| This dissertation focuses on defining how the chemical and physical complexity of soils shapes the biogeochemical processes controlling the fate and transport of arsenic upon anaerobiosis.;I examined the extent to which structural Al influences reductive dissolution/transformations of ferrihydrite, a common and highly reactive Fe (hydr)oxides, and the resulting impacts on As retention. Aluminum impedes reductive transformation of ferrihydrite due to the increased stability and resulting redox insensitivity of Al-ferrihydrite. Higher concentrations (>2.0 mM) of dissolved Fe(II) are required to catalyze the transformation of Al-ferrihydrite to goethite, which is nearly 1000X the concentration needed for pure ferrihydrite transformation. The inability of Al-ferrihydrite to retain Fe(II), and to convert to secondary Fe phases, results in a greater rate and extent of Al-ferrihydrite dissolution relative to pure ferrihydrite in the presence of a model Fe reducer (Shewanella sp. ANA-3). However, the difference in Fe dissolution between Al-ferrihydrite and ferrihydrite is negligible in the presence of adsorbed As. In the long-term, structural Al will promote As desorption during concurrent reduction of Fe(III) and As(V) as As(III) adsorption on surfaces high in Al is limited.;To resolve the role of soil physical structure on As transport in soils, I examined As desorption from constructed aggregates---a fundamental unit of soil structure. Spherical artificial aggregates were made with As(V)-bearing ferrihydrite-coated sand inoculated with Shewanella sp. ANA-3 and placed in a cylindrical reactor; advective flow of anoxic or aerated solutes were then initiated around the aggregates. The greater extent of As desorbs under anoxic advecting flow compared to the aerated counterpart. With aerated advecting solutes, Fe(III) remained oxidized, or was oxidized, in the cortex of the aggregate, forming a 'protective' barrier that limited As release to the advective channel. However, anaerobiosis within the aggregate interior, even with aerated flow, ultimately promotes internal re-partitioning of As to the exterior region; due to As(V) and Fe(III) reduction in the interior, As diffuses and is retained proximal to the advecting flow domain. This finding suggest that As(V) and Fe(III) reduction may be an important processes even in seemingly aerated soil, and may promote As release by moving As to the transport front (the boundary between immobile and mobile water).;In soils and sediments, reductive dissolution and transformation of Fe (hydr)oxides have long been considered to trigger As release under anaerobic conditions; however, recent studies show that reductive biomineralization can sequester As. Formation of Fe(II)-As(III) solids has been proposed as a potential mechanism of As sequestration upon anaerobiosis, and, in fact, a solid of this type has been observed by abiotic reaction of Fe(II) with As(III). The formation of Fe(II)-As(III) precipitate was also observed during Fe(III) reduction of As(III)-sorbed lepidcrocite. However, such a solid phase has never been identified in soils and sediment. To understand the environmental conditions required for the formation of an Fe(II)-As(III) precipitate, the composition, structure and solubility of this solid was investigated.;Overall, the research presented here reveals the importance of chemical and physical complexity on the fate and transport of As within soils. In order to accurately predict the hazards imposed by As (either through biological availability or migration), we must therefore consider biogeochemical processes, particularly Fe and As reduction, in the context of the mineralogical and physical framework of soils. (Abstract shortened by UMI.). |
