High-salinity brines from desalination and industrial processes are challenging to treat due to high osmotic pressure and energy demand. Osmotically assisted reverse osmosis (OARO) offers a promising approach, but salt-based draw solutions still face challenges in energy-efficient recovery.

In a study published in Journal of Membrane Science, a research team led by Prof. WANG Zhongzhen from the Hefei Institutes of Physical Science of the Chinese Academy of Sciences (CAS), along with researchers from the University of Science and Technology of China of CAS and Anhui University, developed a new membrane-based approach for high-salinity brine treatment.

The researchers proposed using thermoresponsive ionic liquids (TRILs) as recoverable draw solutes. These ionic liquids have low volatility and can be regenerated using low-grade heat. The researchers selected a thermoresponsive ionic liquid, [P4444][DMBS], and developed a laboratory-scale TRIL-based OARO system.

The system achieved stable operation under high-salinity conditions. After treatment, the diluted ionic liquid solution could be regenerated through thermal phase separation, allowing most of the ionic liquid to be recovered and reused. Nanofiltration membranes were demonstrated to effectively remove residual ionic liquid from the water phase.

By using a transport model and energy assessment, the researchers analyzed this process. The results showed that OARO performance was influenced by several factors, including membrane selectivity, solute leakage, and draw-solution recovery.

Besides, the researchers found that although thermoresponsive ionic liquids performed well in individual steps, the overall TRIL-based OARO system required more energy than a salt-based system under the studied conditions. The additional energy mainly came from the extra separation processes needed for ionic liquid recovery and water purification.

The findings of this study demonstrate the use of thermoresponsive ionic liquids as recoverable draw solutes in OARO, highlight the need for system-level optimization of membrane technologies, and provide guidance for developing more efficient OARO systems for sustainable brine treatment.