Anti-wetting mechanism of negative pressure mode in direct contact membrane distillation using hollow fiber membrane

Membrane wetting is one of the key challenges that needs to be overcome to realize operational stability in membrane distillation. In this work, we showed that negative pressure mode can be applied into direct contact membrane distillation (DCMD) using PVDF hollow fiber membrane to achieve such stability for low surface tension (42.2 mN m−1) organic solvent (represented by ethanol)-containing saline solution feed but is less useful for surfactant (represented by Tween-20)-containing saline solution feed with similar surface tension. While the conventional positive pressure DCMD showed rapid deterioration of flux and increase of permeate conductivity during 6-hour continuous operation when treating a 25 vol% ethanol in 0.6 M NaCl solution, negative pressure DCMD could maintain stable high flux of 10.4 kg m−2 h−1 and low permeate conductivity of 3 μS cm−1 throughout 6-hour duration. Using time-dependent droplet evaporation experiments and membrane morphology observations, we demonstrated the different membrane wetting and fouling mechanisms of ethanol and Tween-20-containing solutions. We also demonstrated that negative pressure can effectively prevent the membrane wetting in ethanol solution feed case by keeping the contact angle of the liquid on the pore wall larger than 90o, which promotes the formation of a Cassie-Baxter state on the liquid-vapor interface.