Published: September 15, 2011 r2011 American Chemical Society 21120 dx.doi.org/10.1021/jp204633g | J. Phys. Chem. C 2011, 115, 21120–21127 ARTICLE pubs.acs.org/JPCC Trends in Ln(III) Sorption to Quartz Assessed by Molecular Dynamics Simulations and Laser-Induced Fluorescence Studies Jadwiga Kuta, † Matthew C. F. Wander, †,§ Zheming Wang,* ,‡ Siduo Jiang, ‡ Nathalie A. Wall,* ,† and Aurora E. Clark* ,† † Department of Chemistry, Washington State University, Pullman, Washington 99164, United States ‡ Fundamental and Computational Science Directorate, Pacific Northwest National Laboratory, Richland, Washington 99352, United States b S Supporting Information ’ INTRODUCTION Trivalent lanthanides [Ln(III)] and actinides [An(III)] are present in nuclear wastes that have not been reprocessed, and Ln(III) are commonly considered chemical analogs for An(III), due to the similar bonding character of the trivalent f-block elements. 1,2 Moreover, Ln(III) are less susceptible to solution phase and surface oxidation and reduction (redox) reactions that can complicate chemical interpretation of surface-bound species. As such, the geochemical behavior of Ln(III) has been exten- sively investigated with respect to the performance assessment of nuclear waste repositories and remediation of sites contaminated with both Ln and An. 3,4 Evidence suggests that oxide minerals are the key sorbents responsible for controlling the fate and trans- port of Ln elements in natural systems, 5 and although significant advances have been made in our understanding of their reactivity with oxides, major deficiencies remain. Although batch sorption and column experiments provide useful information on the effects of solution chemistry and mineral properties on Ln sorp- tion behavior, they describe only the macroscopic aspects of ion interactions with the mineral surface and give no direct informa- tion on the structure and local chemical environment of sorbed species. More information can be gleaned from surface com- plexation models (SCMs) that use thermodynamic and kinetic databases combined with mathematical algorithms to fit metal sorption experimental data. 6À8 However, the primary limitation of SCMs is that the site-specific structural parameters for these models are too strongly correlated to small changes in pH and other global paramaters, thus reducing or eliminating their geochemical value. In recent years, molecular spectroscopy has emerged as a vital component to elucidate molecular scale structures sorbed to mineral surfaces, providing insight into the types of surface com- plexes and precipitations formed as a function of the metal ion, mineral surface, pH, surface coverage, etc. 9 Molecular simula- tions have also proven a useful complement to sorption and spectroscopic experiments. In this case, the sorption behavior of specific ions can be studied directly through the simulations of realistic models of molecular complexes at mineral surfaces. 10 However, these computational studies typically do not account for the influence of ionic strength (I), are performed without consideration of pH, and lack meaningful comparison with experimental observations under similar conditions. Thus, the Received: May 18, 2011 Revised: September 5, 2011 ABSTRACT: Molecular dynamics simulations were performed to examine trends in trivalent lanthanide [Ln(III)] sorption to tSiOH 0 and tSiO À sites on the 001 surface of α-quartz across the 4f period. Complementary laser-induced fluorescence studies examined Eu(III) sorption to α-quartz at a series of ionic strengths from 1 Â 10 À4 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 tSiO À site. The coordination shell of the ion contains 6À7 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 2.0À3.0 Å of the plane formed by the silanol O atoms (∼3.5 Å from an individual tSiO À 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.