Highly Permeable and Selective Polymer Inclusion Membrane for Li+ Recovery and Underlying Enhanced Mechanism

Abstract

Polymer inclusion membrane (PIM) has become a potential alternative in efficient Li+ separation from brines typically with Mg2+ and Li+, due to its exceptional stability and selectivity. However, the practical application of PIMs is still greatly restricted by its slow mass transfer and related unclear transport mechanism. Here, we fabricated an efficient PIM with tributyl phosphate (TBP) along with sodium tetraphenylborate (NaBPh4) as carriers and cellulose triacetate (CTA) as a polymer skeleton for highly selective and permeable Li+ separation from high Mg2+/Li+ aqueous solutions, offering an initial flux of 3 ~ 20 times higher than other PIMs. With increasing TBP content in the PIM, a nonmonotonical trend (sharp increase-leveling off-slow decrease) of its initial flux was observed. The improvement of the solubility (compatibility) of NaBPh4 in CTA matrix by introducing TBP was demonstrated by scanning electron microscope (SEM) coupled with X-ray energy dispersive spectroscopy (EDS) images, which was highly consistent with binding energy simulation. X-ray diffraction (XRD) and positron annihilation lifetime spectroscopy (PALS) results verify that moderate TBP content in the PIM could enlarge the distance of polymer chains and enhance the initial flux of PIMs, while excessive TBP can occupy the free volume sites, leading to an adverse effect on the permeability. Our study provides valuable information on the compatibility effect on the mass transfer through the polymer matrix including PIMs, which exhibits prospect in a wide range of applications of metal separation.

Keywords: Li+ separation, polymer inclusion membrane, high Mg2+/Li+ ratio, transport mechanism