Mixed draw solutions for improved forward osmosis performance

• Addition of divalent ions into NaCl draw solution did not affect water flux.
• Addition of divalent ions into NaCl draw solution reduced reverse salt flux.
• MgCl2 was found to be a better draw solution additive than other salts.

Reverse salt flux (RSF) of ions from the draw solution (DS) to the feed is a major drawback of forward osmosis (FO). RSF is reduced when divalent ion salts such as MgCl2 and MgSO4 that have larger hydrated radius are used instead of salts with monovalent ions only (e.g., NaCl). However, using divalent ion DSs comes with a cost – namely lower water flux compared to NaCl DS at similar osmotic pressures. The objective of this study was to enhance FO by lowering RSF while maintaining high water flux by adding low concentrations of divalent ions or organic ions to NaCl DS. We have demonstrated that water flux was similar for pure NaCl DS and mixed salts DS having low concentrations of divalent or organic ions at the same osmotic pressure of pure NaCl DS. Simultaneously, the average RSF was lower for all mixed salts DSs tested compared to pure NaCl DS. Results from a student t-test comparing the average RSF of the mixed salts DSs to the pure NaCl DS revealed that although the average RSF was lower for all the mixed DSs tested, only the mixed salts DS containing MgCl2 had RSF significantly lower than the pure NaCl DS.

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