title: Ferrihydrite Formation: The Role of Fe13 Keggin Clusters.
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.
authors.forename: Joshua S
authors.initials: JS
authors.lastname: Weatherill
authors.forename: Katherine
authors.initials: K
authors.lastname: Morris
authors.forename: Pieter
authors.initials: P
authors.lastname: Bots
authors.affiliation: School of Earth and Environment, University of Leeds , Leeds LS2 9JT, U.K.|German Research Centre for Geosciences, GFZ , 14473 Potsdam, Germany.
authors.forename: Tomasz M
authors.initials: TM
authors.lastname: Stawski
authors.forename: Arne
authors.initials: A
authors.lastname: Janssen
authors.affiliation: National Nuclear Laboratory, Chadwick House, Warrington Road, Birchwood Park, Warrington WA3 6AE, U.K.
authors.forename: Liam
authors.initials: L
authors.lastname: Abrahamsen
authors.affiliation: Sellafield Ltd., Hinton House, Birchwood Park Avenue, Risley, Warrington, Cheshire WA3 6GR, U.K.
authors.forename: Richard
authors.initials: R
authors.lastname: Blackham
authors.forename: Samuel
authors.initials: S
authors.lastname: Shaw
mesh_terms: D000143:Acids; D005290:Ferric Compounds / Q000737:chemistry*; D053838:Scattering, Small Angle; D012996:Solutions; D014961:X-Ray Diffraction
chemical_list: D000143:Acids; D005290:Ferric Compounds; D012996:Solutions