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

Abstract

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 (SO42−, 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 cCu2+ ≤ 0.1 mM, caused by the coverage of active sites on calcite surfaces. As the Cu2+ concentration increases (1 mM ≤ cCu2+ ≤ 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.