Oil–water separation has recently become a worldwide challenge due to the frequent occurrence of oil spill accidents and increasing industrial oily wastewater. In this work, the multifunctional mesh films with underwater oleophobicity and certain bacteriostatic effects are prepared by layer-by-layer assembly of graphene oxide-silica coatings on stainless steel mesh. The mesh film exhibits excellent environmental stability under a series of harsh conditions. The new, facile and reusable separation system is proposed to achieve deep treatment of oily wastewater, and the oil collection rate can reach over 99%.
Oil–water separation is an urgent environmental issue because of the increasing industrial oily wastewater, as well as the frequent oil spill accidents (Hu et al. 2014). Hence, materials with special wettability have caused broad attention in recent years, especially the novel interfacial materials with superhydrophobicity and superoleophobicity used as oil filtration or absorbent membrane (Teng et al. 2014), such as polyurethane foam (Zhang et al. 2013b), reduced graphene oxide (GO) foams (Zhang & Seegar 2011), metal/metal oxide nanocrystals with thiol modification coating fabric (Wang et al. 2013), and Polytetra fluoroethylene coating mesh (Feng et al. 2004). Although these methods can obtain a high rate of oil–water separation, those superoleophilic materials are plugged by the adhered oil and easily fouled, which limits their practical applications (Teng et al. 2014).
Inspired by the wetting behavior of fish scales, which present underwater superoleophobicity due to water-phase micro–nanohierarchical structures (Liu et al. 2009, 2015), artificial hydrophilic and underwater superoleophobic surfaces have been prepared in recent years. Zhang et al. (2013a) reported a self-cleaning underwater superoleophobic mesh that can be used for oil–water separation is prepared by the layer-by-layer (LbL) assembly of sodium silicate and titanium dioxide (TiO2) nanoparticles on the stainless steel mesh. Xue et al. (2011) fabricated a superhydrophilic and underwater superoleophobic polyacrylamide hydrogel-coated mesh at a three-phase oil–water–solid interface, which could selectively separate water from oil–water mixtures with a high separation efficiency and resistance to oil fouling.
Very recently, GO has attracted tremendous research interest in designing super-wetting materials for effective separation of various oil–water mixtures because it has many hydrophilic functional groups such as carboxyl and hydroxyl, and further chemical modification can change these functional groups and get different interface properties (Dong et al. 2013). Kou & Gao (2011) synthesized graphene oxide--silicon dioxide (GO–SiO2) nanohybrids in a water–alcohol mixture at room temperature and can be directly applied as a general kind of building blocks. In fact, oily wastewater often contains a high concentration of salts, different kinds of acid and alkali, even microorganisms such as Escherichia coli (E. coli). Although several oil–water separation materials stay stable in harsh conditions, to the best of our knowledge, there has been no report about multifunctional mesh films with a certain antibacterial effect and high stability.
In the current work, multifunctional mesh films with underwater oleophobicity and certain bacteriostatic effects were fabricated via LbL assembly of GO–SiO2 nanohybrids on stainless steel mesh. The property of the multifunctional mesh films on oil–water separation and antibacterial activity was investigated. The steel mesh was chosen as the base material because of its inherent porous structure and good mechanical and chemical stability, as well as its easy availability and low cost (Lu et al. 2014). GO–SiO2 were chosen as the coating due to their hierarchical micro–nano structures, hydrophilic nature, excellent dispersity and the ability to form thin film via solution-casting.
MATERIAL AND METHODS
Synthesis of GO–SiO2 nanohybrids
GO was derived from natural graphite ﬂakes through the modified Hummers method (Hummers & Offeman 1958). Silica nanoparticles were deposited on GO by in situ hydrolysis of tetraethoxysilane (TEOS) (Kou & Gao 2011). In a typical procedure, GO powder was dispersed in alcohol–water (5:1, v/v) solution by sonication. After that, the pH was adjusted to 9.0 with ammonia solution and then TEOS was added to the solution. After being vigorously sonicated, the mixture was kept for 24 h at room temperature. Finally, the GO–SiO2 suspension was centrifuged and washed with alcohol and the resulting product was stored in alcohol.
LbL assembly of GO–SiO2 coatings on stainless steel mesh
A pre-cleaned stainless steel mesh was etched with copper(II) chloride (CuCl2) (0.2 mol/L) solution (Lu et al. 2014). In each cycle, the cleaned mesh was immersed in poly(dialkyl-dimethyl- ammonium chloride) (PDDA, Mw = 200,000–350,000, 20 wt %) solution (2.0 mg/mL) for 10 min to render surface positively charged, followed by rinsing with water and drying with nitrogen gas (N2) flow. Then the mesh were immersed in GO–SiO2 suspension for 10 min, followed by rinsing with water. Finally, the as-prepared coatings were dried under vacuum at 60 °C overnight. In the process of the experiment, pore sizes of the stainless steel mesh are 50, 80, 100, 150, and 200 meshes with different coating cycles of 10, 15, 20, 25, and 30, respectively. Eventually, the 100 mesh size with 20 coating cycles was selected for further experiment by comparing the contrast oil–water separation effect and membrane flux (data not shown).
Characterization of mesh film
Freshly fabricated coatings were examined by scanning electron microscope–energy dispersive X-ray spectrometer (SEM-EDS, S-4800, Hitachi, Japan). Water contact angle (CA) and oil CA of the meshes were measured at ambient temperature on a CA–interface system (DSA25, KRÜSS, Germany), where 4 μL liquid volume was used for proper observation if not otherwise indicated.
Oil–water separation test
The antibacterial activities of mesh films loaded with GO–SiO2 were tested by an inhibition zone method (Ravindra et al. 2010). In this method, E. coli was taken as the model bacteria. For this study, the films were cut into small pieces, which had three samples each. The plates were examined for possible clear zone formation after incubation at 37 °C for 16 to 18 h. The presence of a clear zone around ﬁlms on the plates was recorded as an inhibition against E. coli.
Stability and recyclability experiments
The stability of as-fabricated films was investigated by immersing the film into corrosive solution of 1 M sodium hydroxide (NaOH) and 1 M hydrogen chloride (HCl) for 48 h separately, and 1 M sodium chloride (NaCl) for 1 month. After that, the CAs of the immersed mesh film were measured. Recyclability was studied by the parameter that oil–water separation rate of different oil (soybean oil and diesel) after films being used for 5 or 10 cycles.
RESULTS AND DISCUSSION
Characterization of mesh film
Wettability behavior of mesh film
Stability and recyclability
In summary, multi-functional mesh films with oleophobicity underwater and certain bacteriostatic effects have been fabricated via LbL assembly method. The mesh films can separate various oils (soybean oil, diesel, chloroform) from water efficiently, effectively avoiding or reducing the possibility of membrane fouling and clogging. The as-fabricated mesh is highly stable in the harsh conditions, such as highly acidic, alkali, and salt conditions. The oleophobicity underwater and certain bacteriostatic films may open a new avenue for applications in industry and everyday life, for example, deep treatment of waste oil, oil fences for oil spill accidents, etc.
This research has been supported by Jiangsu Province Research Joint Innovation Fund-Prospective Joint Research Project (BY2014123-08), Scientific Research Innovation Project of Jiangsu University (KYXX-0028), the Scientific Research Foundation for Talented Scholars of Jiangsu University (No. 10JDG039), and the Open Fund of State Key Laboratory of Soil and Sustainable Agriculture, Institute of Soil Science, Chinese Academy of Sciences (No. Y052010043).