# Novel Materials for Heavy Metal Removal in Capacitive Deionization
## 3 Materials for Heavy Metal Removing Using CDI
### Others  
Aside from carbon-based materials, polymer-based materials have attracted great interest owing to their ion-exchanging ability and conductivity. Polypyrrole is one of the most studied polymers due to its high specific capacitance, high conductivity, and simple preparation. Polyaniline/attapulgite composites (PANI/ATP) were prepared via in situ oxidative polymerization [123]. The calculated equilibrium adsorption capacity was 15.42 mg g\({}^{-1}\). The facile modified paper electrode with polymer attached to the surface of ATP contributed to the high removal efficiency of lead ions (66.5%) in wastewater, with an initial concentration of 2.5 mM Pb\({}^{2+}\) and stable cyclability after 10 runs (64.2%). [PERSON] et al. deposited polypyrrole on chitosan, a stable adhesive agent functionalized with -OH and -NH\({}_{2}\) groups, and proved that a polypyrrole/chitosan composite (Py/CS) exhibits considerably stable electroadsorption and desorption behavior and shows good performance regarding the removal of heavy metal ions including Ag\({}^{+}\), Cu\({}^{2+}\), Pb\({}^{2+}\), and Cd\({}^{2+}\)[124]. Transition metal sulfide has been viewed as a promising modified candidate for CDI. [PERSON] et al. decorated ZnS and FeS on a carbon felt electrode. The ZnS on the carbon material increased the wettability and electrochemical capacitance. The enhanced adsorption ability of Cu\({}^{2+}\) and removal capacity of Cu\({}^{2+}\) was 27.4 mg g\({}^{-1}\) in the presence of Cr\({}^{6+}\) and natural organic matter [124]. The materials used for heavy metal electrosorption are primarily carbon-based materials due to their tunable properties such as surface area, pore size, and distribution as well as their surface functionalities. Overall, polymer carbon composites have been investigated for Cu and Pb removal, and their physical and chemical adsorption abilities were found to be excellent owing to the chelation process of capturing ions. However, a few unusual properties, such as restricted specific area and relatively poor conductivity, limit their per  
Figure 4: Mechanism of metal-oxide-supported carbon composites for heavy metal removal. formance regarding the electrosorption of HMs. Their adsorption capacity is relatively low, and a higher adsorption capacity was achieved for Pb using PTFE/CNTs. An adsorption capacity of 16.83 mg g\({}^{-1}\) for Cu was achieved using polypyrrole/chitosan/CNT due to the use of functional polymer-like polypyrrole, a common conducting polymer, which showed ultra-high specific capacitance and conductivity [112; 113]. Moreover, synergy between polypyrrole and chitosan increased adsorption capacity through the chelation process [125; 126]. Metal-oxide carbon composites have been proven to show excellent performance in the removal of Pb, Cu, and Cr. Fe\({}_{3}\)O\({}_{4}\)/peanut-shell-derived activated carbon material exhibited a 99.9% removal rate and 24.5 mg g\({}^{-1}\) Cr\({}^{6+}\) adsorption rate in wastewater [116]. The redox ability of transition metal oxide was useful in deducing the toxicity of HMs during the electrosorption process. The removal of Cu by MnO\({}_{2}\)/carbon fiber demonstrated a 96% removal rate with a 172.9 mg g\({}^{-1}\) adsorption rate [127]. The enhancement of adsorption via the polarization potential and unique nanoflower microstructure of deposited MnO\({}_{2}\) may equally lead to the superior adsorption ability of MnO\({}_{2}\)/CF electrodes. TiO\({}_{2}\)-nanotube-coated CNT electrodes have been proven to efficiently remove Cu and Pb with adsorption capacities of 282.25 mg g\({}^{-1}\) and 241.65 mg g\({}^{-1}\), respectively. The excellent activity of metal oxide/CNT composite relies on its mesoporous features, good ion-exchanging properties, and very high conductivity. The removal rates and adsorption capacities for Pb by Fe\({}_{2}\)O\({}_{3}\)/C were 90.3% and 830.17 mg g\({}^{-1}\), respectively; this improvement was accredited to its considerably large specific surface area (1832 m\({}^{2}\) g\({}^{-1}\)) and high dispersion at the near quantum scale.