Hierarchical-Porous Gas-Permeable Membrane toward Efficient and Robust Ammonia Recovery

The ammonia recovery from wastewater via a concentration-driven membrane stripping process is highly promising yet constrained by the trade-off issue between ammonia permeability and antiwettability of hydrophobic gas-permeable membranes (GPMs). We designated a tailored hierarchical-porous GPM (HGPM) via a facile fabrication method to address the above issue and established its transfer model, clearly illustrating the ammonia diffusion across the distinct layers. Molecular diffusion across the finger-like layer of HGPMs was responsible for a superior overall mass transfer coefficient of 3.3 × 10⁻⁵ m/s. Meanwhile, its nanoscale pores in the top-skin and sponge-like layers conferred commendable wetting resistance against neighboring alkaline or acidic liquids, with the liquid entrance pressure measured at 182 kPa. The hierarchy of HGPMs together achieved the best-ever balance between ammonia permeability and antiwettability (both increased up to ∼150-200% of the commercial GPMs), significantly breaking the upper bound limit. Dealing with the real wastewater from anaerobic digestate, HGPMs demonstrated a robust ammonia recovery flux of 75-80 g N/m² h over 50 h, 3 times higher than that of the commercial GPMs. This work sheds light on the effective recovery of volatile valuable compounds from complex wastewater, with the as-tailored membrane of great upscalability holding great potential toward a future circular society.