Electrochemical analysis of changes in iron oxide reducibility during abiotic ferrihydrite transformation into goethite and magnetite

Electron transfer to ferric iron in (oxyhydr-)oxides (hereafter iron oxides) is a critical step in many processes that are central to the biogeochemical cycling of elements and to contaminant dynamics. Understanding these processes requires analytical approaches that allow for characterizing the reactivity of iron oxides towards reduction under thermodynamically controlled conditions. Here, we used mediated electrochemical reduction (MER) to follow changes in iron oxide reduction extents and rates during abiotic ferrous iron-induced transformation of six-line ferrihydrite. Transformation experiments (10 mM ferrihydrite-FeIII) were conducted over a range of solution conditions (pHtrans=6.5 to 7.5 at 5 mM Fe2+ and for pHtrans =7.00 also at 1 mM Fe2+) that resulted in the transformation of ferrihydrite into thermodynamically more stable goethite and/or magnetite. The mineralogical changes during the transformation were quantified using X-ray diffraction analysis. MER measurements on iron oxide suspension ali...