One-step modification of PVDF membrane with tannin-inspired highly hydrophilic and underwater superoleophobic coating for effective oil-in-water emulsion separation

Highlights

• PVDF membrane was modified via a facile dip-coating in TA/NaIO₄-mixed solution.

• Highly hydrophilic and underwater superoleophobic membranes were obtained.

• The membrane exhibited outstanding performance while treating oily emulsions.

• The membrane possessed satisfactory antifouling property and stability.

Abstract

Herein, hydrophobic PVDF membranes were facilely transformed into highly hydrophilic and underwater superoleophobic ones via one-step dip-coating in tannic acid (TA)/sodium periodate (NaIO₄)-mixed solution. The employment of NaIO₄ could not only facilitate TA oxidation and deposition, but also introduce abundant hydrophilic carboxyl groups on membrane surface. Consequently, compared with the pristine PVDF membrane, the modified membrane showed much higher hydrophilicity and underwater oleophobicity with water contact angle decreasing from 121° to 32°, oil contact angle increasing from 116° to 162° and a tilt angle of just 2°. In contrast to the two-step modification (deposition of TA followed by post-oxidation by NaIO₄), our approach endowed the membrane with dramatically improved permeability (>2400 LMH/bar) and oil rejection (>98%) when dealing with different types of oil-in-water emulsions. Additionally, the membrane displayed excellent antifouling performance with flux recovery ratios of above 95% and favorable reusability after 5-cycle filtration tests. Moreover, the membrane possessed a stable TA coating even being subjected to harsh pH solutions or sonication. This work provides a simple, robust and cost-effective strategy for membrane modification.

Introduction

With the rapid development of industry and intensification of human activities, oily wastewater, mainly originated from industrial emissions, daily life and oil spill, is increasingly accumulated in nature and severely threatening the ecosystem and human health [1]. Therefore, developing oil/water separation technologies for water remediation and reclamation has aroused general concern. However, traditional techniques, such as air floatation [2], gravity settling [3], adsorption [4] and centrifugation [5], might suffer from drawbacks of inefficiency or high energy consumption. Moreover, the above technologies usually fail to deal with emulsified oily wastewater owing to the high stability and comparatively small droplet size of emulsions (0.1–10 μm) [6]. Thus, it is of paramount importance to develop high-efficient and low-cost technologies for emulsified oily wastewater purification. In recent years, membrane technology has been recognized as one of the most promising alternative separation technologies [7], [8], [9], [10], [11] attributed to its unique advantages such as high efficiency, low energy consumption, ease to scale-up and low operational cost, which can be well applied in various emulsions treatment based on the size exclusion effect.

Membrane fouling is a ubiquitous phenomenon in the filtration process due to the strong hydrophobic interactions between foulants and membrane materials, leading to undesired flux drop and shortening of membrane lifetime [12], [13], [14], [15]. When it comes to the oil/water emulsions separation, oil droplets tend to attach, deform and spread on membrane surfaces [16], resulting in dramatic pore blockage and a sharp decrease in membrane permeability. Hence, it is crucial to fabricate antifouling membranes for the high-level and long-term application. Over the past decades, enhancing the hydrophilicity and underwater oleophobicity of membranes has been proved to be an effective approach to achieve this target [17], [18], [19], [20]. Up to now, a great deal of hydrophilic membranes have been prepared via various strategies embracing surface coating [21], surface grafting [22], physical blending [23], surface segregation [24], [25], [26] and surface bio-adhesion [27]. For instance, in the study of Matsuura’s group, a hydrophilic surface modifying macromolecule (LSMM) was synthesized and blended into the casting solutions, which imparted improved hydrophilicity and fouling resistance to the as-prepared membranes [15]. Matsuyama et al. [9] fabricated a superior antifouling PVDF membrane with a superhydrophilic and underwater superoleophobic poly(tetrafluoride ethylene-r-vinylpyrrolidone) (F-VP) coating, which displayed high permeability and oil rejection when treating oil-in-water emulsions. Jin et al. [22] grafted a zwitterionic nanohydrogel on PVDF membrane, after which the surface was endowed with superhydrophilicity and excellent anti-oil-adhesion property.

Among the aforementioned methods, surface bio-adhesion stands out as a competitive strategy thanks to its simplicity, versatility, low toxicity and robustness [28]. As the most commonly used bio-adhesive agent, dopamine exhibits extraordinary attaching ability to a wide range of materials through its oxidative polymerization in alkaline solutions [29], which has been broadly employed for surface antifouling modification. Additionally, the oxidized dopamine is able to react with functional molecules containing amine or thiol groups via Michael addition or Schiff base reactions [30], which serves as a versatile platform to further increase the hydrophilicity of membrane surfaces. For example, Li et al. [31] transformed hydrophobic PVDF membranes into superhydrophilic ones by the surface coating with dopamine and polyethyleneimine. Chen’s group [32] developed an antifouling PVDF membrane for oil-in-water emulsions separation through the deposition of polydopamine on membrane surface and subsequent grafting of thiol-terminated zwitterionic polymers. Nevertheless, the high cost of dopamine and the unwelcome dark color as well as poor uniformity of the formed coatings have restricted its large-scale applications [33], [34], [35], [36], [37].

Tannic acid (TA), a kind of plant polyphenol which can form a smooth and light-colored coating on a variety of substrates, has been considered as a new generation of “bio-glue” because of its low cost, nontoxicity and easy storage [6], [38], [39], [40]. In the study of Li’s group [20], a superhydrophilic and underwater superoleophobic PVDF membrane was fabricated by the one-step co-deposition of TA and 3-aminopropyltriethoxysilane (APTES), during which a hierarchical coating was constructed on the surface. Recently, Peng et al. [41] reported a superhydrophilic PVDF membrane prepared by the rapid deposition of TA on membrane surface followed by its immersion in NaIO₄ solution. As a kind of powerful oxidant, NaIO₄ can not only facilitate the oxidation of catechol groups but also lead to the formation of hydrophilic carboxyl groups in the coating in acidic solutions (pH = 5) [42], [43], [44]. Although the hydrophilicity of membrane was well improved through the above two-step synthesis, the permeate flux still remained low (40–400 L/m²·h, 0.8 bar) when filtrating oil-in-water emulsions, which would hinder its practical use. So far, the TA-decorated membrane derived from the in situ oxidation of NaIO₄ (one-step synthesis) has not been studied for oil/water separation.

Herein, PVDF microfiltration membrane was selected as the substrate owing to its excellent mechanical strength, high thermal stability and extraordinary chemical resistance [45], [46], [47]. In this work, PVDF membrane was modified through a simple dip-coating in TA/NaIO₄-mixed solution for the separation of various oil-in-water emulsions. The amount of NaIO₄ employed and deposition time of TA were optimized, respectively. As a control, the TA-coated PVDF membrane stemmed from air oxidation was prepared to explore the impact of NaIO₄ on TA deposition. Besides, a two-step modification of PVDF membrane with TA deposition followed by NaIO₄ oxidation, was also conducted to investigate the impact of modification manner on membrane performance. The surface chemistry, morphology, roughness and wettability of the as-prepared membranes were characterized in detail. The membrane separation performance, antifouling property as well as stability were systematically investigated. As far as we know, this is the first work adopting one-step approach to realize the TA deposition on membrane surface triggered by NaIO₄ for oil/water separation.