# Adsorption of Lead and Nickel on to Expanded Graphite Decorated with Manganese Oxide Nanoparticles
## 3 Results and Discussion
### Adsorption Isotherms  
Experimental adsorption isotherm data for Pb\({}^{2+}\) and Ni\({}^{2+}\) on the prepared MONPs-EG in single and binary metals systems, and their fitting with the selected isotherm models, are presented in Figure 4. The corresponding parameters of the models are summarized in Table 3. The experimental data were better fitted to the Langmuir model, which provided much higher correlation coefficient values compared to those given by the Freundlich model, suggesting that the Langmuir model can effectively described the experimental results in both single and binary metals adsorption systems including the presence of HA. As a consequence, the estimated maximum adsorption capacities (qmax) agree well with the experimental data (qmax, exp.) at the highest level of initial metal concentrations.  
The prepared MONPs-EG exhibited much higher adsorption capacity for Pb\({}^{2+}\) than for Ni\({}^{2+}\). The maximum adsorption capacities at 298 K were calculated at 0.278 and 0.113 mmol/g for Pb\({}^{2+}\) and Ni\({}^{2+}\), respectively. There is evidence that the effectiveness of an adsorption process is dependent on the size of the ions in the solution; higher ionic radius (or lower hydrated radius) of heavy metals generally led to higher maximum adsorption capacity [42; 43; 44; 45]. In this respect, because the ionic radius of Pb\({}^{2+}\) (119 pm) is larger than that of Ni\({}^{2+}\) (70 pm), higher adsorption capacity of MONPs-EG for Pb\({}^{2+}\) can be anticipated (Table 3). In addition, higher molecular weight metals (such as Pb) can be removed more easily than lower weight ones (such as Ni) [43].  
In binary metal systems, the adsorption capacity of MONPs-EG for metal ions was reduced in the presence of other metal species (Figure 4). The numbers of heavy metal ions in binary systems are high compared to the availability of adsorption sites. It was observed that the removal efficiency of metal ions depends on the initial concentration and decrease as the initial metal ion concentration increases (Figure 4b). The maximum adsorption capacity of MONPs-EG for Pb\({}^{2+}\) decreased slightly in the presence of the same level of Ni\({}^{2+}\). However, the MONPs-EG adsorption ability for Ni\({}^{2+}\) was seriously inhibited when it coexisted with Pb\({}^{2+}\) in the system. The maximum adsorption capacity for Ni\({}^{2+}\) in a single metal system was 0.113 mmol/g, while it dropped to 0.021 mmol/g in the binary metal system. The existence of Pb\({}^{2+}\) in the solution had a stronger impact on Ni\({}^{2+}\) adsorption than vice versa. This suggested that competitive adsorption occurred when two metal ions coexisted in the solution.  
\begin{table}
\begin{tabular}{c c c c} \hline \hline
**Material** & **BET Surface Area (m\({}^{2}\)/g)** & **Pore Volume (cm\({}^{3}\)/g) \({}^{1}\)** & **Mean Pore Size (nm)** \\ \hline EG & 98.55 & 0.16 & 6.20 \\ MONPs-EG & 87.60 & 0.13 & 13.91 \\ \hline \hline \end{tabular} \({}^{1}\) Pore volume was calculated at P/P\({}_{0}\) = 0.990.  
\end{table}
Table 2: Elemental compositions of EG and MONPs-EG.  
Figure 3: X-ray diffraction (XRD) patterns of EG, Mn\({}_{3}\)O\({}_{4}\) nanoparticles, and MONPs-EG composite.