As water scarcity continues to impact communities around the world, inland desalination will become increasingly necessary to meaningfully expand water supply. Unfortunately, inland desalination concentrate, i.e., the waste stream that results from the separation of fresh water from the saline feed, often contains high concentrations of salts and contaminants and can be toxic if discharged into the environment. The current management of such concentrates is prohibitively expensive, stifling the growth of inland desalination plants. Temperature swing solvent extraction (TSSE) is an emerging technology that is able to desalinate the hypersaline concentrates, thereby reducing their volume and facilitating disposal while simultaneously producing additional freshwater to further alleviate water scarcity. TSSE is a thermomorphic multiphase system approach that uses a switchable solvent with thermally-responsive polarity to extract water from the hypersaline brine while rejecting salts. The application of low temperature heat triggers the solvent to switch from a hydrophilic to a hydrophobic state, causing the product water to disengage from the extract and regenerating the solvent to be recycled into the process. The opportunity to employ low-grade heat as the sole energy source and the recyclability of the solvent offer greener reaction conditions compared to conventional hypersaline desalination techniques. This study is the first to systematically investigate the partitioning of water, salt, and amine solvents in the TSSE multiphase system over a range of temperatures, providing fundamental insights into key molecular interactions in the process. We reveal that, as temperature decreases, the amine solvent can extract more water but also intrinsically extracts additional salt, resulting in a tradeoff between water recovery and salt rejection. Van’t Hoff plots are constructed to analyze the energetics of both water recovery, from the brine feed into the solvent phase, and solvent loss into the product water. We further demonstrate how amine structures with increased steric hindrance on the nitrogen atom can lead to a decrease in the thermoresponsiveness of solvent loss to the product water. These insights will aid in in the design of greener, next-generation solvent chemicals, which can maximize water recovery and salt rejection while minimizing solvent loss and energy consumption, advancing the technology towards actual implementation.

Sustainable hypersaline desalination via solvent extraction: Molecular insights from thermodynamic analysis

ACS Spring 2022

The official archive and repository for all ACS Meetings & Expositions abstracts and select posters or presentations.