## 2 Removal of Lead
### Removal of Lead using GO-Magnetite's
The initial higher sorption within 60 min. was attributed to high surface reactivity of MCGO due to sufficient number of exposed active sites followed by the large surface area, which decreased later on due to a decrease in the number of bonding sites on the surface of the adsorbent and aggregation between particulates. Langmuir and Freundlich isotherms are the fundamental and most frequently used adsorption isotherms to describe the adsorption phenomenon and behavior of metal ions onto the surfaces. The maximum adsorption capacity of MCGO determined from the Langmuir isotherm was found to be 82.10 mg/g, which was almost similar with the calculated value of 76.94 mg/g for monolayer chelation adsorption. Higher adsorption capacity for Pb(II) ions when compared with other adsorbents were attributed with the additional interactions of axial hydroxyl and amino functional groups with metal ions. For the possibility of regeneration and reusability of MCGO, experimentally desorption efficiency was found to be 90.3% and 90.1% at pH 1 and 2, respectively, indicating that adsorbed metal ions could be removed efficiently by desorption from the adsorbent surface. Adsorption percentage almost remained steady in the first four regeneration cycles and then decreased to 75% after fifth regeneration cycles. Hence, regeneration and reusability indicated that biodegradable and biocompatible MCGO has long term stability for the adsorption of Pb(II) ions. A composite of hydrated manganese oxide and GO (HMO-GO) with excellent settling ability (<2 min) was fabricated by _in-situ_ growth of nanosized hydrated manganese oxide (HMO) onto the GO nanosheet surface [224]. Typically, GO was suspended into MnCl\({}_{2}\) solution at pH 7 for 24 h at room temperature followed by centrifugation to obtained Mn(II)-GO which was further added to NaClO-NaOH solution at the alkalinity of 5% and active chlorine of 6%. The mixed solution was stirred for 24 h to oxidize Mn(II) to Mn(IV). Finally, the solution was centrifuged and solid was repeatedly washed with HCl and deionized water until neutral pH and freeze dried under vacuum. HMO grafted GO i.e., HMO-GO adsorbent substructure revealed that electron dense single grain HMO nanoparticles were uniformly distributed onto the GO surface. To increase the oxygenated abundance onto the GO surface, polymer or surfactants as stabilizers were dispersed using similar approaches [175; 225]. Adsorption behavior of HMO-GO composite and GO as an adsorbent were compared at pH from 2.5 to 6.7. The adsorption capacity of HMO-GO and GO increased with increasing pH of the medium because at higher pH, deprotonation increases, which results more negative active sites to adsorb positively-charged Pd(II) ions. A sharp decrease in adsorption capacity in strong acidic media indicated the regeneration possibilities of HMO-GO adsorbent for reuse. The leaching of Mn from HMO-GO was negligible during the adsorption experiment over the entire pH range. Selective adsorption of Pb(II) ions in presence of Ca(II) and Mg(II) co-existing ubiquitous versatile alkaline earth metal cations onto the surface of HMO-GO was compared with that of GO. Presence of Ca(II) and Mg(II) cations (concentration of Ca(II), Mg(II)/Pb(II) = 50) do not affected the selective adsorption of Pb(II) ions by HMO-GO adsorbent whereas a significant reduction in the adsorption capacity of GO was observed. Selective Pb(II) ions adsorption by HMO-GO was attributed with the specific, complex adsorption process by nanostructured homogenous distribution HMO onto the surface of GO [226]. Reduced desorption of Pb(II) ions onto GO was attributed with the nonspecific, electrostatic attraction of oxygenated functional groups and greatly affected due to the presence of Ca(II) and Mg(II) cations. Due to lower \(\Delta G_{\text{hydration}}^{0}\) (Ca(II) = \(-\)1656 kJ/mol, than Mg(II) = \(-\)2049 kJ/mol) and greater ion exchange ability, Ca(II) had greater influenced on the adsorption of Pb(II) ions onto GO surface [227].