Effect and mechanism of an anionic surfactant on membrane performance during direct contact membrane distillation

This study aimed to investigate the effect of a typical anion surfactant sodium dodecyl sulfate (SDS) on direct contact membrane distillation (DCMD) process using three different membranes, including polyvinylidene fluoride (PVDF) and polytetrafluoroethylene (PTFE) hydrophobic membranes and an omniphobic membrane. The PVDF membrane was the most likely to be wetted, and the pores wetting got more serious for both the PVDF and PTFE membranes with SDS concentration increasing. The omniphobic membrane presented robust anti-surfactant-wetting ability, even facing the saline feed with 0.5 mM SDS. The calcium ions can promote the surfactant SDS micellization, and the formed SDS colloids could cause membrane fouling. After 50 h continuous DCMD operation, the permeate flux of the hydrophobic membranes declined more than 50% due to membrane fouling, while the loss of permeate flux for the omniphobic membrane was only about 20%. The mechanism of membrane fouling was quantitatively investigated by the extended Derjaguin-Landau-Verwey-Overbeek theory. The calculated results manifested that the omniphobic membrane had the lowest attractive force to the foulant, indicating a less risk of being fouled. The energy barrier of the omniphobic membrane was the highest, resulting in the strongest resistance against the SDS colloids deposition on membrane surface.