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 zeta 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 membrane surface chemistry on membrane flux and rejection. The RO membranes evaluated were a thin-film composite polyamide membrane and an asymmetric cellulose acetate membrane. The solution chemistries investigated include Suwannee River humic acid and sodium dodecyl sulfate (an anionic surfactant). Zeta potential measurements revealed that in the presence of an indifferent electrolyte, both membranes had a positive zeta 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 high pH range, Suwannee River humic acid and sodium dodecyl sulfate were found to readily adsorb to the membrane surface and markedly influence the membrane surface charge. Suwannee River humic acid had a significant influence on salt rejection at low pH where adsorption of the organic macromolecules caused a change in sign of the zeta potential (from positive to negative) and therefore a change in co-ion exclusion effects. The effects of the sodium dodecyl sulfate, also more apparent at low pH, were attributed to the formation of hemimicelles which caused decreased flux and increased salt rejection.