The effects of membrane surface wettability on pore wetting and scaling reversibility associated with mineral scaling in membrane distillation

The application of membrane distillation (MD) to hypersaline wastewater treatment is constrained by mineral scaling. Although tuning membrane surface wettability has been recently used to mitigate membrane scaling in MD, the effectiveness of this strategy for different scaling types has not been well understood. Furthermore, two important factors that determine the performance of MD membrane, namely wetting resistance and scaling reversibility, have been rarely discussed in MD scaling studies. In this work, we investigated the effects of membrane surface wettability on pore wetting and scaling reversibility associated with gypsum and silica scaling in MD. We challenged a hydrophobic membrane, a Janus membrane with a hydrophilic top layer, and a superhydmphobic membrane with gypsum- and silica-containing feed solutions. Compared to the hydrophobic and Janus membranes, the superhydrophobic membrane delayed scaling induction and enhanced scaling reversibility in gypsum scaling tests, in which the total water recoveries achieved by the tested membranes were inversely correlated to membrane surface hydrophilicity. Also, the superhydrophobic membrane uniquely resisted pore wetting induced by gypsum scaling, probably by preventing feedwater intrusion into membrane pores. In contrast, altering membrane surface wettability was ineffective to improve either scaling reversibility or total water recovery under silica scaling, which did not induce pore wetting for all the tested membranes. We attributed the differences in scaling behaviors as well as the varied responses to membrane surface wettability between gypsum and silica scaling to their distinct scaling mechanisms. The oriented and intrusive gypsum crystallization resulted in pore deformation and discrete crystals that could be detached by physical cleaning. In contrast, the slow kinetics and lack of orientation of silica polymerization formed a thin and crosslinked scaling layer, which was not intrusive but firmly attached to the membrane surface. The results of our study provide valuable insights on the interplays among membrane surface wettability, scaling type, and membrane performance in MD desalination, and suggest that the development of scaling mitigation strategies in MD should be tailored to the varied mechanisms of different scaling types.