EFFECT OF SOLUTION CHEMISTRY ON THE SURFACE-CHARGE OF POLYMERIC REVERSE-OSMOSIS AND NANOFILTRATION MEMBRANES

A streaming potential analyzer has been used to investigate the effect of solution chemistry on the surface charge of four commercial revers e osmosis and nanofiltration membranes. Zeta potentials of these membr anes were analyzed for aqueous solutions of various chemical compositi ons over a pH range of 2 to 9. In the presence of an indifferent elect rolyte (NaCl), the isoelectric points of these membranes range from 3. 0 to 5.2. The curves of zeta potential versus solution pH for all memb ranes display a shape characteristic of amphoteric surfaces with acidi c and basic functional groups. Results with salts containing divalent ions (CaCl2, Na2SO4, and MgSO4) indicate that divalent cations more re adily adsorb to the membrane surface than divalent anions, especially in the higher pH range. Three sources of humic acid, Suwannee River hu mic acid, peat humic acid, and Aldrich humic acid, were used to invest igate the effect of dissolved natural organic matter on membrane surfa ce charge. Other solution chemistries involved in this investigation i nclude an anionic surfactant (sodium dodecyl sulfate) and a cationic s urfactant (dodecyltrimethylammonium bromide). Results show that humic substances and surfactants readily adsorb to the membrane surface and markedly influence the membrane surface charge.