The development of oil extraction technologies usually accompanies with the formation water, which causes the ultimate production to exist in the form of an emulsion. In addition, the development of petroleum industry generates a large amount of oily wastewater(Fang et al. 2016, Peng et al. 2019, Sun et al. 2020, Teng et al. 2019). The oil-water emulsion can cause pipeline blockage, equipment corrosion, and serious water environmental pollution(Yuan et al. 2020, Zhang et al. 2013, Zhao et al. 2020). Therefore, the demulsification of oil-water emulsion is of great necessity.
Typically, oil-water emulsion is fairly stable. It is stabilized by natural surface active substances (asphaltenes, waxes, and resins) and inorganic solids through the vigorous π-π interactions(Zhang et al. 2020b). Furthermore, steric effect and electrostatic force can also affect the stability of oil-water emulsion(Ma et al. 2020). In recent decades, chemical and physical methods are two main demulsifying approaches. Typical physical treatment techniques include centrifugation, electric field, membrane separation and microwave radiation(Chen et al. 2019a, Ghanbari &Esmaeilzadeh 2018, Ichikawa et al. 2004, Tan et al. 2007, Xiong et al. 2018, Yi et al. 2019). However, they have many disadvantages such as high energy, low processing efficiency and complex equipment, etc. The chemical method is used to demulsify the emulsion by adding the demulsifier. It has the advantages of low energy consumption, fast processing speed and low cost. Therefore, it is widely adopted at present.
Chen et al.(Chen et al. 2019b) synthesized a magnetically responsive demulsifier called Fe3O4@hyperbranched polyamidoamine-graphene oxide (MKh-GO), and ORR in emulsion reached 96.0% when the dosage was 20 mg/L at 40 ℃. Furthermore, MKh-GO can be recycled seven times without obvious decrease in efficiency. Kuang et al.(Kuang et al. 2020) prepared a hyperbranched demulsifier (PTC) with trimethyl citrate as centronucleus. LTA could reach 91.5% with 50 mg/L of PTC at ambient temperature.
The surface of carbon-based materials has a huge π conjugate system. The oil-water interfacial film in crude oil emulsion is easily destroyed with the aid of π-π or p-π interactions between carbon-based materials and asphaltenes/resin(Cote et al. 2010). As a result, the droplets can gather at the interface and achieve the separation of oil and water(Liu et al. 2015a). Liu et al.(Liu et al. 2015a, Liu et al. 2015b, Wang et al. 2016) reported some carbon-based demulsifiers such as functionalized multiwalled carbon nanotubes, graphene oxide and reduced graphene oxide, which can initiate and achieve the oil-water separation in O/W emulsion. Moreover, the carbon-based materials are environmentally friendly and easy to obtain. Recently, Chen et al.(Chen et al. 2015) used a two-step coating process to prepare a demulsifier. In their work, amorphous SiO2 coated Fe3O4 particles were further functionalized by KH-1231. It showed a great demulsifying performance. Furthermore, the demulsifier can be recycled and reused. Wang et al.(Wang et al. 2011) prepared a demulsifier for the oil-in-water emulsion by grafting nano-SiO2 onto TA1031, and the efficiency could be improved by 20% and reached 97%. In our previous work, some carbon-based materials such as SiO2@CS, Ox-CB@SiO2 and MCNT@β-CD were used to demulsify the oil-water emulsions(Ye et al. 2019, Ye et al. 2020b, Yuan et al. 2020). Although all of them had an excellent demulsifying performance, there are still some disadvantages such as overdose, high operating temperature, or low applicability only applies to one type of emulsion (O/W or W/O emulsion).
For the purpose of improving the demulsifying performance and broadening the application scope, nano-SiO2 coated graphene oxide (GO@SiO2) was prepared by the sol-gel method in current work. The products are environmentally friendly, non-toxic and efficient. Especially, it can treat both W/O and O/W emulsions. It is expected to be applied to break the oil-water emulsion in petroleum and chemistry industry.