# Novel Materials for Heavy Metal Removal in Capacitive Deionization
## 3 Materials for Heavy Metal Removing Using CDI
### Pure Carbon Materials  
Generally, adsorption capacity is governed by the electrical properties of electrodes, especially specific capacitance, and surface areas is believed to be relevant to the removal efficiency of CDI cells [82]. Carbon has abundant allotropes and has been extensively studied in the past several decades for a variety of applications. Owing to its high surface area, excellent conductivity, and tunable structure, carbon materials are the most popular materials applied for CDI electrodes. Large surface areas and mesoporous carbon materials exhibit good characteristics and excellent performance regarding electrochemical activity, the adsorption/sorption of toxic gases, and the removal of HMs [83]. Carbon materials can reversibly adsorb and desorb significant amounts of HMs using double-layer mechanisms without affecting their mechanical and electrical properties. Specific surface areas and pore sizes were found to closely correlate with ion electroadsorption at the electrical double layer [84]. High-surface-area porous electrodes must be optimized for both high surface area (capacity) and pore size (kinetics), and electrosorption performance was found to be primarily determined by hydrated size, ionic charge, and initial molar concentration in solutions. Nanoporous carbons are used as a key component in the electrosorption process due to their good electrical conductivity, high surface area, and remarkable sorption capacity. Carbon aerogels, activated carbons, carbon nanofibers, carbon nanotubes, graphene, mesoporous carbon, and carbon-based composites are recognized as promising electrodes for CDI [78]. Since the first report on the electroadsorption of Cr(VI) using carbon aerogel electrodes in 1997, increasing attention has been paid to developing new carbon-based materials for heavy metal electroadsorption [85]. Activated carbon is a notable material used for electrosorption because of its strong mechanical stability; large pore volume; high surface area; electrochemical stability; and low manufacturing cost. Many investigations have revealed that activated carbon is effective in removing ions such as Na, Mg, Ca, Cl, etc., from water. For example, in one study, the removal efficiencies were 32%, 43%, and 52% for Cd\({}^{2+}\), Pb\({}^{2+}\), and Cr\({}^{3+}\), respectively, initially formed heavy metal-containing water with a concentration of 0.5 mM [86]. Electrosorption rates and ionic hydrated radii were not linear for heavy metal but in the order Cu\({}^{2+}\)\(>\) Pb\({}^{2+}\)\(>\) Cd\({}^{2+}\), attributed to the complex mechanisms occurring in electrosorption, such  
Figure 3: Summary of materials applied for CDI. as electrodeposition, and OH\({}^{+}\) complex formation during charging [86]. Cu\({}^{2+}\) and Pb\({}^{2+}\) showed notably low desorption efficiency during electrode regeneration, implying high fouling potential during CDI treatment because of the facial deposition of Cu (+0.34 V) and Pb (\(-\)0.13 V). [PERSON] et al. investigated the electrosorption and reduction behavior of Cu\({}^{2+}\) and Zn\({}^{2+}\) in a wide voltage window on activated carbon fiber electrodes [87]. It was shown that removal of Cu\({}^{2+}\) worked better than that of Zn\({}^{2+}\) at 0.4-0.6 V, while better removal efficiency for Zn\({}^{2+}\) occurred in the range 0.8-1.2 V in wastewater containing a single metal ion. During the membrane CDI process, Cu\({}^{2+}\) could be preferentially electrodeposited while Zn\({}^{2+}\) reduction was prevented at the voltage of 0.8 V [87]. As(III) and As(VI) removal action is known, and the removal capacity of activated carbon electrodes strongly depends on applied voltage and initial arsenic concentration. The sorption capacity for treating As(V) is superior to that for As(III) due to the negative charges of prevalent As(V) species. The removal of As(V) occurs mainly via electrosorption, whereas As(III) removal using the CDI system involves the oxidation of As(III) to As(V) before it is electrostatically adsorbed on the anode surface [88].