Trends in Ln(III) Sorption to Quartz Assessed by Molecular Dynamics Simulations and Laser-Induced Fluorescence Studies

J Kuta and MCF Wander and ZM Wang and SD Jiang and NA Wall and AE Clark, JOURNAL OF PHYSICAL CHEMISTRY C, 115, 21120-21127 (2011).

DOI: 10.1021/jp204633g

Molecular dynamics simulations were performed to examine trends in trivalent lanthanide Ln(III) sorption to SiOH0 and SiO- sites on the 001 surface of alpha-quartz across the 4f period. Complementary laser- induced fluorescence studies examined Eu(III) sorption to alpha-quartz at a series of ionic strengths from 1 x 10(-4) M to 0.5 M such that properties of the surface-sorbed species could be extrapolated to zero ionic strength, the conditions under which the simulations are performed. Such extrapolation allows for a more direct comparison of the data and enables a molecular understanding of the surface-sorbed species and the role of the ion surface charge density upon the interfacial reactivity. Potential of mean force molecular dynamics as well as simulations of presorbed Ln(III) species agrees with the spectroscopic study of Eu(III) sorption, indicating that strongly bound inner-sphere complexes are formed upon sorption to an SiO- site. The coordination shell of the ion contains 6-7 waters of hydration, and it is predicted that surface silanol OH groups transfer from the quartz to the inner coordination shell of Eu(III). Molecular simulations predict less- strongly bound inner-sphere species in early lanthanides and more strongly bound species in late lanthanides, following trends in the surface charge density of the 4f ions. Hydroxyl ligands that derive from the surface silanol groups are consistently observed to bind in the inner coordination shell of surface-sorbed inner-sphere Ln(III) ions, provided that the ion is able to migrate within 2.0-3.0 angstrom of the plane formed by the silanol O atoms (similar to 3.5 angstrom from an individual SiO- group). Sorption to a fully protonated quartz surface is not predicted to be favorable by any Ln(III), except perhaps Lu. The present work demonstrates a combined theoretical and experimental approach in the prediction of the fate of trivalent radioactive contaminants at temporary and permanent nuclear waste storage sites.

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