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    Unraveling the Kinetics and Mechanism of Surfactant-Induced Wetting in Membrane Distillation: An In Situ Observation with Optical Coherence Tomography

    Year: 2022

    Journal: Environ. Sci. Technol., Volume 56, JAN 4, page 556–563

    Authors: Shao, Senlin; Shi, Danting; Hu, Jiangshuai; Qing, Weihua; Li, Xianhui; Li, Xue; Ji, Bin; Yang, Zhe; Guo, Hao; Tang, Chuyang Y.

    Organizations: Natural Science Foundation of China [52070147]; Hong Kong Scholar Program [XJ2017039]

    Keywords: membrane distillation; membrane wetting; optical coherence tomography; surfactant transport; autophilic effect

    In this study, we performed a direct contact membrane distillation and successfully demonstrated the non-invasive imaging of surfactant-induced wetting using optical coherence tomography. This method enabled us to investigate the wetting kinetics, which was found to follow a three-region relationship between the wetting rate and surfactant concentration: the (i) nonwetted region, (ii) concentration-dependent region, and (iii) concentration-independent region at low, intermediate, and high surfactant concentrations, respectively. This wetting behavior was explained by the autophilic effect, i.e., the wetting was caused by the transfer of surfactants from the water-vapor interface to the unwetted membrane and rendered this membrane hydrophilic, and then the wetting frontier moved forward under capillary forces. At region-(i), the surfactant concentration in the water-vapor interface (C-iv) was too low to make the unwetted membrane sufficiently hydrophilic; thereby, the membrane could not be wetted. At region-(ii), due to the fast adsorption of the surfactant on the newly wetted membrane, the wetting rate was determined by the advection/diffusion of surfactants from the feed stream. Consequently, the wetting rate increased with the increases in the water flux and surfactant concentration. At region-(iii), the advection/diffusion provided excess surfactants for adsorption, and thus C-iv reached its upper limit (maximum surface excess) and the wetting rate leveled off.
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