Diffusion Dialysis Anion Exchange Membranes Based on a Polymer Blend with Compatibility Transition

Abstract

Partially compatible polyphenylene oxide - polyvinyl pyrrolidone (PPO-PVP) dense membranes have been investigated for recovering sulfuric acid from salt-containing waste acid via diffusion dialysis. The compatibility of PPO-PVP blends with varying PVP (K90) contents was studied, and micro-phase separation behaviors were observed by transmission electron microscopy. In sulfuric acid solution, the carbonyl groups of pyrrolidone moieties react spontaneously with H⁺, rendering the membrane positively charged and enabling anion exchange functionality. The mass transfer performance of membranes with different PVP mass ratios was studied. PPO-PVP blends are fully miscible at PVP contents ≤ 15 wt%, whereas distinct micro-phase separation occurs at 25–55 wt% PVP. The resultant membranes exhibit three micro-domains: PPO-rich domain, PVP-rich domain, and PVP aggregates, which are functionally distinguishable micro-domains rather than thermodynamic phases. The PVP aggregates act as ion transport channels within the membrane, and crucially, the membrane’s mass transfer performance is highly dependent on the quantity and size of such PVP aggregates. The membrane with initial 45 wt% PVP delivers optimal performance (sulfuric acid permeability coefficient 34.9×10⁻⁹ m²·h⁻¹ and sulfuric acid/ferrous sulfate separation factor 17), arising from PPO-PVP compatibility transition-induced micro-phase separation that forms distinct micro-domains to mediate selective acid transport. This research reports an interesting compatibility transition phenomenon for the fabrication of micro-phase separated anion exchange membranes.