What is it about?
Manganese oxides are common minerals in soils and sediments that control the cycling of carbon, nutrients, and contaminants. We measured how fast three types of manganese oxides — birnessite, manganite, and hausmannite — dissolve when electrons are transferred to them, using chemical compounds that mimic natural electron shuttles. We found that a thermodynamic quantity called the Pourbaix potential, which accounts for pH, redox conditions, and mineral stability, predicts reduction rates across all three oxides without requiring detailed knowledge of the reaction pathways. We also developed a model that separates intrinsic electron transfer rates from physical transport effects, and used classical nucleation theory to explain why the three oxides react at different speeds.
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Why is it important?
Manganese oxides are powerful natural oxidants, yet their reactivity has been much harder to predict than that of iron oxides. This study provides a generalizable thermodynamic framework — based on the Pourbaix potential — that predicts manganese oxide reduction rates across different mineral phases and solution conditions without requiring predefined reaction stoichiometry. This is especially valuable in natural environments where the exact redox pathways are unknown. The approach is broadly applicable to other redox-active minerals and could significantly improve our ability to model nutrient cycling, contaminant transformation, and organic matter degradation in soils and aquatic systems.
Perspectives
This paper grew out of a simple question: can we predict how fast a mineral dissolves just from thermodynamics? What started as a follow-up to our earlier work on iron oxides turned into a much richer story once we brought in the Pourbaix potential and nucleation theory. Manganese is notoriously complex with its three oxidation states and diverse oxide structures, so it was genuinely satisfying to see a single thermodynamic framework capture the reactivity of such different minerals.
Meret Aeppli
Swiss Federal Institute of Technology Lausanne (EPFL)
Read the Original
This page is a summary of: Predicting rates of manganese oxide reduction from thermodynamic driving forces and structural properties, Proceedings of the National Academy of Sciences, May 2026, Proceedings of the National Academy of Sciences,
DOI: 10.1073/pnas.2525899123.
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