Iron oxyhydroxide formation in the enhanced actinide removal plant

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. ■ INTRODUCTION Ferrihydrite is a nanoparticulate iron oxyhydroxide which is ubiquitous in the natural near-surface environment. It is also present in the core of the protein ferritin and has numerous industrial applications. Due to its high adsorptive capacity and ability to coprecipitate aqueous ions, ferrihydrite is a crucial sequester of contaminants in both natural and industrial systems. The properties of ferrihydrite which control its reactivity are sensitive to the aqueous conditions and mechanisms of formation; therefore, it is critical to have an understanding of ferrihydrite formation processes from solution at the molecular scale to underpin its environmental behavior and technological applications. Ferrihydrite is the first product of induced ferric hydrolysis and the phase that typically initially forms in natural aqueous environments. Hydrolysis of ferric iron and subsequent precipitation of ferrihydrite from solution have been studied extensively e.g. refs 8−13. The classic model indicates formation proceeds by successive polymerization steps: solvated Fe(III) ions undergo hydrolysis to form low molecular weight hydrated Fe(III) species (dimers, trimers), which go on to interact via olation and oxolation to form ferric species of higher nuclearity, leading to nucleation (i.e., formation) of ferrihydrite nanoparticles from solution. However, details of the ferric hydrolysis mechanisms are complex, and the extensive literature does not provide a unified view of ferrihydrite formation. Due to the high charge density of Fe(III), the hydrolysis reactions occur rapidly, rendering the isolation and characterization of intermediate hydrolysis products challenging. As such, the pathways from monomer to ferrihydrite and the structure of any intermediate species are unclear. Recent studies have indicated that the μ-oxo Fe(III) dimer is the dominant species in partially hydrolyzed ferric solutions, with no larger Fe(III) oligomers detected. Other studies have similarly concluded that no polycations larger than Received: May 18, 2016 Revised: August 1, 2016 Accepted: August 2, 2016 Published: August 2, 2016 Article