## 2 Removal of Lead
### Removal of lead using functionalized GO/RGO/GO-aerogel
Lower zeta potential until pH 8.0 indicated that there were enough negative charge density to provide the strong electrostatic attraction between adsorbent's functional group and Pb(II) ions. However, as the pH of the medium increased from 8.0, zeta potential rise indicated that negative charge density decreased and Pb(II) ions hydrolyze to give Pb(OH)\({}^{+}\), Pb(OH)\({}_{2}\), [Pb(OH)\({}_{3}\)]\({}^{-}\), [Pb\({}_{3}\)(OH)\({}_{4}\)]\({}^{2+}\) [Pb\({}_{2}\)(OH)\({}_{2}\)]\({}^{2+}\) [Pb\({}_{6}\)(OH)\({}_{8}\)]\({}^{4+}\) and [Pb\({}_{4}\)(OH)\({}_{4}\)]\({}^{4+}\)[171, 172, 173]. As it is desirable to have appropriate both the adsorption and desorption capacities to be an economical and ideal adsorbent, desorption experiments at different pH were carried out using Pb(II) pretreated EDTA-GO to determine desorption capacity. Desorption rate of Pb(II) was found to be maximum in acidic medium as consistent with zeta potential indicated that at lower pH, neutralization of functional groups leads to the competition between proton and Pb(II) ions adsorption [174]. Atomic absorption spectroscopy results were found to be in good agreement indicated that Pb(II) ions could be repeatedly desorbed from EDTA-GO adsorbent even after 10 cycles. [PERSON] et al. prepared Mg-Al-layered double hydroxide and partially reduced GO (LDH/pRGO) nanocomposites by a two-step process, resulting a three-dimensional highly porous nanocomposite adsorbent [174]. GO nanosheets were obtained using the modified Hummer's method and in situ homogeneously LDH nanosheets were grown onto the surface of GO by a crystallization and facile supersaturation coprecipitation technique at pH 10 [175]. Typically, a mixed metal solution of Mg(NO)\({}_{3}\).6H\({}_{2}\)O and Al(NO\({}_{3}\))\({}_{3}\).9H\({}_{2}\)O and alkaline solution of Na\({}_{2}\)CO\({}_{3}\) and NaOH were added dropwise simultaneously to GO ultrasonicated solution at 10 pH. After the stipulated period of time, precipitate was filtered and washed with deionized water and dried at 100 \({}^{\circ}\)C for 24 h. It was supposed that Mg\({}^{2+}\) and Al\({}^{3+}\) ions form coordination complexes with the carboxylic, hydroxyl and carbonyl functional groups of GO upon simultaneous alkaline precipitating agents and weakened the C=O str. absorption peak in FT-IR (Fourier transform infrared spectroscopy) spectrum at 1729 cm\({}^{-1}\) indicating the partial reduction of GO in Mg-Al-LDH/pRGO composite and was in consistent with 0.65 nm shifting of (001) basal spacing X-ray diffraction peak. Disappearance of pRGO diffraction peaks from Mg-Al-LDH-pRGO composite exhibited the complete exfoliation of pRGO sheets. Further, GO and Mg-Al-LDH/pRGO were exploited under nitrogen adsorption-desorption studies which revealed that meso-porous structural morphology related to narrow slit-like pores for GO and interconnected pore network for Mg-Al-LDH/pRGO composite. BET (Brunauer-Emmett-Teller) surface area, pore volume and pore size indicated increasing trend as the metal ion concentration of LDH increases. An increase surface area with crosslinked network is supposed to offer more active sites for better metal ions adsorption from the aqueous solution. Figure 2 presents pH based adsorption efficiency for Pb(II) for 24 h contact time at 100 mg dm\({}^{-3}\) as an initial Pb(II) ion concentration. The highest adsorption efficiency was observed for the Mg-Al-LDH/pRGO composite adsorbent as compared to LDH, GO, and pRGO, which might be correlated with the synergistic effects of LDH and GO. The adsorption efficiency of the adsorbent was found to be lowest in acidic medium and increased with increasing pH of the medium. The lower adsorption in the acidic medium was attributed due to the competitive presence of H\({}^{+}\) ions. As the basicity of the medium increased, the concentration of H\({}^{+}\) ions decreased, resulting in higher Pb(II) ion adsorption.