## 2 Removal of Lead
### Removal of lead using functionalized GO/RGO/GO-aerogel
pH is an important factor which greatly affects the adsorption of Pb(II) ions hence, studied first. pH study range was set from 3.0 to 6.0, since, Pb(II) ions get precipitated as [Pb(OH)], [Pb(OH)\({}_{2}\)], [Pb(OH)\({}_{3}\)]-, [Pb\({}_{3}\)(OH)\({}_{4}\)]\({}^{2+}\) [Pb\({}_{2}\)(OH)]\({}^{2+}\) [Pb\({}_{6}\)(OH)\({}_{8}\)]\({}^{4+}\), and [Pb\({}_{4}\)(OH)\({}_{4}\)]\({}^{4+}\) above pH 6.0 [189] Electrosorption capacity of GO/NF was found to be increased with decreasing medium acidity. pH 5.0 was found the appropriate condition for electrosorption as the steady adsorption capacity was observed above 5.0 pH. Lower adsorption capacity of GO/NF at low pH was correlated with the competitive adsorption between H\({}^{+}\) and Pb(II) ions as reported in literature [189; 190]. Whereas, on increasing pH, H\({}^{+}\) ions competition weaken and Pb(II) ions adsorption increased. It was interesting to note that GO coating largely enhanced the adsorption capacity of GO/NF (193 mg/g at 100 mg/L as initial concentration) for Pb(II) ions without applying any voltages when compared with virgin NF owing that GO had large negatively-charged oxygenated surface which favored Pb(II) ions adsorption. Electrosorption capacity of GO/NF (663 mg/g, approx. three times) increased when electric current of 1.2 V was applied at the same initial Pb(II) ions concentration indicated that external electric field driven effectively the adsorption of Pb(II) ions adsorption. Langmuir adsorption isotherms of Pb(II) on GO/NF electrode were found to be better fitted with higher correlation coefficient (R\({}^{2}\) = 0.9795) and maximum adsorption capacity (3690.37 mg/g) than Freundlich. GO/NF showed extremely higher electrosorption capacity, indicating the great potential for Pb(II) ions adsorption from aqueous system in comparison to other adsorbents. Electrosorption mechanism for Pb(II) ions onto the surface of GO/NF surface was analyzed using XPS indicating new Pb 4f peak at 183.3 eV after successful adsorption. Noticeable decreased in relative C-OH bond area from 26.98% to 7.98% and increased bond area ratio of -C=O from 11.92% to 13.05%, clearly indicated, surface adsorption of Pb(II) ions, facilitated by hydroxyl and carboxylic functional groups. Mono or multilayers coating of GO not only increased surface area of GO/NF but also provided more space for double layer electrosorption/chemisorption of Pb(II) ions. Electron beam irradiation is another cost-effective and environment friendly method that can be applied to all material phases (i.e., solid, liquid, and gas,) at a low temperatures and can be scaled up easily [172]. Therefore, [PERSON] et al. synthesized GO with a controlled oxygen content using electron beam irradiation and used Pb(II) metal ions adsorption [191]. Before the preparation of GO, graphite was placed on stainless steel plate uniformly and irradiated with different intensity dosages of 6.4 and 19.2 kGy by changing irradiation period of time. GO was prepared using modified Hummer's method and dried under vacuum for 48 h at 40 \({}^{\circ}\)C. Irradiated GO was then examined under the effects of the contact time, pH of the medium, initial concentration of Pb(II) ions (75 mg/L), and ionic strength for Pb(II) ions sorption. After the sorption process, solution was filtered and Pb(II) ion concentrations remaining were examined by inductively coupled plasma with an optical emission spectrometer. The removal capacities of the adsorbent q\({}_{e}\) (mg/g) were calculated using the equation, q\({}_{e}\) = (C\({}_{i}\) - C\({}_{e}\))V/m, where C\({}_{i}\) and C\({}_{e}\) are the initial and equilibrium Pb(II) ion concentrations (mg/L), respectively; V is solution volume in mL; and m is amount of the adsorbent in mg. The amount of Pb(II) adsorbed onto the GO surface increased remarkably with increasing adsorption time and the equilibrium was reached within 20 min. Hence, a maximum of 2 h of contact time was found to be adequate to achieve sorption equilibrium.