Hierarchically Structured Janus Membrane Surfaces for Enhanced Membrane Distillation Performance

Abstract

Commercial hydrophobic poly(vinylidene fluoride) (PVDF) membranes are vulnerable to membrane fouling and pore wetting, hampering the use of membrane distillation (MD) for the treatment of surfactant-and oilcontaining feed streams. To address these challenges, we designed novel Janus membranes with multilevel roughness to mitigate foulant adhesion and prevent pore wetting. Specifically, fouling-and wetting-resistant Janus MD membranes with hierarchically structured surfaces were tailored via a facile technique that involved oxidant-induced dopamine polymerization followed by in situ immobilization of silver nanoparticles (AgNPs) on commercial PVDF hollow fiber substrates. These membranes demonstrated outstanding antifouling properties and salt rejection performances in comparison to membranes with single-level structures. We ascribed the membranes' excellent performances to the coupled effects of improved surface hydrophilicity and self-healing mechanism brought about by AgNPs. Furthermore, the newly engineered membranes exhibited antibacterial properties in Bacillus acidicola solutions as evidenced by clear inhibition zones observed on a confocal laser scanning microscope. The development of hierarchically structured Janus MD membranes with multilevel roughness paves a way to mitigate membrane fouling and pore wetting caused by low-surface-tension feed streams in the MD process.

Asymmetric Surface Engineering for Janus Membranes

“…In this way, the modification depth could be controlled to some extent via the deposition time. [ 49 ] The modifiers used for the single‐side interface‐confined modification include dopamine, [ 49–59 ] hydroxyapatite, [ 60 ] catechol, [ 61 ] starch paste, [ 62 ] KOH solution, [ 63,64 ] 1H,1H,2H,2H‐perfluorodecyltriethoxysilane, [ 65 ] octadecylamine, [ 66 ] fluoropolymer emulsion foam, [ 67 ] paraffin, [ 68 ] polydimethylsiloxane, [ 69 ] and TiO 2 nanoparticle slurry. [ 70 ] Note that the prerequisite of this method is that the membrane surface should be nonwettable to the modifier solution for ensuring a nonuniform modification.…”

“…[ 70 ] Note that the prerequisite of this method is that the membrane surface should be nonwettable to the modifier solution for ensuring a nonuniform modification. [ 49–70 ] It is thus not suitable for the wettable system because the modification solution would easily penetrate through membrane pores due to capillary effect, leading to an indiscriminate decoration on both sides. This substrate‐dependent issue could be addressed to some extent through the single‐side spraying technique.…”

“…The introduction of hydrophilic layer effectively depresses the fouling phenomenon for both vacuum membrane distillation [ 109 ] and direct contact membrane distillation. [ 26–28,45,52,55,63,101,110 ] Besides, a hydrophilic layer with a certain thickness shortens the pathway for mass transfer from the hot feed fluid to the cold permeate for achieving a high steam flux. Further increasing the thickness of hydrophilic layer would compromise the salt rejection and even cause a breakthrough by water for the membranes.…”

Acoustophoretic assembly of millimeter-scale Janus fibers

“…The equilibrium material balance can be expressed by substituting eqn (4) and (7) into eqn (3), 2Ga 2 27…”

Novel Janus Fibrous Membranes with Enhanced Directional Water Vapor Transmission

“…The unique directional water transport can alternatively be achieved by abruptly changing the wettability from hydrophobicity to hydrophilicity along the thickness of the fibrous membrane, such as Janus membrane which is a "double-side" membrane with asymmetric properties on either side of the membrane or at membrane surfaces. These properties are primarily derived from different components and/or structures, and are expressed in the form of hydrophilic/hydrophobic or positive/negative surface charges [24][25][26][27][28][29].…”