Predicting rates of manganese oxide reduction from thermodynamic driving forces and structural properties

Manganese (oxyhydr)oxides are abundant redox-active minerals that regulate carbon and nutrient cycling in the environment. Predicting the environmental reactivity of these oxides remains challenging due to the structural diversity and varying Mn oxidation states. We quantified the reduction kinetics of three geochemically relevant manganese oxides—birnessite, manganite, and hausmannite—using extracellular electron shuttles with varying redox potentials to systematically modulate the thermodynamic driving force for electron transfer. Rate-Gibbs free energy (ΔrG) relationships for individual manganese oxides could be established using our previously developed approach used to characterize iron oxide reduction. While ΔrG correlated with reduction kinetics for individual oxide phases, it failed to explain trends across different minerals. To address this challenge, we used the Pourbaix free-energy difference (Δ𝚿). It allowed us to predict reactivity across all three Mn oxides without requiring detailed knowledge of exact reaction stoichiometry, which is often unknown in natural systems. We further developed a coupled kinetic–mass transport model that we demonstrated that the three oxides share similar mass-transfer coefficients while their intrinsic electron-transfer rate constants differ significantly. Classical nucleation theory was applied to contextualize these differences, indicating that the balance between surface and bulk energies controls the dissolution barrier. Our work provides a predictive framework applicable to a variety of redox-active minerals, facilitating the modeling of redox fluxes in complex geochemical environments where mineral complexity previously hindered accurate predictions.