high positive, a high negative or in a net zero charge of the appropriate membrane material. The absolute permeate flux and its drop with increasing filtering time strongly depend on the charge conditions of the membrane surface and the charge of the particles, molecules or ions in the feed solution. Improved filtering conditions can be achieved if the membrane and the content of the feed solution show a similar charge, since the electrostatic repulsion decelerates the formation of a flux reducing fouling layer on the membrane surface. (C) 2001 Elsevier Science B.V. All rights reserved.