Abstract
In this study, we synthesized core-shell γ-Fe2O3/MnOx magnetic nanoparticles with core diameter of ~31 ± 3 nm and saturation magnetization ranging from 55 to 70 emu/g, using an inexpensive, facile and environmentally friendly approach. The synthesis is done through acid catalyzed reduction of KMnO4 and carboxymethyl cellulose (CMC) capped iron oxide in water. The synthesized particles were characterized by X-ray powder diffraction, transmission electron microscopy, X-ray photoelectron spectroscopy, vibrating sample magnetometer, Fourier transform infrared spectroscopy, thermo gravimetric analysis and differential scanning calorimeter. The continuous oxidation of carboxymethyl cellulose on Fe3O4 in the presence of KMnO4 enabled the formation of MnOx shell. The thickness of MnOx shell is tuned by varying the amount of acid and the ratio of KMnO4: iron oxide precursor. The shell thickness increased from ~2.9 nm to 4.6 nm with increase in HCl concentration because of rapid formation of large number of nuclei and their deposition on iron oxide. Interestingly, MnOx shell was amorphous at lower [H+] but was crystalline at higher [H+]. The magnetic particles with amorphous MnOx nanoshell showed superior stability for a range of pH. A change in particle morphology is (flower to square shaped) due to the competitive adsorption of oxidized organic products of carboxymethyl cellulose polymer and MnOx nuclei at higher temperature, and the higher Gibbs free energy of the flower shaped particles. XPS results show that the iron oxide coated with CMC has 2:3 relative concentration of the Fe2+:Fe3+ ions, while oxidation state of Fe in MnOx capped one was +3. The percentage of hydroxyl group content decreased from 14 to 11% when the Fe3O4:KMnO4 ratio was changed from 1:0.2 to 1:0.5 due to the change in Mn3+:Mn4+ ratio. It is hypothesized that oil sorption occur through exchange of surface hydroxyl groups on MnOx shell. The oil removal efficiency of the prepared particle was >93% after six cycles.
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