Ferrihydrite (Fh) is the most bioavailable form of Fe(III) in the environment. In suboxic conditions, aqueous Fe(II) rapidly catalyzes Fh transformation to more stable phases such as lepidocrocite (Lp) and goethite (Gt), but mass transfer pathways have remained unclear. Here, using a selective extractant, we report isolation and quantification of a critical intermediate Fe(III) species involved, one that is more labile than the Fe(III) in Fh, that forms by oxidation of aqueous Fe(II) on the Fh surface. Comparison of time-dependent concentrations of solid-associated Fe(II) and labile Fe(III) against the kinetics of phase transformation showed that the accumulation of labile Fe(III) is constitutively related to Lp/Gt formation in a manner consistent with classical nucleation theory. <sup>57</sup>Fe isotope tracer experiments confirmed the origin of labile Fe(III) from the oxidation of sorbed Fe(II). A transformation mechanism entailing Lp/Gt product crystallization through olation and oxolation reactions that construct dense ordered networks from labile Fe(III) is implied, the rate of which is greatly accelerated by Fe(II).

Labile Fe(III) from sorbed Fe(II) oxidation is the key intermediate in Fe(II)-catalyzed ferrihydrite transformation

Macromolecular Chemistry: The Second Century

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ACS Spring 2020 National Meeting & Expo

The official archive and repository for all ACS Meetings & Expositions abstracts and select posters or presentations.