One-step plant-inspired reaction that transform membrane hydrophobicity into high hydrophilicity and underwater super oleophobicity for oil-in-water emulsion separation

Highlights

• A universal wetting coating was designed by cost-effective one-step reaction.

• The coating possesses both high hydrophilicity and underwater super oleophobicity.

• The coating endows membrane excellent oil water emulsion separation efficiency.

Abstract

Here we report a universal and cost-effective strategy via a one-step levodopa (l-DOPA)/3-amino-propyltriethoxysilane (APTES) reaction modified hydrophobic membrane for highly efficient separation of oil-water emulsions. Interestingly, the newly designed composite membrane possesses both high hydrophilicity and underwater super oleophobicity. Under gravity-driven separation conditions, the membrane displays superior efficiency to separate oil-water emulsions with an ultrahigh water flux. The low cost, the simplicity of fabrication, and versatility of the l-DOPA/APTES coating may open up new insights to molecularly design commercial membranes with superior hydrophilicity and underwater super oleophobicity for oily water separation and purification.

Introduction

The frequent occurrence of crude oil leakage and the discharge of industrial waste water and domestic sewage are threatening human health and ecological environment [[1], [2], [3], [4]]. The oily sewage treatment is extremely difficult, low efficiency and high cost. Sewage medium oil according to the physical state is divided into four kinds: floating oil, dispersed oil, emulsified oil, dissolve oil, different types of oil-bearing wastewater treatment in different ways, emulsified oil processing is the most difficult. Therefore, much more efforts should be made toward designing advanced separation materials with high efficiency and extreme flexibility to guard emulsified oil processing. Under such circumstances, various membrane technologies from microfiltration, ultrafiltration, nanofiltration, reverse osmosis, forward osmosis to membrane distillation and different membrane configurations such as planar membranes, hollow fibers, nanofibers and 3D porous materials have been explored [[5], [6], [7], [8], [9], [10]]. Among them, planar oil-water separation membrane has been widely concerned because of its own advantages, such as low energy consumption, high single-stage separation efficiency, flexible and simple process, low environmental pollution, strong versatility [[11], [12], [13], [14]]. Base on water and many oils are intrinsically immiscible, the essence of oil-water separation is interface problem. By designing the special wettability on the surface of the material, the separation membrane with the characteristics such as super hydrophilic, super oleophobic, super oleophilic and super hydrophobic is undoubtedly the most effective means to improve its oil-water separation performance [[15], [16], [17]].

Generally, oil-water separation membrane based on special wettability is divided into two kinds: hydrophobic oleophilic and hydrophilic oleophobic membrane. When the mixture of oil and water touches the surface of the hydrophobic oleophilic membrane, the oil droplets rapidly spread and permeate on the surface of the membrane, water could not be wetting the membrane surface, and be intercepted on the impermeable membrane, so-called oil-removing method [18,19]. However, due to the potential oleophilicity of the membrane, oil droplets and other impurities are easily adsorbed irreversibly on the surface of the super hydrophobic oleophilic membrane, the membrane is seriously polluted which leads to a fast flux attenuation, short membrane life, and difficult cleaning. In contrast, the hydrophilic oleophobic membrane can form a water isolation layer on the membrane surface which can effectively prevent the adhesion of oil droplets. When oily wastewater touches the surface of the membrane, water can be continuously permeated downward, while the surface remains super oleophobicity, oil is intercepted on the surface to achieve the effect of oil-water separation, so-called water-removing method [20,21]. Because of the potential oil-repellency of the membrane, oil can't pollute membrane surface all the time. The hydrophilic oleophobic membrane is a real anti-pollution, low energy consumption, long life, high-efficiency separation membrane, thus becoming the main development direction of oil-water separation membrane in the future.

Polypropylene (PP) has been widely used as membrane materials due to their excellent flexibility, mechanical strength, and thermal stability [22,23]. However, PP is essentially hydrophobic and it can easily absorb the oil in the emulsion and cause the flux extreme to drop. Transform PP membranes hydrophobicity into high hydrophilicity and underwater superoleophobicity has been proved to be an extremely effective approach to solve this problem. A variety of methods have been adopted to endow membrane wettability including chemical etching, surface coating, surface grafting, layer by layer assembly, and hydrothermal treatment [[24], [25], [26], [27], [28], [29]]. Unfortunately, PP is chemically inert, and hydrophilic chemicals can hardly be introduced to PP surfaces through these strategies. The general strategy is to treat the surface of the PP membrane with corona treatment or plasma treatment to produce active sites and then hydrophilic treatment. However, these processes are often complex, expensive and time-sensitive. Therefore, it is highly desired to develop a new research direction to convert hydrophobic membrane directly into superhydrophilic membrane, especially high hydrophobic membrane under simple conditions. The catechol chemistry stimulated by mussels provides a good opportunity for the decoration of various substrates with its remarkable adhesion, which opens a new way for the preparation of superhydrophilic surfaces [[30], [31], [32], [33], [34]]. It is a pity that dopamine is not widely used because of the high price of its reagents. As a precursor of dopamine, levodopa (l-DOPA) has similar chemical properties to dopamine and can be extracted directly from legumes. In 1913, biologist Guggenhiem extracted l-Dopa from the broad bean, and then scientists found the existence of l-DOPA in many plants, such as cat bean and litmus bean [35]. The content of l-Dopa in cat bean reaches 9% and becomes the main raw material for extraction of l-Dopa. In addition, chemical synthesis, enzyme catalysis, microbial fermentation and metabolic engineering are also the main methods for obtaining l-Dopa. Compared with DA, it has the advantages of low cost, a wide range of sources and easy industrial production. Herein, based on the surface-independent adhesion properties of l-DOPA, we report a facile one-step reaction between l-DOPA and 3-amino-propyltriethoxysilane (APTES) to transform membrane hydrophobicity (PP) into high hydrophilicity and underwater superoleophobicity for high-efficiency separation of oil-in-water emulsions (Fig. 1). This strategy has obvious advantages, including the following: i) Room temperature one-step modification get a super wetting interface, avoid multistep reactions, sophisticated equipment and rigorous reaction conditions. ii) Modification reaction based on plant extract of l-DOPA and commercially available silane coupling agent APTES, low cost is more suitable for industrial application. iii) Based on the particular surface adhesion performance of l-DOPA, this strategy can be used to construct a highly hydrophilic coating at any hydrophobic interface.