Preparation of smart and reversible wettability cellulose fabrics for oil/water separation using a facile and economical method

Highlights

• A durable micro/nano-structure was built by cellulose solvents of NaOH/urea and ZnCl₂ aqueous solution.

• A switched superhydrophobic/superoleophobic underwater smart CF was fabricated.

• The wettability of smart CF could switch with an extra short period of time.

• The smart CF was controllable to separation weight-/light-oil and water mixtures efficiently.

• The smart CF exhibited excellent stability and reusability.

Abstract

Successful fabrication of smart membrane based on cellulose fabric for controllable oil/water separation was reported. Sheet hexagonal ZnO was in situ synthesized on surface of cellulose fibers using NaOH/urea and ZnCl₂ aqueous solution which were also exploited to fix ZnO steadily due to their swelling and dissolving effects on cellulose. Reversible wettability transition between superhydrophobicity/superoleophilicity and superhydrophilicity/superoleophobicity underwater was manipulated easily by dipping into a lauric acid ethanol solution and NaOH/ethanol water solution in turn for 2min. In detail, the as-prepared functionalized membranes can separate weight or light oil/water mixtures with separation efficiency higher than 98% of water removing and 96.5% of oil removing and high oil flux of 2900–3200 L h⁻² m^-2 and water flux of 3100–3400 L h⁻² m⁻². Simultaneously, the modified fabrics exhibited good stability and excellent recyclability via 20 times cycle operations of wettability transition and separation.

Introduction

Development of technologies for selective separation of organic pollutants from water is attracting global attention along with increasingly accidental oil spills and industrial wastewater (Gossen & Velichkina, 2006; Li, Xu et al., 2017; Xue, Cao, Liu, Feng, & Jiang, 2014). Various methods are currently employed for effective cleaning oil pollution, such as oil skimmers, oil-absorbing materials, gravity, centrifuging, and filtration (Barnett, 2001; Klasson et al., 2005; Lee et al., 2013). These conventional oil/water separation methods are limited by numerous drawbacks, such as lower efficiency, stability and selectivity, high costs and complicated process and equipment requirements compared with super wetting interfacial materials (Barnett, 2001; Hai & Guo, 2016; Xue et al., 2014).

Inspired by the natural water repellency ability of lotus, various superhydrophobic and superoleophilic materials with water contact angle higher than 150° and oil contact angle less than 5° have aroused great attention and have been efficiently applied for separating oil/water mixtures (Lee, Johnson, Drelich, & Yap, 2011; Liu et al., 2016; Wang, Liang, Guo, & Liu, 2015). However, considering the demands of the economy and environmentally friendly properties, it is necessary to fabricate controllable of superwetting functional materials that can selectively separate oil and water (Li, Yan et al., 2016). At present, “smart” interfaces with the reversible wettability transition between superhydrophobicity and superhydrophilicity have received much attention (Li, Zhao et al., 2017) and have been developed by external stimulation such as light irradiation (Tian et al., 2012, 2014; Uyama et al., 2011; Zhu et al., 2016), pH (Ren et al., 2017; Wang & Guo, 2013), temperature (Xue, Gao, Hou, Liu, & Jiang, 2013; Yao, Ju, Yang, Wang, & Jiang, 2014), voltage (Zheng, Guo, Tian, Zhang, & Jiang, 2016), solvent (Lu, Peng, Li, Zhang, & Han, 2010), etc. (Tian, Zhai, Song, & Jiang, 2011). The surface wettability switching triggered by the aforementioned approaches require the time consuming procedure (Liu & Liu, 2016) or special equipment to work (Chen, He, Fan, Yang, Zeng et al., 2017). Hence, it would be greatly acceptable to manipulate the wettability transition with a timesaving and convenient method. Typically, superhydrophobic fabrics have been regarded as good candidates to realize the separation of a mixture of water and oils due to the advantages of fabrics such as good flexibility and easily scalable fabrication (Li, Li et al., 2015). Na Liu developed a controllable oil/water separation copper mesh. The reversible wettability transition between superhydrophobicity and surperhydrophilicity was manipulated easily by immersion in a stearic acid ethanol solution and tetrahydrofuran in turn for 5 min (Liu et al., 2014). Nevertheless, the fabrication of superhydrophobic copper was often complicated and not cost-efficient.

As one of the most abundant and important materials, cellulose fabric has been utilized in fabricating super wetting surfaces on account of its porous character, good elasticity and easily scalable fabrication (Cheng et al., 2017; Deng et al., 2010; Li et al., 2010). However, only a few studies have reported the utilizations of cellulose fabric to make the “smart” interfaces, which can achieve oil removing and water removing. Basically, chemical composition and micro/nanoscale hierarchical structures are two key points that govern wettability on solid surfaces (Feng et al., 2002b; Guo, Liu, & Su, 2011; Tian, Zhang, Wang, Zhai, & Jiang, 2011). ZnO was extensively applied to increase surface roughness of cellulose fabrics due to its more abundant structures which can be deposited controllably on surface of cellulose fabrics such as hexagonal, rod, wire, flake and flower-like (Ao, Li, Yang, Zeng, & Ma, 2006; Kołodziejczakradzimska & Jesionowski, 2014; Mohan et al., 2013; Wang, Xin, Xiao, & Daoud, 2004; Xu & Cai, 2008). Furthermore, ZnO is biocompatible, biodegradable and bio safe for environmental applications. Currently, the main methods coating ZnO on surface of cellulose fabrics are hydrolytic sol-gel and hydrothermal process (Shao, Gao, Cao, & Wei, 2016; Vandenboer et al., 2012). However, a durable fixation of ZnO on cellulose fabrics is the major challenge as there is no affinity between the fabrics and ZnO. In our recently study, various commercial nanoparticles were fixed on surface of cellulose fabrics to obtain functional cellulose fabrics via the micro-dissolving technologies (Fan, Hu, Zhao, Liu, & Lu, 2017, 2018; Li, Fan, Hu, Liu, & Lu, 2016). The process was achieved by solvent systems which can dissolve cellulose as NaOH/urea aqueous solution and ZnCl₂ aqueous solution (Ang & Zhang, 2007; Li, Wang, Lu, & Zhang, 2015; Zhao & Lai, 2006).

Herein, we developed a facile and efficient method for fabricating durable ZnO-coated cellulose fabrics (ZnO-CFs). ZnO was fixed on the surface of cellulose fabrics utilizing swelling and micro-dissolving effect of NaOH/urea and ZnCl₂ aqueous solution on cellulose fabrics. The smart and superhydrophobic surfaces were obtained by grafting a lauric acid. The reversible wettability switching was achieved easily by dipping into a NaOH/ethanol water solution and lauric acid ethanol solution in turn for 2 min. Furthermore, the smart surfaces were applied to manipulate oil/water separation, exhibiting a high separation efficiency and excellent reusability. This study may provide a novel strategy to fix hexagonal ZnO on surface of cellulose fabrics and develop controllable oil/water separation membrane with a facile, environment friendly and effective process in future work.