Surface modification of polypropylene membrane by helium plasma treatment for membrane distillation

Abstract

Helium plasma was used to modify the surface of commercial polypropylene capillary membranes applied for membrane distillation. The membranes were manufactured for microfiltration and had a thin skin layer with low porosity. The plasma reactions such as etching and re-deposition resulted in a substantial modification of the skin layer. The surface porosity and pore diameters increased due to plasma treatment and resulted in 25% enhancement of the permeate flux during membrane distillation (MD). Although the plasma treatment caused hydrophilization of the membrane surface, the resistance of membranes to wetting did not deteriorate and the permeate conductivity at a level of 5 μS/cm was retained for 200 h of the MD process with NaCl solution. The model calculations confirmed that wetting only of the surface pores did not cause a significant decrease in MD efficiency, while the calculated efficiency decreased by 30% when the wetted layer reached half the thickness of the membrane wall. The modified membranes demonstrated good stability and the permeate purity was similar during the 300 h desalination of Baltic Sea Water after three years of their storage. The separation of water contaminated by oil (50 ppm) revealed that hydrophilic compounds on the membrane surface formed in helium plasma treatment significantly reduced oil fouling.