Potential sequestration of toxic elements: the specific example of cadmium and carbonates

Coupled dissolution-precipitation reactions have been studied extensively recently for their ability to retain elements of interest into a stable solid form that can sequester potentially toxic elements. This is achieved through the initial dissolution of a substrate mineral in a fluid containing the target (often toxic) element. The dissolution leads to the supersaturation of a boundary layer at the mineral surface with respect to another solid phase containing the element of interest1. When the relative solubilities of the different minerals (in the aqueous fluid at the reaction interface) and their molar volume difference allow it, a coupled dissolution-precipitation can lead to the pseudomorphic replacement of the original substrate2. We tested this reaction for CaCO3 and cadmium (Cd) containing solutions as calcite (CaCO3) and otavite (CdCO3) form an almost perfect solid solution. We compared the reaction in similar solutions with different types of CaCO3: calcite single crystal, Carrara marble (polycrystalline calcite) and aragonite single crystals. For single calcite crystals, the reaction in a Cd-solution passivates the crystal’s surface due to the epitaxial growth of a (Ca,Cd)CO3 solid solution layer of low solubility. However, the random orientation of the grains in the Carrara marble samples and the change of crystal structure for the aragonite crystals modified the mechanism and allow the replacement of CaCO3 by (Ca,Cd)CO3 to take place3. Hydrothermal experiments and in situ fluid-cell atomic force microscopy (AFM) were used to observe the reaction both at room temperature and high pressure and temperature (200°C). In addition to SEM, BSE and EDX observations, synchrotron X-ray microtomography images were acquired on Carrara marble and aragonite samples at different stages of the reaction in order to gather more information about the replacement mechanism and the extent of Cd-uptake achievable through this process. The extent of the reaction was shown to be similar for the different solution concentrations used and limited in the case of Carrara marble. The porosity closes fast after the start of the reaction blocking the fluid pathways necessary for the reaction to proceed. The reaction in the aragonite samples seems to progress mainly through a reaction-induced fracture network probably created by the stress caused by the crystallographic structural differences between parent and product phases. Overall, these results demonstrate the capacity of CaCO3 to trap and store cadmium into a solid phase by a mechanism of coupled dissolution-precipitation.   (1)        Putnis, C. V.; Putnis, A. A Mechanism of Ion Exchange by Interface-Coupled Dissolution-Precipitation in the Presence of an Aqueous Fluid. Journal of Crystal Growth 2022, 600, 126840. https://doi.org/10.1016/j.jcrysgro.2022.126840. (2)        Pollok, K.; Putnis, C. V.; Putnis, A. Mineral Replacement Reactions in Solid Solution-Aqueous Solution Systems: Volume Changes, Reactions Paths and End-Points Using the Example of Model Salt Systems. American Journal of Science 2011, 311 (3), 211–236. https://doi.org/10.2475/03.2011.02. (3)        Julia, M.; Putnis, C. V.; King, H. E.; Renard, F. Coupled Dissolution-Precipitation and Growth Processes on Calcite, Aragonite, and Carrara Marble Exposed to Cadmium-Rich Aqueous Solutions. Chemical Geology 2023, 621, 121364. https://doi.org/10.1016/j.chemgeo.2023.121364.