Abstract
► Cr(VI) adsorption on mixed maghemite–magnetite is spontaneous and endothermic. ► Theoretical multiplet analysis shows that during Cr adsorption, the amount of maghemite increases (from 70 to 89%). ► Fe(II) was transformed into Fe(III) by the redox reaction. ► Cr(VI) species were reduced to Cr(III) species on the mixed maghemite–magnetite.
Mixed maghemite–magnetite has been used as adsorbent for Cr(VI) removal in this study. Results show that the adsorption capacity is enhanced with an increase in reaction temperature and decrease in free energy change. Thermodynamic study shows that Cr(VI) adsorption on the mixed maghemite and magnetite is endothermic in nature and is dependent on solution pH between 3 and 6. X-ray photoelectron spectroscopy (XPS) results demonstrate the theoretical multiplet peaks for iron and chromium adsorbed iron at the surface of the γ-Fe2O3 and Fe3O4 mixture. Theoretical multiplet analysis shows that during Cr adsorption, the amount of maghemite increases (from 70 to 89%). In magnetite spectra, the relative content of Fe(II) decreases from 8.2 to 3.6% indicating the reduction of magnetite in the mixture particles. In Raman spectroscopy studies, clear peaks of chromium on iron oxide were generated at 826cm−1, which could be attributed to chemical interactions between chromium compound and iron oxide. From the results of Raman and XPS studies, electrostatic attraction and oxidation–reduction between chromium and mixed maghemite–magnetite are postulated as mechanisms for the removal of Cr(VI) from aqueous solutions.