Transport implications resulting from internal redistribution of arsenic and iron within constructed soil aggregates

Soils are an aggregate-based structured media that have a multitude of pore domains resulting in varying degrees of advective and diffusive solute and gas transport. Consequently, a spectrum of biogeochemical processes may function at the aggregate scale that collectively, and coupled with solute transport, determine element cycling in soils and sediments. To explore how the physical structure impacts biogeochemical processes influencing the fate and transport of As, we examined temporal changes in speciation and distribution of As and Fe within constructed aggregates through experimental measurement and reactive transport simulations. Spherical aggregates were made with As(V)-bearing ferrihydrite-coated sand inoculated with Shewanella sp. ANA-3; aerated solute flow around the aggregate was then induced. Despite the aerated aggregate exterior, where As(V) and ferrihydrite persist as the dominant species, anoxia develops within the aggregate interior. As a result, As and Fe redox gradients emerge, and the proportion of As(III) and magnetite increases toward the aggregate interior. Arsenic(III) and Fe(II) produced in the interior migrate toward the aggregated exterior and result in coaccumulation of As and Fe(III) proximal to preferential flow paths as a consequence of oxygenic precipitation. The oxidized rind of aggregates thus serves as a barrier to As release into advecting pore-water, but also leads to be a buildup of this hazardous element at preferential flow boundaries that could be released upon shifting geochemical conditions.