title: Formation of a heteronuclear hydrolysis complex in the Th(IV)-Fe(III) system.
abstract: The speciation in the mixed Th(IV)-Fe(III) system has been studied in aqueous solution in the pH range of 2.0-4.8. In the individual systems iron(III) and thorium(IV) hydrolyze easily and hydrolysis products precipitate at approximately pH ≥ 2.0 and 4.0, respectively, at the metal concentrations used in this study, 0.02-0.05 mol dm(-3). In the mixed Th(IV)-Fe(III) system precipitation of ferrihydrite takes place after months of storage at low pH values, 2.0 (six-line ferrihydrite) and 2.3 (two-line ferrihydrite), as identified by X-ray powder diffraction. In the pH range 2.9-4.5 no precipitation was observed after 24 months. Two thorium(IV)-iron(III) solutions with pH = 2.9, C(Th) = 0.02 and 0.05 mol dm(-3) and C(Fe) = 0.02 mol dm(-3), were studied by extended X-ray absorption fine structure, EXAFS, using the Fe K and Th L(3) edges, and a third solution with pH = 2.9 and C(Th) = C(Fe) = 0.40 mol dm(-3) by large angle X-ray scattering, LAXS, to determine the structure of the predominating species. A heteronuclear hydrolysis complex with the composition [Th(2)Fe(2)(μ(2)-OH)(8)(H(2)O)(12)](6+) is proposed to form in solution, with Th···Th, Th···Fe and Fe···Fe distances of 3.94(2) and 3.96(2), 3.41(3) and 3.43(2), 3.04(2) and 3.02(4) Å, as determined by EXAFS and LAXS, respectively.
authors.affiliation: Department of Chemistry, Swedish University of Agricultural Sciences, P.O. Box 7015, SE-756 51 Uppsala, Sweden.
authors.forename: Natallia
authors.initials: N
authors.lastname: Torapava
authors.forename: Artsiom
authors.initials: A
authors.lastname: Radkevich
authors.forename: Ingmar
authors.initials: I
authors.lastname: Persson
authors.forename: Dmitri
authors.initials: D
authors.lastname: Davydov
authors.forename: Lars
authors.initials: L
authors.lastname: Eriksson