Insights into gypsum and silica scaling behaviors of dense Janus membranes in membrane distillation: The effect of varied surface properties

Dense Janus membranes (JMs) have emerged as potential candidates to address the tough scaling issue in hypersaline wastewater treatments for membrane distillation (MD). However, behaviors and mechanisms of various scalants on dense JMs have not been systematically investigated. Herein, we designed three dense JMs to simultaneously compare their anti-scaling properties against gypsum and silica in MD and focus on the critical role of varied surface property in scaling control. Results showed that dense JMs with highly positive gypsummembrane interfacial energy and with strong electrical repulsion against silica species were more effective in resisting gypsum and silica, respectively. Scaled membrane characterization, quartz crystal microbalance monitoring, interfacial energy analysis and AFM force measurement revealed the distinct anti-scaling mechanisms. Highly hydrophilic surfaces without specific interactions with scaling ions provided robust thermodynamic barriers and less kinetically affected local supersaturations, preventing gypsum nucleation and growth. In contrast, positively charged surfaces favorably attracted negatively charged silica species with high reactivity, promoting subsequent polymerization with silicic acid that kinetically dominated silica scaling. Moreover, coexisting oversaturated gypsum and silica influenced the scaling behavior of each other, and surface properties of the preferentially formed scaled layer might also matter in scaling process. This study shall provide usefulinsights into the anti-scaling mechanisms of dense JMs in MD and highlight the potential of dense surface design as an effective scaling control strategy to pave the way for potential MD applications.