Cr(VI) Adsorption and Reduction by Humic Acid Coated on Magnetite

Easily separable humic acid coated magnetite (HA-Fe3O4) nanoparticles are employed for effective adsorption and reduction of toxic Cr­(VI) to nontoxic Cr­(III). The adsorption and reduction of Cr­(VI) is effective under acidic, neutral, and basic pH conditions. The chromium adsorption nicely fits the Langmuir isotherm model, and the removal of Cr­(VI) from aqueous media by HA-Fe3O4 particles follows pseudo-second-order kinetics. Characterization of the Cr-loaded HA-Fe3O4 materials by X-ray absorption near edge structure spectroscopy (XANES) indicates Cr­(VI) was reduced to Cr­(III) while the valence state of the iron core is unchanged. Fe K-edge extended X-ray absorption fine structure spectroscopy (EXAFS) and X-ray diffraction measurements also indicate no detectable transformation of the Fe3O4 core occurs during Cr­(VI) adsorption and reduction. Thus, suggesting HA on the surface of HA-Fe3O4 is responsible for the reduction of Cr­(VI) to Cr­(III). The functional groups associated with HA act as ligands leading to the Cr­(III) complex via a coupled reduction–complexation mechanism. Cr K-edge EXAFS demonstrates the Cr­(III) in the Cr-loaded HA-Fe3O4 materials has six neighboring oxygen atoms likely in an octahedral geometry with average bond lengths of 1.98 Å. These results demonstrate that easily separable HA-Fe3O4 particles have promising potential for removal and detoxification of Cr­(VI) in aqueous media.