Chemical states in XPS and Raman analysis during removal of Cr(VI) from contaminated water by mixed maghemite-magnetite nanoparticles

► 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.