# Application of Fly Ash as an Adsorbent for Removal of Air and Water Pollutants
## 3 Physical and Chemical Properties of Fly Ash
### Chemical and Mineralogical Composition  
The chemical and mineralogical composition of FA plays an important role in the analysis of the application and disposal of FA, including its use as a cement replacement material. The chemical composition of FA can be determined by X-ray fluorescence (XRF) and spectrometry techniques. Researchers [66; 67; 68; 69; 70] have reported that FA is composed of SiO\({}_{2}\), Al\({}_{2}\)O\({}_{3}\), Fe\({}_{2}\)O\({}_{3}\), CaO, MgO, Na\({}_{2}\)O, K\({}_{2}\)O, TiO\({}_{2}\), P\({}_{2}\)O\({}_{5}\), and small amounts of MnO, BaO, and SO\({}_{3}\). The detailed chemical composition of fly ash is given in Table 2. The chemical composition of FA may differ based on geographical origin and  
Figure 8: SEM images of fly ash fractions. FBC ash, fluidized bed combustion ash (reproduced with permission from [15], Elsevier, 2017). Figure 7: SEM images of: non-carbonated fly ash (**a**,**b**) and carbonated fly ash (**c**,**d**) (reproduced with permission from [57], Elsevier, 2018). Note: The fly ash was collected from coal burning power plants in the Mpumalanga province of South Africa. combustion conditions. Some heavy metals are also found in FA, such as Fe, Ca, Zn, Cd, Co, Ni, Cu, Pb and Mn [71], as shown in Table 3. FA can also be divided into high calcium ash (\(\geq\)10% CaO) and low calcium ash (<10% CaO), according to the content of CaO. High calcium ash can remove acidic gases and fix hazardous heavy metals, such as Cd and Cu. CaO is present in FA as free lime and embedded in glassy spheres. Usually, the CaO content increases with decreasing particle size [57]. Loss on ignition (LOI) has long served as a standard method to measure unburned carbon in FA [72; 73]. All types of FA have a certain LOI value indicating their unburnt carbon content (Table 2). Unburnt carbon in FA powder has properties similar to activated carbon, such as good adsorption capacity. Some minerals in FA can be measured by X-ray diffraction (XRD) spectroscopy techniques. [PERSON] et al. [57] analyzed the XRD spectra of non-carbonated and carbonated FA, as shown in Figure 9. Minerals in non-carbonated FA were found to mainly consist of amorphous, CaO, hematite, mullite, and quartz (see Figure 9a). The minerals in carbonated FA are mainly composed of amorphous CaCO\({}_{3}\), magnetite, mullite, plagioclase, quartz, bassanite, and anhydrite (see Figure 9a). The major crystalline phases are mullite and quartz for most ash [74; 75; 76; 77]. The CaO in non-carbonated FA is converted into calcite (main component: CaCO\({}_{3}\)) when it is carbonized. These results have been described in SEM spectrum analysis, and the \"cubic-like\" structures in Figure 7c are calcites [57]. Many other researchers have also confirmed these results [78]. Among the many oxides in FA, recovery of alumina has attracted interest in recent decades. Therefore, as science and technology progresses, the recovery and utilization of industrial waste to benefit humans and the environment will help reduce environmental impacts. \begin{table}
\begin{tabular}{c c c c} \hline
**Component (wt.\%)** & **Bituminous** & **Sub-Bituminous** & **Lignite** \\ \hline SiO\({}_{2}\) & 20–60 & 40–60 & 15–45 \\ Al\({}_{2}\)O\({}_{3}\) & 5–35 & 20–30 & 10–25 \\ Fe\({}_{2}\)O\({}_{3}\) & 10–40 & 4–10 & 4–15 \\ CaO & 1–12 & 5–30 & 15–40 \\ MgO & 0–5 & 1–6 & 3–10 \\ SO\({}_{3}\) & 0–4 & 0–2 & 0–10 \\ Na\({}_{2}\)O & 0–4 & 0–2 & 0–6 \\ K\({}_{2}\)O & 0–3 & 0–4 & 0–4 \\ LOI a & 0–15 & 0–3 & 0–5 \\ \hline \end{tabular} a  
\end{table} 
Table 2: Primary chemical compositions for fly ash produced from different coal types (adapted with permission from [5], Elsevier, 2017).