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.