Interactions between manganese oxides and dissolved organic matter in soil

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Manganese oxides are ubiquitous soil minerals. They are effective sorbents for heavy metals and have a high oxidation potential for organic and inorganic compounds. The knowledge of factors that influence the reactivity of manganese oxides is crucial for predicting the cycling of nutrients and pollutants as well as organic matter in soils. An important but rarely studied factor is dissolved organic matter (DOM). There is currently little systematic information on (1) the adsorption of terrestrial DOM on manganese oxides, (2) redox reactions between DOM and manganese oxides, and (3) the influence of DOM on manganese oxide transformation reactions. For this reason, we plan batch experiments to explore the adsorption of different types of DOM (aqueous extracts from soil organic layers, bacterial exopolymers) to soil-relevant manganese oxides (delta-MnO2, cryptomelane, lithiophorite, todorokite) as a function of chemical variables such as pH, ionic strength and solution composition. Using modern, predominantly spectroscopic, spectrometric and chromatographic methods, we will investigate speciation changes of manganese as well as organic matter at the molecular level. In addition, we will evaluate the influence of DOM on the extent and kinetics of manganese(II)-catalyzed manganese-oxide transformation under reducing soil conditions. Overall, the project will significantly improve the understanding of the environmental reactivity of manganese oxides and the coupling between manganese and carbon cycles in soils.