# Novel Materials for Heavy Metal Removal in Capacitive Deionization
## 3 Materials for Heavy Metal Removing Using CDI
### Heteroatom Doped Carbon  
Heteroatom doping is the process of substituting some carbon atoms with other atoms in the carbon structure, primarily P block elements [103]. Bringing heteroatoms (e.g., N, B, S, P, etc.) into carbon nanomaterials can alter the electron configurations of the adjacent carbons due to their sizes and electronegativity differences, hence enhancing electrical conductivity and wettability [103; 104]. Moreover, this process can induce a huge number of defects, which can create a more accessible surface area and is beneficial for charge accumulation. A sheet-like N, O-enriched hierarchical porous carbon material was developed by [PERSON] et al. [105], adopting eggplant as the carbon source and KHCO\({}_{3}\) as the activation agent. The largest capacitance was up to 172.5 F g\({}^{-1}\) in NaCl solution. Pb\({}^{2+}\)-and Cd\({}^{2+}\)-containing wastewater mainly occurred due to a high concentration of N-doped oxygen functional groups on the surface and a 3D porous structure leading to greater mass transport efficiency and electroconductibility [83]. Further, with wastewater containing Pb\({}^{2+}\) and Cd\({}^{2+}\) at concentrations of 0.5 ppm and 0.1 ppm, removal rates of 99.1% and 97.9%, respectively, were achieved in rechargeable Zn-air battery-coupled CDI devices. The self-powered system displayed excellent charge efficiency (60.3%) for CDI, whereby the residual concentrations of Pb\({}^{2+}\) and Cd\({}^{2+}\) in water dropped to 4.5 ppb and 2.2 ppb in 90 minutes. In another study, 3D honeycomb-like porous carbon was synthesized via the hydrothermal carbonization of corncob combined with KOH activation [106]. The newly prepared porous carbon-based electrodes exhibited a high capacitance of 452 F g\({}^{-1}\) in 1 M Na\({}_{2}\)SO\({}_{4}\). A batch-mode CDI experiment demonstrated the removal of Cr\({}^{6+}\) through a non-faradic process, achieving a removal rate of 91.85% when the initial Cr\({}^{6+}\) concentration was 30 mg\(\cdot\)L\({}^{-1}\). Moreover, a highly porous N-doped graphene-based CDI device exhibited high removal efficiency (90-100%), fast removal (<30 min), and good regeneration performance (10 cycles, 99% retention) for multiple HMs (Pb\({}^{2+}\), Cd\({}^{2+}\), Cu\({}^{2+}\), Fe\({}^{2+}\), etc.) in water at a wide range of concentrations (0.05-200 ppm) [94]. [PERSON] et al. developed an N-doped graphene nanosheet material via the one-step pyrolysis of a graphene oxide and cyanamide mixture [94]. [PERSON] et al. developed an N-doped and fluorine co-doped graphene oxide as an electrode material, and a specific capacitance of 245.6 F g\({}^{-1}\) was achieved with a high removal percentage of 95% and a specific electrosorption capacity of 52.4 mg g\({}^{-1}\) at 100 mg L\({}^{-1}\) Cu(II) [107]. It is worth noting that highly prepared porous carbon was firstly used as an electrode for membrane CDI to remove multiple HMs, e.g., Pb\({}^{2+}\), Cd\({}^{2+}\), Fe\({}^{2+}\), and Cu\({}^{2+}\), simultaneously, as the prepared carbon material showed a high specific surface area of 695 m\({}^{2}\) g\({}^{-1}\). At 200 ppm, the adsorption capacity of NG for Pb\({}^{2+}\) and Cd\({}^{2+}\) can, dramatically, reach 521 mg g\({}^{-1}\) and 498 mg g\({}^{-1}\), and high removal efficiencies of 96%, 80%, 100%, and 100% for Fe\({}^{2+}\), Cd\({}^{2+}\), Pb\({}^{2+}\), and Cu\({}^{2+}\), respectively, have been achieved with a wide range of initial concentrations (0.05-200 ppm). [PERSON] et al. developed a three-dimensional reduced graphene oxide/nitrogen-doped carbon quantum dot composite. In this work, the electrosorption capacity of an rGO/25% NCDs electrode was 19.26 mg/g when the initial concentration of Pb\({}^{2+}\) was 5 mg/L [108]. [PERSON] et al. developed two kinds of N-doped carbon with distinct configurations: pyrrolic N-dominated graphene (N-5-G) and pyridinic N-dominated graphene (N-6-G) [109].