## 2 Removal of Lead
### Removal of lead using GO-polymer composites
Adsorption kinetics for Pb(II) ions adsorption was evaluated using Lagergren pseudo-first order (Equation (1)) and second-order (Equation (2)) modes:  
\[\ln\Bigl{(}q_{\mathrm{e}}-q_{\mathrm{t}}\Bigr{)}=\ln q_{\mathrm{e}}-k_{1}t \tag{1}\]  
\[\frac{t}{q_{t}}=\frac{1}{\left(k_{2}q_{\mathrm{e}}\right)^{2}}+\frac{t}{q_{ \mathrm{e}}} \tag{2}\]  
where \(q_{\mathrm{e}}\) and \(q_{\mathrm{t}}\) are the amounts adsorbed in mg/g at equilibrium and after time t in min., respectively; k\({}_{1}\) and k\({}_{2}\) are the adsorption rate constants for pseudo first- and second-order reactions respectively. The experimental results did not showed a linear fit with the calculated values; however, the t/q, plots showed good agreement with the pseudo second-order kinetics for Pb(II) ions adsorption. The correlation coefficients (R\({}^{2}\)) for Pb(II) ion adsorption suggested that the pseudo second-order adsorption mechanism was dominant using RGO/PAM adsorbent. To better understand the adsorption mechanism, the Langmuir and Freundlich adsorption isotherms were studied and results are listed in Table 2. The R\({}^{2}\) values for Pb(II) ions adsorption indicated that adsorption mechanism is well fitted with Langmuir isotherms and the maximum adsorption capacity of Pb(II) ions with RGO and RGO/PAM were found to be 500 and 1000 mg/g, respectively. Separation factor calculated from Langmuir isotherms for Pb(II) ions adsorption onto the surface of RGO-PAM was in the range of 0.313-0.901 indicating that the adsorption of Pb(II) ions was favorable and the adsorption capacity was highest as per literature reports [164]. The superior adsorption capacity of the RGO-PAM composite was attributed with the good dispersion of GO in an aqueous medium due to the introduction of highly functionalized PAM polymer. The adsorption capacity of the RGO-PAM composite was 2-3 times higher than the adsorption capacities of RGO and GO. Thermodynamic studies indicated the negative enthalpy changes, which were responsible for the spontaneous and exothermic adsorption of Pb(II) ions at different temperatures. [PERSON] et al. prepared ternary lignocellulosic-GO-polyaniline (LS-GO-PANI) nanocomposites via an in situ oxidative polymerization of aniline in the presence of lignocellulosic and GO and examined the effect of time, pH of the medium, adsorbent, and adsorbate concentration in an aqueous medium [72]. Scheme 4 represents the possible surface adsorption mechanism for Pb(II) ions from aqueous solution. LS is water soluble and has a large number of functional groups that are supposed to provide good adsorption interactions in the combination of GO and PANI. An air-tight oxidation method was used for GO preparation using sealed stainless steel autoclave [202]. In brief, expandable graphite, KMnO\({}_{4}\) and sulfuric acid containing Teflon reactor was placed in stainless steel autoclave and heated at 110 \({}^{\circ}\)C for 2 h. Resulted slurry was diluted and treated with H\({}_{2}\)O\({}_{2}\) followed by repeatedly washing with dilute HCl and deionized water. LS-GO-PANI nanocomposite was prepared using suspension of GO, LS and aniline followed by the oxidative polymerization at 0 \({}^{\circ}\)C for 24 h using ammonium persulfate as an initiator. Due to enormous active edges and different surface functionalities, GO is compatible with polymer matrix and provide various nucleation sites for PANI chains anchoring. Moreover, \(\pi\)-\(\pi\) interactions also favor surface polymerization of PANI onto the surface of GO [203; 204; 205]. Due to higher surface area, resulted LS-GO-PANI ternary composite showed higher adsorption capacity (98.1 mg/g) than GO-PANI (85.70 mg/g) and virgin PANI (95.70) for Pb(II) ion adsorption.