Biotic and Abiotic Contributions to the Reduction of Fe-Oxides at Circumneutral pH.

Although the formation of Fe-oxides at circumneutral pH in streams and soils was previously believed to be an abiotic process, increasing attention has been drawn to microbial effects on metal cycling and biomineralization at these conditions. In this study, we investigate the biotic and abiotic factors controlling the reduction of environmental bacteriogenic Fe-oxides (EBIOS) at circumneutral pH and any accompanying phase changes. Composite EBIOS and synthetic 2-line ferrihydrite (2LFh) were incubated for 2 weeks under one aerobic (~20% O₂) and two anoxic (100% N₂ and 95/5% N₂/H₂) conditions. Total Fe and Fe(II) concentrations in solution were measured on days 1, 3, 7, 10, and 14, and changes in Fe-oxide mineralogy were measured by using X-ray diffraction (XRD). Dissolved Fe(II), resulting from reduction of EBIOS samples, was detected under all conditions by trapping with the Fe(II) complexing agent 1,10 phenanthroline. The rate of reduction was comparable for all aerobic and anoxic conditions, averaging a 32 μM/day increase in Fe(II), and the addition of a microbial metabolism inhibitor did not significantly affect reduction rates in the presence of 1,10 phenanthroline. Additionally, XRD of EBIOS samples revealed slight development of the mixed-valence Fe mineral magnetite under 95/5% N₂/H₂ conditions, as well as the formation of the Fe(III) oxide-hydroxide mineral goethite in the presence of 1,10 phenanthroline under both aerobic and 95/5% N₂/H₂ conditions. In contrast, 2LFh samples experienced minimal reduction and did not exhibit similar phase changes. Based on these results, we hypothesize that reduction of EBIOS at circumneutral pH may occur abiotically through reactions between Fe and organic matter within the biofilms. Understanding the reduction mechanism(s) of EBIOS will enhance our understanding of both Fe biogeochemical cycling and the bioavailability of environmental contaminants.