Eco-efficient design of vinyl carbazole derived anion exchange membranes with controlled d-spacing for high-performance acid recovery and metal ion exclusion

Advanced membrane design with high proton selectivity is essential in industry for effective acid recovery and the elimination of multivalent metal ions, thereby reducing chemical consumption and environmental impact. Here, we present Vinyl carbazole (Vc)-functionalized membranes with adjustable interchain spacing (d-spacing) and hydrophilic channels to enhance acid recovery via diffusion dialysis. Adjusting Vc content optimizes the membrane structure for balanced proton conduction and metal-ion transport rejection. Moderate Vc incorporation (2–8 mol%) expands interchain spacing and simultaneously facilitates in situ crosslinking, thereby forming dense networks that suppress water uptake. This form of interconnected ionic channels significantly increases U acid flux: U acid values are 18.21 × 10 −3  m h −1 (HCl/FeCl 2 ) and 14.02 × 10 −3  m h −1 (H 2 SO 4 /FeSO 4 ), while metal ion transport (U salt ) is suppressed to 2.85 × 10 −6  m h −1 (HCl/FeCl 2 ) and 4.5 × 10 −6  m h −1 (H 2 SO 4 /FeSO 4 ). In the HCl/FeCl 2 system, the membrane achieved high selectivity (S ≈ 997.81–4515.99). However, further increasing the Vc content up to 10 mol% (PT 0.9 -Vc 0.1 ) enhances crosslinking, which reduces selectivity (S ≈ 1612.59) and proton flux (U acid  = 9.20 × 10 −3 , and 8.86 × 10 −3  m h −1 for HCl/FeCl 2 and H 2 SO 4 /FeSO 4 ). These findings suggest that moderate Vc levels optimize high H + conductivity and prevent metal-ion crossover through steric effects, Donnan exclusion, and charge-assisted hopping via the carbazole nitrogen, thereby controlling Fe 2+ crossover. The adjustable microstructure also minimizes swelling, creating durable membranes suitable for industry use.