Momentary increase in reactive surface area in dissolving carbonate

TL;DR

This study reveals that reactive surface area in dissolving carbonate minerals can temporarily increase due to flow-dissolution coupling, which is crucial for environmental reactive transport modeling and cannot be explained solely by surface roughening or accessibility.

Contribution

The paper provides direct in situ observation of reactive surface generation and links it to flow and dissolution coupling, introducing a macroscopic Damköhler number as an indicator.

Findings

Reactive surface area can temporarily increase during dissolution.

Flow-dissolution coupling influences surface generation.

Damköhler number correlates with reactive surface dynamics.

Abstract

The quantification of surface area between mineral and reactive fluid is essential in environmental applications of reactive transport modelling. This quantity evolves with microstructures and is difficult to predict because the mechanisms for the generation and destruction of reactive surface remain elusive. The challenge of accounting for the inherent heterogeneities of natural porous media in numerical simulation further complicates the problem. Here we first show a direct observation of reactive surface generation in chalk under circumneutral to alkaline pH using in situ X-ray microtomography. The momentary increase of reactive surface area cannot be explained by a change in fluid accessibility or by surface roughening stemming from mineralogical heterogeneity. We then combine greyscale nanotomography data with numerical simulations to show that similar temporal behaviour can be observed over a wide range of pH as porous media dissolve in imposed flow field. We attribute the observation to the coupling between fluid flow and mineral dissolution and argue that the extent of surface generation is strongly correlated with the advective penetration depth of reactants. To conclude, we demonstrate the applicability of using a macroscopic Damk\"ohler number as an indicator for the phenomenon and discuss its environmental significance beyond geologic carbon storage.