Abstract:
Previously published microscopic observations of the heterogeneous oxidation of Mn(II) at hematite and albite surfaces were used to develop rate expressions that model the changes in solution composition during oxidation experiments. A geometric approach, which approximates the evolution of reactive surface area over the course of an experiment, was used to expand the right side of the following equation:rhet = d[Mn(II)]dt = - d[β - MnOOH]dt such that a theoretical link between macroscopic (d[Mn(II)]dt) and microscopic observations (d[β - MnOOH]dt) was created. Through this analysis, macroscopic rate constants, which are most commonly determined through measurements of the changes in solution composition, are broken down into smaller components that represent a microscopically measured rate constant and the reactive surface area present during an oxidation experiment. These microscopic rate constants can be used to directly compare the reactivities of minerals used in Mn(II) oxidation experiments.The new kinetic models, based on two cases of precipitate development, provide good fits of published solution data (Davies and Morgan, 1989), suggesting that these models successfully scale the microscopic processes to the macroscopic observations. However, it was also found that very different microscopic mechanisms (e.g., adsorption vs. precipitation) can produce equally good fits of the same macroscopic data. This situation demonstrates the importance of upgrading kinetic models to include more detailed microscopic observations that were not possible to collect just a few years ago.