# Adsorption of Lead and Nickel on to Expanded Graphite Decorated with Manganese Oxide Nanoparticles
## 3 Results and Discussion
### Effect of Humic Acids (HA)  
Humic acids (HA) are the major components of dissolved organic matter in natural surface water. They can have excellent binding characteristics and electrostatic interactions with metal ions in solution, leading to the formation of metal-organic complexes, which might affect the adsorption of metal ions on the prepared adsorbent [51; 52; 53]. The effect of HA on the adsorption behaviors of metal ions onto the prepared MONPs-EG in single and binary systems are shown in Figures 5 and 7. It can be seen that the presence of HA in the systems reduced the adsorption rates of metals ions onto the MONPs-EG. This reduction is clearly observed for single metal systems, while it is indistinct in binary metal systems (Figure 5). However, with the supporting data shown in Table 4, the changes in the adsorption rates are noticeable in binary metal systems with the coexistence of HA. Indeed, wide variations in the pseudo-second order kinetic rate constants (k\({}_{2}\)) are observed in these systems, which are decreased from 24.37 to 16.73 and 4486.45 to 991.41 (g/mmol/h) for Pb\({}^{2+}\) and Ni\({}^{2+}\), respectively. Interestingly, despite the fact that the adsorption rate is hindered with the presence of HA, the adsorption capacity was promoted, especially in single systems (Figure 7). The maximum adsorption capacity of MONPs-EG for Pb\({}^{2+}\) in single metal systems was 0.278 mmol/g at 298 K; this value increased to 0.298 mmol/g in the presence of 10 mg/L HA (Table 3). A similar trend was observed for Ni\({}^{2+}\): the adsorption capacity increased from 0.113 to 0.174 mmol/g when HA coexists in the system. The presence of HA in  
Figure 6: Effect of temperature on adsorption behaviors of prepared MONPs-EG for: (**a**) Pb\({}^{2+}\); (**b**) Ni\({}^{2+}\) in single metal solutions. \begin{table}
\begin{tabular}{c c c c c c} \hline \hline
**Metal ion** & **Temperature (K)** & **InK\({}_{0}\)** & \(\Delta\)**G\({}^{0}\) (kJ/mol)** & \(\Delta\)**S\({}^{0}\) (kJ/mol/K)** & \(\Delta\)**H\({}^{0}\) (kJ/mol)** \\ \hline \multirow{3}{*}{Pb\({}^{2+}\)} & 298 & 14.42 & \(-\)35.16 & \multirow{3}{*}{\(-\)0.27} & \multirow{3}{*}{\(-\)115.65} \\  & 308 & 12.21 & \(-\)31.90 & & \\  & 323 & 10.47 & \(-\)27.82 & & \\ \hline \multirow{3}{*}{Ni\({}^{2+}\)} & 298 & 8.92 & \(-\)21.83 & \multirow{3}{*}{\(-\)0.08} & \multirow{3}{*}{\(-\)46.68} \\  & 308 & 8.04 & \(-\)20.81 & & \\ \cline{1-1}  & 323 & 7.32 & \(-\)19.54 & & \\ \hline \hline \end{tabular}
\end{table} 
Table 5: Thermodynamic parameters of adsorption of Pb\({}^{2+}\) and Ni\({}^{2+}\) onto the prepared MONPs-EG in single systems. binary metal systems can also promote the overall adsorption process, but increases in the adsorption capacities of MONPs-EG in these cases are insignificant, probably due to adsorption competition between metal ions, as discussed above. However, the results also verified the positive effect of HA coexistence in the systems on overall adsorption processes. The increase of metal ion adsorption capacities in the presence of HA might be due to the chelation of Pb\({}^{2+}\) and Ni\({}^{2+}\) with functional groups on HA, and adsorption of HA onto exposed carbon and manganese oxides structure on the MONPs-EG composite [53; 54; 55; 56]. The comparison of adsorption performances of Pb\({}^{2+}\) and Ni\({}^{2+}\) by the prepared MONPs-EG achieved in this study with other similar adsorbents under various experimental conditions are presented in Table 6. The MONPs-EG exhibited superior adsorption capacities, which are much higher than those of previous studies that used various adsorbent such as expanded graphite (EG) and its modifications (G-EG, P-EG) [57], manganese oxide-coated sand (MOCS) [21; 24], manganese oxide coated crushed brick (MOCB) [21], and manganese oxide-coated zeolite (MOCZ) [22; 23]. This finding suggested the applicability of the coating process and the prepared MONPs-EG can serve as a competent adsorbent for removing heavy metals from the aquatic environment.