Impact of Ion-Ion Correlations on the Adsorption of M(III) (M = Am, Eu, Y) onto Muscovite (001) in the Presence of Sulfate

The environmental fate of metal ions is influenced by their interactions with natural organic and inorganic ligands, which modify the ions' structure and charge and thus influence their interactions with mineral phases. We investigate the impact of ubiquitous sulfate on the retention of trivalent f-element cations (M(III) = Am, Eu, Y) by muscovite. We combine ex situ a spectrometry and in situ surface X-ray diffraction (i.e., crystal truncation rod and resonant anomalous X-ray reflectivity) to determine M(III) coverages and interfacial structures at the molecular level. M(III) cations adsorb as two distinct outer-sphere (OS) complexes (i.e., adsorbed and extended OS complexes whose coverages vary with increasing sulfate concentration, [SO42-]. When [SO42-] <= 0.4 mM, M(III) coverages increase with increasing [SO42-] and exceed the amounts needed for surface charge compensation of muscovite by a factor of similar to 3. This overcompensation is likely controlled by ion-ion correlations at the mineral/water interface rather than adsorption of MSO4+, which has a lower thermodynamic stability in the solutions and weaker electrostatic attraction to the mica surface than M3+. For higher [SO42-], MSO4+ and M(SO4)(2)(-) dominate solution speciation, leading to a strong decrease of the M(III) coverage due to their lower sorption affinity and weaker ion-ion correlations compared to M3+. These results indicate that interactions between electrolyte anions and metal ions at charged interfaces need to be explored for a more realistic prediction of contaminant transport in the environment.