Full Length ArticleA bifunctional melamine sponge decorated with silver-reduced graphene oxide nanocomposite for oil-water separation and antibacterial applications
Graphical abstract
A bifunctional melamine sponge decorated with the Ag/RGO nanocomposite for oil-water separation and antibacterial applications.
Introduction
Oil contains many toxic compounds such as benzene and toluene and thus, its spills can cause severe environmental problems that threaten every species in the ecological system, from algae and plankton to higher mammals [1], [2], [3]. In the Deepwater Horizon oil spill in the Gulf of Mexico, 4.9 million barrels of crude oil was released, resulting in a devastating effect on marine life in the Gulf and along the circumjacent coastline [4]. Therefore, separation of the spilled oil from water is of great importance for marine ecosystems. To date, several strategies including in situ burning, mechanical absorption, biodegradation by oil-loving microorganisms, dispersion treatment, and skimming systems have been employed for oil-water separation [5], [6], [7], [8], [9], [10]. Among them, mechanical absorption, which uses absorbent materials to remove and recycle oil is considered to be one of the most promising methods [11]. Conventional absorbent materials such as wood sawdust, straw mat, membranes, meshes, and polymer films can separate oil from water conveniently without energy consumption. However, these materials have drawbacks of low absorption capacity and poor recyclability, restricting their industrial application. The development of novel absorbent materials for efficiently removing the oil from water remains a challenge.
Three-dimensional (3D) porous materials that are superhydrophobic and oleophilic offer an alternative solution for oil-water separation and have received tremendous concerns [12], [13], [14], [15], [16], [17]. A facile, effective, and low-cost route to fabricate 3D superhydrophobic porous materials is to modify chemicals with low surface energy on the surfaces of the commercial sponges that intrinsically possess highly porous open-cell structure. So far, several sponges such as polyurethane (PU) sponge, melamine sponge (MS), melamine-formaldehyde sponge, silica sponge, ethylene-propylene-diene monomer sponge, and polyvinyl alcohol sponge have been utilized to construct 3D porous materials for water remediation and oil-water separation applications [18], [19], [20], [21], [22], [23], [24], [25]. Among them, MS has gained particular interest owing to its very high porosity (>99%), high thermal stability, ideal biosafety, and easy processing [26], [27], [28], [29], [30], [31], [32], [33]. Peng et al. prepared a superhydrophobic MS coated with striped polydimethylsiloxane (PDMS) by UV-assisted thiol-ene click reaction, and the PDMS-coated MS exhibited a desirable absorption capacity of 103–179 times its own weight with various oils [31]. Ding et al. reported a metal-ion induced hydrophobic MS prepared by a one-step solution immersion process [32]. The as-obtained MS exhibited excellent oil absorption capacities, 71–157 times of its weight for a wide range of oils and organic solvents (n-hexane, ethanol, acetone, DMF, chloroform, pump oil, vegetable oil, and silicone oil). Gao et al. prepared a hydrophobic sponge by decorating the silica nanoparticles onto the surface of the MS skeleton, followed by a silanization process with vinyltrimethoxysilane as reagent [33]. The modified MS showed hydrophobicity and oleophilicity and exhibited excellent absorption capacity (60–109 g/g) for various oils and organic solvents.
Due to the hydrophobicity, unique 2D structure, and large specific surface area, graphene (G) and reduced graphene oxide (RGO) are very suitable materials for modifying MS [34], [26], [35], [36], [37], [38]. For example, Song et al. prepared an RGO modified MS via an ultrasonic-microwave method, and this sponge was superhydrophobic and showed high selectivity for collecting various oils and organic solvents (diesel, petroleum, lubricate oil, soybean oil, chloroform, hexamethylene, n-hexane, dichloromethane, DMF, and methylbenzene) from water [36]. Zhao et al. fabricated a G coated MS by immersing the MS in mixed solutions with GO and NH3 H2O/C2H5OH for 3 h. The as-obtained sponge showed extremely high absorption capacity of 105 g/g for gasoline and diesel oil [37]. Zhu et al. reported an RGO modified absorbent by squeezed/released MS in the GO suspension for three times, followed by thermal reduction at 180 °C for 6 h [38]. The as-fabricated sponge not only exhibited outstanding absorption performance for various oils (silicone oil, pump oil, and crude oil) and solvents (toluene, chloroform, methanol, acetone, dimethylformamide, and dichloromethane), but also possessed very high efficiency (99.98%) for oil-water separation. To date, most of the work has been devoted to the surface modifications of MS by neat G or RGO. Little attention has been paid to the coupling of the G or RGO-based nanocomposites (NCs) with MS. In addition to superhydrophobicity, the G or RGO-based NCs own other functional properties such as photoconductive, gas-sensing, energy-storage, and antibacterial properties, depending on the individual components constituting the NCs [39], [40], [41], [42]. Therefore, the combination of the G or RGO-based NCs and MS could endow the modified sponge with novel functionalities. For example, the Ag/RGO NCs exhibited excellent antibacterial properties with long-term effects [43], [44], [45], [46]. One can expect that the coupling of the Ag/RGO NCs with MS could generate bifunctional 3D porous material featured with superhydrophobicity and antibacterial activity. As is known, the spilled oil can be easily contaminated by the marine bacteria. When the retrieved oil from seawater is transported by the pipeline or tanker, some of the marine bacteria such as sulfate reducing bacteria can accelerate the microbially-influenced corrosion of the pipeline or tanker. The bifunctional 3D porous material can simultaneously realize the oil-water separation and antibacterial processes, which is of great importance for both retrieving and purifying the spilled oil.
Herein, followed the successful synthesis of the RGO sheets and Ag/RGO NC via a direct reduction method, the RGO-MS and Ag/RGO-MS were facilely prepared using an immersion process. Surface topographies, absorption capacities for oils and solvents, oil-water separation efficiencies, and antibacterial properties of the as-obtained functionalized MSs have been studied. The functionalized MSs (i.e. RGO-MS and Ag/RGO-MS) not only exhibited desirable absorption capacities and recyclability for a wide range of oils and organic solvents but also possessed excellent oil-water separation efficiencies (>98%). Moreover, the Ag/RGO-MS demonstrated ideal antibacterial activities, making it a promising candidate for potential applications in oil-spill disposal and water disinfection.
Section snippets
Chemical materials
Silver nitrate (AgNO3, ≥99.8%), p-Phenylenediamine, N, N-dimethylformamide (DMF, ≥99.5%), dopamine hydrochloride (DA-HCl, 98%), ethanol, methanol, acetone, dichloromethane, Sudan red III, and distilled water were provided by Sinopharm Chemical Reagent Co., Ltd. GO sheets were provided by Nanjing XFNANO Materials Tech Co., Ltd. MSs were obtained from MaiYuan New Energy Technology Co., Ltd. Soybean oil, blending oil, and olive oil were purchased from Yihai Kerry Group. Machine oil, diesel, and
Characterization
The synthesis procedures of the RGO-MS and Ag/RGO-MS are schematically illustrated in Fig. 1. The RGO sheets and Ag/RGO NC were firstly obtained via the direct reduction of the GO sheets and the mixture of Ag+ ions and GO sheets by p-Phenylenediamine, respectively. The oxygen-containing groups of the GO sheets provide the suitable sites for the nucleation and growth of the Ag crystals, leading to the in situ growth of the Ag nanoparticles (NPs) on the surfaces of the GO sheets. The GO supports
Conclusions
In summary, we presented a facile and inexpensive approach to fabricate the MSs coated with RGO sheets and Ag/RGO NC. The RGO-MS and Ag/RGO-MS were hydrophobic, with a water contact angle of 140° and 145°, respectively. The RGO-MS exhibited higher absorption capacities than those of the Ag/RGO-MS for various oils and organic solvents, which can be attributed to the rougher surface and higher porosity of the former. Besides the outstanding absorption capacity and excellent oil-water separation
Acknowledgements
This work is financially supported by the National Key Research and Development Program (No. 2016YFB0300700 and 2016YFB0300704), the Natural Science Foundation of Shanghai (17ZR1440900), and the Natural Science Foundation of China (Nos. 51602195, 51202142 and 51202144).
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