Evaluating and predicting surfactant-induced hydrophilization of pore channels in membrane distillation

Membrane distillation (MD) processes are susceptible to membrane pore wetting, which compromises the membrane's salt rejection efficiency and deteriorates the quality of the distillate. However, current understanding of the dynamics of membrane wetting remains incomplete, and the conventional indicators used to detect post-wetting, such as conductivity and distillate flux, have limitations. We develop a theoretical model for the wetting front (l) to elucidate the adsorption of surfactant molecules and their role in the hydrophilization of commercial polyvinylidene fluoride (PVDF) membranes under various sodium dodecyl sulfate (SDS) concentrations and different membrane pore sizes. Subsequently, we utilize an in-situ impedance-based monitoring technique to quantitatively track the progression of wetting and validate our model's ability to accurately describe the relationship between the wetting front and the square root of time (l=Kt) to obtain the wetting progression coefficient K within the first 30 min of the slow wetting stage. Hence, K, through the combination of impedance measurement and modelling, offers the advancement of wetting until the critical wetting front is reached. This critical wetting front, located at the half point of the membrane thickness, signifies the occurrence of partial wetting, alerting the need for membrane cleaning to mitigate full wetting before it occurs. Additionally, the potential for membrane recoverability can be assessed by examining the variation of K after cleaning, thereby facilitating the evaluation of the effectiveness of different membrane cleaning strategies. The theoretical model and applicable parameters presented in this study contribute to the understanding and quantification of mechanisms slow wetting in surfactant-induced scenarios and provide a promising method for determining the appropriate membrane for the desired feed and the optimal membrane cleaning strategy for extending the lifespan of membranes.