Gelatin-tannic acid coating for high flux oil-water separation

Highlights

• The coating can be applied to different substrates such as stainless steel mesh, copper mesh, glass, etc.

• A super-hydrophilic and underwater super-oleophobic mesh was fabricated by simple and eco-friendly dipping-coating method.

• The super-hydrophilic and underwater super-oleophobic mesh demonstrate high flux separation of oil-water mixtures.

• The coated mesh show excellent oil-repellent properties and chemical stability in salt solution and acidic environment.

Abstract

Special wettability material with super-hydrophilicity/underwater super-oleophobicity is one of the best materials for treating oily wastewater, developing an urgent demand for simple, green, low cost, and efficient in practical application. Both gelatin and tannic acid are natural and low-cost renewable resources. In this study, gelatin-tannic acid coating(FOGE-TA) that can be applied to different substrates was prepared by a simple and green dip-coating method using the film-forming properties of gelatin as well as the hydrophobic interactions and hydrogen-bonding interaction between gelatin and tannic acid. The coating can be applied to different substrates such as glass, PP melt-blown nonwoven fabric, copper mesh, and stainless steel mesh. After dipping FOGE-TA, the underwater oil contact angle of the mesh surface all reach over 150 °, and the water contact angle is 0 °. Among them, the prepared gelatin-tannic acid-coated stainless steel mesh(FOGE-TA-4 L-SSM) has a high water flux of over 1.74 × 10⁵ L∙m^-2∙h^-1 and separation efficiency of over 99.0% for oil/water separation. Due to its excellent oil repellency only needs to be soaked and rinsed with deionized water when the flux decays to continue the oil-water separation. After 90 separations, the water flux and separation efficiency did not significantly decrease. In addition, the FOGE-TA-SSM has good stability in salt solutions and an acidic environment, making it an ideal material for solving oil-water separation problems.

Introduction

The process of petroleum extraction and utilization can lead to a large amount of oily wastewater, giving rise to severe environmental pollution and waste of resources [1], [2], [3]. There is a growing body of literature that recognizes the importance of the treatment of oily wastewater. Scientists have developed various methods and materials to recycle oily wastewater [4], [5], [6]. Filtration is of interest because of its high separation efficiency, excellent selectivity, and low energy consumption [7]. A key aspect of excellent selectivity and high separation efficiency is the construction of special wettability of the material surface. Special wettability materials for oil-water separation are generally classified into two types: super-hydrophobic/super-oleophilic materials, also known as oil-removing materials, and super-hydrophilic/super-oleophobic materials or water removing materials. Super-hydrophilic/super-oleophobic materials tend to have greater anti-fouling performance than super-hydrophobic/super-oleophilic materials due to their oil repellency. In contrast, super-hydrophobic/super-oleophobic materials are prone to irreversible materials contamination due to adsorption of the oil phase [8]. Nevertheless, the surface tension of water is normally higher than that of oil. Therefore, it is difficult to build a surface that is both super-hydrophilic and super-oleophobic in the air [9], [10]. Liu et al. fabricated super-hydrophilic/underwater super-oleophobic materials with fish-scale structures inspired by nature. This material exhibits super-hydrophilicity/underwater super-oleophobicity in air while super-oleophobicity in water. This is because a stable hydration layer is formed on the surface of the material when the surface is pre-wetted with water [11]. Since it was reported in 2009, super-hydrophilic/super-oleophobic materials have attracted a lot of interest. Hydrophilic materials with strong water-binding capacity require to be introduced in order to achieve the construction of this stabilized hydration layer. So far, a series of methods such as chemical oxidation [12], [13], hydrothermal crystallization [14], [15], coating of hydrophilic polymers [16], [17], [18], surface grafting of zwitterionic polymers [19], electrostatic spray deposition [20], [21], electrospinning [22], [23], layer-by-layer assembly [24] have been developed to prepare super-hydrophilic/underwater super-oleophobic materials. It is still a challenge to design the oil/water separation materials using low-cost, green, renewable raw materials and simple, economical operation methods. Recently, researchers have shown an increased interest in using green and low-cost biomass as raw materials [25], [26]. The use of green materials is environmentally sustainable, and they are relatively safer because they require less manufacturing and safety legislation to process [27].

Gelatin (GE) is a renewable, biodegradable, non-bio-toxic, green resource with high film-forming capacity, which is widely applied in medical tissue engineering [28], [29]. Gelatin is a macro-molecular hydrophilic colloid with a thermally induced gelation ability [30]. Physical or chemical cross-linking of the hydrogel can improve the swelling performance and mechanical properties of the hydrogel [31]. Formaldehyde is a chemical cross-linking agent commonly used in gelatin modification applications [32]. Physical cross-linking is usually developed based on van der Waals forces or hydrogen bonds. Tannic acid (TA) is a polyphenolic compound widespread in plants and is a rich natural and renewable resource [33]. Tannic acid is capable of binding tightly to the peptide chain of gelatin through hydrophobic interactions as well as hydrogen bonds [34]. The Gel-TA-Ag NW hydrogel with simple preparation applied for self-powered strain sensor was reported by Wang et al. in 2020. It is worth noting that the composite hydrogel's self-healing and highly stretchable mechanical properties were derived from the cross-linking of gelatin with tannic acid [35]. Furthermore, the anti-microbial contamination properties of the gel are a classic problem in the practical application process. Gelatin is susceptible to microbial contamination, but tannic acid can give hydrogels intrinsic anti-inflammatory, antioxidant and antibacterial properties [36]. Based on this property, Ahmadian et al. used gelatin-tannic acid hydrogels as multifunctional extracellular matrix mimetic hydrogels that can be used as wound dressings to accelerate wound healing at the site of skin injury [37]. The addition of tannic acid to gelatin also improves its hydrophilicity. Zhao et al. alternately deposited gelatin and tannic acid on polyacrylonitrile (PAN) ultrafiltration membranes, and the synthesized membranes exhibited high water permeability along with excellent ethanol-water separation performance [38]. Gelatin-tannic acid coatings with simple preparation have the potential to be super-hydrophilic/underwater super-oleophobic, however, there are few studies applied to the oil-water separation.

Herein, we obtained FOGE-TA coated mesh using the hydrophobic interaction and hydrogen bond between gelatin and tannic acid for assembly on stainless steel mesh. The effects of different GE/TA ratios and the number of dipping on the separation performance of FOGE-TA-SSM, including separation water flux and separation efficiency, were investigated. The wettability, surface morphology, and chemical structure of FOGE-TA-SSM were characterized. In addition, the chemical stability, mechanical stability, and long-term cycling stability of the FOGE-TA-SSM were evaluated. Furthermore, the broad applicability of FOGE-TA oil-water separation coating was explored, and FOGE-TA coated mesh based on copper mesh or melt-blown cloth were prepared and tested for their oil-water separation performance.