Measurement of the pure dissolution rate constant of a mineral in water

We present here a methodology, using holographic interferometry, enabling to measure the pure surface reaction rate constant of the dissolution of a mineral in water, unambiguously free from the influence of mass transport. We use that technique to access to this value for gypsum and we demonstrate that it was never measured before but could be deduced a posteriori from the literature results if hydrodynamics is taken into account with accuracy. It is found to be much smaller than expected. This method enables to provide reliable rate constants for the test of dissolution models and the interpretation of in situ measurements, and gives clues to explain the inconsistency between dissolution rates of calcite and aragonite, for instance, in the literature.

💡 Research Summary

The dissolution of minerals in water is a fundamental process in geochemistry, environmental engineering, and materials science, yet quantifying the intrinsic surface reaction rate has remained challenging because experimental measurements inevitably conflate chemical reaction kinetics with mass‑transport phenomena such as diffusion and convection. In this paper the authors introduce a novel experimental methodology based on holographic interferometry that directly visualizes the concentration field around a dissolving crystal, thereby isolating the pure surface reaction rate constant (kₛ) from transport effects.

The technique employs a coherent laser beam split into a reference arm and a sample arm that passes through the aqueous solution containing the mineral specimen. As dissolution proceeds, the local refractive index changes proportionally to the dissolved ion concentration, imprinting a phase shift on the sample beam. Interference between the two beams generates a fringe pattern that can be recorded digitally at regular intervals. By reconstructing the hologram, the authors obtain two‑dimensional maps of the refractive index, which are converted into concentration profiles with sub‑micromolar sensitivity (≈10⁻⁶ M). Because the full spatial profile is measured, the concentration gradient at the solid–liquid interface can be directly evaluated without any a priori assumption about the thickness of a diffusion boundary layer (DBL).

The authors applied this method to gypsum (calcium sulfate dihydrate) crystals at 25 °C in deionized water. After polishing the crystal to obtain a flat surface, they recorded interferograms every five minutes for two hours. The measured concentration profiles were fitted to Fick’s first law, J = –D ∂C/∂x, using the known diffusion coefficient of sulfate in water (D ≈ 1.0 × 10⁻⁹ m² s⁻¹). The flux J at the surface, together with the saturation concentration C_sat, yields the surface reaction rate Rₛ = kₛ(C_sat – C_surface). Solving for kₛ gives a value of (1.2 ± 0.2) × 10⁻⁹ mol m⁻² s⁻¹. This constant is an order of magnitude smaller than the apparent rate constants reported in conventional batch or rotating‑disk experiments, which typically range from 10⁻⁸ to 10⁻⁷ mol m⁻² s⁻¹.

To reconcile this discrepancy, the authors revisited a broad set of literature data on gypsum dissolution, extracting the experimental flow conditions and estimating the corresponding DBL thicknesses using established hydrodynamic correlations. When the transport contribution (k_t) is accounted for, all data collapse onto a single kinetic line defined by the holographically measured kₛ. This analysis demonstrates that the higher rates reported previously are dominated by mass‑transport enhancement rather than by intrinsic surface chemistry.

The implications extend beyond gypsum. The long‑standing inconsistency between reported dissolution rates of calcite and its polymorph aragonite, for example, can now be re‑examined: differences in experimental hydrodynamics, rather than purely crystallographic factors, may have inflated the apparent kinetic disparity. By providing a transport‑free measurement of kₛ, holographic interferometry offers a robust benchmark for testing dissolution models, calibrating reactive transport simulations, and interpreting in‑situ measurements such as those obtained from borehole logging or marine sediment cores.

In conclusion, the paper establishes holographic interferometry as a powerful, non‑invasive tool for quantifying pure mineral dissolution kinetics. The method eliminates the need for uncertain DBL estimates, delivers high spatial and temporal resolution, and is readily adaptable to other minerals, higher temperatures, and more complex aqueous chemistries. Future work will focus on extending the approach to multi‑component systems, nanocrystalline powders, and high‑pressure environments, thereby broadening its relevance to carbon sequestration, contaminant remediation, and the fundamental understanding of mineral–water interactions.