Switchable Polarity Solvent (SPS) As Draw Solutes for Forward Osmosis

Switchable polarity solvent (SPS) 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.

In addition a series of tertiary amines have been screened for their function as switchable polarity solvents (SPS). The relative ratios of tertiary amine and carbonate species as well as maximum possible concentration were determined through quantitative ¹H and ¹³C NMR spectroscopy. The viscosities of the polar SPS solutions were measured and ranged from near water in the dilute systems through to the formation of a gel in specific systems at high concentrations. The van‘t Hoff indices for SPS solution were measured through freezing point depression studies as a proxy for the solutions osmotic strengths. A new form of SPS with an amine:carbonate ratio significantly greater than unity has been identified. Alkyl tertiary amines that function as SPS at ambient pressures appear to be limited to molecules with fewer than 12 carbons.