Understanding Mechanisms Coupling the Fates of Iron Oxides, Organic Matter, and Micronutrient Metals at Redox Hot Spots
EMSL Project ID
51922
Abstract
Iron (oxyhydr)oxide minerals are ubiquitous in soils and sediments and play an commanding role in nutrient cycling, microbial metabolism, carbon protection, and contaminant immobilization. Among them, ferrihydrite (Fh) is of particular interest because of its high reactivity and large specific surface area. Originating from the rapid precipitation of Fe(III), Fh is poorly crystalline and at ambient conditions tends to slowly transform into more thermodynamically stable ferric minerals such as goethite (Gt) and hematite (Hm). In anaerobic environments, this transformation can be strongly catalyzed by aqueous Fe(II), made commonly available through a variety of biogeochemical processes. Because Fh is a potent sorbent and repository for nutrient and contaminant metals, and dissolved organic matter, its transformations can strongly impact their distribution and transport in soils and aquatic systems. Mechanisms of Fe(II)-catalyzed Fh transformation at the molecular level have been a matter of debate for decades, though it is now well accepted that interfacial electron transfer between surface-associated Fe(II) and Fh is involved.In our recent EMSL supported work we discovered a labile Fe(III) species produced on the Fh surface by oxidation of Fe(II), revealing it to be the key intermediate that seeds nucleation and growth of the more crystalline Fe-(oxyhydr)oxide products. This discovery enables important mechanistic questions to be addressed for the first time. We will advance fundamental understanding in this system with a multi-method experimental and computational effort that capitalizes on the unique ability of EMSL to support an integrated research campaign. In particular, we will focus on quantifying the role of organic matter, and its transformation process and its impacts on incorporation and release of Cr, Mn, Ni, and Co micronutrient metals. Different roles of organic matter on Fe(II)-catalyzed Fh transformation have been proposed: (1) inhibition of interfacial electron transfer between sorbed Fe(II) and Fh due to surface-site blockage; (2) OM-induced changes to the surface charge of Fh and consequent inhibition of growth of more stable mineral phases by suppressing Ostwald ripening and/or oriented aggregation of nuclei; and (3) hindrance of Fh transformation by incorporation of OM into the crystal structure of mineralized products. Our proposal is centered on coupled solids and aqueous analytical characterization over time in detail, exploiting highly resolved electron microscopies, spectroscopies, and isotopically resolved elemental mapping, supported with computational molecular simulation of key underlying interfacial reaction steps. The goal is to unravel how organic matter controls the half-life of Fh in microaerophilic to anerobic soil environments and the extent to which micronutrient metals remain bioavailable.
Project Details
Project type
Large-Scale EMSL Research
Start Date
2021-10-01
End Date
N/A
Status
Active
Released Data Link
Team
Principal Investigator
Team Members