First Principle Investigation of the Incorporation of Trivalent Lanthanides and Actinides in Hydroxycarbonate and Hydroxychloride Green Rust

In this work, we present first principle density functional theory calculations on hydroxycarbonate and hydroxychloride green rust. Green rust is a layered mineral, with brucite-like 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 Fe2+ by Fe3+ creates a layer charge, which is compensated by interlayer anions (e.g., carbonate or chloride). The simultaneous presence of Fe2+/Fe3+ in the brucite layer of green rust is a considerable theoretical challenge due to the open shell ground states of Fe2+/Fe3+. 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 Fe3+ 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 Am3+–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 Am3+–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.