Robust Graphene/Poly(vinyl alcohol) Janus Aerogels with a Hierarchical Architecture for Highly Efficient Switchable Separation of Oil/Water Emulsions

Abstract

Given the complexity and diversity of actual oily sewages, developing multifunctional separation materials with features of high separation efficiency and low energy consumption for separating diverse oil/water emulsions is urgently needed, yet it remains a formidable challenge till now. Herein, a superior graphene/poly­(vinyl alcohol) Janus aerogel (J-CGPA), showing an intriguing three-dimensional (3D) hierarchical architecture (a dense skin-layer and a larger internal cell network) and desirable asymmetric wettability, was exploited via a simple direct freeze-shaping technique and subsequent mussel-inspired hydrophilic modification. Benefiting from the controlled unilateral decoration of dopamine, the resultant aerogels displayed completely opposite superwettability on two antithetic sides, i.e., one side is highly hydrophobic (water contact angle (WCA), 143°), whereas the other side is superhydrophilic. On the basis of the favorable 3D hierarchical structure and binary cooperative superwetting properties, the Janus aerogels achieved a remarkable switchable separation performance for both highly emulsified oil-in-water and water-in-oil emulsions as well as stratified oil/water mixtures accompanied with outstanding separation efficiencies. Particularly, an ultrahigh permeation flux of 1306 L m–2 h–1 along with a high rejection efficiency of 99.7% was acquired solely under the driving of gravity (<1 kPa), which is 1–2 order of magnitude higher than that of pioneering two-dimensional Janus polymeric/inorganic membranes recently reported. Moreover, together with robust reusability, this novel 3D Janus aerogel indicates a promising practical application for high-performance oily wastewater remediation.

Janus Membranes with Asymmetric Wettability Applied in Oil/Water Emulsion Separations

“…Similarly, the asymmetric wettability of Janus membranes prepared by gel also has outstanding advantages in the separation of oil/water emulsions. The robust graphene/poly(vinyl alcohol) Janus aerogels with a hierarchical architecture prepared by Li et al [69] can efficiently separate the water-in-oil and the oil-in-water emulsion by switching the feed-facing surface, with a high separation efficiency of 99.7% and ultrahigh permeability flux.…”

“…d) Unidirectional permeation simulation and mechanism of the directional water transport on the Janus fabric. Reproduced with permission [69]. Copyright 2018, Springer Science+Business Media, LLC, part of Springer Nature.…”

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.…”

“…Accordingly, the combination of thick lyophilic layer and thin lyophobic layer is able to achieve a rapid one‐way delivery of liquid. Such particular gating property has endowed Janus membranes with competitive advantages in the applications of liquid manipulation, including oil collection, [ 29,40,49,53,106,107 ] oil−water separation, [ 34,37–39,44,46,54,59,65,69,100,108 ] fog harvesting, [ 16,20,32,78,82,85,88 ] and biofluid management. [ 22,23,25,35,36,60 ]…”