Aggregate‐Scale Heterogeneity in Iron (Hydr)oxide Reductive Transformations

Abstract

There is growing awareness of the complexity of potential reaction pathways and the associated solid‐phase transformations during the reduction of Fe (hydr)oxides, especially ferrihydrite. An important observation in static and advective‐dominated systems is that microbially produced Fe(II) accelerates Ostwald ripening of ferrihydrite, thus promoting the formation of thermodynamically more stable ferric phases (lepidocrocite and goethite) and, at higher Fe(II) surface loadings, the precipitation of magnetite; high Fe(II) levels can also lead to green rust formation, and with high carbonate levels siderite may also be formed. This study expands this emerging conceptual model to a diffusion‐dominated system that mimics an idealized micropore of a ferrihydrite‐coated soil aggregate undergoing reduction. Using a novel diffusion cell, coupled with micro‐x‐ray fluorescence and absorption spectroscopies, we determined that diffusion‐controlled gradients in Fe2+(aq) result in a complex array of spatially distributed secondary mineral phases. At the diffusive pore entrance, where Fe2+ concentrations are highest, green rust and magnetite are the dominant secondary Fe (hydr)oxides (30 mol% Fe each). At intermediate distances from the inlet, green rust is not observed and the proportion of magnetite decreases from approximately 30 to <10%. Across this same transect, the proportion of goethite increases from undetectable up to >50%. At greater distances from the advective‐diffusive boundary, goethite is the dominant phase, comprising between 40 and 95% of the Fe. In the presence of magnetite, lepidocrocite forms as a transient‐intermediate phase during ferrihydrite‐to‐goethite conversion; in the absence of magnetite, conversion to goethite is more limited. These experimental observations, coupled with results of reactive transport modeling, confirm the conceptual model and illustrate the potential importance of diffusion‐generated concentration gradients in dissolved Fe2+ on the fate of ferrihydrite during reduction in structured soils.