Switchable polarity solvents as draw solutes for forward osmosis

Switchable polarity solvents (SPS), mixtures of carbon dioxide, water, and tertiary amines, are presented as viable forward osmosis (FO) draw solutes allowing a novel SPS FO process. In this study substantial osmotic strengths of SPS are measured with freezing point osmometry and were demonstrated to induce competitive fluxes at high salt concentrations on a laboratory-scale FO unit utilizing a flat sheet cellulose triacetate (CTA) membrane. Under the experimental conditions the SPS degrades the CTA membrane; however experiments with polyamide reverse osmosis (RO) membranes display stability towards SPS. Once the draw is diluted the major fraction of the switchable polarity solvent can be mechanically separated from the purified water after polar to nonpolar phase shift induced by introduction of 1 atm carbon dioxide to 1 atm of air or nitrogen with mild heating. Trace amounts of SPS can be removed from the separated water with RO in a process that avoids solution concentration polarization. The separated nonpolar phase can be regenerated to a full strength draw and recycled with the re-addition of 1 atm of carbon dioxide.

► Switchable polarity solvents (SPS) are novel draw solutes for forward osmosis.
► SPS draws provide high osmotic strengths, > 13 Osm/kg.
► SPS draws provide high FO fluxes.
► SPS draws provide positive flux even at high feed concentrations (5.0 mol/kg NaCl).
► SPS draws are easily recycled and trace amounts of SPS removed with reverse osmosis.