Coupled reverse draw solute permeation and water flux in forward osmosis with neutral draw solutes

Understanding the simultaneous forward water flux and reverse draw solute flux in osmotically driven membrane processes is essential to the development of these emerging technologies. In this work, we investigate the reverse fluxes of three neutral draw solutes-urea, ethylene glycol, and glucose-across an asymmetric forward osmosis membrane. Our experiments reveal that for the rapidly permeating draw solutes (urea and ethylene glycol), an additional resistance to mass transfer develops due to external concentration polarization on the feed side of the membrane. A model to predict the reverse flux for these highly permeable solutes is derived and then validated using independently determined transport parameters. The experimentally measured water fluxes generated by these solutes are consistently lower than those predicted by theories for calculating the water flux in osmotically driven membrane processes-even when the effects of external concentration polarization are taken into account. These results are indicative of a coupling between the forward water flux and reverse solute flux, and a reflection coefficient is introduced to account for the solute–solvent coupling. Interestingly, our experiments demonstrate that solute–solvent coupling does not significantly impact the reverse flux of draw solute. The effect of this solute–solvent coupling on the reverse flux selectivity (i.e., the ratio of the forward water flux to the reverse solute flux) is demonstrated.