MEASURING THE ZETA (ELECTROKINETIC) POTENTIAL OF REVERSE-OSMOSIS MEMBRANES BY A STREAMING POTENTIAL ANALYZER

The use of a novel streaming potential analyzer to measure the zeta potential of cellulose acetate and composite polyamide reverse osmosis membranes is reported. Zeta potentials of these membranes were measured at various solution chemistries. These include effects of salt (NaCl) concentration, solution pH, and the presence of dissolved humic substances. It is demonstrated that streaming potential is a useful tool to measure zeta potential of reverse osmosis membrane surfaces. Results indicate that solution chemistry has a marked effect on the electrokinetic properties of reverse osmosis membranes. Humic substances strongly adsorb onto the surface of reverse osmosis membranes and thus alter the surface charge of the membranes. Furthermore, the zeta potential of reverse osmosis membranes becomes more negative as the NaCl concentration in solution increases, in a marked contrast to conventional electric double layer theories. It appears that the zeta potential of reverse osmosis membranes is strongly influenced by the presence of unreacted chemical substances or impurities on the membrane surface. Various explanations for the behavior of the membranes at the above solution chemistries are evaluated and discussed.