No abstract

The uptake of iodide and chloride during the synthesis of green rust (GR), the Fe endmember of the layered double hydroxide (LDH) group, was investigated. GR compounds were prepared by aerial oxidation of Fe­(OH)2 in suspension, considering various I/Cl ratios at constant ionic strength. Only GR compounds formed in all experiments, and the associated I/Cl ratio increased with that of the starting suspension. No preferential uptake of any halide could be detected, and all compounds had comparable morphology. Furthermore, the height of the interlayer gallery increased with the I/Cl ratio from ∼7.7 Å for the chloride endmember to ∼8.3 Å for the iodide endmember, and the observed linear increase was attributed to increasing interlayer iodide content. In all compounds, Fe K-edge X-ray absorption spectroscopy evidenced the presence of sixfold coordinated iron with a Fe2+/Fe3+ ratio of 3, homogeneously distributed within flattened octahedral sites, with six Fe as next-nearest neighbors. The Fe short-range environment was not affected by the interlayer composition, and no halide from the interlayer could be detected. Furthermore, iodide and chloride anions are located in a water-like environment, being loosely bound by weak electrostatic interactions to the octahedral sheet likely above ferric iron. Results consistently hint at the formation of a solid solution between chloride and iodide GR endmembers, certainly facilitated by the crystallization of both compounds in the same space group. This study provides further insights into the ability of LDH to simultaneously accommodate several anionic species of various sizes. The formation of such LDH compounds in a deep geological repository for nuclear waste thus represents a possible retention barrier to the migration to the far field of anionic species like 36Cl– and 129I– mobilized from the waste matrix. The extent of retention in disposal sites will depend, among others, on the availability of GR and on the concentration of competing anions.

In this work, we present first principle density functional theory calculations on hydroxycarbonate and hydroxychloride green rust. Green rust is a layered mineral, with brucitelike layers of Fe(OH) 2 . It is an important corrosion product of iron present in the near field of a nuclear waste disposal site. Substitution of a part of the Fe 2+ by Fe 3+ creates a layer charge, which is compensated by interlayer anions (e.g., carbonate or chloride). The simultaneous presence of Fe 2+ /Fe 3+ in the brucite layer of green rust is a considerable theoretical challenge due to the open shell ground states of Fe 2+ /Fe 3+ . We fully characterized the lattice parameters and the internal coordinates of pure hydroxycarbonate and hydroxychloride green rust and reproduced the available experimental structural data to a very high accuracy. Based on these results, we investigated the incorporation of trivalent lanthanides and actinides into the brucite layer of green rust by replacing Fe 3+ and obtained internuclear distances in agreement with available experimental results. We show that the incorporation in all investigated green rust variants is structurally possible. The Am 3+ −O distances are in good agreement with experimental data [Finck, N.; Nedel, S.; Dideriksen, K.; Schlegel, M. L. Trivalent Actinide Uptake by Iron (Hydr)oxides. Environ. Sci. Technol. 2016, 50, 10428], whereas the agreement of the calculated and measured Am 3+ −Fe distances is less satisfactory. We demonstrated that DFT+U is a very reliable theoretical method for the theoretical investigation of hydroxycarbonate and hydroxychloride green rust and the incorporation of trivalent lanthanides and actinides into these layered double hydroxides.

Green rust (GR), a layered double hydroxide (LDH) containing Fe, and magnetite can be found in natural and engineered environments. The ability of chloride GR (GR-Cl) and magnetite to retain iodide as a function of various parameters was investigated. Sorption equilibrium is achieved within 1 day of contact time between iodide and preformed GR-Cl in suspension. pH m variations (7.5−8.5) have no significant influence, but the iodide sorption decreases with increasing ionic strength set by NaCl. Sorption isotherms of iodide suggest that the uptake operates via ionic exchange (IC), which is supported by geochemical modeling. The shortrange binding environment of iodide associated with GR is comparable to that of hydrated aqueous iodide ions in solution and is not affected by pH m or ionic strength. This finding hints at an electrostatic interaction with the Fe octahedral sheet, consistent with weak binding of charge balancing anions within an LDH interlayer. The presence of sulfate anions in significant amounts inhibits the iodide uptake due to recrystallization to a different crystal structure. Finally, the transformation of iodide-bearing GR-Cl into magnetite and ferrous hydroxide resulted in a quantitative release of iodide into the aqueous phase, suggesting that neither transformation product has an affinity for this anionic species.