Polymeric Microcapsules as Robust Mimics of Emulsion Liquid Membranes for Selective Ion Separations.

Selective ion separations are increasingly needed to combat water scarcity, recover resources from wastewater, and enable the efficient recycling of electronics waste. Emulsion liquid membranes (ELMs) have received interest due to rapid kinetics, high selectivities, and low solvent requirements but are too unstable for industrial usage. We demonstrate that polymeric microcapsules can serve as robust, solvent-free mimics of ELMs. As a proof of concept, we incorporated the copper-selective ligand Lix 84-I in the walls of microcapsules formed from a commercial polystyrene--polybutadiene--polystyrene triblock polymer. The microcapsules were formed from a double-emulsion template, resulting in particles typically 20-120 μm in diameter that encapsulated even smaller droplets of a dilute (≤0.5 M) HSO solution. Batch experiments demonstrated facilitated-transport behavior, with equilibrium reached in as little as 10 min for microcapsules with 1% ligand, and with ∼15-fold selectivity for Cu over Ni. Furthermore, the microcapsules could be packed readily in columns for flow-through operation, thus enabling near-complete Cu removal in ∼2 min under certain conditions, recovery of Cu by flowing through fresh dilute HSO, and reuse for at least 10 cycles. The approach in this work can serve as a template for using selective ligands to enable robust and simple flow-through processes for a variety of selective ion separations.