Reverse osmosis separations of some organic and inorganic solutes in aqueous solutions using aromatic polyamide membranes

The reverse osmosis separations of some ethers, ketones, aldehydes, monocarboxylic acids, and inorganic salts in single-solute (55–454 ppm) aqueous solution systems using aromatic polyamide membranes have been studied at 250 psig. It was found that reverse osmosis separation was essentially a function of steric parameter for ethers, and of both polar and steric parameters for ketones, aldehydes, and alcohols. Solute separations for monocarboxylic acids passed through a minimum at a pK_a value of ～4.8. The values for the free-energy parameter for Li⁺, Na⁺, K⁺, Rb⁺, Cs⁺, and F^?, Cl^?, Br^?, I^?, and IO₃^? ions have been calculated for the polyamide membranes used. The above values for cations are negative and those for anions are positive, suggesting that the polyamide membrane surface behaves as if it is positively charged. Further, the data show that the polyamide membrane material is only about 40% as polar as that of the cellulose acetate membrane material studied earlier.