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.
Highlights
Ferrihydrite is a nanoparticulate iron oxyhydroxide which is ubiquitous in the natural near-surface environment
transmission electron microscopy (TEM) on samples collected at pH 1, 3, and 9 (Figures 4 and S13) show that the composition, stucture, and morphology of the particles were consistent with 2-line ferrihydrite, with no other phases detected
The thermodynamic calculations are based on the principles of classical nucleation theory (CNT), and as such Fe(III)aq is predicted to remain constant until the solution becomes saturated with respect to ferrihydrite (Fe(OH)3), at which point instantaneous precipitation is predicted to occur
Summary
Ferrihydrite is a nanoparticulate iron oxyhydroxide which is ubiquitous in the natural near-surface environment. At pH 3.1, a df of 2.3 indicates more densely packed aggregates with mass fractal structure[39] and shows the aggregates undergo densification with increasing pH, consistent with previous studies which showed densification of iron oxyhydroxide nanoparticles with changing chemical conditions.[69,70] This is concurrent with the rapid decrease in Fe(III)aq and TEM observation of mass fractal aggregates of ∼3 nm ferrihydrite nanoparticles at pH 3 (Figure S13) This highlights that during this stage of the reaction (pH up to 3.1) all dissolved Fe(III)aq i.e. Fe13 Keggins and smaller hydrolysis products (e.g., monomers and dimers) precipitate to form aggregates of ferrihydrite nanoparticles. This localized formation model is supported by the absence of Fe13 clusters in pH 1 ferric nitrate solution which was prepared by directly dissolving Fe(NO3)[3] in 0.1 M HNO3 (Figure S15)
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