Mixed Mineralogical and Biogeochemical Controls On Arsenic Fate in Diffusively Controlled and Physically Complex Media.

Manganese (Mn) and iron (Fe) oxides are ubiquitous minerals that occur under similar redox conditions in terrestrial systems and have high sorptive capacities for many trace metals, including arsenic (As).  Although numerous studies have characterized the effects of As adsorption onto Fe and Mn oxides individually, the fate of arsenic within mixed systems representative of natural environments is unresolved.   Here, we examine competitive retention of As on goethite and birnessite using a Donnan reactor, where each oxide is isolated by a semi-permeable membrane through which arsenic can migrate.  To initiate the Donnan reactor experiments, As(III) is simultaneously added to both chambers.  Arsenic(III) injected into the birnessite chamber is rapidly oxidized to As(V) and then slowly redistributes across both chambers, while that added to the goethite chamber undergoes rapid adsorption;  oxidation of As(III) on goethite is then controlled by desorption and diffusion into the birnessite chamber. With increased reaction time, As(V) is generated and preferentially partitioned onto goethite at low As concentrations.  To further explore the role of Fe and Mn oxides in controlling the fate and transport of arsenic, we investigate arsenic dynamics in an aerobic aggregate composed of ferrihydrite and birnessite coated quartz sand fused by an agarose polymer.  Mn and Fe oxide coated sands, having pre-adsorbed As(V), are cast into cohesive spheres and inoculated with Shewanella sp. ANA-3, a bacterial strain capable of reducing As(V) and Mn and Fe oxides.  Arsenic(III) produced by bacterial reduction of As(V) diffuses into the aggregate exterior (proximal to advecting, aerated solutes), where it is re-oxidized to As(V) by Mn-oxides; following oxidation, As(V) is repartitioned onto the Fe oxides.  These results illustrate the dynamic interplay or biogeochemical transformation, physical heterogeneity of natural systems, and mixed sorbents on the fate of arsenic.