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.

Amino-functionalized silica magnetite nanoparticles for the simultaneous removal of pollutants from aqueous solution

“…Unfortunately, most magnetic sorbents that are based on MNPs have several deficiencies such as leaching under acidic conditions, being susceptible to autoxidation and toxicity. Therefore, protection of the surface of the bare MNPs can be performed to reduce these undesirable features [17]. For this purpose, MNPs are embedded in a polymeric, hybrid or inorganic matrix.…”

Chemical Processes Affecting the Mobility of Heavy Metals and Metalloids in Soil Environments

“…Very stable Cr(III)-organic complexes form when Cr(VI) is reduced by soil organic matter [28,30]. Effective sorption and reduction of Cr(VI) by humic acid coated on magnetite has been recently demonstrated by XANES [31]. This work demonstrated that humic acid (HA) was responsible of Cr(VI) reduction while the valence state of the iron in magnetite remained unchanged.…”

Effect of Organic Matter on Cr(VI) Removal from Groundwaters by Fe(II) Reductive Precipitation for Groundwater Treatment

“…Due to this, the removal of Cr(VI) was found to be effective at Fe(II) dosages lower than the aforementioned 1:3 molar ratio ([Cr(VI)]:[Fe(II)]) [21,22]. The kinetics of a combined reduction by the co-presence of Fe(II) and HAs are hereby different from the kinetics of experiments where only Fe(II) or only HAs were available [22,23]. It is claimed that this effect is dependent on the specific type of humic substance [22] and it is also limited by the lower solubility of Fe(III) at higher pH values, which would lead to a precipitation of ferric hydroxide(s), so that no further recycling of Fe can take place [24].…”

“…These results are not in good agreement with the results reported by other researchers. Jiang et al [23] showed that HAs by themselves are capable of reducing Cr(VI) to Cr(III). Prior to the tests with Fe(II), several batch tests were conducted to verify if the presence of HAs influences the oxidation state of chromium.…”