Tunable thermoresponsive UCST-type alkylimidazolium ionic liquids as a draw solution in the forward osmosis process

Abstract

Recently, forward osmosis (FO) has been attracting attention as a new energy-saving water treatment technology. To put the FO process to practical use, it is indispensable to develop the optimum draw solution (DS) and construct its low energy regeneration process. In this study, various types of alkylimidazolium tetrafuluoroborate salts were synthesized, and their capabilities as novel thermoresponsive DSs for the FO desalination process were assessed using solution parameters such as phase diagram and osmotic pressure. The carbon number of a cation tunables the upper critical solution temperature (UCST)-point of alkylimidazolium-based ionic liquids' phase transition temperature. Considering the effect on the membrane material, the cooling temperature according to the climate, and the temperature range of low-grade waste heat in FO process, the ionic liquids having UCST points in the range of 25 °C to 50 °C, 1-pentyl-3-methylimidazolium tetrafluoroborate ([C5mim][BF4]), 1-pentyl-2,3-dimethylimidazolium tetrafluoroborate ([C5dmim][BF4]), 1-butyl-3-ethylimidazolium tetrafluoroborate ([C4eim][BF4]) and 1,3-dipropylidazolium tetrafluoroborate ([C3pim][BF4]), were judged to be suitable. The osmolalities of its water mixture varied with temperature, with a larger value at 50 °C than at 25 °C, with ILs with a lower UCST point exhibiting higher osmolality. The concentrated phase of [C4eim][BF4], which was phase-separated at 25 °C, had an osmotic pressure at 50 °C that was 1.6 times that of seawater, implying that it might be used as a DS in the FO seawater desalination process. Moreover, the osmolality at 25 °C dilutive phases of the [C5mim][BF4], [C5dmim][BF4], and [C4eim][BF4] aqueous solutions were all extremely low, these indicating that the osmotic resistance in the post-treatment membrane process can be minimized. The tunable thermoresponsive materials enabled a flexible process design, depending on the environmental and waste heat temperatures. Further, they can be used in various reaction solvents, extraction solvents, and other ingredients of the process.