## 2 Removal of Lead
### Removal of lead using GO-polymer composites  
To prevent the aggregation of GO nanosheets, poly(3-aminopropyltriethoxysilane) (PAS) oligomers were cross-linked three dimensionally with GO nanosheets to obtain a high performance adsorbent, PAS-GO [193]. Three-dimensional crosslinking network was meant to prevent the aggregation of GO nanosheets, to provide easy accessibility for foreign molecules and to introduce a large amount of amino functional groups for better adsorption of Pb(II) ions. Two new absorption peaks in FT-IR spectrum of PAS-GO at 3430 cm\({}^{-1}\) for -NH\({}_{2}\) stretching and at 2927 cm\({}^{-1}\) for -CH\({}_{2}\) stretching vibrations indicated oligomers crosslinking. Additionally, a distinct Raman peak at 1120 cm\({}^{-1}\) supported the presence of Si-O-Si antisymmetric stretching present in composite material. Pb(II) ions adsorption studies were carried out at different temperature, pH and concentration of adsorbate and the filtrate was used for adsorption kinetics. Figure 3 represents the adsorption performance of PAS-GO, AS-GO, and GO for Pb(II) ions adsorption in aqueous solution. The Langmuir adsorption isotherms represent the maximum adsorption capacities for PAS-GO as compared to AS-GO and pure GO samples. The highest adsorption capacity of PAS-GO was 312.5 mg/g, which was attributed due to the following factors:  
1. Multi-armed, three dimensional cross-linked functionality prevented GO nanosheets from aggregation, which may provide easier accessibility for trapping of foreign molecules. 2. PAS oligomers chains, providing higher functionality/binding sites in PAS-GO adsorbent, were supposed to be responsible for the higher adsorption of metal ions. pH of the medium not only influences the adsorption capacities of the adsorbent but also facilitate selective metal ions adsorption [194; 195; 196]. Hence, Pb(II) ions adsorption at different pH from 2 to 6 were studied. As it is already discussed that in acidic media there is competition between H\({}^{+}\) and Pb(II) ions adsorption, hence, lower adsorption capacity for Pb(II) ions was observed at pH from 2 to 3, while, as the pH of the medium increases to 4, adsorption capacity of PAS-GO reached to 100% because at higher pH, the effect of protonation becomes weak and negatively-charged hydroxyl and carboxylic acid functional groups leads higher Pb(II) ions adsorption. PAS-GO may be the best adsorbent for the removal of Pb(II) ions from acidic media (pH 4.0-7.0) because these ions get precipitated in basic media [189; 190]. Langmuir, Freundlich, and Dubinin-Redushkevich adsorption isotherms were used to explain the adsorption mechanism of Pb(II) ions onto the surface of PAS-GO adsorbent. Langmuir and Freundlich isotherms are applicable for homogenous and heterogeneous surface adsorption respectively. Calculated results from adsorption isotherms revealed that the adsorption fitted well to Langmuir isotherm with a correlation coefficient (R\({}^{2}\)) values of 0.9996, 0.9989 and 0.9874 at 293 K, 303 K and 313 K, respectively. Highest correlation coefficient and better fitted Langmuir isotherms confirmed the homogenous adsorption with all the adsorption sites of equal adsorption affinity. Maximum adsorption capacity increased from 200.00 mg/g to 344.80 mg/g as the temperature increased from 293 K to 313 K indicating endothermic adsorption process. Figure 3: PAS-GO, AS-GO, and GO Langmuir isotherms for Pb(II) adsorption in aqueous solution at 303 K [193]. [PERSON] et al. reported the one step synthesis of polyacrylamide-grafted graphene (PAM-GO) composites via in situ radical polymerization by 300 kGy doses \(\gamma\)-ray irradiation of the acrylamide monomer in an aqueous solution at room temperature, leading to GO single layer exfoliation into the PAM polymer [197]. The resulted PAM-GO adsorbent showed a 24.2% degree of grafting of PAM onto the graphene surface with a thickness of 2.59 nm that was a more powerful adsorbent for Pb(II) ions adsorption.