Uranyl sorption onto birnessite: A surface complexation modeling and EXAFS study

- A structural and chemical model for the sorption of U onto birnessite is proposed.
- The inner-sphere mononuclear complexation evolves with the pH.
- Prevailing influence of bidentate edge sites in diluted acidic conditions.
- At low pH no difference between the coordination of UVI on birnessite or δ-MnO2.
- U sorption on birnessite exceeds or compares to clays, zeolite or iron oxides.

This work investigates the mechanism of the uranyl interaction with birnessite, one of the most common layer-type MnO2 mineral at the Earth's surface, by coupling macroscopic (surface complexation experiments) and microscopic (EXAFS measurements) approaches. The sorption of uranyl on synthetic hexagonal birnessite, the low-pH birnessite form, was studied under various conditions of pH (3-6), electrolyte backgrounds (0.1 M NaClO4, NaNO3 and Na2CO3), and solid/liquid ratios (from 0.27 to 4.5 g/L). Sorption isotherms exhibit a complex form indicative of at least two types of sorption sites. EXAFS data reveal the presence of two equatorial O shells at ca. 2.32 Å and 2.46 Å for all the samples, and a Mn shell at ca. 3.38 Å in the low-pH (≤ 5) samples only. No U-U pair was detected, despite the presence of polynuclear dissolved species in some of the samples.From the combination of the sorption isotherms and EXAFS results, a structural model for the sorption of uranyl onto hexagonal birnessite is proposed, in which two energetically different sites are involved. At low pH (≤ 5) a bidentate edge-sharing complex with Mn octahedra of the mineral edges can be inferred, whereas bidentate corner-sharing and/or monodentate complexation to layer vacancies would most likely describe EXAFS features of higher pH samples. A diffuse double layer model of surface complexation was developed for describing within the same framework the uranyl sorption against pH, involving both high-affinity (Mn octahedra edge) and low-affinity (above layer vacancies) sites.The comparison of the uranyl sorption onto hexagonal birnessite and various related environmental minerals shows that the affinity of uranyl for birnessite largely exceeds the sorption observed on montmorillonite and zeolite and turns out to be comparable to iron oxides, confirming the potential role of phyllomanganates to the control of uranyl mobility in post-oxic acidic environments.