IRON ELECTRON TRANSFER AND ATOM EXCHANGE IN FERRIHYDRITE-NOM PRECIPITATES AND THE EFFECT ON CARBON STABILITY

Organic matter in soils and sediments is often associated with iron (Fe) minerals and this association can impact the rate of carbon cycling in soils, which has implications for soil health and global climate change. Ferrihydrite, a widespread Fe mineral commonly found with organic matter, is known to transform in the presence of Fe(II) raising the question of carbon stability associated with ferrihydrite. Here we explore the effect of different natural organic matters, as well as different Fe-C ratios and Fe(II) concentrations on Fe(II)-catalyzed ferrihydrite transformation. Specifically, we coprecipitated Suwannee river natural organic matter (SRNOM) with ferrihydrite and reacted it with Fe(II). Not surprisingly, we found that SRNOM-Fh at an Fe-C ratio of 1.2 stabilized ferrihydrite against secondary mineral transformation. Surprisingly, however, even in the absence of any measureable secondary mineral transformation, we observed extensive electron transfer and atom exchange between Fe(II) and Fe(III) in SRNOM-Fh co-precipitates similar to what we have previously observed in goethite and hematite. Our results suggest that inhibition of Fe(II)-Fe(III) electron transfer may not explain why ferrihydrite transformation is inhibited in the presence of SRNOM. Additional work is being done to determine whether the organic carbon redistributes during the exchange of Fe atoms and whether exchange alters carbon availability.