Mineral Dissolution Rates: Importance of Surface Reactivity

Mineral dissolution plays a key role in many environmental and technical fields, e.g., weathering, building materials, as well as host rock characterization for potential nuclear waste repositories. The rate of mineral dissolution in water is controlled by two parameters: (1) transport of dissolved species over and from the interface determined by advective fluid flow and diffusion (transport control) and (2) crystal surface reactivity (surface reactivity control). Current reactive transport models (RTM) simulating species transport commonly calculate mineral dissolution by using rate laws [1]. These rate laws solely depend on species concentration in the fluid and therefore do not include intrinsic variability of surface reactivity. Experimental studies under surface-controlled conditions have shown a heterogeneous distribution of reaction rates [2]. This rate heterogeneity is caused by nanotopographical structures on the crystal surface, such as steps and etch pits that are generated at lattice defects. At these structures, the high density of reactive kink sites is leading to a local increase in dissolution rates.
In this study, we test whether experimentally observed rate heterogeneities can be reproduced by using current RTMs. We apply a standard RTM approach combined with the measured surface topography of a calcite single crystal [2]. Calcite is one of the larger mineral components in the sandy facies of the Opalinus clay formation, that is under consideration for nuclear waste storage. The calculated surface dissolution rate maps are compared to experimentally derived rate maps. The results show that the measured rate heterogeneities cannot be reproduced with the existing RTM approach. To improve the predictive capabilities of RTMs, the surface reactivity that is intrinsic to the mineral needs to be implemented into rate calculations. Investigating calcite surface reactivity in the context of dissolution can also yield information about other kinetic surface processes such as the adsorption of radionuclides. We discuss parameterization of surface reactivity via proxy parameters, such as surface roughness or surface slope. The implementation of these proxy parameters will allow for a more precise prediction of host rock-fluid interaction over large time scales in RTMs, relevant for safety assessment.
[1] P. Agrawal, A. Raoof, O. Iliev and M. Wolthers, Evolution of pore-shape and its impact on pore conductivity during CO2 injection in calcite: Single pore simulations and microfluidic experiments, Advances in Water Resources, 136, 103480 (2020).
[2] I. Bibi, R.S. Arvidson, C. Fischer and A. Luttge: Temporal Evolution of Calcite Surface Dissolution Kinetics, Minerals, 8, 256 (2018).