Aqueous Mn(III) species as geochemical vectors of manganese

Manganese (Mn) is widely distributed across the earths surface. Mn acts as an essential micronutrient for all organisms and its redox cycle impacts numerous environmental processes in terrestrial and aquatic environments. Thus, a comprehensive and quantitative understanding of the Mn redox cycle is important to maintain and improve environmental quality and sustainability. In nature, Mn exists in three oxidation states (Mn(II), Mn(III) and Mn(IV)). Mn readily changes its oxidation state and thus acts as an electron transfer agent in oxidation-reduction reactions. Mn(II) is known as the primary aqueous form of Mn and can be oxidized to Mn(III) and Mn(IV) via biotic and abiotic oxidation reactions. Solid phase Mn(III, IV) species are among the strongest natural oxidants that readily oxidize trace metals and organic compounds. Until recently, the natural occurrence of aqueous Mn(III) was largely ignored due to its low solubility and redox instability. However, several top-ranked research journals in analytical chemistry, environmental sciences and biogeochemistry fields have reported substantial amounts of aqueous Mn(III) complexed by organic and inorganic ligands in natural systems, and emphasized its ubiquitous presence in soil, sediment, marine and estuarine systems. As a redox intermediate, the Mn(III)-complexes have the potential to acts as efficient catalysts and powerful reactants in different environmental processes such as soil carbon (lignin) degradation, depolymerization of microplastics, oxidation of metal(loid) contaminants. However, knowledge of aqueous Mn(III) chemistry is sparse both with respect to the kinetics of which high affinity ligands dissolve and stabilize soluble Mn(III) species, and the mechanism through which the reactivity of soluble Mn(III) species modulates the redox activity in various natural environments. The key aim of this project is to examine the formation and the reactivity of abiotically produced Mn(III)- ligand complexes under environmentally relevant conditions that will be simulated through experimental research using a mixture of soil minerals and biogenic organic ligands. This project will take place in the department of Civil and Environmental Engineering at the University of California, Davis under the supervision of Professor Jasquelin Peña from January 2021 to December 2022. During this period of time, the Earth and Environmental Science Area at the Lawrence Berkeley National Laboratory, where Prof. Peña is secondly affiliated, will act as a research collaborator. In the return phase the project will be continued under the supervision of Priv.-Doz. Dr. Markus Puschenreiter at University of National Resources and Life Sciences (BOKU) in 2023.

Manganese (Mn) is widely distributed across the earths surface. In nature, Mn exists in three oxidation states (Mn(II), Mn(III) and Mn(IV)) and readily changes its oxidation state by undergoing biological and mineral surface catalyzed processes. Until recently, the natural occurrence of aqueous Mn(III) was largely underappreciated due to its redox instability (e.g. disproportionation). However, recent studies have demonstrated the existence of substantial amounts of aqueous Mn(III) complexed by ligands, and emphasized its ubiquitous presence in various environments including soil, sediment, marine and estuarine systems. The goal of this project is to develop a mechanistic and quantitative understanding of the formation, stability and reactivity of aqueous Mn(III) species in order to better understand their role in nutrient and contaminant cycling. As a redox intermediate, the Mn(III)-complexes have the potential to acts as efficient catalysts and powerful reactants in different environmental processes such as soil carbon (lignin) degradation, depolymerization of microplastics, oxidation of metal(loid) contaminants. However, knowledge of aqueous Mn(III) chemistry is sparse both with respect to the kinetics of which high affinity ligands dissolve and stabilize soluble Mn(III) species, and the mechanism through which the reactivity of soluble Mn(III) species modulates the redox activity in various natural environments.