## 2 Removal of Lead
### Removal of lead using functionalized GO/RGO/GO-aerogel
Compared to EDTA solution used, better adsorbent regeneration was achieved with 0.1 mol dm\({}^{-3}\) HCl solution, which demonstrated the effective ability of HCl to regenerate the spent adsorbent which could be further reused for other industrial applications, such as electroplating. Nitrogen-doped graphene aerogels (NGAs) were prepared via a facile one-pot hydrothermal route and used as the electrode material for the electrosorption of Pb(II) ions as depicted in Scheme 3 [180]. The insertion of heteroatoms (strong coordination ability with metal cations) onto carbon and graphene-based nanomaterials enhance the adsorption capacity because heteroatoms can coordinate strongly with heavy metal ions [181]. NGAs was prepared by hydrothermal treatments of GO and urea homogenous suspension for 12 h at 180 \({}^{\circ}\)C in Teflone lined autoclave. Resulted samples were immersed in pure water for 72 h to remove impurities by changing water at every 4 h and freeze-dried at -52 \({}^{\circ}\)C to obtain NGAs which was used for the electrosorption of Pb(II) ions as paper electrode. Appearance of new XPS (X-ray photoelectron spectroscopy) peaks at 400.0 eV and 285.8 eV indicated the incorporation of nitrogen and C-N bond formation after hydrothermal treatment respectively. Deconvolution spectrum fitted for N 1s into three XPS peaks at 398.1, 399.5, and 401.4 eV were found to be corresponded with pyridinic N, pyrrol N and graphitic N, respectively. The electrosorption experiment for Pb(II) was performed using a conventional three-electrode system consisting of NGAs paper, saturated calomel electrode (SCE), and platinum foil as working, reference, auxiliary electrodes, respectively. NGAs paper electrode showed highest electrosorption of Pb(II) ions in the shortest adsorption equilibrium time (within 5 s) at -0.3V (1 mM, pH 4.2) potential applied when compared with GAs, and RGO. The superior electrosorption behavior for Pb(II) could be attributed due to the synergistic effects of both NGAs (with a larger surface area -434.4 m\({}^{2}\) g\({}^{-1}\)) and incorporated heteroatoms which result electrosorption through the electrostatic attractions derived from an external electric field, electrostatic attraction caused by intrinsic charges on the NGAs and Pb(II), and strong coordination between the doped nitrogen atoms and Pb(II) ions. The highest metal ion removal ratio (%R) was found to decline continuously with an increased initial concentration of Pb(II) ions indicating NGAs as a promising electrode material for Pb(II) ions electrosorption at low concentration [182; 183]. The electrosorption data were fitted with Langmuir and Freundlich models. The correlation coefficient (R\({}^{2}\)) for the Freundlich model was closer to 1 as compared with Langmuir model (0.9778 vs. 0.8651), which suggests that the electrosorption of Pb(II) by the NGAs electrode could be modelled more accurately by the Freundlich model. The R\({}^{2}\) value obtained for pseudo first-(0.8465) and second-order kinetics (0.9984) showed that the electrosorption kinetics for Pb(II) onto the NGAs electrode was a better fit to the pseudo-second-order model. Recyclability of used electrode is another important characteristics of electrosorption, i.e., reversible electrosorption and electrodesorption for the same electrode material could be easily achieved using variable potential. NGAs exhibited a highest metal ions removal ratio even after 100 electrosorption/electrodesorption than the previous reports [184; 185; 186; 187]. Graphene oxide/ nickel foam (GO/NF) synthesized via vacuum impregnation method was used for Pb(II) ions electrosorption by [PERSON] et al. [188]. To prepare electrode, the appropriate size of NF sheet was washed with acetone followed by hydrochloric acid and deionized water which then immersed in GO solution for 2 h at 40 \({}^{\circ}\)C under vacuum. The composite was washed with deionized water and dried under vacuum. GO/NF electrode was installed in Pb(NO\({}_{3}\)) tank for Pb(II) ions electrosorption studies.