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

Einfluss der Oberflächenladung auf den Permeatfluss in der Dead-End-Filtration mit keramischen Membranen
: Moritz, T.; Benfer, S.; Arki, P.; Tomandl, G.


Separation and purification technology 25 (2001), Nr.1-3, S.501-508
ISSN: 1383-5866
Fraunhofer IKTS ()
ceramic membrane; dead-end filtration; streaming potential; surface charge measurment

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 d. become more and more important with regard to its retention and selectivity. Typical parameters like permeate flux and retention of salt ions, particles or mols. are governed by a so-called effective pore diam. depending on the surface charge and the thickness of the electrochem. 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 liq. satd. oxide ceramic membranes, a surface charge results from dissocd. surface groups and adsorbed charge-detg. ions. Ceramic ultrafiltration membranes made of TiO2, ZrO2, .alpha.- and .gamma.-Al2O3 were investigated by streaming potential measurements. For these measurements an Electrokinetic Analyzer EKA from Anton Paar, Austria, was used. The membranes were characterized by a tangential flow method, i.e., an electrolyte soln. 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 d. can be expressed by the .zeta.-potential. The permeate fluxes of polyethylenimine solns. and dextrane solns. 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 expts. result either in a high pos., a high neg. or in a net zero charge of the appropriate membrane material. The abs. 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, mols. or ions in the feed soln. Improved filtering conditions can be achieved if the membrane and the content of the feed soln. show a similar charge, since the electrostatic repulsion decelerates the formation of a flux reducing fouling layer on the membrane surface.