Development of thermoresponsive star oligomers with a glycerol backbone as the draw solute in forward osmosis process

Abstract

Recently, forward osmosis (FO) is attracting research attention once again. In a FO process, it is important to develop a draw solution (DS) with a high osmotic pressure and low solute leakage. In this study, thermoresponsive star-shaped oligomers with a glycerol backbone were developed as new draw solutes for FO. A series of glycerol-oligo(ethylene oxide)-block-oligo(butylene oxide) (GEB) oligomers were systematically designed and synthesized. The average degrees of polymerization of ethylene oxide (EO; m) and butylene oxide (BO; n) units of GEmBn were varied to control the hydrophilic/hydrophobic balance of the molecule. Aqueous solutions of GEBs were evaluated in terms of their osmotic pressures, phase diagrams, and viscosities. Most of them showed a lower critical solution (LCST)-type phase separation at temperatures below 60 °C. The osmotic pressure of 68 wt% GE7B3 (concentration of the dense phase after phase separation at 60 °C) was 74 bar, about 2.6 times higher than that of seawater. Moreover, the leakage of GE7B3 was much lower than that of conventional draw solutes. The osmotic pressure of the dilute phase of a GE7B3 solution at 60 °C was less than 2 bar, implying reduced energy consumption during post-processing by low-pressure reverse osmosis to collect pure water.