# Application of Fly Ash as an Adsorbent for Removal of Air and Water Pollutants
## 1 Introduction  
Along with unceasing economic development, people's living standard enhances, and humans currently live in a time of rapid development, with daily life constantly becoming more high-tech and intelligent. However, the continuous improvement of human life will inevitably increase the demand for energy and other substances, and it will cause some environmental pollution problems, such as air and water pollutants including nitrogen oxides (NOx) [1], volatile organic compounds (VOCs) [2], sulfur dioxide (SO\({}_{2}\)) [3], and heavy metal pollution [4]. Fly ash (FA), a type of industrial waste, is a coal combustion product composed of particulates [5]. With rapid industrial development, large demand for electricity has led to hundreds of millions of tons of FA (around 80% of which is fly ash) being discharged every year worldwide. The annual coal consumption of China in 2015 was about 50% of the global coal consumption [6]. Power generation using coal provides energy for daily life and industry, but it also produces atmospheric and water pollution, which disrupts ecological cycles. FA pollutes the atmosphere and threatens human health if it is discharged directly from power plant chimneys into the atmosphere without treatment [7; 8]. This is a significant contributor to recent air quality and haze problems in China [9; 10]. However, with developing science and technology, FA from power plants is changing from industrial waste into an industrial raw material [11; 12; 13]. FA is a large porous structure exhibiting an irregular shape, with large specific surface areas and high adsorption activities. The characteristics of FA mainly depend on the type of coal and the combustion conditions [14; 15]. [PERSON] et al. pointed out that the average diameter of FA in the form of fine particles is less than 20 \(\upmu\)m, and their bulk density and specific surface area are in the range of 0.54-0.86 g/cm\({}^{3}\) and 300-500 m\({}^{2}\)/kg, respectively [15]. In addition, FA contains valuable oxide components, such as SiO\({}_{2}\), Al\({}_{2}\)O\({}_{3}\), CaO, MgO, Na\({}_{2}\)O, and TiO\({}_{2}\), and essential elements, including P, K, Mg, Zn, Fe, Mn, and others [5; 7]. Therefore, FA has attracted the attention of many researchers. FA is a pozzolanic mixture formed by high temperature combustion of pulverized coal. Its chemical composition is similar to clay, with a good lime binding capacity. Thus, it is primarily used for production of cement [16], clay bricks [17], hollow blocks [18], aerated concrete [19], and other building materials [20; 21; 22]. It can also be used as an agricultural fertilizer and soil conditioner due to its low bulk density, high water retention capacity, favorable pH, and other properties [15; 23; 24]. [PERSON] et al. [25] studied the effects of FA on rice crop production from 1996 to 2000 and found that FA improved texture, fertility, and crop productivity of mine spoil. Figure 1 shows the various applications of FA in different fields. FA can also be used as a low-cost adsorbent because of its excellent specific surface area, porosity, particle size [26], and other natural characteristics [27]. [PERSON] et al. [28] found that FA could be used as an adsorbent in a cascade with a plasma reactor to NOx emission from biodiesel engines. [PERSON] et al. [29] showed that FA carbon captures Hg efficiently. Other researchers have found that, after simple modification, FA is a good adsorbent of SO\({}_{2}\)[30] and VOCs [31]. Furthermore, one-step fabrication of FA into a fibrous membrane by electrospun technology to adsorb VOCs from air has been successfully reported by our research group [32; 33]. On the other hand, FA can also be combined with TiO\({}_{2}\)[34] or AgNO\({}_{3}\)[35] to produce a multifunctional fibrous membrane under electrospinning for purifying water, such as adsorption of heavy metals (Hg and Pb), removal of dyes (methylene blue), antibacterial activity, carcinogenic arsenic (As) and toxic organic dyes.