EFFECT OF HUMIC SUBSTANCES AND ANIONIC SURFACTANTS ON THE SURFACE-CHARGE AND PERFORMANCE OF REVERSE-OSMOSIS MEMBRANES

Streaming potential measurements were used to determine the surface ze ta potential of two reverse osmosis (RO) membranes and to investigate the effect of solution chemical composition on membrane surface charge . Performance measurements were performed to evaluate the effect of me mbrane surface chemistry on membrane flux and rejection. The RO membra nes evaluated were a thin-film composite polyamide membrane and an asy mmetric cellulose acetate membrane. The solution chemistries investiga ted include Suwannee River humic acid and sodium dodecyl sulfate (an a nionic surfactant). Zeta potential measurements revealed that in the p resence of an indifferent electrolyte, both membranes had a positive z eta potential in the low pH range, passed through an isoelectric point between pH 2 and 3, and had a negative zeta potential in the mid to h igh pH range, Suwannee River humic acid and sodium dodecyl sulfate wer e found to readily adsorb to the membrane surface and markedly influen ce the membrane surface charge. Suwannee River humic acid had a signif icant influence on salt rejection at low pH where adsorption of the or ganic macromolecules caused a change in sign of the zeta potential (fr om positive to negative) and therefore a change in co-ion exclusion ef fects. The effects of the sodium dodecyl sulfate, also more apparent a t low pH, were attributed to the formation of hemimicelles which cause d decreased flux and increased salt rejection.