Manganese Redox Geochemistry in Soils

Résumé du livre

Manganese is highly redox-sensitive, and it occurs in three oxidation states, e.g., Mn(II), Mn(III), and Mn(IV), in many geochemical settings such as soils and sediments. Depending on the oxidation state and speciation, Mn plays multiple crucial roles in a variety of environments. While the oxidation state and speciation of Mn are thermodynamically controlled by pH and Eh that are often restrained by climate in soils, it remains largely unknown how climate affects the soil Mn redox geochemistry. By examining the Mn oxidation states and bioavailability in soils of multiple ecosystems at different spatial scales, the Mn oxidation state of soils was found to be predominantly controlled by the soil water balance. Specifically, in a well-controlled soil system with the only variable of mean annual precipitation (MAP), rainfall was found as the controller of oxidation state and availability of Mn. The increase in MAP caused the increase of Mn(II) fraction but the decrease of Mn(IV) fraction, because of the shifts of soil redox from oxic to anoxic conditions favoring the reduction of Mn(III,IV). The fraction of Mn(III) reached its maximum in soils at intermediate rainfalls because of the suboxic soil conditions. Reductive dissolution and rainfall leaching loss jointly led to the general decrease in total soil Mn, but a maximized exchangeable Mn(II) in soils at intermediate rainfalls. Soil water balance was the primary regulator of soil Mn redox chemistry even in the complex soil system at a continental scale, despite the great variances in many climatic and ecosystem factors that could affect the soil Mn redox chemistry as well. Soil redox can be defined as oxic, suboxic, and anoxic regimes with increasing the effective soil moisture. A suboxic regime occurred in soils near the neutral soil effective moisture, where Mn(III) reached the maximum. The release of Mn(II) from parent materials and carbonates due to weathering and the subsequent oxidation can contribute to an increase in Mn(IV) fraction and Mn average oxidation state, but a decrease in Mn(II) fraction. Sample drying does not change the overall pattern of Mn oxidation state as a function of soil water balance but induces alterations in oxidation state and availability of Mn depending on soils, suggesting that drying treatment of soils should be conducted with caution when studying highly redox sensitive metals. Due to the high redox sensitivity and the multiple oxidation states, Mn can also be utilized as a proxy to study paleoenvironments. While Mn oxides are predominantly formed through the oxidation of Mn(II) by O2 with assistance of microbes, the occurrence of Mn oxides on Mars has been interpreted to indicate the presence of O2 on early Mars. However, a recent study proposed that Mn oxides can form through the oxidation of Mn(II) by bromate (BrO3−), a strong oxidant widely distributed on Mars. It suggests that the occurrence of Mn oxides does not necessarily indicate an O2-rich paleoenvironment on early Mars. However, the reaction is strongly kinetically limited in solution phase and not observable with low concentrations of reactants, that may be more relevant to the environments on the surface of Mars. Our laboratory simulations showed that Fe oxides, the abundant semiconductor minerals on Mars, can catalyze and initiate oxidation of Mn(II) by bromate, with much lower concentrations of Mn(II) and bromate. Our study suggests that the mineral-surface catalyzed oxidation of Mn(II) by bromate is favorable from both thermodynamic and kinetic perspectives, and can be a major pathway for the occurrence of Mn oxides on Mars where lacks microorganisms to catalyze the reactions. This study further improves our understanding of the thermodynamic and kinetic controls on Mn(II) oxidation. All findings in this dissertation widen our views for understanding the geochemical behavior and multiple roles of Mn in various geochemical environments, from the laboratory molecular scale to the field continental scale. These findings also provide promising implications for the study of redox geochemistry of redox sensitive elements and elemental cycling of trace metals and nutrients.