Unique Behaviors and Mechanism of Highly Soluble Salt-Induced Wetting in Membrane Distillation

Abstract

Scaling-induced wettinggreatly limits the application of membrane distillation (MD) for the desalination of high-salinity feed. Although highly soluble salts (e.g., NaCl) have high concentrations in this water, their scaling-induced wetting remains overlooked. To unravel the elusive wetting behaviors of highly soluble salts, in this study, we systematically investigated the scaling formation and wetting progress by in situ observation with optical coherence tomography (OCT). Through examining the influence of salt type and vapor flux on the wetting behavior, we revealed that highly soluble salt-induced wetting, especially under high vapor flux, shared several unique features: (1) occurring before the bulk feed reached saturation, (2) no scale layer formation observed, and (3) synchronized wetting progress on the millimeter scale. We demonstrated that a moving scale layer caused these interesting phenomena. The initial high vapor flux induced high concentration and temperature polarizations, which led to crystallization at the gas–liquid interface and the formation of an initial scale layer. On the one hand, this scale layer bridged the water into the hydrophobic pores; on the other hand, it blocked the membrane pores and reduced the vapor flux. In this way, the decreased vapor flux mitigated the concentration/temperature polarizations, and consequently led to the dissolution of the feed-facing side of the scale layer. This dissolution prevented the membrane pores from being completely blocked, facilitating the transportation and crystallization of salts at the distillate-facing side of the scale layer (i.e., the gas–liquid interface), thus the proceeding of the wetting layer.

Progress on Improved Fouling Resistance-Nanofibrous Membrane for Membrane Distillation: A Mini-Review

“…In MD applications, the hydrophobic membrane acts as a barrier, preventing feed liquid from directly entering membrane pores while allowing vapor transfer. MD is not limited by the high salt concentration and is suitable for the treatment of high-salinity wastewater [ 2 ]. However, commercial hydrophobic membranes are mostly made of low-surface-energy polymers such as polyvinylidene fluoride (PVDF), polypropylene (PP), and polytetrafluoroethylene (PTFE), which cannot avoid pore fouling, especially for the treatment of challenging wastewaters [ 3 , 4 ].…”

Tailoring Structural Property and Surface Wettability of Omniphobic Membranes for Superhigh Flux and Competent Wetting Resistance in Membrane Distillation

“…Both factors lead to undesirably decreased LEP values of MD membranes. In addition, hypersaline feed in MD might also result in membrane wetting due to the large amounts of hydrophilic mineral deposition on the membrane surface or inner pores. , Subsequently, the exploitation of the wetting-resistant MD membrane is critical to mitigate the above issues and expand its practical applications.…”