Nanoscale investigation of calcite dissolution processes in Cd-bearing solutions

Mineral dissolution is a key process driving the evolution of porous structures in natural environments. Among all minerals, calcite is the most widespread in the Earth crust. Moreover, due to its high affinity for divalent metals, calcite plays a prominent role in the studies of heavy-metal sequestration and groundwater remediation techniques. Cadmium (Cd) is among the most toxic and persistent heavy metals detected in industrial wastewater. Its interaction with carbonate minerals is crucial to understand contaminant mobility and retention in natural systems. A comprehensive understanding of the kinetics of Cd interaction with calcite is essential to unravel the fundamental mechanisms governing these phenomena.

In this work, we rely on in-situ, real time measurements of calcite surface topography acquired via Atomic Force Microscopy (AFM) at nano-scale resolution. The main objectives of this study are: (i) to quantitatively assess the spatial heterogeneity of calcite dissolution; (ii) to evaluate the temporal evolution of the reaction kinetics; (iii) to investigate the effects of dissolved Cd ions on characteristic reaction patterns and on the spatial distribution of rates.

Freshly cleaved calcite crystals are exposed to deionized water and Cd-bearing solutions in a flow-through cell, where AFM acquisition is performed simultaneously with the continuous flow of the liquid phase. This set up allows spatial distributions of dissolution rates to be obtained by comparing topographic maps acquired at successive times.

Stochastic models based on multimodal Gaussian and sub-Gaussian random fields successfully reproduce the statistical behavior of nano-scale dissolution-rate datasets. The temporal evolution of the model parameters provides insights into the key mechanisms controlling mineral surface dynamics and its interaction with Cd.