A highly hydrophobic metal mesh has great potential for its application in oil/water separation due to its special wettability. However, most current oil/water separation devices are simple with limited separation capacity. A separation device based on a highly hydrophobic metal mesh was constructed for different types of oil/water mixtures. Experimental results show that the device not only can be used for the continuous separation of binary oil/water mixtures of any density ratios but also can realize the simultaneous separation of heavy oil/water/light oil ternary mixtures. This achievement is meaningful for practical applications, which will gain great interest in the future.
A universal oil/water separation device based on a highly hydrophobic metal mesh was constructed.
The device can be used for sustainable separation of binary oil/water mixtures and heavy oil/water/light oil ternary mixtures.
The constructed device allows simultaneous recovery of light and heavy oils in different channels.
The frequent oil spills and incessant discharge of industrial oily wastewater have caused great damage to water resources and the ecological environment, which seriously threaten human health (Zhang et al. 2017; Li et al. 2018). Traditional methods for the treatment of oil/water mixtures, such as gravity sedimentation, centrifugation, and biological and chemical methods (Xue et al. 2014; Xu et al. 2021) seem workable. However, most of them are high in cost and low in efficiency, and some even cause secondary pollution, impelling researchers to explore more effective countermeasures.
Recently, membrane separation technology based on special wettability has offered a promising and cost-effective route for treating oily wastewater (Liu et al. 2017; Wang et al. 2022; Shi et al. 2021). Through constructing rough surface on the originally amphiphilic (oil-wet and water-wet) metal meshes, ‘oil-removing’ (super) hydrophobic meshes can be obtained for selective oil/water separation (Li et al. 2016; Wang & Wang 2019; Sun et al. 2022; Zhang et al. 2022). However, little attention has been paid to the separation device. Most researchers simply sandwich the mesh between two containers, which is normally only suitable for the removal of heavy oils (ρoil > ρwater) rather than light oils (ρoil < ρwater) from the binary oil/water mixtures (Xue et al. 2011; Gao et al. 2016, 2019; Wang & Wang 2019). By tilting the mesh, the removal of light oils seems possible, but this tilting strategy is limited to separating light oil/water mixtures with a small amount of water (Cao et al. 2013, 2017). To solve the above problems, we have proposed 2D → 3D conversion of the mesh (‘Taylor cone’ container) together with the pumping technology to achieve continuous separation of large-volume oil/water mixtures of any density with high efficiency and high purity (Lei et al. 2020). However, this device requires good control of the outlet pipe during the separation and it cannot be applied to separate heavy oil/water/light oil ternary mixtures and the disturbing binary oil/water mixtures.
In this work, we construct a universal oil/water separation device based on a highly hydrophobic (water droplet contact angle ≥ 120°) metal mesh, which not only can be used for the continuous separation of binary oil/water mixtures of any density ratios both in static and disturbing states but also for the simultaneous separation of heavy oil/water/light oil ternary mixtures.
Polyvinylidene fluoride (PVDF, MW ≈ 625,000) powder was purchased from Sunshui Chemical Co., Ltd. Analytically reagents, including N,N-dimethylformamide (DMF), acetone, hexadecane, and chloroform were obtained from Sinopharm Chemical Reagent Co., Ltd. All chemicals and solvents were used directly without further purification.
Preparation of a highly hydrophobic metal mesh
The cocktail solution was prepared by pouring 3 g of SiO2 powder and 2 g of PVDF into mixed solvents of 60 mL of DMF and 40 mL of acetone with continuous stirring at 30 °C for 12 h. Before coating, the commercial brass mesh (200 mesh) was tailored and cleaned by acetone and ethanol. The tailored mesh was then dip-coated in the PVDF–SiO2 cocktail for 5 min and air-dried for further use.
The surface morphology and elemental distribution of the as-prepared samples were studied using field-emission scanning electron microscopy (FE-SEM, Zeiss, Sigma-HD-01–36, Germany) combined with energy-dispersive X-ray spectroscopy (EDS, 51-XMX1003, Oxford Instruments, UK). The wettability characterization was conducted on a contact angle analyzer (JC2000D3, Shanghai, China) using a droplet (9 μL) of water or hexadecane as an indicator at room temperature. The contact angle data were figured out based on the ellipse fitting method.
RESULTS AND DISCUSSION
Figure 3(d) shows the effect of the light oil amount on the separation efficiency under static conditions. It is found that increasing the oil amount in the mixture can bring a higher efficiency and our experiments demonstrate that η of 98.5% or higher can be realized when the oil amount exceeds 80 g. This is due to the fact that the coated mesh and pipes in the separation device adsorb (or adhere) a small portion of the oil, and the larger the starting amount of light oil, the smaller the effect (Lei et al. 2020). Figure 3(e) shows the effect of the mesh number on the light oil recovery from the oil/water mixture under static conditions. It can be seen that when the starting amount of the oil is the same as 100 g, the oil recovery rate from 120 mesh is significantly faster than that from other mesh numbers, while for 180 and 200 mesh ones, the oil recovery rates of both are very similar. This is due to the fact that the large number of mesh facilitates the adhesion filling of the coating material, which in turn decreases the passage rate of the wettable liquid (Wang et al. 2015; Lei et al. 2020).
Figure 4(d) shows that the separation efficiency of heavy oil from the static oil/water mixture increases with increasing the starting oil amount in the mixture, the reason of which has been mentioned above. It is noted that the separation efficiency for the heavy oil/water mixture is much lower as compared with the light oil/water mixture, especially noticeable when the starting oil amount is less than 100 g. This difference should arise from the volatility and density of the oils. Figure 4(e) plots the influence of the mesh number on the recovery of heavy oil from the chloroform/water mixture under static conditions. Unlike the separation of light oil/water mixture, the mesh seems more vulnerable to heavy oils as evidenced by the damage of meshes with small numbers of 120 and 150 during the separation (Figure 4(e)). The failure reveals that the coating (Figure 2(b)) on the mesh with a small number (120, 150) is more vulnerable to the corrosion of the heavy oils (chloroform) (Ping & Lu 2019). Therefore, in the following separation experiment of heavy oil/water/light oil ternary mixtures, the mesh with a big number of 200 was used.
In summary, a universal oil/water separation device based on the highly hydrophobic metal mesh has been constructed for sustainable separation of binary and ternary oil/water mixtures. Results from the separation of binary oil/water mixtures show that the separation efficiency under static conditions is better than that under stirring for both light and heavy oil/water mixtures. As compared with heavy oil/water mixtures, light oil/water mixtures normally show higher separation efficiency although with slow recovery rate. In terms of the separation of heavy oil/water/light oil ternary mixtures, the constructed device also allows the simultaneous recovery of light and heavy oils in different channels.
This work was supported by the Natural Science Foundation of Fujian Province (2021J01298 and 2020J01709) and Xiamen City Science and Technology Youth Innovation Fund Project (3502Z20206010).
DATA AVAILABILITY STATEMENT
Data cannot be made publicly available; readers should contact the corresponding author for details.
CONFLICT OF INTEREST
The authors declare there is no conflict.
These authors contributed equally to this work.