In soils and sediments, iron (hydr)oxides are ubiquitous and exert a pronounced effect on the fate and transport of nutrients and contaminants. Iron cycling depends on a tight interplay between hydrodynamic transport (advection and diffusion) and (bio)geochemical reactions. Their relative magnitudes may vary significantly over small but environmentally-relevant spatio-temporal scales, leading to mass transfer limitations and the establishment of redox gradients at the aggregate scale. Such biogeochemical compartmentalization can, in turn, impact the distribution of microorganisms, as well as dictate the characteristics of predominant iron solid phases. To elucidate the iron cycling within soils and sediments, inclusive of phase distributions and micro-scale variation in transformation, we examine the intricate coupling of physical and biogeochemical processes driving iron transformations. Multi-pore domain experimental systems are coupled with reactive transport modeling to determine mass transfer and biogeochemical redox controls on the cycling of iron ranging from pore- to aggregate-scales.

The coupling of physical, chemical, and biological processes affecting iron cycling is determined through parallel experimentation using a novel aggregate-based reaction cells and reactive transport modeling. The outcome of our research will (1) provide unprecedented information on magnitude and small-scale variability in iron transformation within soils and sediments, (2) deduce the importance of mass transfer and biogeochemical reaction controls on iron cycling, and (3) reveal the importance of physical structure on biogeochemical heterogeneity within natural systems. Furthermore, this research will improve our understanding and predictive capability concerning the fate and transport of nutrients and contaminants which correlate with that of iron. (Collaboration with C. Meile, University of Georgia)