Connecting Observations of Hematite (α-Fe₂O₃) Growth Catalyzed by Fe(II)

Abstract

Electron exchange between aqueous Fe(II) and structural Fe(III) in iron oxides and oxyhydroxides is important for understanding degradation of environmental pollutants through its apparent constitutive role underlying highly reactive "sorbed Fe(II)" and by catalyzing phase interconversion among these minerals. Although a mechanistic understanding of relationships between interfacial Fe(II)(ads)-Fe(III)(oxide) electron transfer, bulk electron conduction, Fe(II) release, and phase transformation behavior is emerging, much remains unclear, in part due to poorly interconnected investigations. The focus of this study is on reconciling two mutually similar observations of Fe(II)-catalyzed hematite growth documented spectroscopically and microscopically under substantially different chemical conditions. Here, we employ iron isotopic labeling to demonstrate that hematite grown on the (001) surface in Fe(II)-oxalate solution at pH 2.10 and 348 K has magnetic properties that closely correspond to those of hematite grown in Fe(II) solution at pH 7.4 and room temperature. The temperature evolution and extent of the Morin transition displayed in these two materials strongly suggest a mechanistic link involving trace structural Fe(II) incorporation into the growing hematite. Our findings indicate that Fe(II) catalyzed growth of hematite on hematite can occur under environmentally relevant conditions and may be due to bulk electron conduction previously demonstrated for hematite single crystals.

Kinetics of Fe(II)-Catalyzed Transformation of 6-line Ferrihydrite under Anaerobic Flow Conditions

“…This is likely the reaction mechanism for ferrihydrite reacting with 0.36 mM injection FeSO 4 solution. It is possible that the release of Fe(II) and transformation to goethite was with a result of conduction through the bulk ferrihydrite and involved a series of redox-driven "conveyor belt" reactions as demonstrated in recent studies by Yania and Rosso, Handler et al and Rosso et al (36)(37)(38). The formation of magnetite in this case is likely to be accomplished by a more conventional heterogeneous nucleation and growth mechanism from the Fe(III) released during ferrihydrite structure breakdown and dissolution.…”

The effect of pH on stable iron isotope exchange and fractionation between aqueous Fe(II) and goethite

“…The coexistence of Fe(II) and Fe(III), where Fe(II) generally occurs as the aqueous component and Fe(III) as the solid component, is common during weathering of Fe(II)-bearing primary crystalline rocks (Fantle and DePaolo, 2004), biological and abiotic oxidation of Fe(II)-rich fluids (e.g., groundwater seeps, mine drainage) (Frierdich et al, 2011a;Burgos et al, 2012;Roden et al, 2012), or during dissimilatory iron reduction (DIR) (Crosby et al, 2005(Crosby et al, , 2007Johnson et al, 2008). These processes may drive secondary, abiotic reactions between aqueous and sorbed Fe(II) and solid Fe(III) phases via Fe(II)-Fe(III) electron transfer and atom exchange (ETAE) (Williams and Scherer, 2004;Pedersen et al, 2005;Yanina and Rosso, 2008;Handler et al, 2009;Jones et al, 2009;Catalano et al, 2010;Rosso et al, 2010;Schaefer et al, 2010;Frierdich et al, 2011b;Neumann et al, 2013).…”

<strong>Cr release from Cr-substituted goethite during the aqueous Fe(II)-induced recrystallization</strong>

“…Consequently, electron transfer between Fe(II)aq and Fe(III)oxide accelerates, resulting in increased rates of Fe atom exchange [32,45,47]. However, increases in sorbed Fe(II) beyond the capacity of the monolayer coverage can result in Fe(II) surface passivation, which prevents electron inject from outer-sphere to inner-sphere geometry and neutralizes the potential gradient on the surface of the goethite, and further inhibits the exchange between Fe(II)aq and Fe(III)oxide [46][47][48][49][50]. Based on the above results concerning Cr(III) release, the release of Cr(III) from Cr-goethite is considered to be dependent on Fe atom exchange and the efficiency of Fe(II)aq-induced recrystallization of Cr-goethite [49].…”

“…During this process, oxidative Fe(II) aq uptake occurs on the surface of iron (hydr)oxide. The transferred electrons migrate within the goethite structure, leading to the dissociation of iron (hydr)oxides [16,[45][46][47]. Sorbed Fe(II) is one of the most important driving forces for the interplay between Fe(II) aq and Fe(III) oxide [32,33,46].…”

“…The transferred electrons migrate within the goethite structure, leading to the dissociation of iron (hydr)oxides [16,[45][46][47]. Sorbed Fe(II) is one of the most important driving forces for the interplay between Fe(II) aq and Fe(III) oxide [32,33,46]. When the amount of sorbed Fe(II) is below the capacity of the monolayer coverage of goethite, Fe(II) sorption is proportional to the amount of Fe(II) aq added [32].…”