# Novel Materials for Heavy Metal Removal in Capacitive Deionization
## 3 Materials for Heavy Metal Removing Using CDI
### Pure Carbon Materials
The fundamental behavior of Cu\({}^{2+}\) removal in the CDI process over activated carbon electrodes has been investigated [89]. It was shown that electrical double-layer charging assisted Cu\({}^{2+}\) removal, and electrodeposition can be curbed at a low applied voltage. The equilibrium electrosorption capacity of Cu\({}^{2+}\) was 24.57 mg g\({}^{-1}\) by the electrical double layer at a charge potentially less than 0.8 V, while electrodeposition of Cu was seen when the potential was over 0.8 V.  
The adsorption capacity of electrodes also depends on co-existing organic and inorganic pollutants. Cr(VI) electroadsorption was raised from 155.7 to 190.8 mg g\({}^{-1}\) when KCl concentration inclined in an electrolyte in the range of 100-500 mg L\({}^{-1}\), but notably declined to 90.2 mg g\({}^{-1}\) when the KCl concentration reached 1000 mg L\({}^{-1}\). High Cr\({}^{6+}\) removal efficiency occurred in the absorption of Cr(VI) onto electrodes, the reduction of Cr(VI) into Cr(III), and the formation of precipitation onto the electrode surface [90]. Co-existing Sr and Cs ions in aqueous solution largely inhibited the removal efficiency of Co ions from 36.54% to 8.37% [91]. [PERSON] et al. used a carbon aerogel as the electrode for the CDI removal of Cu\({}^{2+}\)[92]. The 3D network-like carbon structure was composed of small particles with mesopores and macropores having a pore volume of 3.41 cm\({}^{3}\) g\({}^{-1}\) nm in the carbon materials and had a great impact on the adsorption of diverse ions and the anion adsorption effect in the following order: SO\({}_{4}\)\({}^{2-}\)\(>\) NO\({}_{3}\)\({}^{-}\)\(>\) Cl\({}^{-}\). The highest specific capacity of 156 F g\({}^{-1}\) in 6 M KOH solution, reaching a Cu\({}^{2+}\) adsorption rate of 29.7 mg g\({}^{-1}\), was achieved in an optimized condition in which the Cu\({}^{2+}\) concentration was 100 mg L\({}^{-1}\) and the applied voltage was 1.2 V.  
Graphene is a two-dimensional material with a high specific surface area and intrinsic electrical conductivity along with mechanical robustness. It is considered an excellent candidate for CDI electrodes. Graphene-based materials have been widely studied for water desalination [93; 94]. However, graphene sheets are prone to stack and agglomerate, resulting in low surface area and uncontrollable pore size. This decreases the electrical double layer capacitance of the electrodes and reduces the CDI performance [95]. [PERSON] et al. fabricated carbon nanotube-graphene hybrid aerogels as electrode materials through the CO\({}_{2}\) supercritical drying of graphene-CNT hybrid hydrogel [96]. This novel material has the impressive properties of a large surface area (435 m\({}^{2}\) g\({}^{-1}\)), large pore volume (2.58 m\({}^{3}\) g\({}^{-1}\)), high conductivity (7.5 S m\({}^{-1}\)), and being light weight (41.1 mg cm\({}^{-3}\)). The hierarchically porous structure was found to be crucial to the electro-binding of four different HMs with adsorption capacities of 104.9 mg g\({}^{-1}\) for Pb\({}^{2+}\), 93.3 mg g\({}^{-1}\) for Hg\({}^{2+}\), 64 mg g\({}^{-1}\) for Ag\({}^{+}\), and 33.8 mg g\({}^{-1}\) for Cu\({}^{2+}\)[96]. Graphene-oxide-modified carbon felt electrodes have been used to dispose of both low-concentration and high-concentration heavy metal pollution using either direct current (DC) or alternating current (AC) electrodeposition. Graphene oxide (GO) provides a high density of surface oxygen functional groups (C-O, COOH, COH, etc.) to assist in electrodeposition. The electrodeposition method showed a capacity two orders of magnitude of a higher capacity (>29 g heavy metal for 1 g of GO) than traditional adsorption methods [97]. DC electrodeposition with GO-modified carbon -felt electrodes can reduce single-heavy-metal-ion pollution (Cu, Cd, and Pb) and multiple ion mixtures to below safe water drinking water levels. By tuning the AC frequency and voltage, the electrodeposition method can further selectively recover Cu, Cd, and Pb separately, which adds value to the heavy metal removal process [97].