Unraveling the Role of Competitive Oxidation and Adsorption On Arsenic Cycling.

Transport of arsenic (As) through soil environments is controlled by a complex web of competing sorption and redox reactions imparted by metal oxide sorbents and dissimilatory metal reducing microbes.  Here, we examine the combined roles of As(V)-reducing bacteria, Fe oxides, and Mn oxides  onAs cycling, and we explore their combined contributions within a diffusively-controlled system using a Donnan reactor.  Our results from competitive redox cycling of As in the presence of birnessite and As(V)-reducing bacteria, Shewanella sp. ANA-3, showed that As oxidation by birnessite is inhibited by the formation of rhodochrosite due to the production of carbonate during microbial respiration.  We also found that competitive adsorption of As onto Fe and Mn oxides is dependent on the relative surface coverage.  Iron oxides are the principle adsorbent at low surface loadings; however, Mn oxides have greater adsorptive capacity at higher As(III) concentrations due to surface alterations induced by surface redox reaction.  Furthermore, we used the reactive transport model MIN3P to describe reaction scenarios that may impact As cycling in complex biogeochemical systems including decreased electron donor availability, various oxidant and sorbent ratios, and varying As concentrations.