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Ferrihydrite Formation: The Role of Fe 13 Keggin Clusters

DOI: 10.1021/acs.est.6b02481 DOI Help

Authors: Joshua Weatherill (University of Manchester) , Katherine Morris (University of Manchester) , Pieter Bots (University of Manchester) , Tomasz Stawski (University of Leeds. Helmholtz Centre Potsdam GFZ German Research Centre for Geosciences) , Arne Janssen (University of Manchester) , Liam Abrahamsen (University of Manchester) , Richard Blackham (Sellafield Ltd.) , Sam Shaw (University of Manchester)
Co-authored by industrial partner: Yes

Type: Journal Paper
Journal: Environmental Science & Technology

State: Published (Approved)
Published: August 2016
Diamond Proposal Number(s): 11075 , 12704

Abstract: Ferrihydrite is the most common iron oxyhydroxide found in soil and is a key sequester of contaminants in the environment. Ferrihydrite formation is also a common component of many treatment processes for cleanup of industrial effluents. Here we characterize ferrihydrite formation during the titration of an acidic ferric nitrate solution with NaOH. In situ SAXS measurements supported by ex situ TEM indicate that initially Fe13 Keggin clusters (radius ∼ 0.45 nm) form in solution at pH 0.12–1.5 and are persistent for at least 18 days. The Fe13 clusters begin to aggregate above ∼ pH 1, initially forming highly linear structures. Above pH ∼ 2 densification of the aggregates occurs in conjunction with precipitation of low molecular weight Fe(III) species (e.g., monomers, dimers) to form mass fractal aggregates of ferrihydrite nanoparticles (∼3 nm) in which the Fe13 Keggin motif is preserved. SAXS analysis indicates the ferrihydrite particles have a core–shell structure consisting of a Keggin center surrounded by a Fe-depleted shell, supporting the surface depleted model of ferrihydrite. Overall, we present the first direct evidence for the role of Fe13 clusters in the pathway of ferrihydrite formation during base hydrolysis, showing clear structural continuity from isolated Fe13 Keggins to the ferrihydrite particle structure. The results have direct relevance to the fundamental understanding of ferrihydrite formation in environmental, engineered, and industrial processes.

Subject Areas: Environment, Chemistry, Energy
Collaborations: Diamond Manchester
Diamond Offline Facilities: I22 support lab
Instruments: I22-Small angle scattering & Diffraction