## 2 Removal of Lead
### Removal of lead using GO-polymer composites
\begin{table}
\begin{tabular}{c c c c c c c c} \hline \multicolumn{6}{c}{**Langmuir Model**} & \multicolumn{6}{c}{**Freundlich Model**} \\ \hline Adsorbents & q\({}_{\text{m}}\)(mg/g) & K\({}_{\text{L}}\) (L/mg) & R\({}^{2}\) & RSD & K\({}_{\text{F}}\) ((mg\({}^{1-}\)n\({}^{1}\)n)/g) & n & R\({}^{2}\) & RSD \\ RGO & 500 & 0.00149 & 0.989 & 0.0196 & 13.14 & 1.84 & 0.960 & 0.013 \\ PAM/GO & 1000 & 0.00137 & 0.987 & 0.0173 & 7.42 & 1.88 & 0.950 & 0.011 \\ \hline \end{tabular}
\end{table} 
Table 2: Langmuir and Freundlich adsorption results for Pb(II) onto the surface of RGO and RGO/PAM composite [201]. Figure 4A represents the adsorption capacity (\(Q\)) and adsorptivity (\(q\)) effects with adsorption time for 1.6 g/L ternary LS-GO-PANI adsorbent at 200 mg/L Pb(II) ions initial concentration an initial pH of 5.0 at 30 \({}^{\circ}\)C. Adsorption capacity and adsorptivity increased rapidly with time and reached 90.70 and 72.50% after 2 h. After 4 h, the adsorption capacity reached 98.30%, which indicates the excellent adsorption capacity of LS-GO-PANI composite as compared with other adsorbents [172; 206]. After reaching to saturation, there was a gradual decrease in the adsorption rate, which was correlated with the adsorption and desorption process at equilibrium onto the surface of the adsorbent. Figure 4B represents the effects of pH variation on the adsorption capacity using 1.6 g/L ternary LS-GO-PANI adsorbent and 200 mg/L initial Pb(II) ions concentration. Figure 4B represents the increasing adsorption efficiency of LS-GO-PANI adsorbent with the increasing pH from 2 to 5. When the pH of the medium increased from 2 to 5, the adsorption efficiency of LS-GO-PANI composite ternary adsorbent increased because as the pH increases, the H\({}^{+}\) concentration decreases and other functional groups (carboxyl, epoxy, hydroxyl, and amino groups) get ionized, resulting in the higher adsorption for Pb(II) ions and reached to maximum 98.1 mg/g [207]. Hence, pH 5.0 was set for the further studies for Pb(II) ions adsorptions. As shown in Figure 4C adsorption capacity decreased with LS-GO-PANI adsorbent contents while adsorptivity increased. Decreased adsorption capacity was attributed with the decreased surface area exposed to Pb(II) ions when the content of LS-GO-PANI adsorbent increased. Since, increasing adsorbent dosages for Pb(II) ions adsorption found to be invalid, hence, the effect of initial Pb(II) ions concentration was studied at fixed adsorbent dosage. Figure 4D represents the effect of sorbate concentration on the adsorption efficiency of LS-GO-PANI adsorbent. Adsorption efficiency of LS-GO-PANI adsorbent increased with the increased sorbate Pb(II) ions concentration. While adsorptivity initially increased then decreased after 100 mg/g sorbate concentration because on the increasing amount of sorbate there would not be enough adsorbent content to adsorb Pb(II) ions from the solution as LS-GO-PANI adsorbent gets saturation before 100% adsorption capacity. For Pb(II) ions adsorption, Langmuir adsorption isotherm was fitted well with higher coefficients of 0.9916. Langmuir isotherms were correlated with the homogenous intermolecular forces between the sorbate and equally charged active sites of the adsorbent [172]. Higher adsorption capacity were attributed with the high  
Figure 4: Possible Pb(II) ion adsorption with LS-GO-PANI ternary composite adsorbent [72]. surface area and large number of active functionalities which provided numerous nucleation sites for PANI growth and enhanced the surface for Pb(II) ions adsorption.