PNNL

Abstract

The comparative review of the selected range of studies describing abiotic Tc(VII) reduction with iron oxides and oxyhydroxides, such as magnetite, hematite, goethite, and ferrihydrite, revealed co-existence of several processes upon reduction of pertechnetate anion (Tc(VII)O4-) that can be detected simultaneously: polymerization of Tc(IV) into chains of Tc(IV) oxide that might create clusters and precipitate on the surface of iron mineral as TcO2 mono or dimer complexes; surface sorption of Tc—Tc dimers; and incorporation of Tc(IV) into lattice structure of iron oxides, i.e. predominantly magnetite. The mechanism for mononuclear incorporation of Tc(IV) into magnetite is discussed, and it is demonstrated that Tc(IV) substitution of Fe(II) with consequent loss of Fe(II) for the charge mismatch is more evident, despite commonly accepted assumption of Tc(IV) for Fe(III) substitution due to similarity in atomic radii. Oxidation of Fe(II) and creation of cavity during Fe(II) loss is described by the natural process of maghematization (transfer of magnetite to its fully oxidized state, maghemite), and Tc(IV) incorporation during this process is more energetically favorable, as it was supported before through computational studies. Clarification of the Tc local environment in iron minerals via EXAFS data analysis does not produce a definitive answer, as the local environment in different iron oxides and hydroxides is very similar and may not be taken alone for the conclusive statements.