Iron-Organic Matter Coprecipitates: Reactivity and Morphology.

Iron (Fe) is the 4th most abundant element of the Earth’s crust, and Fe (oxy)hydroxide minerals can account for up to 50% of the bulk mass of soils. In contrast to lab-synthesized ferrihydrite (Fhy), natural ferrihydrite is often formed by oxidation of Fe(II) in the presence of humic substances (HS). Despite the abundant presence of Fhy-HS coprecipitates in natural systems, there is a significant gap in our understanding of the (crystal) structure and reactivity of Fhy-HS coprecipitates. Thus, the objectives of this project are to investigate the impact of coprecipitated HS on the structure and bioreducibility of ferrihydrite. Fhy-HS coprecipitates were synthesized by hydrolyzing Fe(III) in the presence of different concentrations of soil humic acid. The Fhy-HS coprecipitates were characterized using electron microscopy, X-ray diffraction, X-ray absorption spectroscopy, scanning X-ray microscopy and their surface area and zeta-potential were determined. Fhy-HS coprecipitates with different C:Fe ratios (i.e., 0 to 1.2) were incubated with Fe reducing bacteria (i.e., Shewanella putrefaciens) under anoxic conditions for either 7 or 20 days. The production of Fe(II) was determined in the aqueous and solid phase over time to examine the effect of coprecipitated HS on the extent and kinetics of Fhy reduction as well as the secondary Fe phases formed. The C:Fe ratio influenced the Fe(III) reduction in a nonlinear fashion showing little impact of low HS concentrations (e.g., C:Fe = 0.1 to 0.3) but significantly increased total Fe(II) production at high C:Fe ratios (i.e., C:Fe = 0.5 to 1.2). However, HS at all concentrations resulted in higher solid phase Fe(II) concentrations relative to pure Fhy. Therefore, the effect of coprecipitated HS on microbial iron mineral transformation and reactivity has to be included in the framework of environmental iron biogeochemistry.