Short communicationIn situ one-step fabrication of durable superhydrophobic-superoleophilic cellulose/LDH membrane with hierarchical structure for efficiency oil/water separation
Graphical abstract
Introduction
With the ever-increasing industrial oily wastewater and the frequent oil spill accidents, the treatment of oily wastewater has become a worldwide problem [1]. Thus, it has been an urgent demand to seek a suitable way to solve this problem. Traditional cleanup techniques, including air flotation [2], oil-absorbing materials [3], coagulation [4], and flocculation [5], have been applied in recent years for the separation of oil from oily wastewater. However, these techniques have some undesirable sides such as low efficiency, high operation cost, secondary pollution and poor recyclability [6], [7]. Membrane separation technology, which has advantages of simple separation equipment and low energy consumption, is considered to be more effective and advantageous in large-scale processing [8]. Due to the distinguished properties, oil/water separation membranes with both superhydrophobicity and superoleophilicity, which are called “oil-removing” type of materials [9], realized absorption or filtration of oils from water effectively and selectively without generating pollution. To date, abundant oil-removing type of membranes have been studies to separate oil/water mixtures, such as metallic mesh-based materials [10], ceramic membranes [11], foam-based materials [12] and carbon-based materials [13], etc.
It is known that the superhydrophobic surfaces of oil/water separation membranes play a crucial role in removing oils and organics from oily wastewater. Up to now, a large amount of works in the field of oil/water separation membranes have been presented, mainly focused on the fabrication of superhydrophobic surfaces [14], [15], [16]. Two dominant factors of superhydrophobic surfaces, surface roughness and surface energy state, should be taken into account. Generally, the formation process of superhydrophobic surface can be divided into two stages: (1) fabricating micro-nanostructures to enhance surface roughness; (2) surface modification to reduce the surface energy [17], [18], [19]. Based on this principle, various techniques have been developed by exploiting the synergistic effect of surface roughness and low energy including chemical vapor deposition [20], electrochemical methods [21], hydrothermal methods [22] and spray methods [23], followed by surface modification with appropriate surface functional groups. However, the complicated fabrication techniques and uncontrollable oils separation procedures are the major obstacles that hinder their large-scale applications of oil/water separation membranes. As a result, many attempts by different research groups have been made towards the simple fabrication of superhydrophobic surfaces, including one-pot solvothermal strategy and one-step sonochemistry irradiation method. For example, Li et al. [24] reported robust polydivinylbenzene-PDMS decorated superhydrophobic filter membrane via a one-pot solvothermal strategy, in which polydivinylbenzene-PDMS polymer was immobilized on the surface of substrate randomly. Li et al. [25] fabricated a superhydrophobic and superoleophilic cotton fabric membrane by deposition of SiO2 nanoparticles functionalized with octadecyltrimethoxysilane, and the SiO2 nanoparticles were random and close-packed over cotton fabrics. These strategies, however, ignored the control of surface morphology of membranes that is of great significance to affect the surface wettability. Therefore, it is highly desirable to develop a simple one-step method to fabricate separation membranes with controlled morphology and stable superhydrophobic properties.
In this communication, a simple but efficient strategy was developed to fabricate the cellulose-based membrane with controlled morphology and stable superhydrophobic properties in an open oil/water two-phase system via integrating hydrothermal reaction with surface hydrophobic modification. In the aqueous phase, the rough surface of cellulose fibers was formed by utilizing controlled crystal growth, and in the oil phase, a self-assembled layer of stearic acid was deposited on the surface of cellulose fibers by hydrophobic modification, resulting in a superhydrophobic surface. The oil/water separation membranes can separate multiple types of oil/water mixtures driven by gravity without an external force. More importantly, the membrane exhibits a high separation efficiency, excellent recyclability and stable superhydrophobicity under acid conditions, making them cast for oily water remediation.
Section snippets
Materials
Qualitative filter paper (moderate speed, nominal pore size 15–20 μm, thickness 340 ± 20 μm) was provided from special paper Co., Ltd (Hangzhou, China). nPentadecane (C15H32), stearic acid (C18H36O2), ammonium hydroxide (NH4OH, 25.0–28.0 wt%), ammonium chloride (NH4Cl), and zinc acetate dihydrate (Zn(CH3COO)2·2H2O) were purchased from Sinopharm Chemical Regent Co., Ltd (Shanghai, China).
Fabrication of superhydrophobic membrane
In a typical experiment, NH4Cl (10 mmol) and Zn(CH3COO)2·2H2O (5 mmol) were dissolved in deionized water (100 mL)
Fabrication procedure and mechanism
As a green resource in nature, cellulose has properties of high chemical durability, biodegradation, low toxicity due to the strong intra- and inter-molecular hydrogen bonding and it can be used as a potential separation membrane or adsorbent for oil separation or absorption from oil/water mixtures. In this study, an open oil/water two-phase system is developed to fabricate the superhydrophobic surface, which relies on the joint effort of in situ technology, cation-anion double hydrolysis
Conclusions
In summary, we have developed a facile one-step strategy to fabricate superhydrophobic LDH/cellulose membranes in an open oil/water two-phase system by combining the merits of hydrothermal reaction and hydrophobic modification. In the aqueous phase, the rough surface of cellulose fibers is formed by utilizing controlled crystal growth, while the superhydrophobic surface is obtained by using the self-assembly of stearic acid in the oil phase. The as-prepared superhydrophobic LDH/cellulose
Acknowledgements
The authors are grateful for the China Postdoctoral Science Foundation of Jiangsu Province (1601016A), the Natural Science Foundation of Jiangsu Province (BK20161362, BK20161264 and BK20160500), the National Nature Science Foundation of China (U1507115), the Jiangsu University (15JDG142), High-Level Personnel Training Project of Jiangsu Province (BRA2016142), the China Postdoctoral Science Foundation (2016M600373) and the Scientific Research Foundation for Advanced Talents.
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