Continuum between Sorption and Precipitation of Fe(III) on Microbial Surfaces

Bacteria are a widespread, abundant, geochemically reactive component of aquatic environments. However, their role in the formation of secondary reactive surface phases such as iron oxides or in the direct sorption of metal contaminants has yet to be quantitatively described. Here, we compare the formation of iron oxides on bacterial cell surfaces to their formation abiotically (no bacteria present) over a range of both Fe(III) concentration (10-2−10-4.5 M) and pH (2−4.5) in the laboratory. Iron sorption and subsequent precipitation reactions at bacterial surfaces were modeled using current geochemical approaches. Solid-phase partitioning of Fe(III) as hydrous ferric oxide (HFO) was enhanced in the presence of a variety of bacteria over that seen in abiotic controls. The onset of HFO formation occurred at lower pH values and in greater quantities at any given pH in the bacterial treatments. Fe(III) reactions at bacterial surfaces follow a clear continuum between sorption and precipitation that can be quantitatively described using geochemical principles and modeled using surface precipitation theory; to date only demonstrated for inorganic surfaces. These results show that the reactions at biological surfaces are likely to be important in determining the spatial distribution of iron oxides in nature and thus the reactive transport of metals in aqueous environments.