Fate of Arsenic In the Presence of Birnessite and Dissimilatory Metal Reducing Bacteria.

Manganese oxides are ubiquitous minerals in terrestrial systems with high sorptive and oxidative capacity, which can impact the cycling of trace metals including arsenic (As).  Multiple studies have examined arsenite (As(III)) oxidation and adsorption onto birnessite while many others have investigated the reduction of arsenate As(V) by dissimilatory metal reducing bacteria (DMRB).  However, few studies have combined both reductive and oxidative pathways in a single study and often the redox pathways and fate of arsenic have been difficult to decipher.  Here, we investigate competitive redox dynamics controlling the fate or arsenic by using a Donnan reactor where solid phase oxidants (Mn oxides) and reductants (Shewanella sp. ANA-3) are isolated by a semi-permeable membrane through which arsenic can migrate.  As(III) is simultaneously added to both chambers.  Arsenic(III) injected into the birnessite chamber is rapidly oxidized to As(V) which slowly redistributes across both chambers.  As(V) that enters the Shewanella chamber is then reduced by the bacteria to As(III).  The As(III) from the bacterial reduction is then able to diffuse back into the Mn oxide containing chamber and re-oxidized, leading to a gradual increase in aqueous Mn(II) concentrations due to reductive dissolution of the Mn oxide solid phase.  The rate of As(III) oxidation in the birnessite chamber decreases as the Mn oxide solid transforms in the presence of increasing Mn(II) concentrations.  Our results illustrate the complex interplay between oxidative and reductive processes on the fate and partitioning of arsenic in soils.