Direct Determination of Dissolution Rates at Crystal Surfaces Using 3D X-ray Microtomography

Abstract

Investigations of mineral surface reactivity have recently challenged the classical approach of determining dissolution rates from mineral powders as crystals often exhibit heterogeneous and/or anisotropic reactivity. However, face-specific measurements are restricted to small areas at the surface and limited depth and ignore the contribution of the crystal edges to the whole process. Here, we provide a detailed characterization of the dissolution kinetics at pH 4.0 of a single calcite crystal in 3D using X-ray microtomography with a resolution less than 1 μm. The imaging method allows 3D mapping of the crystal surface topography, providing a description of the time-dependent local dissolution fluxes all over the crystal surface, and the calculation of the crystal dissolution rates. The global rate determined at the crystal scale integrates the contribution of all the crystal features, including the faces, edges, and corners, which can be detailed in the local rate distributions. Under acidic conditions, pits develop at the {101̅ 4} surface before dissolution tends to smooth out both the crystal surface asperities and the edges and corners. In addition, a high rate variability is noticed over the crystal surface. The heterogeneous dissolution rates at the crystal surface first led to a local increase of the surface roughness due to pit formation and coalescence, followed by a decrease of the global crystal roughness due to smoothing of the large-scale surface asperities, crystal edges, and corners. Etch pits dominate initially the surface topography, whereas the evolution of the crystal morphology is dominated by the reactivity of edges and corners, whose contribution to dissolution is on average 1.7−3.6 times higher than that of the crystal faces. These results suggest that the dissolution reaction preferentially occurs at the crystal edges and corners, something not considered in most studies of mineral dissolution.

A Statistical Approach for Analysis of Dissolution Rates Including Surface Morphology

“…The average of all the calcite cement rates, excluding the time lapses the samples were influenced by polishing, is 0.6 ± 0. Such as in 3D studies (e.g., [19]), we show that VSI data also permits the study of different crystal faces, edges and corners and provide total material fluxes in the units mol•s −1 . This 3D study [19], reported three orders of magnitude higher calcite dissolution rates (total rates) than what we measured here, but the authors used an acidic solution (pH = 4) for the experiments, much lower than one used here (pH = 8.7).…”

“…Such as in 3D studies (e.g., [19]), we show that VSI data also permits the study of different crystal faces, edges and corners and provide total material fluxes in the units mol•s −1 . This 3D study [19], reported three orders of magnitude higher calcite dissolution rates (total rates) than what we measured here, but the authors used an acidic solution (pH = 4) for the experiments, much lower than one used here (pH = 8.7). As reported before [12], due to the uncertainty related in defining "reactive surface area", the highest differences in rate magnitude are found in studies that measured rates from fluid chemistry.…”

“…An alternative treatment recently offered is based on representation and quantitative description of rate spectra [15,17], a statistical frequency distribution of rates. The spectra have been shown to vary over spatial and temporal domains, as it was shown in site-specific experiments [15,[18][19][20]. The systematic analysis of these variations and their quantitative description may potentially provide very important kinetic information for reactive transport models at the pore scale.…”

Examination of crystal dissolution in 3D: A way to reconcile dissolution rates in the laboratory?

“…The transformation was applied to the other VOIs, so that the topography evolution of the surface extracts is directly comparable. This technique is preferred over the extraction of the VOIs from the segmented images, the direct registration of which, in a horizontal plane, would highlight interpolation steps at the crystal surface, which can be confused with crystal surface steps (see for instance Figure 2 shown in Noiriel et al, 2019). After segmentation of the grayscale VOIs using the same procedure as described above (see section 2.3.1), the topography of the fluid-solid interface was extracted.…”

“…Although it has long been assumed that crystal edges provide a source of reactive sites (Schott et al, 1989), it is only very recently than the quantitative contribution of crystal edges to dissolution rates has been evaluated experimentally (Noiriel et al, 2019), using X-ray microtomography imaging (XMT). By enabling true 3D space, XMT should permit a significant extension to current methods and models by singling out the reactivity of crystal corners and edges, and to fill the gap between fine-scale (nm-to µm-scale) measurements of surface reactivity and macroscopic determination of bulk rates on powders.…”