Manganese oxides (MnO2) are ubiquitous in the environment and play an important role in the transformation and transport of contaminants in the environment and the biogeochemical cycling of carbon. This study investigated both aspects of these roles using a combination of laboratory experiments and field work. First, the kinetics of oxidation of the more toxic arsenic form, arsenite (As(III)), to the less mobile form, arsenate (As(V)), by MnO2 was determined and the mechanism of the reaction investigated. Subsequently, the effect of low As(V) concentrations on the anaerobic microbial respiration of MnO2 and iron oxides, Fe(OH)3, were investigated to shed light into the mechanism of As(V) toxicity in aquatic environments. At low As(V) concentrations, microbial reduction rates were increased compared to arsenic-free treatments suggesting As(V) at low concentrations stimulates metal reduction (MnO2, Fe(OH)3) in sediments as a result of a catalytic cycle in which arsenic is constantly reduced via microbial detoxification and abiotically re-oxidized by MnO2. The cumulative findings from this study suggest that arsenic is not simply immobilized in sediments and that a suite of complex abiotic and microbial processes including adsorption, intracellular transport, and redox reactions may affect arsenic transport in the environment, and ultimately the quality of drinking water resources.

In parallel, manganese cycling, and the dominant redox pathways involved in carbon remineralization processes were characterized in marine sediments across the Louisiana shelf and slope that are exposed to mineral and terrestrial organic inputs from the Mississippi River. Although sulfate reduction dominates on the continental shelf, denitrification and microbial manganese reduction appear equally significant anaerobic respiration processes along the continental slope the closest to the Mississippi River. This study emphasizes how the differential oxidation kinetics of Mn2+ (slow) and Fe2+ (fast) as well as sediment transport processes influence the main pathways of carbon degradation in these regions. Overall this thesis emphasizes the importance of characterizing the fundamental molecular processes of complex systems to better understand large scale global biogeochemical cycles.