Effects of surfactant types on membrane wetting and membrane hydrophobicity recovery in direct contact membrane distillation

As a common pollutant in wastewater, surfactant is a major factor for membrane wetting in direct contact membrane distillation (DCMD) process. Many studies have been reported to develop anti-wetting membranes, but few study focused on hydrophobicity recovery of membranes wetted by surfactants. Membrane wetting caused by different surfactants in DCMD was evaluated through net and liquid flux, and hydrophobicity recovery of the wetted membranes using water washing and drying was systematically assessed. The results showed that wetting rates and degrees varied, with hexadecyl trimethyl ammonium bromide (CTAB) inducing the fastest and the most serious wetting, followed by Tween 20 and sodium dodecyl sulfate (SDS). With respect to cleaning via washing and drying, the membranes wetted by SDS and CTAB exhibited good hydrophobicity recovery, but the hydrophobicity of the membranes wetted by Tween 20 was not effectively restored. Alternatively, a good hydrophobicity recovery was also achieved when the membranes wetted by Tween 20 were soaked in CTAB before washing and drying. The hydrophobicity recovery was improved with the increase in CTAB ratio during filtration of the mixture of CTAB and Tween 20. The results could provide guidance for restoring hydrophobicity of the membranes wetted by surfactants via optimized cleaning approaches.