Electron transfer at birnessite/organic compound interfaces: mechanism, regulation, and two-stage kinetic discrepancy in structural rearrangement and decomposition

Electron transfer between birnessite and organic compounds (OC) plays a dominant role in the coupling cycle of manganese (Mn) and carbon across diverse environmental settings. While previous studies have extensively investigated individual processes of interface Mn reduction, surface Mn²⁺ adsorption, and surface-to-interior electron transfer, the dynamic interplay among these reactions and the mechanisms regulating subtle changes in surface and interior Mn states remained poorly understood. Additionally, existing models have not adequately captured electron transfer kinetics in multivariable systems involving pH, Mn²⁺ concentration, electron donor type, etc. In this study, we investigated the reduction kinetics of birnessite under the influence of multiple environmental variables by employing three typical OC: formic acid (HCOOH), formaldehyde (HCHO), and methanol (CH₃OH). Time-series analysis revealed kinetic discrepancy and time lag between the alteration of the average Mn oxidation state (AMOS) within the solid and the release of Mn²⁺ from the reductive dissolution of birnessite, indicating a two-stage electron transfer mechanism occurring at the interface between birnessite and OC. X-ray absorption fine structure spectra revealed a rapid increase in corner-sharing MnO₆ octahedra and a decline in AMOS during the initial stage, followed by a slight decrease in AMOS and substantial mineral dissolution to release Mn²⁺ in the subsequent stage. The transition point between the two stages is primarily influenced by the concentration of surface Mn^II under pH regulation, as confirmed by soft X-ray absorption spectroscopy and density functional theory calculations. Based on these findings, the adsorption equilibrium and electron transfer rate were modeled by a machine learning framework (JAX), which is influenced by three main factors: pH, Mn²⁺ concentration, and OC types. The adsorption equilibrium constant for HCHO was one order of magnitude lower than for HCOOH, yet displayed a faster reaction rate due to higher electron transfer rates. Competitive adsorption of OC and Mn²⁺ on reactive sites was influenced by both pH and Mn²⁺ concentrations. Combining these parameters, we created a 3D surface plot that comprehensively considered the interplay between different elementary reactions, including competitive adsorption and redox reaction rates, thereby visualizing the kinetic regulation mechanisms in multivariable systems. Furthermore, a comprehensive rate equation for the reduction of birnessite by OC was developed to predict its behavior in natural settings. With an electron storage capacity of 2.7×10²³ electrons/mol Mn before structural decomposition or dissolution, we propose that birnessite can act as a geobattery driving cryptic elemental biogeochemical cycling. Our findings also suggest that the highly reversible redox reactivity of birnessite and the kinetics discrepancy in multi-step electron transfer reactions enable it to facilitate energy conversion among OC, sunlight, and microbes across a variety of temporal and spatial scales.