Macroscopic and microscopic perspectives on dissolution of carbonate minerals

Carbonate mineral dissolution and precipitation partially regulate the pH and alkalinity of natural waters, affecting the fate and transport of contaminants. A detailed understanding of the microscopic behavior of these mineral surfaces is essential to the development of quantitative geochemical models. To that end, a series of studies investigating the dissolution of an isostructural series of carbonates, including calcite (CaCO₃), magnesite (MgCO₃), rhodochrosite (MnCO₃), siderite (FeCO ₃), and smithsonite (ZnCO₃), is conducted by simultaneous atomic force microscopy/flow-though reactor experiments under oxic and anoxic conditions.; Macroscopically, carbonate dissolution occurs by parallel proton- and water-promoted dissolution reactions under anoxic conditions. These reactions are cast mathematically as mechanistic or surface complexation models. Circumneutral dissolution rates vary from 10−5.5 for CaCO₃ to 10−8.9 mol m−2 s−1 for MgCO₃. Trends in dissolution rates as a function of composition are rationalized by literature water exchange rates and calculated energies.; Microscopically, dissolution occurs by crystallographically-controlled pit formation of and subsequent step retreat. For type I carbonates (CaCO ₃ and MnCO₃), near rhombohedral pits form at all pH values. For type II carbonates (FeCO₃ and MgCO₃), rhombohedral pits form for pH > 4. However, for pH < 4, steps reorient, resulting in a change in pit morphology. ZnCO₃, a type III carbonate, forms distorted pits at pH > 4 and triangular pits at pH < 4, possibly due to the formation of tetrahedral Zn2+ surface complexes.; Microscopic dissolution rates derived from the observation of surface microtopographic changes are compared to macroscopic dissolution rates derived from changes in aqueous metal ion concentration. Macroscopic and microscopic dissolution rates agree within a factor of four for type I carbonates at all pH values and for type II carbonates for pH > 4. However, macroscopic and microscopic dissolution rates for type II carbonates diverge for pH < 4 to a maximum 30-fold separation.; Oxygen is a critical parameter for the dissolution of redox active carbonates. For MnCO₃ under oxic conditions, a tabular Mn₂O ₃ precipitate forms for 5.8 < pH < 7.7, while MnOOH hillocks form at pH > 7.7. For FeCO₃ for 6.0 < pH < 10.3, hillock Fe(OH) ₃ precipitates form at steps, accompanied by a decrease in macroscopic dissolution rates.