Tracking the dissolution of calcite single crystals in acid waters: a simple method for measuring fast surface kinetics

Abstract

Although the dissolution kinetics of calcite in acid waters has been studied for more than a century, the process is not fully understood, and for particles and microcrystals the process is often assumed to be diffusion-controlled. Herein, the dissolution kinetics of calcite single microcrystals in aqueous solution (pH ca. 3) has been investigated for the first time by a combination of real-time optical microscopy coupled with numerical simulations. The small size and well-defined geometry of rhombohedral calcite single crystals enables the measurement of the dissolution rates of the individual crystal faces exposed to the solvent and an assessment of the relative importance of corners and edges compared to the {104} faces. Data are used to parameterise finite element method (FEM) models for the quantitative analysis of dissolution kinetics. The simulations provide an accurate determination of the near-interface concentration of solution species during dissolution, as well as concentration gradients. The intrinsic first-order dissolution rate constant for the attack of protons on the exposed {104} faces, k = (6.4 ± 2.8) × 10 m s, is in good agreement with previous microscopic and macroscopic measurements, corroborating the method. This study is a further demonstration of the power of simple in situ optical microscopy for quantitative interfacial (dissolution/growth) kinetic measurements, using a configuration of practical relevance for processes as diverse as the remediation of acid water and scale removal.

Mapping Three-dimensional Dissolution Rates of Calcite Microcrystals: Effects of Surface Curvature and Dissolved Metal Ions

“…Although the dissolution experiments on single crystal calcite in the size of millimeter to centimeter range have been performed, the local dissolution rate variations in three dimensions (3D) have not been reported. 20,21 The presence of metal ions and organic molecules is wellknown to modify the morphology of growth hillocks observed on calcite (104) surfaces. 22−24 Depending on the functional group and chirality of the organic molecules, they can selectively adsorb to either acute or obtuse steps, resulting in changes of the step energies and step growth velocities.…”

“…However, these methods do not reveal the morphological controls over dissolution at macroscopic edges and corners, which can account for a significant portion of the reactive area of a sub-micron sized crystalline grain. Although the dissolution experiments on single crystal calcite in the size of millimeter to centimeter range have been performed, the local dissolution rate variations in three dimensions (3D) have not been reported. , …”

Multiphase Assembly of Small Molecule Microcrystalline Peptide Hydrogel Allows Immunomodulatory Combination Therapy for Long‐Term Heart Transplant Survival

“…Although defining the exact mechanism of crystal dissolution is beyond the scope of this paper, it is most likely a combination of surface‐kinetic and mass‐transport controlled processes as is the case with most small molecule and inorganic crystals. [ 48,49 ]…”

The New Era of High-Throughput Nanoelectrochemistry

“…A key general feature, when studying dissolution/growth at microscopic single entities, is that mass transport (diffusion) is well-defined and high, thus enabling the measurement of fast kinetics with simple platforms. 406 Catalysis and Motion. The accumulation of products at the vicinity of a nanoscale electro-catalyst sometimes provokes a change in the local refractive index of the surrounding medium that can further be detected by optical microscopy.…”