Bio-inspired membrane with adaptable wettability for smart oil/water separation

Highlights

• Facile synthesis of mussel-inspired amphiphilic copolymer.

• Universal surface modification methods for constructing membranes with adaptable wettability.

• Selective separation of oil/water mixtures and surfactant-stabilized emulsions at different modes.

• Promising candidate in oil spill cleanup and industrial wastewater treatment.

Abstract

Adaptable wettability is an intriguing property found in nature yet rare in synthetic materials. Separation membranes bearing this adaptable wettability would be of great significance for practical applications because they can achieve on-demand oil/water separation according to the intrinsic properties of the targeting oil/water mixtures other than external stimuli. Herein, we report one novel type of such smart separation membranes by functionalizing steel mesh membrane with polydopamine (PDA) and an amphiphilic synthetic copolymer via a feasible mussel-inspired dip-coating method. The resulting membrane possesses excellent adaptable wettability and can switch their special oil/water wettability to selectively activate “water-removing” or “oil-removing” modes for on-demand collection of the desired component (water or oil) from the oily water, i.e., upon pre-wetting the surfaces with water or oil. It exhibits outstanding performance in separating various immiscible oil/water mixtures and miscible surfactant-stabilized emulsions under ambient pressure. Moreover, the bio-inspired membrane can be regenerated and reused facilely through simple washing and drying process without loss in performance. The facile surface functionalization approach developed in this work is universal and substrate-independent, which can be extended to a variety of materials and membrane systems. Such universal surface modification method and the novel membrane developed have great application potential in various engineering and environmental areas including oil spill cleanup and industrial wastewater treatment.

Introduction

The global occurrence of oil spills and discharge of industrial oily wastewater has posed great risk to the environment and human health. To protect the limited water resources and recover oil from wastewater, developing advanced technologies and materials for efficient and sustainable oil/water separation has been of vital significance. With the development of interfacial science and bionics during the past decade, special wettable materials bearing intrinsic distinct affinities towards oil and water have emerged as highly promising separation materials [[1], [2], [3], [4], [5], [6]]. Based on their wettability, these membranes are classified into superhydrophilic/superoleophobic “water-removing” type [[7], [8], [9]] and superhydrophobic/superoleophilic “oil-removing” type [[10], [11], [12]], aiming at collecting different desired component from the targeting effluent. In the practical applications, due to the variety of the wastewater component, developing smart separation membranes with controllable/switchable special wettability capable of functioning in both “oil-removing” and “water-removing” processes would be of great significance to reduce operation cost and enhance treatment efficiency.

With the development of stimuli-responsive materials and surface chemistry, a number of such smart separation membranes have been successfully fabricated, which can realize on-demand separation of oil or water via the perfect control of surface wettability on response to external stimulus such as temperature [13,14], pH [[15], [16], [17]], CO₂ [18,19], ionic liquid [20], specific organic [21], light [[22], [23], [24], [25]] and electric field [26,27]. However, generally these systems still suffer from severe operating issues for the practical manipulations to achieve on-off switches by changing the above-mentioned working conditions. The separation efficiency of some stimuli-responsive smart membranes, such as temperature-, pH-, CO₂-, ionic liquid- and specific organic-responsive ones, are highly dependent on the physic-chemical properties of the oil/water mixtures [28]. Introducing external additives to or heating the bulk solution is inevitable for these systems in order to maintain the specific wettability of the membranes during oil/water separation. Other stimuli-responsive membranes require exposure to external physical stimuli such as high-energy light irradiation or electric field to activate the surface wettability. While the huge energy consumption and requirement of special supporting facilities have restrained their further applications.

Fortunately, nature offers excellent solutions to the aforementioned dilemma. Recent studies on some natural materials such as corn cob [29], diatomite [30], waste potato residue [31], candle soot [32] and coprinus comatus [33] revealed that these natural materials-modified surfaces possessed a unique adaptable wettability to the contacted liquids, i.e., the surfaces show underwater superoleophobicity when first contacted with water but becomes underoil superhydrophobic when first wet by oil. In practical applications, separation membranes bearing this intriguing adaptable wettability would be with great significance as they can realize on-demand oil/water separation according to the intrinsic properties of the targeting oil/water mixtures other than external stimuli, and can function in highly variable, real-world situations. However, the studies on endowing synthetic materials with adaptable wettability have been very limited [[34], [35], [36]].

Herein we have developed a biomimetic smart separation membrane by a facile dip-coating method via consecutively functioning steel mesh membrane with polydopamine (PDA) and an amphiphilic synthetic copolymer. The prepared polymer/PDA-coated membrane can switch their special oil/water wettability to selectively activate “water-removing” or “oil-removing” modes for on-demand collection of the desired component (water or oil) from the oily water, i.e., upon pre-wetting the surfaces with water or oil. It is found that such biomimetic membranes can accomplish the separation for all types of oil/water mixtures under ambient pressure, including immiscible light oil/water mixtures, heavy oil/water ones, miscible oil-in-water and water-in-oil emulsions stabilized by surfactants. Furthermore, the membrane can be easily regenerated and reused many times through a facile washing-and-drying procedure without loss in performance. These findings demonstrate the great potential of the developed bioinspired membrane with adaptable wettability for efficient on-demand oil spill cleanup and selective collection of valuable component from oil/water mixtures or stable emulsions.