title: 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.
authors.affiliation: Department of Chemistry and Biochemistry, Florida International University , Miami, Florida 33199, United States.
authors.forename: Wenjun
authors.initials: W
authors.lastname: Jiang
authors.forename: Quan
authors.initials: Q
authors.lastname: Cai
authors.forename: Wei
authors.initials: W
authors.lastname: Xu
authors.forename: Mingwei
authors.initials: M
authors.lastname: Yang
authors.forename: Yong
authors.initials: Y
authors.lastname: Cai
authors.forename: Dionysios D
authors.initials: DD
authors.lastname: Dionysiou
authors.forename: Kevin E
authors.initials: KE
authors.lastname: O'Shea
mesh_terms: D000327:Adsorption; D002857:Chromium / Q000737:chemistry* / Q000302:isolation & purification*; D003460:Crystallization; D052203:Ferrosoferric Oxide / Q000737:chemistry*; D006812:Humic Substances / Q000032:analysis*; D006863:Hydrogen-Ion Concentration; D007700:Kinetics; D053758:Nanoparticles / Q000737:chemistry; D010084:Oxidation-Reduction; D017550:Spectroscopy, Fourier Transform Infrared; D013696:Temperature; D014867:Water; D014874:Water Pollutants, Chemical / Q000302:isolation & purification; D056928:X-Ray Absorption Spectroscopy; D014961:X-Ray Diffraction
chemical_list: D006812:Humic Substances; D014874:Water Pollutants, Chemical; D014867:Water; D002857:Chromium; C074702:chromium hexavalent ion; D052203:Ferrosoferric Oxide