## 4 Conclusions  
The present study discussed the successful fabrication of magnetic chitosan modified with polyethyleneimine to produce core-shell structure nanocomposite defined as Fe\({}_{3}\)O\({}_{4}\)/CS/PEI. As described in the experimental section, chitosan and magnetite nanoparticles were cross-linked using Na\({}_{5}\)P\({}_{3}\)O\({}_{10}\) while the modification of the magnetic chitosan using PEI was achieved using epichlorohydrin. The successful combination between three parts of the nanocomposite (Fe\({}_{3}\)O\({}_{4}\), CS, and PEI) was confirmed by using different characterization techniques including XRD, FT-IR, TEM, and VSM. Additionally, the spinel structure of magnetite was not changed by the modification process as shown from XRD. The synthesized nanocomposite was investigated for the adsorption of arsenic ions (As(V) and As(III)) from aqueous solution. Different parameters such as pH, contact time, adsorbent dosage, and initial concentration were examined to define the optimum conditions of adsorption. The arsenic adsorption over the Fe\({}_{3}\)O\({}_{4}\)/CS/PEI magnetic nanocomposite \begin{table}
\begin{tabular}{c c c c c} \hline \hline \multirow{2}{*}{**Adsorbent**} & \multirow{2}{*}{**pH Value**} & \multicolumn{2}{c}{**Removal Capacity (mg/g)**} & \multirow{2}{*}{**Ref.**} \\ \cline{3-4}  & & **As(V)** & & \\ \hline Fe\({}_{3}\)O\({}_{4}\)/CS/PEI & 6.70 & 86.50 & 77.61 & This study \\ \hline Fe-Mn binary oxide-impregnated granular activated carbon & 4.0 & 16.0 & 18.0 & [63] \\ \hline Egg shell & 4.10 & 8.0 & 12.0 & [64] \\ \hline Tea waste & 7.0 & 5.0 & 7.30 & [64] \\ \hline MnO\({}_{2}\)-impregnated alginate beads & 6.50 & 6.50 & - & [65] \\ \hline PEI-coated bacterial biosorbent & 4.0 & 63.0 & - & [66] \\ \hline Magnetic pinecone biomass & 8.0 & - & 18.0 & [67] \\ \hline Turbinaria vulgaris sp. & 4.0 & 26.0 & - & [68] \\ \hline Iron oxide/nanoporous carbon & 8.0 & - & 6.69 & [69] \\ \hline MIL-53(Al)-GO nanocomposites & 9.0 & - & 65.0 & [70] \\ \hline Dust ferric hydroxide & 7.9 & 6.90 & 3.50 & [71] \\ \hline Micro-sized tetravalent manganese feroxyhyte & 8.0 & 15.4 & - & [72] \\ \hline Micro-sized granular ferric hydroxide & 8.0 & 22.4 & - & [72] \\ \hline \hline \end{tabular}
\end{table} 
Table 2: Comparison of the adsorption capacity of Fe\({}_{3}\)O\({}_{4}\)/CS/PEI nanocomposite toward arsenic ions with other materials reported in the literature. was found to be pH dependent and the optimum pH was detected to be 6.7. The arsenic removal over Fe\({}_{3}\)O\({}_{4}\)/CS/PEI nanocomposite according to the adsorption isotherm results occurred as a monolayer over energetically symmetrical adsorption sites. Moreover, the experimental data was better fitted with the pseudo 2 nd order kinetics indicating the chemisorption uptake of arsenic ions over the nanocomposite. The magnetic separation of the synthesized nanocomposite enhanced their reusability for arsenic removal up to five successive cycles by using sodium hydroxide as eluent solution. The results indicated the ability to reuse the nanocomposite for arsenic removal with high efficiency that reduces the overall cost of water purification. All results indicated that Fe\({}_{3}\)O\({}_{4}\)/CS/PEI nanocomposite can be considered as a promising adsorbent for the removal of As(V) and As(III) ions from aqueous solution, and must be examined in the near future for the removal of additional pollutants. Conceptualization, M.A.T. and F.B.R. ; methodology, A.A.; software, N.S.A. ; validation, N.S.A., K.M.K., and F.M.A. ; formal analysis, F.M.A. ; investigation, K.M.K. ; resources, A.A.; data curation, F.B.R. ; writing--original draft preparation, F.B.R. ; writing--review and editing, M.A.T. ; visualization, F.B.R. ; supervision, M.A.T. ; project administration, A.A.; funding acquisition, N.S.A. All authors have read and agreed to the published version of the manuscript. This research was funded by Deanship of Scientific Research at King Khalid University. Not applicable.