Abstract：The development of high-performance ion-selective membranes is crucial for acid reclamation from industrial waste, enabling resource recovery while minimizing environmental impact. Herein, we report the preparation of quaternized poly (2,6-dimethyl-1,4-phenylene oxide) (QPPO)-based anion exchange membranes (AEMs) modi-fied with varying loadings of Bis-MDEA-C+, a flexible diamine crosslinker bearing two quaternary ammonium(QA) groups. This study represents the first application of Bis-MDEA-C+ as a bifunctional flexible crosslinker in AEMs for diffusion dialysis. The incorporation of this novel crosslinker enables custom-made ionic channel formation via three-phase structural modulation. The resulting QBM-x membranes exhibit a well-tuned three-phase structure consisting of hydrophilic domains, hydrophobic polymer backbones, and ionic crosslinking in-terfaces enabling efficient dialysis coefficient of acid (U_H⁺) while suppressing the co-diffusion of multivalent cations such as Fe²⁺. Among the series, QBM-0 (crosslinked without Bis-MDEA-C₆²⁺) shows the highest H⁺/Fe²⁺ selectivity (S = 104.7), attributed to enhanced electrostatic repulsion and restricted ionic pathways. Increasing the Bis-MDEA-C₆²⁺ content improves the U = 27-54×10⁻³ m h⁻¹, owing to the formation of semi-continuous ion-conductive networks. However, excessive crosslinker loading compromises selectivity due to channel over- hydration and weakened charge exclusion. Compared to commercial and reported membranes, the QBM-x membranes demonstrate superior performance, achieving a high selectivity (S = 94-104) and competitive U_H⁺. These results highlight the importance of balanced phase engineering in AEM design and highlight the QBM-x membranes as promising candidates for efficient acid recovery via diffusion dialysis (DD).