# Superhydrophobic Polypyrrole-Coated Cigarette Filters for Effective Oil/Water Separation
## 1 Introduction  
With the continuous development of industrialization, the demand for oil is also increasing [1, 2]. From oil extraction, refining, storage, and transportation to industrial use, these processes inevitably produce a large amount of oil/water mixture waste [3, 4]. Accidents such as oil spills and substandard discharge of industrial sewage cause catastrophic damage to marine and lake ecosystems [5]. Thus, effective methods to recycle oil from water are urgently needed. Many methods have been developed to separate oil and organic contaminants from water, such as gravity separation, centrifugal separation, electric de-separation, coarse-grained dehydration of emulsified water, and air-float separation [6]. However, most of these methods are inefficient, poor-safety, high-energy-consumption, and complicated processes [7], making them challenging for use in large-scale applications. Besides the traditional methods, superhydrophobic and oleophilic materials, which can repel water and be wetted by oils, have been considered as suitable candidates for high-efficiency oil/water separation. Recently, researchers have fabricated an enormous amount of superhydrophobic materials for oil/water separation through sol-gel [8], chemical etching [9], spray methods [10], and hydrothermal treatment [11]. Superhydrophobic materials can be divided into separation materials and absorption materials. Separation materials mainly use steel mesh, textiles, and kapok fiber as substrates [12, 13], which simply separate oil and water. [PERSON] et al. [14] reviewed the recentprogress of oil/water separation technologies based on filtration and absorption methods using various materials that possess surface superwetting properties. [PERSON] et al. [15] prepared a pair of stainless steel meshes with hydrophilic and hydrophobic functions to achieve rapid and continuous separation of oil/water mixtures. [PERSON] et al. [16] utilized laser marking technology to fabricate superhydrophobic stainless steel mesh for oil/water separation. [PERSON] et al. [17] utilized the reverse-phase method initiated by an inert solvent to fabricate a superhydrophobic PVDF membrane for high-flux oil/water emulsion separation. [PERSON] et al. [18] utilized the electrostatic spinning deposition method to fabricate a superhydrophobic membrane for selective oil/water separation. The main problems for oil/water separation materials include their absorption capacity, secondary pollution, and separation stability, which limit their industrial application [19]. Absorption materials make use of 3d porous materials as substrates, such as sponges and foams. Oil/water separation is one of the functions of adsorption materials, but another important function is that they can absorb oil from water. The absorbed oil can be easily reused by squeezing or burning, which avoids secondary pollution [20]. [PERSON] et al. [7] fabricated superhydrophobic Fe\({}_{3}\)O\({}_{4}\) melamine sponges for absorbing oil from water. The modified sponges have high absorption capacities and good reusability. [PERSON] et al. [21] fabricated graphene-based superhydrophobic sponges for oil/water separation, which showed excellent absorption properties for various oils and organic contaminants. [PERSON] et al. [22] fabricated a joule-heated graphene-wrapped sponge for viscous oil fast sorption. [PERSON] et al. [23] fabricated a micro/nanostructure layer sponge which can be recycled for more than 70 cycles. Compared to these high-porosity and expensive sponges, cigarette filters can be an economical choice for fabricating superhydrophobic absorbing materials. Cigarette filters are the most troublesome solid waste in the world [24]. Their main constituent material is cellulose acetate, which cannot easily biodegrade [25]. If cigarette filters can be used as an oil/water separation material, this can effectively solve the problem of the difficult recycling and degradation of waste cigarette filters.