# Novel Materials for Heavy Metal Removal in Capacitive Deionization
## 2 A Brief Review of Electrochemical Methods
\begin{table}
\begin{tabular}{c c c c c} \hline \hline
**Metal** & **Method** & **Cathode** & **Anode** & **Ref** \\ \hline Cr6+ & Reverse electrodialysis & Carbon felt & Ti/IrO\({}_{2}\)-Ta\({}_{2}\)O\({}_{5}\) & [44] \\ Cr6+ & Reverse electrodialysis & Carbon felt & Biotic anode & [45] \\ Cr6+ & Electrocoagulation & Cylindrical vessel iron & Vertical rotating impeller & [15] \\ Cr6+ & Electrocoagulation & cathode & Fe anode & [15] \\ Cr6+ & Electrocoagulation & Air cathode & Pure sacrificial iron plate & [16] \\ Cr6+ & Microbial fuel cell & Carbon brush & Carbon felt & [53] \\ Cr6+ & & & & [54] \\ Cr6+ & Microbial fuel cell & Graphite rods & Polyaniline-modified & [55] \\  & & & Carbon-fiber-cloth- & \\ Cr6+ & Alkaline ethanol fuel cell & Carbon fiber cloth & coated & [56] \\ Pb2+ & Fuel cell & Carbon cloth & Carbon-cloth-coated Ni & [57] \\ Cr6+ & Solar electrocoagulation & Perforated Zn & Perforated Zn & [17] \\ Cu2+; Cr6+; Ni2+; & Electrocoagulation– & Stainless steel & Al & [58] \\  & electrofolation & Zn & Zn & [18] \\  & & & & \\ Pb2+; Zn2+; Cd2+; & Electrocoagulation & Stainless steel & Al & [19] \\ As3+ & Electrocoagulation & Fe plate & Al plate & [20] \\ Cd2+; Cu2+ & Electrocoagulation & Al plate & Al plate & [21] \\ Zn2+ & Electrocoagulation & Stainless steel plate & Ti expanded mesh & [59] \\ Cu2+ & Electro-Fenton method & Graphite & Fe & [32] \\ Cr3+; Cu2+; Ni2+; Zn2+ & Electrofolation & Stainless steel & Ti plate & [60] \\ Fe3+; Cu2+; Zn2+ & Electrocoagulation & Iron (carbon steel) plates & Iron (carbon steel) plates & [61] \\ Cu2+; Zn2+; Ni2+; Ag+ & Microbial fuel cell & Activated carbon & Carbon fiber veil & [49] \\ Ni2+; Cu2+ & Microfluidic electrodialysis & Au & Au & [62] \\ Mo4+ & Electro-reduction & Carbon fiber cloth & Pt-coated Ti panel & [63] \\ Cd2+; Cu2+; Zn2+; Ni2+; & Electro-oxidation & Ti/RuO\({}_{2}\)–IrO\({}_{2}\) & Ti/RuO\({}_{2}\)–IrO\({}_{2}\) & [64] \\ Fe2+; Pb2+; As3+ & Electro-reduction and ion & Graphite particle & Ti substrate & [65] \\ Hg2+ & exchange & & & \\ Cd2+ & Electro-reduction & PSAC & Graphite felt & [66] \\ Cu2+ & Microbial electrolysis cells & Stainless steel mesh & Graphite fiber brush & [67] \\ Cu2+ & Electrodeposition & Ti & & [68] \\ Pd2+; Zn2+ Cd2+; Cr6+ & & Ti & & [33] \\ Cu2+; Ni2+ Hg2+ & Electrocoagulation & Conductive carbon cloth & Pt-coated panel & [69] \\ Cu2+; Ni2+ Fe2+ & Electrolysis & Mesh Ti stainless steel plate & Mesh Pt-coated titanium & [70] \\ Cu2+; Zn2+ Pb2+ & Microbial electrolysis cell & Carbon cloth & Graphite brush & [71] \\ Ni2+ & Electrolysis & Graphite plate & Graphite felt & [72] \\ Cu2+ & Electrolysis & Granular graphite & & [73] \\ Pd2+; Ni2+ & Electrolysis & Cu sheet & Pt wires & [74] \\ Cu2+; Cr6+; Ni2+ & Electrodialysis & Pt & Pt & [75] \\ Cu2+ & Electrolysis & Stainless steel & Ti grid & [76] \\ \hline \hline \end{tabular}
\end{table} 
Table 1: Electrochemical methods for heavy metal removal. of CDI cells [78]. The CDI process may specifically remove heavy metal ions using an electric field without adding any chemicals, making the process an energy-efficient technology compared with distillation and reverse osmosis. In recent decades, CDI has grown substantially, considering electrode materials, cell configurations, and process analysis, while parasitic reactions and a relatively low desalination capacity hinder the practical application and further development of CDI [79]. Its side reactions mainly include anode oxidation reactions (such as carbon electrode oxidation and chloride oxidation), cathode reduction reactions (such as oxygen reduction and carbon electrode reduction), and co-ion expulsion (-i.e., ions with similar charge to the electrodes are expelled from the electrodes during the charging process in CDI). These reactions often occur during the operational process, increasing energy loss and leading to water quality fluctuations. The low desalination capacity is mainly due to the relatively low physical charge storage capacitance of the electrode materials and the adverse effects of side reactions.