Sorption of trivalent lanthanides and actinides onto montmorillonite: Macroscopic, thermodynamic and structural evidence for ternary hydroxo and carbonato surface complexes on multiple sorption sites


Sorption of trivalent lanthanides and actinides onto montmorillonite: Macroscopic, thermodynamic and structural evidence for ternary hydroxo and carbonato surface complexes on multiple sorption sites

Marques Fernandes, M.; Scheinost, A. C.; Baeyens, B.

The credibility of long-term safety assessments of radioactive waste repositories may be greatly enhanced by a molecular level understanding of the sorption processes onto individual minerals present in the near- and far-field. In this study we couple extensive macroscopic sorption experiments to surface complexation modelling and spectroscopic tools including extended X-ray absorption fine structure (EXAFS) and time-resolved laser fluorescence spectroscopies (TRLFS), in order to elucidate the uptake mechanism of trivalent actinides and lanthanides (Ln/AnIII) on montmorillonite in the absence and presence of dissolved carbonate. Based on the experimental sorption isotherms, the previously developed 2SPNE SC/CE sorption model needed to be complemented with an additional surface complexation reaction on to a weak site (ºSWOEu2+) for the carbonate-free system. In the presence of carbonate, the previously published model required refinement by reducing the strong-site capacity and by adding the formation of Ln/AnIII-carbonato complexes both on strong and weak sites. EXAFS spectra collected of selected Am sorption samples and TRLFS spectra of selected Cm sorption samples corroborate the model assumptions by showing the existence of different surface complexation sites and evidencing the formation of Ln/AnIII carbonate surface complexes. In the absence of carbonate and at low loadings, Ln/AnIII form strong innersphere sorption complexes through binding to three Al(O,OH)6 octahedra, most likely by occupying vacant sites in the octahedral layers of montmorillonite, which are exposed on {010} and {110} edge faces. At higher loadings, Ln/AnIII bind to only one Al octahedron, forming a weaker, edge-sharing sorption complex. In the presence of carbonate, we identified a ternary mono-carbonato Ln/AnIII complex binding directly to one Al(O,OH)6 octahedron, thereby revealing that type-A ternary complexes form with one or two carbonato groups pointing away from the surface into the solution phase; these complexes form on weak sites only at the observable concentration range, in line with the small amount of strong-site complexes suggested by the complexation model. When the solubility of carbonates was exceeded, formation of an Am carbonate hydroxide could be identified. The excellent agreement between the thermodynamic model parameters developed by fitting a large set of macroscopic data, and the spectroscopically identified mechanisms, demonstrates the mature state of the 2SPNE SC/CE model for predicting and quantifying the retention of Ln/AnIII elements by montmorillonite-rich clay rocks.

Keywords: Americium; Sorption; clay rocks; montmorillonite; TRLFS; EXAFS; surface complexation modeling

Related publications

Permalink: https://www.hzdr.de/publications/Publ-23088
Publ.-Id: 23088