Role of natural deep eutectic solvents (NADESs) in coagulation bath for PVDF-based membranes on enhanced permeation and separation of rare earth ions

Efficient separation of single rare earth element such as neodymium (Nd) has become an important research topic due to the supply crisis which seriously restricts the application and promotion of rare earth permanent magnets. PVDF-based separation membranes prepared by non-solvent induced phase separation (NIPS) method usually exhibited hydrophobic surface properties resulting in performance attenuation and operation cost increase. This article applied an easily available NADES from betaine and lactic acid to regulate and control the coagulation bath of a polymer inclusion membrane (PIM) for the separation of Nd, Sm and Dy ions. Evidences for the formation of DES were characterized via 1H NMR and FT-IR, and betaine showed more obvious changes than lactic acid during the formation of DES via hydrogen bonds. With the help of DES, pores on the surface were much larger and richer, and the migration of PO and P–O groups to the lower epidermal layer of the membrane made the surface layer become more hydrophilic. DES accelerated the liquid-liquid demixing process and led to faster membrane-forming speed and typical finger-like macrovoids and short vertical penetration channels, which could be observed at high DES contents, and a DES content of 5 wt% was found the most optimal considering the overall permeability and separation performances. As an effective means to improve membrane's performances, adding hydrophilic NADESs at a proper ratio in the coagulation bath would be conducive to form more porous structures, higher surface hydrophilicity, optimized permeability and separation performances.