Heterogeneous Sorption of Radionuclides Predicted by Crystal Surface Nanoroughness

Abstract

Reactive transport modeling (RTM) is an essential tool for the prediction of contaminants' behavior in the bio-and geosphere. However, RTM of sorption reactions is constrained by the reactive surface site assessment. The reactive site density variability of the crystal surface nanotopography provides an "energetic landscape", responsible for heterogeneous sorption efficiency, not covered in current RTM approaches. Here, we study the spatially heterogeneous sorption behavior of Eu(III), as an analogue to trivalent actinides, on a polycrystalline nanotopographic calcite surface and quantify the sorption efficiency as a function of surface nanoroughness. Based on experimental data from micro-focus time-resolved laser-induced luminescence spectroscopy (μTRLFS), vertical scanning interferometry, and electron backscattering diffraction (EBSD), we parameterize a surface complexation model (SCM) using surface nanotopography data. The validation of the quantitatively predicted spatial sorption heterogeneity suggests that retention reactions can be considerably influenced by nanotopographic surface features. Our study presents a way to implement heterogeneous surface reactivity into a SCM for enhanced prediction of radionuclide retention.

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

“…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

“…[ 27,28 ] Large calcite sample sections with well‐defined topographic features, e.g., increased kink density in fields of view of several hundred micrometers compared to atomically flat sample surfaces have been studied experimentally and analytically. [ 23 ] Here, surface portions with increased nanoroughness show an adsorption efficiency enhanced by more than one order of magnitude compared to flat cleaved surfaces. The authors also provide a comparison to multiple previous studies that support the influence of surface topography on adsorption.…”

“…However, this effect is also inherent in experimental techniques that map surface adsorption (concentration) and has been observed there. [ 23,27,28 ] The KMC simulation shows that the mineral surface nanotopography influences the adsorption distribution of europium and homologous trivalent actinides. Surface steps originating at crystal defects are identified to be the key structure controlling heterogeneous adsorption.…”

“…However, this effect is also inherent in experimental techniques that map surface adsorption (concentration) and has been observed there. [23,27,28] The KMC simulation shows that the mineral surface nanotopography influences the adsorption distribution of europium and homologous 6d) on the muscovite surface is overlaid with a kernel density estimation of the local adsorbed europium density. An increase in atom density is visible when comparing flat surface sections with surface steps and etch pits.…”

“…[23,27] An elevated surface nanoroughness indicates strong local changes in topography, where a high concentration of highly reactive sites such as steps and kinks can be found. [26] Experimental results show that a high surface roughness on minerals leads to a higher amount of sorption on the examples of mica, [27] calcite, [23] and feldspar. [28] The formation of more strongly bound complexes is reported on feldspar, quartz, and mica.…”