CF4 plasma surface modification of asymmetric hydrophilic polyethersulfone membranes for direct contact membrane distillation

This paper describes the use of CF4 plasma modification of a hydrophilic membrane into a hydrophobic one for membrane distillation. Plasma surface modification conditions were optimized with respect to plasma glow discharge power and treatment duration using a flat sheet PES membrane. The modified membranes were characterized by X-ray photoelectron spectroscopy (XPS), SEM, contact angle measurements, pore size distribution, liquid entry pressure and atomic force microscopy. Results revealed that the plasma modification converted hydrophilic membranes of a contact angle 0° into hydrophobic ones with water contact angle above 120°. Fluorination was ascribed to the wettability change of the membrane from hydrophilic to hydrophobic via insertion and possibly deposition. Direct contact membrane distillation of the hollow fibers using 4% NaCl solution yielded a water flux of 45.4 kg/m2 h at a feed temperature of 63.3 °C. A rather high evaporation efficiency of the membrane distillation process was estimated in comparison with literature results. Direct contact membrane distillation (DCMD) stability test showed a water flux of 42.1 kg/m2 h using 4 wt% NaCl as feed (at the temperature of 60.5 ± 0.2 °C). No leakage was observed for 54 h indicating a stable membrane performance. The high evaporation efficiency and water flux were ascribed most probably to the high porosity of the base membrane.