# Novel Materials for Heavy Metal Removal in Capacitive Deionization
## 4 Challenges and Opportunities
\begin{table}
\begin{tabular}{c c c c c} \hline \hline \multicolumn{1}{c}{**Metal**} & \multicolumn{1}{c}{**Working Electrode**} & \multicolumn{1}{c}{**Counter Electrode**} & \multicolumn{1}{c}{**Removal Rate/Adsorption**} & \multicolumn{1}{c}{**Ref**} \\ \hline Pb\({}^{2+}\) & 3D macroscopic graphene on graphite paper & \multicolumn{2}{c}{3D N-doped macroscopic graphene} & \multicolumn{1}{c}{99.6\%; 134.4 F/g} & [111] \\ Mn\({}^{2+}\) & Graphene-chitosan-Mn\({}_{3}\)O\({}_{4}\) & Pt foil & 12.7 mg/g; 190 F/g & [134] \\ Cu\({}^{2+}\) & Activated carbon & Activated carbon & 69\%; 24.57 mg/g; 164.3 F/g & [89] \\ Pb\({}^{2+}\), Hg\({}^{2+}\), Ag\({}^{+}\), Cu\({}^{2+}\) & Graphene/MWCNT hybrid & Graphene/MWCNT hybrid aerogels on Ni foam & 104.9 mg/g Pb; 93.3 mg/g Hg; 64 mg/g Ag; 33.8 mg/g Cu & [96] \\ Ni\({}^{2+}\), Mn\({}^{2+}\) & \(\alpha\)-MnO\({}_{2}\)/carbon fiber paper & Active carbon & 88.9\% 16.4 mg/g Ni; 98.5\% Mn; & [118] \\ Cu\({}^{2+}\) & Carbon aerogel & Carbon aerogel & 73.6\%; 29.7 mg/g; 156 F/g & [92] \\ Pb\({}^{2+}\) & N-doped graphene & Carbon cloth & 95.8\%; 2.43 mg/m\({}^{2}\); 195.2 F/g & [109] \\  & Chicken-feather-derived & Chicken-feather-derived & 81\%; 4.1 mg/g & [135] \\ Pb\({}^{2+}\) & carbon & carbon & & \\ Co\({}^{2+}\) & Activated carbon cloth & Activated carbon cloth & 36.54\%; 8.3 mg/g & [91] \\ Cu\({}^{2+}\) & Activated carbon fibers & & 108.8 mg/g Cu; 122.6 mg/g Zn & [87] \\ Pb\({}^{2+}\) & PTFE/CNTs & & 2.40 mg/g; 43 F/g & [113] \\ Pb\({}^{2+}\) & Polyaniline/attapulgite & & 66.5\%; 12.39 mg/g & [123] \\ Cu\({}^{2+}\) & MnO\({}_{\rm X}\) & Saturated & 66\%; 372.3 mg/g (electro sob) & [136] \\  & & calomeelectrode & 44.3 mg/g (iso sorb) & \\ \hline \hline \end{tabular}
\end{table} 
Table 2: _Cont._BET area of the electrode. The wettability of the electrode and the porous structure of the materials ensure a greater surface area is engaged in ion charge and discharge. Notably, the specific area is highly dependent on the number of small pores, while more small pores limit ion transportation. The final and ultimate limitation on pore size is the metal ion size. Moreover, large pores provide better transportation pathways but limit surface area, while small pores provide a large surface area and higher adsorption capacity but are constrained by ion transportation. The relationship between these two factors raises vital questions regarding the validity of the gas adsorption models usually used to determine porous electrode surface areas and pore size distributions as well as the effective sizes of ions in pores concerning their degree of solvation. For instance, it is important to discover to what extent and at which potentials the solvation shells of ions entering into pores are partially or completely removed. For example, [PERSON] et al. reported an order of Fe\({}^{3+}\) (0.62 mg FeCl\({}_{3}\)/g) \(>\) Ca\({}^{2+}\) (0.55 mg CaCl\({}_{2}\)/g) \(>\) Mg\({}^{2+}\) (0.52 mg MgCl\({}_{2}\)/g) \(>\) Na\({}^{+}\) (0.45 mg NaCl/g) for mono-component solutions, while the order became Na\({}^{+}\) \(>\) Mg\({}^{2+}\) \(>\) Ca\({}^{2+}\) \(>\) Fe\({}^{3+}\) in mmol/g [137]. Moreover, the removal of certain HMs by CDI was complicated, and an observed order of Cr\({}^{2+}\) \(>\) Pb\({}^{2+}\) \(>\) Cd\({}^{2+}\) in the percentage removal by [PERSON] et al. was observed under the effect of physical adsorption, which introduced uncertainties into the quantification of the sole electrosorption amount [133]. Thus, a good CDI electrode should not only exhibit good porosity and connectivity; the porosity of the electrode material should also allow for a reasonable combination with macropores, mesopores, and micropores to balance the adsorption surface area and metal ion transportation. Further, dead pores that are not accessible to the ions should be avoided. The good electrochemical stability of a material over wide pH and voltage ranges plays a key role in real applications. The oxidation of electrode materials under electrochemical conditions is inimical to their stability.