Effect of Fe(II) and Fe(III) doping levels on the structural and binding mechanism of graphene oxide chitosan:...

Iron (Fe) modification has been recognized as an attractive method to improve the performance of carbon-based materials, but it is still not clear about the effects of Fe modification and doping levels on the structure of carbon-based materials, and the binding mechanism at a molecular level. In this study, Fe(II) and Fe(III) with the molar ratio of 1:1, 2:1, 4:1, and 5:1 are used as modifiers, and graphene oxide and chitosan (GOCS) are used as the carbon-based matrix materials to prepare a series of Fe modified GOCS composites named Fe^{(II, III)}GCs-1/1, 2/1, 4/1, and 5/1, respectively. Batch experiments, characterization analyses, and density functional theory (DFT) calculations are jointly applied to reveal the influence of Fe doping into GOCS and the binding mechanism. Results show that the Fe(II)/Fe(III) ratio greatly affects the formed iron species on the surface of GOCS, further changing the microstructure and stability. When Fe(II)/Fe(III) is 1:1, the formed iron species include Fe₃O₄ and γ-Fe₂O₃. When the ratio exceeds 1:1, the iron species are mainly Fe₃O₄. Fe^{(II, III)}GCs-2/1 shows the maximum specific surface area (44.96 m²/g), which is 10.65 times that of GOCS (4.22 m²/g), while Fe^{(II, III)}GCs-4/1 shows the maximum saturation magnetization of 43.81 emu/g compared to other Fe-modified GOCS composites. Spectral analysis proves that iron doping into GOCS greatly affects the intensities and positions of -COOH, -OH, and -CONH-, and the electronic transitions of the Fe(III) peaks and O-C=O/O-C-O, etc. The DFT calculation results indicate that the electrons flow more easily from the HOMO of the CS molecule (donor) to the LUMO of the GO molecule (acceptor), but the doping Fe changes the electrical properties of GOCS and increases the binding energy loss of GO and CS. When Fe(II)/Fe(III) is lower than 2:1, Fe(III) mainly interacts with GO; on the contrary, Fe(III) mainly interacts with CS. The main binding mechanism includes the coordination interaction between the -COOH group of GO and the -NH₂ group of CS, forming covalent bonds; the electrostatic attraction between Fe ions and negatively charged ions (COO^-, etc.), forming ionic bonds; and the combination of Fe with coordination atoms (O atoms, etc.) in functional groups, forming coordination bonds. Batch experiments show that the removal efficiency values of As(III) and tetracycline of Fe^{(II, III)}GCs-4/1 are 10 times that of GOCS, with 67.54 % and 60.62 %, respectively, indicating that Fe doping also improves the application of carbon-based materials in environmental protection by removing pollutants to various degrees.