title: A XAFS study of plain and composite iron(III) and chromium(III) hydroxides.
abstract: Reduction of hexavalent Cr(VI) to the trivalent state is the common strategy for remediation of Cr(VI) contaminated waters and soils. In the presence of Fe the resulting compounds are usually mixed Fe(III)-Cr(III) phases, while, under iron-free conditions, reduction leads to formation of plain Cr(III) hydroxides. Environmental stability of these compounds depends on their structure and is important to understand how different precipitation conditions affect the local atomic order of resulting compounds and thus their long term stability. In current study, typical Cr(VI) environmental remediation products, i.e. plain and mixed Fe(III)-Cr(III) hydroxides, were synthesized by hydrolysis and redox reactions and their structure was studied by X ray diffraction and X ray absorption fine structure techniques. Plain Cr(III) hydroxide was found to correspond to the molecular formula Cr(OH)3·3H2O and was identified as crystalline in XRD. However, the same compound when examined by EXAFS did not exhibit any clear local order in the range of EXAFS detectable distances, i.e. between 0 and 5Å. Namely, EXAFS spectroscopy detected only contribution from the first nearest neighboring (Cr-O) shell, suggesting that CrO6 octahedra interconnection is loose, in accordance with the suggested anti-bayerite structure of this compound. Mixed Fe(III)-Cr(III) systems resembled 2-line ferrihydrite irrespective of the synthesis route. Analysis of Fe-K-EXAFS and Cr-K-EXAFS spectra indicated that FeO6 octahedra are bonded by sharing both edges and corners, while CrO6 octahedra seem to prefer edge sharing linkage. EXAFS data also suggest that Fe-Cr hydroxide produced by hydrolysis presents a better arrangement of CrO6 octahedra compared to the redox product.
authors.affiliation: School of Mining and Metallurgical Engineering, National Technical University of Athens, Zografou Campus, Athens 15780, Greece. Electronic address: papasiop@metal.ntua.gr.
authors.forename: N
authors.initials: N
authors.lastname: Papassiopi
authors.affiliation: School of Physics, Department of Solid State Physics, Aristotle University of Thessaloniki, Thessaloniki 54124, Greece.
authors.forename: F
authors.initials: F
authors.lastname: Pinakidou
authors.affiliation: School of Physics, Department of Solid State Physics, Aristotle University of Thessaloniki, Thessaloniki 54124, Greece.
authors.forename: M
authors.initials: M
authors.lastname: Katsikini
authors.affiliation: School of Mining and Metallurgical Engineering, National Technical University of Athens, Zografou Campus, Athens 15780, Greece.
authors.forename: G S E
authors.initials: GS
authors.lastname: Antipas
authors.affiliation: School of Mining and Metallurgical Engineering, National Technical University of Athens, Zografou Campus, Athens 15780, Greece.
authors.forename: C
authors.initials: C
authors.lastname: Christou
authors.affiliation: School of Mining and Metallurgical Engineering, National Technical University of Athens, Zografou Campus, Athens 15780, Greece.
authors.forename: A
authors.initials: A
authors.lastname: Xenidis
authors.affiliation: School of Physics, Department of Solid State Physics, Aristotle University of Thessaloniki, Thessaloniki 54124, Greece.
authors.forename: E C
authors.initials: EC
authors.lastname: Paloura
mesh_terms: D002857:Chromium / Q000737:chemistry*; D052918:Environmental Restoration and Remediation; D005290:Ferric Compounds / Q000737:chemistry*; D006878:Hydroxides / Q000737:chemistry*; D010084:Oxidation-Reduction; D056928:X-Ray Absorption Spectroscopy; D014961:X-Ray Diffraction
chemical_list: D005290:Ferric Compounds; D006878:Hydroxides; D002857:Chromium; C092844:ferric oxyhydroxide; C031356:hydroxide ion
keywords: Cr(III) hydroxide; Cr(VI) remediation; EXAFS; Fe(III)–Cr(III) mixed hydroxides; XANES