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

Abstract

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.