Recovering Spent Natural Deep Eutectic Solvents Using Organic Solvent Nanofiltration

Natural deep eutectic solvents (NADES) have been shown to have exceptional abilities for phenolic compound extraction from plant material. However, solvent and solute recovery from NADES to achieve a circular process remains challenging. Herein, we propose an alternative low-energy process for NADES recovery using membrane filtration, specifically “open porous” nanofiltration (NF). In this study, a dead-end stirred cell was used to screen eight NF membranes for flux and selectivity. Solvent flux was tested for unused 1,2-propanediol, a choline chloride and 1,2-propanediol NADES (ChChlPro_13), and a betaine and 1,2-propanediol NADES (BetaPro_14), revealing decreasing flux in the order 1,2-propanediol > ChChlPro_13 > BetaPro_14 for all membranes. The UA60 membrane offered a good balance between polyphenol retention and solvent flux. At pilot scale, at 35.8 bar, from 25 to 45 °C, NF permeate flux increased from 3.5 to 11.6 L m−2 h−1 and 1.6 to 5.5 L m−2 h−1 for the spent 1,2-propanediol and ChChlPro_13, respectively, while total phenolic compounds (TPC) retention increased from 58.6% to 78.3% and 52.4% to 72.2% for spent 1,2-propanediol and ChChlPro_13, respectively. NF was identified as a novel, viable low-energy (<200 kJ kg−1) recovery strategy for NADES and other similar viscous solvents, such as 1,2-propanediol.