## 3 Results and Discussion
### The Characterization of Synthesized Nanomaterials
This indicated the higher size of Fe\({}_{3}\)O\({}_{4}\)/CS/PEI and Fe\({}_{3}\)O\({}_{4}\)/CS nanocomposites than that of pure Fe\({}_{3}\)O\({}_{4}\), similar to other results reported in the literature [49]. The morphology of synthesized nanomaterials was also studied by the TEM analysis as shown in Figure 2.  
Figure 2: TEM images of Fe\({}_{3}\)O\({}_{4}\) nanoparticles (**a**), Fe\({}_{3}\)O\({}_{4}\)/CS nanocomposite (**b**), Fe\({}_{3}\)O\({}_{4}\)/CS/PEI nanocomposite (**c**), and the particle size distribution of the nanocomposite Fe\({}_{3}\)O\({}_{4}\)/CS/PEI (**d**). According to the TEM image of Fe\({}_{3}\)O\({}_{4}\) NPs (Figure 2a), the particles are homogenous with spherical shape, and the nanoparticles are also aggregated. The TEM image of the Fe\({}_{3}\)O\({}_{4}\)/CS nanocomposite (Figure 2b) showed a higher particle size of the nanocomposite than that of magnetite, which is compatible with the results of XRD. Additionally, the TEM image of Fe\({}_{3}\)O\({}_{4}\)/CS/PEI (Figure 2c) showed an increase in the particles' size after the polymer was grafted on the magnetic chitosan. Figure 2b,c showed the core-shell structure of the nanocomposites of Fe\({}_{3}\)O\({}_{4}\)/CS and Fe\({}_{5}\)O\({}_{4}\)/CS/PEI, respectively, that can be seen at the boundary of magnetite spherical NPs. This core-shell structure was reported in many previously synthesized magnetic nanomaterials [52; 53; 54]. Figure 2d showed the particle size distribution of the nanocomposite Fe\({}_{3}\)O\({}_{4}\)/CS/PEI that equals to 12 nm, similar to the results of XRD. The most important characteristic of magnetic adsorbents are their magnetic separation from the experimental solution. Therefore, the magnetization curves of the synthesized Fe\({}_{3}\)O\({}_{4}\), Fe\({}_{3}\)O\({}_{4}\)/CS, and Fe\({}_{3}\)O\({}_{4}\)/CS/PEI are shown in Figure 3.  
According to Figure 3, all synthesized nanomaterials are ferromagnetic with saturation magnetization (M\({}_{\rm s}\)) of 71, 62, and 51 emu g\({}^{-1}\) for Fe\({}_{3}\)O\({}_{4}\), Fe\({}_{3}\)O\({}_{4}\)/CS, and Fe\({}_{3}\)O\({}_{4}\)/CS/PEI, respectively. The clear drop of saturation magnetization by the modification of magnetite with chitosan was attributed to the non-magnetic properties of chitosan. Similarly, the drop occurred when the PEI was added. This was attributed to the diamagnetic properties of the polymer. The drop of magnetic properties after the modification of magnetite NPs was reported in previous studies [55; 56]. This order of magnetism of the three synthesized nanomaterials was reflected on their time of separation with an external magnet that was found to equal 69 s, 93 s, and 119 s for Fe\({}_{3}\)O\({}_{4}\), Fe\({}_{3}\)O\({}_{4}\)/CS, and Fe\({}_{3}\)O\({}_{4}\)/CS/PEI, respectively. Figure 3 (inset) showed the magnetic separation of Fe\({}_{3}\)O\({}_{4}\)/CS/PEI nanocomposite with an external magnet. The synthesized nanomaterials have excellent magnetic properties sufficient for their application in water treatment with the benefit of the adsorbent magnetic separation. Moreover, Brunauer-Emmett-Teller surface area (S\({}_{\rm BET}\)) was analyzed for the three synthesized nanomaterials as shown in Figure 4. The S\({}_{\rm BET}\) values were found to equal 82.5, 83.1, and 74.6 m\({}^{2}\) g\({}^{-1}\) for Fe\({}_{3}\)O\({}_{4}\), Fe\({}_{3}\)O\({}_{4}\)/CS, and Fe\({}_{3}\)O\({}_{4}\)/CS/PEI, respectively. The results indicated the small difference in the S\({}_{\rm BET}\) value between Fe\({}_{3}\)O\({}_{4}\) and Fe\({}_{5}\)O\({}_{4}\)/CS while the value was decreased by the addition of branched PEI indicating the effect of PEI on the surface properties of magnetic chitosan. This means that the core-shell structure of Fe\({}_{3}\)O\({}_{4}\)/CS/PEI nanocomposite was formed by the adsorption of PEI on the surface of magnetic chitosan that will save additional capturing sites for arsenic ions in water.