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 degrees into hydrophobic ones with water contact angle above 120 degrees. 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/m(2) h at a feed temperature of 63.3 degrees 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/m(2) h using 4 wt% NaCl as feed (at the temperature of 60.5 +/- 0.2 degrees 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. (C) 2012 Elsevier B.V. All rights reserved.