Green extractive desalination with natural deep eutectic solvents

Natural deep eutectic solvents are used as novel media for extractive desalination. • Achieves among the lowest salinity and dissolved organics for similar process. • Solvent components create synergic effects to enhance water yield. Extractive desalination is a low-cost, membrane-free process for desalination that can be applied in the presence of low-grade heat sources, such as industrial waste heat and solar thermal energy. However, the solvents previously proposed for extractive desalination of water for human usage are relatively toxic and unsustainable, causing health and environmental concerns. Thus, the use of greener, but still effective, extractants is critical to the viability of this process. In this study, the use of natural deep eutectic solvents (NADESs) for extractive desalination of simulated brackish water, seawater and brine was investigated. Menthol/decanoic acid NADESs demonstrated the most effective desalination performance, producing freshwater with relatively low salinity (140–700 ppm from seawater) compared to other extractants. Moreover, the quantity of these food-based solvents discharged into the water phase was relatively low (around or less than 120 ppm). Consequently, the produced freshwater generally meets international standards and regulations, which is not commonly reported for extracts produced by this process. The presence of menthol enhanced water yields by over threefold, achieved by breaking the strong dimeric hydrogen bonds in decanoic acid molecules that hinder water absorption, and by extending the hydrogen-bond lifetimes between extractant and water. Moreover, the water yield improved with decreasing salinity of the water source, making brackish water and seawater particularly attractive for this process.