Porosity and fluid pathway development during cadmium sequestration by calcium carbonate replacement†

Cadmium contamination of ground water and soil has drastically increased in some areas through the last few decades and remediation strategies are currently being investigated. The coupled dissolution–precipitation of calcium carbonate in cadmium-containing solutions leads to the precipitation of a (Ca,Cd)CO₃ phase of lower solubility, this process trapping cadmium from a solution into a solid phase. The present study analyses the reactions of two types of calcium carbonates (calcite as Carrara marble and aragonite) in cadmium solutions and compares the different reaction pathways and their respective efficiency. X-ray tomography scans of different Carrara marble and aragonite samples reacted in cadmium solutions for 16 to 64 days at 200 °C were acquired and analysed. The reaction in Carrara marble proceeds through a dissolution–precipitation reaction from the surface of the sample. The fluid moves through the porosity developed in the newly precipitated phase and along grain boundaries. Tomograms show that the porosity at the post-reaction time of imaging is mainly disconnected and that the reaction extent decreases with an increase in cadmium concentration of the solution. For aragonite, the main reaction pathway is opened by reaction-induced fracturing, which leads to a faster reaction than for the Carrara marble as the reaction pathways open faster towards the centre of the sample through successive hierarchical fracturing. The reaction rate for aragonite increases with time and cadmium concentration of the solution. Thus, the sequestration of cadmium from solution is potentially more efficient using aragonite due to the reaction-induced fracturing process taking place.