Methods of observing iron mineral transformation in soil

• Catherine Hunt, Environmental Engineering, University of Delaware

• Luiza Notini, Civil, Construction, and Environmental Engineering, University of Delaware

Iron (Fe) minerals in soil heavily influence key biogeochemical cycling such as global carbon cycling (1), contaminant fate (2), and nutrient cycling (3). Ferrihydrite (Fh) is a naturally abundant Fe mineral uniquely important due to its high surface area-to-volume ratio and ability to facilitate sorption processes. For example, through the formation of Fe plaques on the roots of rice plants, Fh can limit arsenic plant uptake and regulate bioavailable arsenic (4). Despite its importance, our current understanding of Fe minerals is often limited to laboratory studies and the use of model organic ligands (5). Therefore, to improve our understanding of Fh transformation under more natural conditions, we synthesized Fh coprecipitated with increasing amounts of organic carbon. Simultaneously, we incubated pure mineral mesh bags of different sizes to investigate diffusion limitations. Both sets of mesh bags were spiked with a small amount of labeled 57Fe to enable the use of 57Fe Mössbauer spectroscopy. This analytical method only detects signal from 57Fe allowing us to track mineral evolution over time of the minerals we added. Preliminary results have shown us that smaller pure mineral mesh bags (50 mg and 100 mg) have similar end transformation products while large mesh bags (600 mg) may limit porewater diffusion, creating an unreacted inner core of the original added Fh. Incubation of coprecipitated Fhs is scheduled for later this summer, but we anticipate Fh transformation to be progressively more limited as organic carbon to iron ratios increase. The carbon acts as a protective layer for Fh, as seen by other studies (5, 6). By improving Fe mineral proxies through incorporation of natural impurities, we can contribute a more holistic understanding of soil cycling under dynamic soil conditions.  

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