Antifouling Plasma-Treated Membranes with Stable Superhydrophobic Properties for Membrane Distillation

Superhydrophobicity has recently garnered significant attention in membrane fabrication and modification. This attention stems from its potential to enhance performance of long-term membrane distillation (MD) operations by providing stability and improved antifouling properties. Herein, we present superhydrophobic polytetrafluoroethylene (PTFE) and cellulose acetate (CA) membranes, modified using plasma micro-nanotexturing followed by plasma deposition, which demonstrate enhanced antifouling properties in two modes of MD, air gap (AGMD), and direct contact (DCMD). The plasma-treated PTFE membranes (initially hydrophobic) exhibit stable performance, with constant fluxes of 3.3 L/(m2 h) in AGMD and 8.5 L/(m2 h) in DCMD, excellent salt rejection (>99.9%), and robust fouling resistance during long-lasting operations and against high foulant concentrations, in this case, bovine serum albumin (BSA) (up to 3 g/L). The same observation is made for the plasma-treated CA membranes (initially hydrophilic), which demonstrate flux values of 3.9 L/(m2 h) in AGMD and 9.3 L/(m2 h) in DCMD. Going one step further, we introduce a method to study the stability of the superhydrophobic state by monitoring the membrane surface in situ during MD, using white light reflectance spectroscopy (WLRS). As revealed by the unaffected reflectance spectrum, no wetting transition occurs for the plasma-treated PTFE membrane for liquids with surface tension as low as 39.1 mN/m, whereas a wetting transition is observed for the pristine PTFE when surface tension decreases below 60 mN/m.