Kinetics and mechanisms of the interaction between the calcite (10.4) surface and Cu²⁺-bearing solutions

Calcite dissolution, occurring in rocks, soils and sediments, is essential to indicate element cycles and local environments in the lithosphere, biosphere, hydrosphere and atmosphere. Calcite dissolution strongly depends on metal ions in aqueous solutions. Previous studies showed that aquatic Cu²⁺, a typical bio-toxic metal ion, can alter the calcite dissolution behavior. However, wide concentration ranges of Cu²⁺ coexisting with ubiquitous anions in local environments, such as waterways in the oxidation zones of copper deposits and soils near metal processing industry, was overlooked. When a considerable amount of aquatic Cu²⁺ ions are released into the environment, they migrate, diffuse, and hence become an environmental pollutant. Therefore, we focused on the interaction between calcite dissolution and wide concentration ranges of Cu²⁺-bearing solutions with different types of anions (SO₄^2-, Cl^- and NO₃^-). Comprehensive approaches including in situ atomic force microscopy (AFM), X-ray photoelectron spectroscopy (XPS), transmission electron microscope (TEM), and density functional theory (DFT) calculations were employed to investigate kinetics and mechanisms of the interaction between the calcite (10.4) surface and Cu²⁺-bearing solutions. Results demonstrated that both anion types and Cu²⁺ concentrations dramatically affect calcite dissolution. The morphology of etch pits generated in CuSO₄ solutions can be fan-shaped but changed to tear-shaped in Cu(NO₃)₂ or CuCl₂ solutions. Calcite dissolution kinetics is inhibited at c_Cu2+ ≤ 0.1 mM, caused by the coverage of active sites on calcite surfaces. As the Cu²⁺ concentration increases (1 mM ≤ c_Cu2+ ≤ 10 mM), calcite dissolution kinetics is enhanced due to the coupling effect of Cu²⁺-incorporated surface structure and solution chemistry. These results revealed the interactive mechanism between calcite dissolution and the migration of toxic Cu²⁺ in waterways, provided a practical consideration in dealing with the local environment.