Atom Exchange between Aqueous Fe(II) and Goethite: An Fe Isotope Tracer Study

Abstract

The reaction of aqueous Fe(II) with Fe(III) oxides is a complex process, comprising sorption, electron transfer, and in some cases, reductive dissolution and transformation to secondary minerals. To better understand the dynamics of these reactions, we measured the extent and rate of Fe isotope exchange between aqueous Fe(II) and goethite using a 57Fe isotope tracer approach. We observed near-complete exchange of Fe atoms between the aqueous phase and goethite nanorods over a 30-day time period. Despite direct isotopic evidence for extensive mixing between the aqueous and goethite Fe, no phase transformation was observed, nor did the size or shape of the goethite rods change appreciably. High-resolution transmission electron microscopy images, however, appear to indicate that some recrystallization of the goethite particles may have occurred. Near-complete exchange of Fe between aqueous Fe(II) and goethite, coupled with negligible change in the goethite mineralogy and morphology, suggests a mechanism of coupled growth (via sorption and electron transfer) and dissolution at separate crystallographic goethite sites. We propose that sorption and dissolution sites are linked via conduction through the bulk crystal, as was recently demonstrated for hematite. Extensive mixing between aqueous Fe(II) and goethite, a relatively stable iron oxide, has significant implications for heavy metal sequestration and release (e.g., arsenic and uranium), as well as reduction of soil and groundwater contaminants.

Laboratory study of spectral induced polarization responses of magnetite — Fe2+ redox reactions in porous media

“…Near complete isotopic exchange between 57 Fe 2+ (aq) and bulk goethite (α FeOOH) was observed at pH 7.5 by Handler et al (2009). They proposed a mechanism of Fe 2+ (aq) adsorption, electron transfer through the bulk mineral and simultaneous mineral growth and dissolution on separate crystal faces, similar to that proposed by Yanina and Rosso (2008) for hematite.…”

“…The adsorption of Ca 2+ , Ni 2+ and Fe 2+ to the magnetite surface progressively increases across this pH range (Vikesland and Valetine, 2002),. In the case of Fe 2+ , previous work on iron (oxyhydr)oxides including magnetite (Yanina and Rosso, 2008;Handler et al, 2009;Gorski et al, 2012) suggests that provided Fe 2+ can interact with the mineral surface, charge transfer can occur to the mineral lattice (see Figure 5b). We therefore infer that the influence of Fe 2+ adsorption on the phase response is likely to arise from redox reactions or charge transfer between adsorbed Fe 2+ and magnetite (such as reactions (1) and (2) described previously); the strong sensitivity of the phase peak frequency to increasing pH results from charge transfer via increasing amounts of adsorbed Fe 2+ ions.…”

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.…”

Experimental determination of the equilibrium Fe isotope fractionation between and FeSm (mackinawite) at 25 and 2°C

“…Experimental data on aqueous Fe species, Fe-oxides and Fe-carbonates have been documented and demonstrate that the largest Fe isotope fractionations are produced during redox reactions in both biologically mediated (Brantley et al, 2001Anbar, 2004;Johnson et al, 2004;Beard et al, 1999Beard et al, ,2003Icopini et al, 2004;Croal et al, 2004) and abiotic systems (Anbar et al, 2000;Skulan et al, 2002Brantley et al, 2004Welch et al, 2003;Matthews et al, 2004;Teutsch et al, 2005;Jang et al, 2008, Handler et al, 2009McAnena, 2009, Beard et al, 2010. Smaller, but significant fractionations have been seen in abiotic non-redox reactions (Wiesli et al, 2004;Wiedehold et al, 2006;Dideriksen et al, 2008;Mikutta et al, 2009), including the ligand-exchange process involved in mackinawite (FeS m ) formation (Butler et al, 2005).…”