Formation of a heteronuclear hydrolysis complex in the ThIV–FeIII 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 L₃ 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 [Th₂Fe₂(μ²-OH)₈(H₂O)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.