Electron Transfer, Atom Exchange, and Transformation of Iron Minerals in Soils: The Influence of Soil Organic Matter

Despite substantial experimentalevidence of electrontransfer,atom exchange, and mineralogical transformation during the reactionof Fe(II)(aq) with synthetic Fe(III) minerals, these processesare rarely investigated in natural soils. Here, we used an enrichedFe isotope approach and Mo''ssbauer spectroscopy to evaluatehow soil organic matter (OM) influences Fe(II)/Fe(III) electron transferand atom exchange in surface soils collected from Luquillo and CalhounExperimental Forests and how this reaction might affect Fe mineralcomposition. Following the reaction of Fe-57-enriched Fe(II)(aq) with soils for 33 days, Mo''ssbauer spectra demonstratedmarked electron transfer between sorbed Fe(II) and the underlyingFe(III) oxides in soils. Comparing the untreated and OM-removed soilsindicates that soil OM largely attenuated Fe(II)/Fe(III) electrontransfer in goethite, whereas electron transfer to ferrihydrite wasunaffected. Soil OM also reduced the extent of Fe atom exchange. Followingreaction with Fe(II)(aq) for 33 days, no measurable mineralogicalchanges were found for the Calhoun soils enriched with high-crystallinitygoethite, while Fe(II) did drive an increase in Fe oxide crystallinityin OM-removed LCZO soils having low-crystallinity ferrihydrite andgoethite. However, the presence of soil OM largely inhibited Fe(II)-catalyzedincreases in Fe mineral crystallinity in the LCZO soil. Fe atom exchangeappears to be commonplace in soils exposed to anoxic conditions, butits resulting Fe(II)-induced recrystallization and mineral transformationdepend strongly on soil OM content and the existing soil Fe phases. Soil OM decreased the extent of Fe(II)-Fe(III)electrontransfer and Fe atom exchange with goethite. The degree of Fe(II)-catalyzedchanges in mineral crystallinity depends on both the nature of initialFe minerals in soils and the presence of soil OM.