12 Jun 2026

Adsorption of the Microbial Exopolysaccharide from Rhizobium Tropici (ATCC 49,672) on Birnessite Facilitates Mn Reduction and Dissolution: Structural Interactions and Morphological Transformation

Abstract: The study addresses the broader question of how microbial exopolysaccharides (EPS) can modulate the reactivity and stability of manganese (Mn) oxides in environmental systems. The objective of this study is to investigate the adsorption process of EPS from Rhizobium tropici on birnessite (MnO2) and mechanisms of induced Mn reduction and dissolution and its further transformation. EPS adsorption isotherms and kinetics on birnessite and the induced dissolution of birnessite were analyzed. Adsorption kinetics could best be described with the Parabolic Diffusion Model while EPS adsorption isotherm was best fitted with the Freundlich model. Adsorbed EPS facilitated Mn reduction and dissolution, releasing Mn²⁺ into the solution. This was also confirmed by fourier transform infrared spectroscopy (FTIR) results with the Mn-O vibrations, O–H groups and C = O carbonyl stretching. The pre-presence of EPS hindered the formation and growth of crystalline structure of birnessite and further hindered its transformation into more stable β-MnO2. The presence of Mn oxide significantly aggregated EPS, forming large aggregates. This study reveals the importance of EPS in influencing both the adsorption behavior and the redox state of Mn, providing significant insights into the interactions of EPS with birnessite in environmental systems.

21 Jun 2024

Iron Oxidation–Reduction Processes in Warming Permafrost Soils and Surface Waters Expose a Seasonally Rusting Arctic Watershed

Abstract: Landscape-scale changes from climate change in the Arctic affect the soil thermal regime and impact the depth to permafrost in vulnerable tundra watersheds. When top-down thaw of permafrost occurs, oxygen and porewaters infiltrate deeper in the soil column exposing fresh, previously frozen material and altering redox conditions. A gap remains in understanding how redox stratifications in thawing permafrost impact the geochemistry of watersheds in response to climate change and how investigations into redox may be scaled by coupling extensive geophysical mapping techniques. In this study, we collected soils and soil porewaters from three soil pits and surface water samples from an Arctic watershed on the North Slope of Alaska and analyzed for trace metals iron (Fe) and manganese (Mn) and Fe oxidation state using bulk and microscale techniques. We also used geophysical mapping and soil thermistors to measure active layer depths across the watershed to relate accelerating permafrost thaw to watershed geochemistry. Overall, evidence showed that Fe and Mn could be useful as geochemical indicators of permafrost thaw and release of Fe(II) from thawing permafrost and further oxidation to Fe(III) could translate to a higher degree of seasonal rusting coinciding with the warming and thawing of near surface-permafrost.