Reverse Permeation of Weak Electrolyte Draw Solutes in Forward Osmosis

The successful development of forward osmosis relies on the identification of draw solutes that can be easily regenerated, produce high water fluxes, and minimize leakage into the feed solution. One promising draw solute currently under investigation, ammonia–carbon dioxide, is a weak electrolyte. Therefore, in this work, we report a fundamental study on the reverse permeation of a model weak electrolyte draw solute, propanoic acid/propanoate ion, through a commercial cellulose triacetate forward osmosis membrane. Reverse solute flux and permeate water flux were monitored as the draw solution pH was varied from pH 4–7. Draw solutions with pH < 4.87, the pK a of propanoic acid, exhibited significantly higher reverse draw solute fluxes. However, pH had little effect on the water flux generated by the draw solutions. A mathematical model for the solute and water fluxes, which accounts for the pH-dependent equilibrium of the propanoic acid dissociation reaction, was developed and compared with experimental data. Using independently determined transport coefficients, strong agreement between theory and experiments was observed over the whole pH range examined in this work.