PNNL

Abstract

Transformation of nanocrystalline ferrihydrite to more stable microcrystalline Fe(III) 10 oxides is rapidly accelerated under reducing conditions with aqueous Fe(II) present. While the 11 major steps of Fe(II)-catalyzed ferrihydrite transformation are known, processes in the initial 12 phase that lead to nucleation and growth of product minerals remain unclear. To track 13 ferrihydrite - Fe(II) interactions during this initial phase, we used Fe isotopes, Mössbauer 14 spectroscopy, and extractions to monitor structural, magnetic, and isotope composition changes 15 of ferrihydrite within ~30 minutes of Fe(II) exposure. We observed rapid isotope mixing 16 between aqueous Fe(II) and ferrihydrite during this initial, lag phase. Our findings from 17 Mössbauer spectroscopy indicate that a more magnetically-ordered Fe(III) phase initially forms 18 that is distinct from ferrihydrite and bulk crystalline transformation products. The signature of 19 this phase is consistent with early stage emergence of lepidocrocite-like lamellae observed in 20 previous transmission electron microscopy studies. Its signature is furthermore removed by 21 xylenol extraction of Fe(III), the same approach used to identify a chemically labile form of 22 Fe(III) resulting from Fe(II) contact that is correlated to the ultimate emergence of crystalline 23 product phases detectable by X-ray diffraction. Our work indicates that the mineralogical 24 changes in the initial, lag phase of Fh transformation initiated by Fe(II)-Fh electron transfer are 25 critical to understanding ferrihydrite