## 3 Results and Discussion
### The Adsorption Kinetics  
The effect of contact time was studied for the removal of As(V) and As(III) ions on the surface of Fe\({}_{3}\)O\({}_{4}\)/CS/PEI nanocomposite as shown in Figure 6a. The effect of time on the adsorption process was studied in the range of 5.0 to 60.0 min. According to Figure 6a, the maximum and rapid capturing of both arsenic ions was achieved in the first 5.0 min and when the time increased, there was no significant increase in the adsorption capacity of the nanosorbent toward the As(V) and As(III) ions. The adsorption equilibrium was achieved during the first 30.0 min. The large number of available and vacant adsorption sites over the surface of the nanocomposite adsorbent is responsible for this rapid kinetic action. After the equilibrium is reached, all adsorption sites were occupied by the pollutants' ions and the contact time had no effect on the adsorption process. The kinetic behavior of arsenic removal over Fe\({}_{3}\)O\({}_{4}\)/CS/PEI nanocomposite was studied using the pseudo 1 st order and the pseudo 2 nd order equations as shown in Figure 6b,c, respectively. According to Figure 6b,c, the pseudo 2 nd order showed better fit to the adsorption kinetics for the uptake of both arsenic ions over Fe\({}_{3}\)O\({}_{4}\)/CS/PEI nanocomposite than the pseudo 1 st order and this was clear from the value of the regression coefficient (R\({}^{2}\)). R\({}^{2}\) value is 0.999 for As(V) and As(III) ions in the case of pseudo 2 nd order fit, while it is 0.630 and 0.603 for As(V) and As(III) ions, respectively, in case of pseudo 1 st order fit. The pseudo 1 st order and the pseudo 2 nd order kinetic parameters for the uptake of arsenic ions over the studied magnetic nanocomposite were introduced in Table 1.  
\begin{table}
\begin{tabular}{c c c c} \hline \hline
**Item** & **Coefficient** & \multicolumn{2}{c}{**Pollutant**} \\ \cline{3-4}  & & **As(III)** & **As(V)** \\ \hline \multirow{2}{*}{Pseudo 1 st order} & R\({}^{2}\) & 0.603 & 0.630 \\ \cline{2-4}  & k\({}_{1}\) & \(-7.90\times 10^{-4}\) & \(-7.70\times 10^{-4}\) \\ \cline{2-4}  & q\({}_{e}\) (mg/g) & 5.1 & 6.1 \\ \hline \multirow{2}{*}{Pseudo 2 nd order} & R\({}^{2}\) & 0.999 & 0.999 \\ \cline{2-4}  & K\({}_{2}\) & 0.43 & 0.35 \\ \cline{2-4}  & q\({}_{e}\) (mg/g) & 15.90 & 23.70 \\ \hline \multirow{2}{*}{[PERSON]’s isotherm} & R\({}^{2}\) & 0.938 & 0.943 \\ \cline{2-4}  & 1/n & 0.5563 & 0.5200 \\ \cline{2-4}  & K\({}_{F}\) & 2.46 & 4.54 \\ \hline \multirow{2}{*}{[PERSON]’s isotherm} & R\({}^{2}\) & 0.999 & 0.999 \\ \cline{2-4}  & R\({}_{L}\) & 0.5649 & 0.4249 \\ \cline{2-4}  & K\({}_{L}\) (L/mg) & 0.0158 & 0.0394 \\ \cline{2-4}  & q\({}_{\rm max}\) (mg/g) & 77.61 & 86.50 \\ \hline \end{tabular}
\end{table}
Table 1: Kinetic parameters for the uptake of arsenic ions over the studied magnetic nanocomposite Fe\({}_{3}\)O\({}_{4}\)/CS/PEI.  
Figure 6: The contact time effect (**a**), pseudo first order plot (**b**), and pseudo second order plot (**c**) for the adsorption of As (III) and As(V) ions over the surface of Fe\({}_{3}\)O\({}_{4}\)/CS/PEI nanocomposite.  
The agreement between the experimental data and the pseudo 2 nd order kinetic model indicated that the capturing of arsenic ions over this polymer-modified magnetic chitosan happened via the chemisorption mechanism. Additionally, the chemical interaction between the arsenic ions and the adsorbent surface was the rate-limiting step [57]. The agreement between experimented data and the pseudo 2 nd order for the treatment of aqueous arsenic ions was previously reported in the literature using another magnetic nanocomposite [58].