Trivalent Actinide Uptake by Iron (Hydr)oxides

Abstract

The retention of Am(III) by coprecipitation with or adsorption onto preformed magnetite was investigated by X-ray diffraction (XRD), solution chemistry, and X-ray absorption spectroscopy (XAS). In the coprecipitation experiment, XAS data indicated the presence of seven O atoms at 2.44(1) Å, and can be explained by an Am incorporation at Fe structural sites at the magnetite surface. Next-nearest Fe were detected at distances suggesting that Am and Fe polyhedra share corners in geometries ranging from bent to close to linear Am-O-Fe bonds. After aging for two years, the coordination number and the distance to the first O shell significantly decreased, and atomic shells were detected at higher distances. These data suggest a structural reorganization and an increase in structural order around sorbed Am. Upon contact with preformed FeO, Am(III) forms surface complexes with cosorbed Fe at the surface of magnetite, a possible consequence of the high concentration of dissolved Fe. In a separate experiment, chloride green rust (GR) was synthesized in the presence of Am(III), and subsequently converted to Fe(OH)(s) intermixed with magnetite. XAS data indicated that the actinide is successively located first at octahedral brucite-like sites in the GR precursor, then in Fe(OH)(s), an environment markedly distinct from that of Am(III) in FeO. The findings indicate that the magnetite formation pathway dictates the magnitude of Am(III) incorporation within this solid.

Plutonium Retention Mechanisms by Magnetite under Anoxic Conditions: Entrapment versus Sorption

“…This experimental data is in clear contradiction to the result of a recent quantum mechanical approach, where structural incorporation into the Oh site of magnetite was predicted . Furthermore, it also does not confirm a postulated 6-fold coordination for Am in magnetite after 2 years of aging at pH ≈ 12, where shell fitting might have been biased by a mix of sorbed and occluded Am species similar to our results before separating their spectral contributions with ITFA. It is well in line, however, with Sm­(III) in Sm-doped magnetite being in 8-fold coordination .…”

“…To the best of our knowledge, there are only two recent studies trying to decipher the local structure of Sm- and Am-doped magnetite by Sm and Am L III -edge XAFS spectroscopy. , In both cases doping was successful, while the local structure was in disagreement with a simple Sm/Am-for-Fe substitution at either the octahedral or (even less likely) the tetrahedral site in magnetite. For Am, the short-range order probed by EXAFS showed a 7-fold coordination of Am and Am–Fe distances largely incompatible with the Fe Oh site in magnetite for a fresh sample at pH 5.7, while in a sample aged for two years at pH 12.5, Am was 6-fold coordinated more commensurate with the Fe Oh site.…”

Retention of Iodide and Chloride by Formation of a Green Rust Solid Solution GR-Cl1–xIx: A Multiscale Approach

“…These results compare well with GR-Cl prepared by direct precipitation from Fe salts. 6 No particle of other morphology could be detected on the micrographs, ruling out the presence of, for example, magnetite in substantial amounts.…”

Risk of colloidal and pseudo-colloidal transport of actinides in nitrate contaminated groundwater near a radioactive waste repository after bioremediation

“…In accordance with previous thermodynamical modelling experiments after microbial processes, ferrous iron in hydroxide, sulphide, and carbonate forms were formed, and precipitation of goethite, pyrrhotite, siderite, troilite, and ferrihydrite mineral phases occurred 56 – 59 . These new sorption phases could cause additional actinide removal from solutions 60 – 64 . Thus, one of the reasons for a significant increase in the size of actinide-containing particles in model samples with the addition of iron and organic matter, as well as in samples of natural water, may be the formation of ferruginous minerals.…”