Electrostatic interaction governed solute transport in forward osmosis

Electrolytes are commonly employed as draw solutes in forward osmosis (FO). This work demonstrates that electrostatic interactions play a key role in ion transport in the FO process. The difference in diffusivity between the constituent ions of the draw electrolyte significantly impact the forward transport of the feed ions. Draw electrolyte composed of low-diffusivity cations and high diffusivity anions promoted forward transport of the feed anions and retarded that of the feed cation, and vice versa. The effects were remarkable even for the most commonly used draw electrolytes (NaCl or MgCl₂), where the forward flux of NO₃^- and NO₂^- was found to increase by a few folds and that of NH₄⁺ was reduced by similar magnitudes than that observed in a nonelectrolyte draw solute (glucose) system. More profound increase/reduction (up to 10 times) was observed for draw electrolytes composed of highly asymmetric cations and anions. An analytical model is developed by considering the electrostatic interaction between the draw and the feed ions, to predict its effect on the forward transport of the feed ions. The normalized diffusivity difference (θD) between the constituent ions of the draw electrolyte is found as a key factor that determines the transport behaviors of the feed ions. These results may have important implications in enhancing our understanding of bidirectional ion transport in FO. The findings may also be useful in the design and development of FO processes for enhanced removal of charged pollutants via draw solute selection and formulation.