## 2 Removal of Lead
### Removal of lead using functionalized GO/RGO/GO-aerogel  
An adduct of GO and EDTA, was prepared using silylation by reacting N-(trimethoxysilpropyl) ethylenediamine triacetic acid (EDTA-silane) with the -OH functionality of the GO layers [164]. [PERSON]'s and [PERSON]'s modified double stage oxidation process was used to produce GO. In the first prec oxidized step, natural graphite was treated with potassium persulfate and phosphorous pentaoxide in sulfuric acid followed by the oxidation with potassium permanganate, conc. sulfuric acid and hydrogen peroxide. GO was filtered and washed with 0.1 M HCl and deionized water. Resulting GO was then reacted through silylation process with EDTA-silane and was filtered followed by repeatedly washing with methanol and water. For comparison, a reduced GO adduct, EDTA-RGO black was prepared by direct thermal reduction treatment at 300 \({}^{\circ}\)C for 30 min. Adsorption isotherms analysis was performed in a plastic vial using 10 mg or 25 mg adsorbent to a 100 or 200 mL solution of Pb(II) ions at room temperature. It is evident that the adsorption of positively-charged metal ions depend on the charge present on the surface of the adsorbents as functionalities [165; 166]. Zeta potential of the adsorbents at different pH was determined using Nano-ZS, Zeta Sizer and correlated with the adsorption of Pb(II) ions. At a particular pH, more negative zeta potential indicated the highest carboxylic and hydroxyl functionalities on the surface of EDTA-GO and GO adsorbents than activated carbon and EDTA-RGO was found to be consistent with [PERSON]'s titration results [167; 168; 169; 170]. The enhanced adsorption for Pb(II) ions from the aqueous solution was observed with an EDTA-GO adduct (Figure 1) when compared with GO and EDTA-RGO adduct. The maximum adsorption capacities of the EDTA-GO adduct and GO for Pb(II) were found to be 479\(\pm\)46 and 328\(\pm\)39 mg/g, respectively, which were greater than those of oxidized carbon nanotubes and activated carbon-based adsorbents [164]. The superior performance of EDTA-GO was correlated with the chelating characteristics of EDTA with functionalized graphene sheets. Mainly two adsorption process were considered to be responsible for the improved efficiency of removal of Pb(II) ions with the EDTA-GO adduct, i.e., the ion-exchange reaction process (Scheme 1) between Pb(II) and carboxylic and hydroxyl functional groups responsible for chelate complex formation between Pb(II) and EDTA onto the GO surface, as shown below. (First adsorption type):  
Interactions of Pb(II) ions onto the surface of GO to form the complex:  
**Scheme 1. ** Adsorption of (Pb)II ions based on ion-exchange process [164]. (Second adsorption type may include):  
Complex formation of Pb(II) with -COOH:  
**Scheme 2. ** Adsorption of Pb(II) ions involving complex forming of EDTA with Pb(II) [164]. The second adsorption mechanism involves the stable complex formation of EDTA with Pb(II) ions for the complete removal of Pb(II) ions from aqueous solution (Scheme 2). After adsorption, Pb(II) equilibrium concentration reached lower than food and drug administration drinking water concentration i.e \(<\) 10 ppm. Higher adsorption capacity was attributed with the higher stability constant (Log K\(\approx\)18.0) of Pb(II)-EDTA complex. [PERSON] and Freundlich adsorption isotherms exhibited  
Figure 1: Heavy metal ions adsorption on EDTA-GO adduct surface [164]. good agreement with the experimental data having a correlation coefficient (R\({}^{2}\)) 0.975 and 0.933 at pH 6.8 respectively. The adsorption phenomenon rate for Pb(II) ions onto the surface of EDTA-GO was found to be pH dependent. It was noted that as pH of the aqueous medium increased from 5.0 to 8.0, adsorption capacity of EDTA-GO also increased due to chelate formation of Pb(II) with the hydroxyl, carboxylic functional groups of GO and EDTA. Low adsorption at high acidic medium was attributed due to neutralization of COO\({}^{-}\) and O\({}^{-}\) surface charges and relative competition between proton and Pb(II) ions.