Janus Membrane with a Dense Hydrophilic Surface Layer for Robust Fouling and Wetting Resistance in Membrane Distillation: New Insights into Wetting Resistance

Abstract

Although membrane distillation (MD) has been identified as a promising technology to treat hypersaline wastewaters, its practical applications face two prominent challenges: membrane wetting and fouling. Herein, we report a facile and scalable approach for fabricating a Janus MD membrane comprising a dense polyvinyl alcohol (PVA) surface layer and a hydrophobic polyvinylidene fluoride (PVDF) membrane substrate. By testing the Janus membrane in direct contact MD experiments using feeds containing a sodium dodecyl sulfate (SDS) surfactant or/and mineral oil, we demonstrated that the dense Janus membrane can simultaneously resist wetting and fouling. This method represents the simplest approach to date for fabricating MD membranes with simultaneous wetting and fouling resistance. Importantly, we also unveil the mechanism of wetting resistance by measuring the breakthrough pressure and surfactant permeation (through the PVA layer) and found that wetting resistance imparted by a dense hydrophilic layer is attributable to capillary pressure. This new insight will potentially change the paradigm of fabricating wetting-resistant membranes and enable robust applications of MD and other membrane contactor processes facing challenges of pore wetting or/and membrane fouling.

Polyamide Thin-Film Composite Janus Membranes Avoiding Direct Contact between Feed Liquid and Hydrophobic Pores for Excellent Wetting Resistance in Membrane Distillation

“…Previous studies suggested that the enhanced wetting resistance originated from the size exclusion of the dense polyamide layer against surfactant molecules. , However, according to the hydraulic pressure-driven filtration tests, the polyamide layer could not completely reject SDS molecules (Figure S3). Therefore, the mechanism of surfactant rejection might be difficult to explain the long-term antiwetting performance of the TFC Janus membrane . When the polyamide layer of the TFC Janus membrane was oriented toward distillate streams, a vapor flux of ∼4 L m –2 h –1 was still achieved (Figure a).…”

“…Therefore, the mechanism of surfactant rejection might be difficult to explain the long-term antiwetting performance of the TFC Janus membrane. 53 When the polyamide layer of the TFC Janus membrane was oriented toward distillate streams, a vapor flux of ∼4 L m −2 h −1 was still achieved (Figure 4a). In this case, if the distillate water entered the polyamide layer and contacted the hydrophobic pores, the vapor would condense in contact with the low-temperature distillate water.…”

“…Therefore, there may be a critical pore size below which water may exist in molecular form rather than in liquid form in the membrane. Even if water exists in liquid form in the dense hydrophilic layer, Feng et al 53 suggested that the high capillary pressure of the small pores of a dense hydrophilic layer (much higher than the transmembrane pressure of MD) trapped the water in it, thereby preventing water from flowing to hydrophobic pores. This might also lead to the noncontact phenomenon.…”

Membrane Distillation Hybrid Peroxydisulfate Activation toward Mitigating the Membrane Wetting by Sodium Dodecyl Sulfate

“…They could readily adhere to the membrane surfaces in MD via hydrophobic and/or electrostatic interactions, gradually sacrificing the pore hydrophobicity during the propagation of the liquid-air interface under the liquid entry scenario [9][10][11]. Despite the development of bespoke MD membranes with special wettability being imperative to overcome these issues, solution polishing to reduce its wetting propensity via adequate pretreatments is also a reasonable strategy [12,13].…”

Understanding the Effect of Membrane Interfacial Wetting Properties on Membrane Distillation Flux

“…If the liquid has intruded into the pores of the membrane, then a non-slip condition applies and the wetting depth might play a role (Figure 1b), which has rarely been discussed. Recently, there have also been proposal of using composite membranes (hydrophilic layer on hydrophobic layer) for MD application (Bhadra et al 2016, Wang et al 2016, Chen et al 2020, Afsari et al 2021, Feng et al 2021, Sun et al 2022. The composite membrane is similar to membranes with partially wetted regions, whose rational design principle is missing.…”