Mitigating Wetting and Scaling in Air Gap Membrane Distillation Crystallization via SiO₂ Seeding

Membrane distillation crystallization (MDCr) is an approach for treating hypersaline wastewaters and enabling zero-liquid-discharge (ZLD) systems. However, its performance is often inhibited by concentration polarization, scaling, and membrane wetting. Heterogeneous seeding has been proposed to shift crystallization into the bulk phase, yet its quantitative influence on flux stability, wetting resistance, and crystal growth remains poorly understood. This study investigates air-gap MDCr (AGMDCr) of 300 g L^-1 NaCl using polypropylene (PP) and polytetrafluoroethylene (PTFE) membranes under seeded and unseeded conditions. Introducing 0.1 g L^-1 SiO₂ seeds (30-60 µm) enhanced steady-state permeate flux by 41% and maintained salt rejection ≥ 99.99%, indicating effective suppression of wetting. Seeding shifted the crystal size distribution from fine (mean 50.6 µm, unseeded) to coarse (230-340 µm), consistent with reduced primary nucleation and preferential growth on seed surfaces. At 0.6 g L^-1, the flux decreased relative to 0.1-0.3 g L^-1, consistent with near-wall solids holdup and hindered transport at high seeding concentration. The PTFE membrane exhibited a 47% higher flux than PP, primarily due to its reduced thermal resistance and optimized module geometry at the same flow rate. These results demonstrate that appropriately sized and dosed SiO₂ seeding effectively stabilizes flux and suppresses wetting in MDCr.