Solid Carrier-Based Membranes Fabricated from Polydimethylsilox-ane for Selective Metal Ion Separations

Abstract

Membrane-based solvent extraction offers a pathway to improve the efficiency and sustainability of hydrometallurgical separations and resource recovery, but conventional supported liquid membranes (SLMs) suffer from limited stability due to carrier loss and pore wetting. Here, we introduce solid carrier-based membranes (SCBMs), in which extractants are embedded within a crosslinked polymer matrix, as a platform for selective metal separations. Using polydimethylsiloxane (PDMS) as a model matrix and Cyanex® 272 as a cobalt-selective extractant, we demonstrate selective Co2+/Ni2+ transport and systematically investigate the effects of crosslink extent, extractant loading, and membrane thickness on transport performance. At an effective thickness of ∼109 μm, SCBMs achieved a Co2+/Ni2+ selectivity of 12.8 and initial cobalt fluxes up to 2.10 ± 0.17 g m−2 h−1, which were approximately 1 order of magnitude lower than SLM fluxes under identical conditions (21.6 ± 4.8 g m−2 h−1) due to reduced diffusivity of metal-extractant complexes in the polymer matrix. Increasing crosslink extent decreased flux by up to 3.4× while improving Co2+/Ni2+ selectivity from 11.5 to 24.0, whereas increasing extractant content enhanced flux but reduced the Co2+/Ni2+ separation factor from 24.9 to 12.8, highlighting a permeability-selectivity trade-off. Using dip coating, we achieved thinner SCBMs (25−35 μm) with higher initial cobalt fluxes (4.7 ± 0.6 g m−2 h−1). While extractant loss remains a key limitation, we showed that membranes can be regenerated with consistent performance over multiple cycles. Due to challenges with low gel fractions for this particular polymer/extractant combination, we were not able to form defect-free SCBMs at the low thicknesses (∼1 μm) typical for polymeric selective layers. To predict performance in this regime, we extrapolated our results using modeling, suggesting that thicknesses below 7.7 μm should enable SCBMs to achieve fluxes comparable to SLMs, and thin films of common thicknesses (∼2 μm or below) are projected to exceed SLM performance, despite lower intrinsic diffusivity. These results establish SCBMs as a possible alternative to SLMs and identify key research questions to achieve high-performance selective separations of metal cations.