Influence of Muscovite (001) Surface Nanotopography on Radionuclide Adsorption Studied by Kinetic Monte Carlo Simulations

Abstract

Mechanistic understanding and prediction of solute adsorption from fluids onto mineral surfaces is relevant for many natural and technical processes. Mineral surfaces in natural systems are often exposed to fluids at non-equilibrium conditions resulting in surface dissolution reactions. Such reactions cause the formation of surface nanotopography and, consequently, the exposure of different types of surface atoms. The quantitative effect of nanotopography on the efficiency of adsorption reactions at crystal surfaces is not known. Using kinetic Monte Carlo simulations, we combined a model of muscovite (001) face dissolution with a consequent model of radionuclide adsorption on the rough mineral surface. The model considers three different adsorption sites based on the muscovite surface cations: silicon, tetrahedral, and octahedral aluminum. Two different nanotopography configurations are investigated, both showing similar adsorption behavior. Octahedral aluminum surface atoms defined by having the highest reactivity toward adsorption are exposed solely on steps and pits on the muscovite (001) face. Thus, their availability directly depends on the surface nanotopography. The model results show the need for a more precise parameterization of surface site-specific adsorption, taking into account the coordination of the involved surface cation such as kink, step, or terrace sites.

Y(III) Sorption at the Orthoclase (001) Surface Measured by X-ray Reflectivity

“…Therefore, the molecular level findings of this study can be transferred to lower metal concentrations as they will be present in the environment, for example, at REE acid mine drainage sites. ,− Although the orthoclase (001) cleavage plane investigated here may not be representative for all naturally existing crystal orientations, it will present a significant extent of the exposed surface area . In consequence, the basal planes control the adsorption of cations, despite the presence of other more energetically favorable sites, as was previously shown in a computational study on muscovite mica . Overall, the results presented here are therefore closely related to phenomena occurring in some natural systems, for example, a crystalline rock fracture exposing millimeter-sized feldspar grains, and provide molecular level structures which can be used to derive reaction equations required for the correct thermodynamic description of the sorption process.…”

“…84 In consequence, the basal planes control the adsorption of cations, despite the presence of other more energetically favorable sites, as was previously shown in a computational study on muscovite mica. 43 Overall, the results presented here are therefore closely related to phenomena occurring in some natural systems, for example, a crystalline rock fracture exposing millimeter-sized feldspar grains, and provide molecular level structures which can be used to derive reaction equations required for the correct thermodynamic description of the sorption process.…”

“…While studies using mineral powder samples give a solid basis for the determination of thermodynamic parameters for surface complexation reactions, − the development of realistic reactive transport models requires an understanding of facet-specific reactivity, the influence of surface roughness, ,, and solid–liquid ratios more similar to those in natural systems. Orthoclase (KAlSi 3 O 8 ), a common polymorph of K-feldspar, has a monoclinic crystal structure ( a = 8.56 Å, b = 12.99 Å, c = 7.19 Å, α = γ = 90°, and β = 116°) with perfect cleavage planes along the (001) and (010) orientations, which are commonly exposed in natural systems .…”

Variability of Radionuclide Sorption Efficiency on Muscovite Cleavage Planes

“…This study aims to investigate the influence of specific surface sites on the efficiency and spatial heterogeneity of adsorption reactions on mica surfaces using a KMC model. The applied KMC algorithm is based on a previously developed model, [ 31 ] that is reparametrized with new energy barriers obtained by DFT calculations for the adsorption reactions of kink, step, and terrace site models. With the new KMC model, we are able to simulate realistic mineral surfaces that exhibit inherent nanotopographies, comparable to those occurring at fracture and fault surfaces of host rocks.…”

“…Schabernack et al developed a KMC model for the adsorption of a generic radionuclide representing trivalent actinides to the muscovite (001) surface to study the influence of the surface nanotopography. [31] A critical requirement in KMC models is the parameterization of the site-specific reactions. Ideally, the activation energies of sitespecific reactions would be used in KMC for establishing the relative reactivity of each site.…”