SURFACE ELECTROCHEMICAL PROPERTIES OF MIXED-OXIDE CERAMIC MEMBRANES -ZETA-POTENTIAL AND SURFACE-CHARGE DENSITY

The surface electrochemical properties of alumina based ceramic microf iltration membranes were studied by measuring electroosmotic rates and surface charge densities obtained from potentiometric titrations. The rate of electroosmosis, which determines the zeta-potential, was meas ured on the membrane itself, whereas the surface charge was titrated o n a suspension obtained by crushing of the membrane. The zeta-potentia l was measured in the presence of salts including NaCl, CaCl2 and Na2S O4, for a wide range of pH values (4-9) at ionic strengths of 0.01 and 0.001 M. The pH value of the isoelectric point (iep) show a specific adsorption of SO42- and Ca2+ ions onto the membrane surface. The iep i n NaCl solutions occurs at pH 4.7 +/- 0.1. The low iep is due to the l arge amount of silicium oxide in the membrane. The surface charge dens ity is relatively high with respect to the low values of zeta-potentia ls. The point of zero charge pH(pzc) determined from surface charge an d pH profiles occurs at pH 8.2 +/- 0.1 in NaCl solutions. The pH(pzc) value was also determined by two 'addition' methods. Similar pH(pzc) v alues were obtained. The difference between the pH(pzc) and pH(iep) ma y be correlated to a loss of acidity that is due to using crushed-memb rane powder to perform potentiometric measurements.