# Application of Fly Ash as an Adsorbent for Removal of Air and Water Pollutants
## 4 Application of Fly Ash for Treatment of Pollutants
### Removing Water Pollutants  
#### 4.2.1 Removal of Heavy Metals  
Industrial development is likely to cause water pollution, especially heavy metal pollution, because of its inherent toxicity, large sources, persistence, non-degradability, and other factors and can cause serious harm to humans, animals, plants, and the environment [137; 138]. Some of the common sources of anthropogenic heavy metals are displayed in Table 5. The treatment of waste water containing heavy metal can be divided into three categories: physical methods, chemical methods, and biological methods [5; 139]. Physical methods include adsorption (activated carbon, carbon nanotubes, kaolinite and montmorillonite, FA, and other low-cost adsorbents), membrane filtration (ultrafiltration, reverse osmosis, nanofiltration, and electrodialysis), coagulation and flocculation, flotation, and other methods [5; 140]. Chemical methods include chemical precipitation (hydroxide precipitation, sulfide precipitation, chelating precipitation, and chemical precipitation combined with other methods), ion-exchange, electrochemical treatment technologies, and others [5; 139]. Biological methods include bio-adsorbents and use of microorganism, as well as others [141; 142]. Among them, ion-exchange, adsorption and membrane filtration are the most commonly used method, and use of modified FA is an economical and efficient method for treatment of heavy metal wastewater because FA is derived from industrial waste. Figure 14: Comparison of SEM images before (**a**) and after (**b**) SO2 adsorption (adapted with permission from [134], Elsevier, 2018). Figure 13: (**a**) Desulfurization activity of different adsorbents; and (**b**) relationship between desulfurization capacity and specific surface area (reproduced with permission from [134], Elsevier, 2018). FA can be used directly or indirectly (synthesized as zeolites) to adsorb heavy metal from polluted water [144; 145; 146]. It is a potential adsorbent that can remove Cu, Pb, Zn, Mn, Cd, Cr, and Ni from wastewater [147; 148]. On the other hand, the pH of the aqueous solution directly affects adsorption of metal ions due to changes in surface charge of the adsorbent and the degree of ionization [26; 144]. FA has a high adsorption capacity for Cu\({}^{2+}\) and Zn\({}^{2+}\) ions at pH 8. In addition, using FA to adsorb Cu\({}^{2+}\) and Zn\({}^{2+}\) ions was faster than that of natural zeolite and peanut husk [149]. The SEM images of the FA before and after the adsorption of heavy metals are shown in Figure 15. As shown in Figure 15b, the porosity between the FA particles is significantly reduced. This could be attributed to that the surface of FA is covered by complexes formed by heavy metal ions [26]. Moreover, FA has a stronger adsorption capacity for Pb\({}^{2+}\) and Cd\({}^{2+}\) than other heavy metal ions. Adsorption of Pb\({}^{2+}\) and Cd\({}^{2+}\) can be attributed to chemical sorption or chemisorption involving valence forces through sharing or exchange of electrons between the sorbent and sorbate, because their adsorption kinetics followed second-order reaction kinetics [26; 150]. With the development of nanotechnology, FA can be fabricated into a functional fibrous membrane with a high capacity for adsorbing heavy metals [34]. FA particles can be perfectly combined with polyurethane fibers via electrospinning, and the FA fibrous membrane has higher adsorption capacity and faster adsorption rate for adsorption of Pb than that of Hg. This is attributed to the affinity of formation of PbOH\({}^{+}\) being higher than to HgOH\({}^{+}\)[34]. The results are encouraging for the removal of heavy metals using FA fibrous membrane from industrial wastewater. FA fibrous membrane can be used as a variety of filters with low-cost and high performance. These are a great help to improve the utilization of FA and reduce the pressure of environmental pollution.