## 2 Removal of Lead
### Removal of lead using functionalized GO/RGO/GO-aerogel
Owing to the subsequent precipitation of Pb(II) ions as [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)\({}_{4}\)]\({}^{4+}\) in basic medium, the pH of the medium was set to 4.5 for all other experimental studies to avoid precipitation and to confirmed the removal of Pb(II) ions through adsorption [173]. Higher adsorption efficiency was obtained with higher \"\(x\)\" values of LDH due to the increasing high surface area and pore volume with increasing \"\(x\)\" factor. The pore sizes of 10.97-21.23 nm of the resulting interconnected 3D network allows the easy flow of hydrated Pb(II) ions, which have -0.4 nm radius. In addition, the availability of more active sites due to the high surface area accounted for the higher adsorption efficiency of the 3D interconnected nano-porous adsorbent. As the adsorption efficiency depends not only on the surface area but also on the interactive strength of the surface functional groups, hence zeta potential was used to explain electrostatic interactions between the adsorbent and adsorbate. Zeta potential decreased as the pH of the medium increased suggesting that there were higher availability of negative charges which were responsible for the strong electrostatic interactions between positively-charged metal ions and negatively-charged adsorbent's surface. Effect of temperature is an important factor of study on which the adsorption phenomenon depends because the adsorption rate depends on the solution viscosity, electrostatic interactions between the metal ions and the adsorbent surface, diffusion of the adsorbents and structural morphology of the adsorbent at a particular temperature. These factors are responsible for maximizing Pb(II) ions uptake due to the increased mobility of Pb(II) ions towards active sites of the adsorbents [166; 176; 177]. It was noted that as the temperature increased, the adsorption capacities of GO and pRGO increased which were ascribed with the decreased viscosity of the medium, increased mobility of Pb(II) ions, and increased pore volume and structural porosity at a higher temperature indicating endothermic nature of adsorption whereas temperature rise sometime may lead to decreased in adsorptive forces affecting adsorption rate [166]. Langmuir adsorption isotherms assumed uniform monolayer surface adsorption. Pb(II) ions adsorption was found to be increased with temperature. Increasing adsorptive strength of adsorbents indicated increasing strong interactions between adsorbents reactive sites and Pb(II) ions with increasing adsorbate temperature [163; 166; 178]. Thermodynamic parameters viz. Gibbs free energy (\(\Delta\)G\({}^{\circ}\)), enthalpy (\(\Delta\)H\({}^{\circ}\)) and entropy (\(\Delta\Sigma^{\circ}\)) obtained indicated spontaneous adsorption process [179]. Negative values for \(\Delta\)G\({}^{\circ}\) demonstrated the spontaneity of adsorption process and adsorption was found to be increased as the temperature increased. Positive values for \(\Delta\)H\({}^{\circ}\) and \(\Delta\Sigma^{\circ}\) for 0.5 Mg-Al-LDH/pRGO denoted endothermic adsorption and increased in the disorderliness at solid-solution interface. It was interested to note that the physiosorption was predominated for low \"\(x\)\" factor i.e., 1.0 and 3.0 Mg-Al-LDH/pRGO nanocomposites whereas adsorption for GO, pRGO and 0.5 Mg-Al-LDH/pRGO was facilitated by both chemisorption and physiosorption process. Desorption experiments were carried out using 0.1 M HCl and 0.01M  
Figure 2: (**a**) Adsorption efficiency of LDH, GO, pRGO, and \(x\) Mg–Al LDH/pRGO adsorbents at pH 1 to 10. (**b**) Speciation of Pb(II) as a function of pH [174]. EDTA solutions to get insight into the recovery of metal ions on the spent adsorbents and the possible regeneration of the adsorbent for reuse [174]. Both insights may help to minimize the disposal of spent adsorbent, and reduce the production of secondary pollutants.