Ferrihydrite Formation: The Role of Fe13 Keggin Clusters

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 Fe-13 Keggin clusters (radius similar to 0.45 nm) form in solution at pH 0.12-1.5 and are persistent for at least 18 days. The Fe-13 clusters begin to aggregate above similar to 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 (similar to 3 nm) in which the Fe-13 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 Fe-13 clusters in the pathway of ferrihydrite formation during base hydrolysis, showing clear structural continuity from isolated Fe-13 Keggins to the ferrihydrite particle structure. The results have direct relevance to the fundamental understanding of ferrihydrite formation in environmental, engineered, and industrial processes.