Micromechanism Underlying Wetting Behavior of the Vacuum Membrane Distillation during Desalination

Abstract

Membrane wetting is a crucial issue in the desalination process by vacuum membrane distillation (VMD). However, the micromechanism of the wetting behavior is far from clear. Here, electrospun nanofiber membranes (ENMs) had been utilized to clarify the pore variations inside the membrane for desalination from NaCl aqueous solution, where the distinct open pore structure of ENMs could address the characterizing limitations for the change occurring in the membrane pores. A wetting formation model had been depicted on the basis of characterized results, illustrating that the basic reason for wetting was the initial NaCl crystallization caused by supersaturation, which was different from the membrane pores blocking or the structure damage caused by scaling. Moreover, membrane regeneration experiments also showed that the wetting caused by the crystallization of NaCl was reversible. By a reasonable regeneration strategy, VMD used for desalination was expected to achieve continuous operation with rejection over 99.9%.

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

“…Also, in the terminal of the tests, no scale layer was observed with the SEM technique, and only a few crystal particles were found (Figure 2g1). This result was also reported in a previous study, 23 that no NaCl scale layer was observed in terminal SEM images of the membrane. We also tested other highly soluble salts, including KCl and Na 2 SO 4 , and a poly(tetrafluoroethylene) (PTFE) membrane in MD (Figure S3).…”

“…Also, in the terminal of the tests, no scale layer was observed with the SEM technique, and only a few crystal particles were found (Figure g1). This result was also reported in a previous study, that no NaCl scale layer was observed in terminal SEM images of the membrane.…”

“…The kinetics of NaCl-induced wetting (wetting thickness vs time) showed that the wetting velocity gradually decreased during the wetting process (Figure a). According to previous studies, − the evaporation of water at the gas–liquid interface led to crystallization near this interface, and the formed crystals could act as a water bridge to introduce water to propagate forward to the distillate side. Therefore, it was necessary to investigate the solute concentration at the gas–liquid interface ( c g‑l ) to better understand the wetting dynamic.…”

“…These highly soluble salts can also crystallize on the membrane surface after concentration and then potentially induce pore wetting . For example, several studies have reported the wetting phenomena in MD fed with pure NaCl solution. − Because of the high concentration, the crystal growth rate of highly soluble salt is much faster than that of sparingly soluble salt . This rapid growth of the crystal, on the one hand, facilitates the formation of scale layer, and, on the other hand, may quickly block the membrane pore and hinder the continuous growth of crystals into membrane pores.…”

Development of Robust Fluorinated SiO2/PVDF Composite Hollow Fiber Membrane for Bromine Resources Recovery from Brine via Membrane Distillation

“…28−31 MD has similar advantages to membrane absorption but is more efficient in bromine extraction by imposing extra vacuum forces. 32,33 The hydrophobicity of the membrane prevents the infiltration of the aqueous solution, because only vapor can be transported through the pore channels, so MD can theoretically achieve a nearly complete rejection of nonvolatile salts. 34,35 Tang et al 36 employed a membrane distillation process to separate bromine−water mixtures with different flat polyurethane membranes.…”