Reverse osmosis separation of organic acids in aqueous solutions using porous cellulose acetate membranes

Reverse osmosis data obtained using porous cellulose acetate membranes and aqueous feed solutions involving one of 22 monocarboxylic acids, seven dicarboxylic acids, and four hydroxycarboxylic acids have been analyzed. The operating pressure used was 250 psig in all cases, and the solute concentration used was ～100 ppm in most cases. The results yield the following physicochemical criteria for preferential sorption at the membrane–solution interface with respect to the un-ionized acid. At pK_a = 4.2 (for monocarboxylic acids), or Taft number (σ^*) = 0.6 or Hammett number (σ) = 0, neither the un-ionized acid nor water is preferentially sorbed at the membrane–solution interface; at pK_a < 4.2 (for monocarboxylic acids) or σ^* > 0.6 or σ > 0, the un-ionized acid is preferentially sorbed at the membrane–solution interface. For practical purposes, preferential sorption of water at the membrane–solution interface may be considered negligible in the σ^* region of 0 to 0.6. The results also show that the criterion of acidity of the molecule governing the extent of its repulsion or attraction at the membrane–solution interface is valid for both ionized and un-ionized acid. Further, when the acid molecule contains three or more straight-chain carbon atoms not associated with a ? COOH group, the nonpolar character of the molecule also affects its separation in reverse osmosis.