Crown ether containing polyelectrolyte multilayer membranes for lithium recovery

Achieving solute selectivity has always been a goal of membrane development studies. The continuing growth of global consumption of scarce metals by different industries has put a strain on traditional sources of these species. Achieving cation selectivity in membranes, especially among monovalent cations, is a major step in introducing alternative sources for scarce metals such as lithium. Polyelectrolyte multilayer membranes (PEMMs) are a novel class of membranes, offering great potentials in monovalent/bivalent ion selectivity. On the other hand, crown ethers are a well-studied family of macrocyclic ligands capable of forming stable complexes with cations. In the current study, for the first time, we report on a PEMM nanofiltration membrane with crown ether moieties embedded in its structure for the goal of achieving monovalent salt selectivity. The crown ether 15-crown-5 was successfully incorporated in the polycation polyethylenimine (PEI), which was then used as the polycation in PEMM formation through layer by layer deposition. Both the synthesized polymer and the polyelectrolyte multilayer (PEM) were characterized and the performance of the resulting membrane was studied. It was determined that crown ether containing polymer forms more stable complexes with lithium than potassium. This was explained by the limitation put on 2:1 potassiumcrownether complexes by steric hindrance from polymer chain. The manufactured membranes showed Li/K selectivity for a period of around 90 min, after which the crown ethers became saturated and selectivity was lost. The modified membranes became non selective after this point, but possessed high salt rejection potential.