Preparation of smart and reversible wettability cellulose fabrics for oil/water separation using a facile and economical method
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.
Section snippets
Materials and solvents
The cellulose fabrics (CFs) (110 g m−2, 52 picks cm-1, 27 ends cm-1, thickness of 0.26 mm) were available from China market and desized in the 10 g L-1 NaOH solution at 98 °C for 60 min. The Zinc chloride (ZnCl2), anhydrous sodium carbonate (Na2CO3), sodium sulfate (Na2SO4), sulfuric acid (H2SO4), sodium hydroxide (NaOH), urea, lauric acid (LA), ethyl alcohol (EtOH), Rhodamine B (RHB) and Methylene blue (MB) were purchased from Nanjing Daoning Chemical Reagent Co., Ltd. All the chemicals were
Morhologies
It was well-known that surface geometrical structure played an important role in construction of the superhydrophobic surface, similar to the lotus leaf’s self-cleaning property resulting from its micro- and nanostructure (Feng et al., 2002a; Wang, Zhao, & Deng, 2008). This mechanism can be explained by Eq. (3), first derived by Wenzel to describe the CA for a liquid droplet at a rough solid surface (Wenzel, 1936).
In [Eq. (3)], was the intrinsic CA on a smooth surface, was that
Conclusion
In summary, a durable micro/nano-structure of sheet ZnO was built to endow the cellulose fabric with super wetting properties by cellulose solvents of NaOH/urea aqueous solution and ZnCl2 aqueous solution. The controllable oil/water separation of cellulose fabrics had been prepared successfully and easily manipulated by dipping into a lauric acid alcohol solution and NaOH/ethanol water solution with an extra short period of time. The reversible switching of superhydrophobicity and
Acknowledgements
This work was financially supported by the Fundamental Research Funds for the Central Universities (XDJK2014B005), Southwest university Undergraduate science and Technology Innovation Fund Project (20171603004 and 20171603003) and National Training Programs of Innovation and Entrepreneurship for Undergraduates (201810635021).
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