Analysis of technetium immobilization and its molecular retention mechanisms by Fe(II)-Al(III)-Cl layered double hydroxide

• Published in: Chemical engineering journal (Lausanne, Switzerland : 1996) Vol. 408; p. 127265
• Main Authors: Mayordomo, Natalia, Rodríguez, Diana M., Rossberg, André, Foerstendorf, Harald, Heim, Karsten, Brendler, Vinzenz, Müller, Katharina
• Format: Journal Article
• Language: English
• Published: Elsevier B.V 15.03.2021
• Subjects: ATR FT-IR; Hematite; LDH; Reduction; Retardation; XAS; Hematite; ITFA; Retardation; MCTFA; XANES; KARA; MS; XAS; EXAFS; TFA; TGA; LDH; Reduction; DSC; ATR FT-IR
• ISSN: 1385-8947; 1873-3212
• DOI: 10.1016/j.cej.2020.127265

[Display omitted] •Fe(II)-Al(III)-Cl LDH removes > 90% of aqueous Tc at pH > 6.•Tc retention is primarily due to Tc(VII) reduction to Tc(IV).•Tc(IV) forms inner-sphere complexes and incorporates in secondary formed hematite. Layered double hydroxides (LDH) play a decisive role in regulating the mobility of contaminants in natural and engineered environments. In this work, the retention of an Fe(II)-Al(III)-Cl LDH towards pertechnetate (TcO4–), which is the most stable and highly mobile form of Tc under aerobic conditions, is investigated comprehensively as a function of pH, Tc concentration and ionic strength. For a technetium initial concentration of 5 µM, its retention yield is higher than 80% from pH 3.5 to pH 10.5, especially at NaCl concentration below 0.1 M. A combination of vibrational and X-ray absorption spectroscopy provides structural information on the retention mechanism on a molecular scale. X-ray absorption near edge spectroscopy (XANES) confirms that most of the Tc uptake is due to an initial Tc(VII) reduction to Tc(IV), and consecutive Tc(IV) interaction with the solid. The analysis of the extended X-ray absorption fine structure (EXAFS) reveals two different mechanisms of Tc(IV) interaction with hematite (sub-product of the LDH oxidation and confirmed by Raman microscopy). At low pH, sorption of Tc(IV) dimers via inner-sphere monodentate complexation on hematite dominates. In contrast, under alkaline conditions, Tc(IV) is incorporated into the structure of hematite. Additionally, in situ attenuated total reflection Fourier-transform infrared spectroscopy (ATR FT-IR) evidences a small contribution of the total uptake corresponding to Tc(VII) anion exchange. The derived molecular structures increase confidence in predictive modelling of Tc migration patterns in subsurface environments, e.g. in the vicinity of a radioactive waste repository and treatment sites or in polluted areas due to other anthropogenic Tc sources.