(388h) Treating Brine Solutions By Osmotically-Assisted Reverse Osmosis (OARO) and Thermal-Responsive Ionic Liquids

Brines such as the shale gas-produced water are highly concentrated (NaCl > 1.5 M), which exhibits high osmotic pressure that cannot be further concentrated by normal reverse osmosis. Conventional solar pond evaporation is energy-efficient, but such a process is time intensive (> 1 year). Osmotically-assisted Reverse Osmosis (OARO) uses both transmembrane pressure and a draw side sweep solution as the driving force, which is suitable for the brine concentration, but multiple stages of separations and the associated energy consumption are required. In this work, instead of using NaCl as draw solutions, we developed a more sustainable OARO process by using thermal responsive ionic liquids as draw solutions. An LCST-type ionic liquid, [P4444] DMBS, with a critical temperature of around 36 °C, was used as the draw solution at different concentrations for the water recovery from NaCl solutions between 1.5 M to 2.5 M, under 65 bar transmembrane pressure. The commercially available FilmTecâ„¢ SW-30 membrane was used for OARO applications, and a bench-scale OARO system is developed. The flux is increased from 1.4 LMH to 2.1 LMH when employing 70 wt% [P4444] DMBS as draw solution and treating 1.5 M NaCl solution. The NaCl rejection is 85%, and the back-flux of [P4444] DMBS is only ~ 1.5 g m-2 h-1. The ionic liquids can be recovered by low-grade waste heat at 50 °C, with most of the NaCl partitions into the water phase. The successful development of such an IL-based OARO process provided a new pathway towards the sustainable treatment of brine solutions such as the produced water.