Ferrihydrite is a nanosized ferric oxy-hydroxide that is pervasive in near-surface aqueous environments and important in bio-geochemical element cycling. Its laboratory formation involves precipitation and aggregation of small hydrolysis products. The average sizes (diameter ~0.9 nm) reported for products formed at more acidic conditions were shown to be consistent with Fe13 Keggins (Weatherill et al., 2016). However, the atomic structural characteristics of the early-formed hydrolysis products are not yet known and important for understanding their relationship to ferrihydrite. We examined the size and structural characteristics of ferrihydrite precursors synthesized using a new method that involves mixed flow reactors (MFR) and in situ laboratory and synchrotron scattering measurements. Acidified Fe(NO3)3 and NaOH solutions were pumped continuously into a stirred 5-mL MFR at different rates to reach target pH conditions ranging from 0.5 to 4.5. The resulting suspension was then passed through an in situ x-ray scattering cell at &lt;5 min reaction time for laboratory small-angle x-ray scattering (SAXS) and synchrotron total scattering for pair distribution function (PDF) analysis. The average size of the hydrolysis products formed at pH &lt;2.5 are ~1.1 nm, consistent with prior studies using SAXS. In situ PDF data show that the products share similarities to ferrihydrite in terms of local structure, but are smaller and more structurally disordered with a coherence range &lt;1 nm. The average Fe-O bond length (~2.25 Å) of the precursors is also significantly longer than ferrihydrite (~2.0 Å). These results are consistent with a large fraction of singly-coordinated oxygens in the structures and agree well with what has been proposed based on computational simulations of Fe13 Keggin clusters (Das 2018). This new information about the atomic structures of synthetic ferrihydrite precursors will be important for understanding ferrihydrite formation and the roles of highly reactive nanoparticles in natural and applied system.

Size and structure of ferrihydrite precursor clusters

ACS Fall 2021

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