# Novel Materials for Heavy Metal Removal in Capacitive Deionization
## 3 Materials for Heavy Metal Removing Using CDI
### Metal Oxide Supported Carbon Composites  
Many solutions have been developed to solve the aforementioned challenges. For example, one of the best of such solutions involves using faradaic electrode materials as the electrode. Unlike non-faradaic materials such as porous carbon electrodes, which store salt ions capacitively in an electric double layer along surfaces or nanopores, faradaic electrodes store ions via intercalation or conversion reactions [114]. Incorporated transited metal oxide could efficiently enhance the capacitance and electrosorption ability of carbon-based materials due to the conversion of heavy metal ions and the adsorption of HMs on defects as well as the intercalation of metal ions into the metal oxide layer structure. [PERSON] et al. synthesized highly mesoporous peanut-shell-derived activated carbon/Fe\({}_{3}\)O\({}_{4}\) nanocomposite materials [115]. Cr(VI) was reduced to Cr(III) by Fe(II) ions of Fe\({}_{3}\)O\({}_{4}\) and was then directly electro-captured on carbon-based composite materials in acidic conditions. The synergetic effect of both physical adsorption and electrosorption combined with a reduction pathway was crucial to Cr(VI) removal. Nearly 99.9% of the Cr(VI) was removed with a high electroadsorption rate up to 24.5 mg g\({}^{-1}\) at pH 6.5 with an initial concentration of 50 mg L\({}^{-1}\). [PERSON] et al. induced highly distributed \(\alpha\)-Fe\({}_{3}\)O\({}_{4}\) of a uniform size on carbon matrixes through the carbonization of MIL101(Fe) [116]. The porous carbon yield exhibited surface areas of \(\sim\)1832 m\({}^{2}\) g\({}^{-1}\) and showed excellent adsorption capacity for Pb\({}^{2+}\) (830.17 mg g\({}^{-1}\)) and a high removal efficiency of 90.3% at an initial concentration of 500 mg L\({}^{-1}\) Pb(II)-containing water through the CDI process with high durability within 10 cycles [116]. This high adsorption capacity and removal efficiency resulted from the electroadsorption and electrodeposition of the Pt(II) on the electrode [116]. In another study, a graphene sheet exfoliated by potassium ions from graphite was prepared [117]. Fe\({}_{3}\)O\({}_{4}\) nanoparticles were successfully supported on porous graphene sheets via the hydrothermal process. The Fe\({}_{3}\)O\({}_{4}\)/porous graphene nanocomposites showed great potential for the removal of both organic pollutants and heavy metal ions, with the removal rates for Pb\({}^{2+}\), Cu\({}^{2+}\), and Cd\({}^{2+}\) found to be 90.6%, 92%, and 90.2%, respectively. Moreover, the results revealed ultrahigh electrosorption capacities for Pb\({}^{2+}\), Cu\({}^{2+}\), and Cd\({}^{2+}\) ions which were found to be 47, 40, and 49 mg g\({}^{-1}\), respectively, in NaCl solution with an initial concentration of 500 mg L\({}^{-1}\), a cell potential of 1.6 V, and a 0.03 mM initial metal ion concentration. \(\alpha\)-MnO\({}_{2}\) nanoneedles were grown on carbon fiber paper using an ordinary hydrothermal method [118]. Both Ni(II) and Mn(II) ions reduced easily and electrosorbed on the \(\alpha\)-MnO\({}_{2}\)-carbon-paper-based electrode due to its mesoporous structure, contributing to fast electron charge transfer. Birnessite-type MnO\({}_{2}\) on carbon nanotubes can effectively remove both Zn\({}^{2+}\) and Ni\({}^{2+}\) via constant potential electrolysis. CNTs offer a larger specific surface area and higher conductivity to birnessite. Their removal capacities for Zn\({}^{2+}\) and Ni\({}^{2+}\) have been found to reach 155.6 and 158.4 mg g\({}^{-1}\) in 50 mg L\({}^{-1}\) origin wastewater, respectively. Hexagonal birnessite is proposed to consist of edge-sharing Mn(IV)O\({}_{6}\) octahedral layers, and these layers are interlayered with water molecules. There are Mn(IV) vacancies in Mn(IV)O\({}_{6}\) octahedral layers due to the repulsion between neighboring Mn(IV). Heavy metal ions can be adsorbed on or below the vacancies during the isothermal adsorption process [119]. Figure 4 depicts the mechanisms of metal-oxide-supported carbon composites in heavy metal removal.