Iron oxide minerals are ubiquitous and host of number of interfacial reactions critical to the geological and environmental sciences, including iron isotopic fractionation, microbial iron reduction, contaminant retention, and trace element cycling. Recent advances in the use of stable iron isotopic tracers and 57Fe M�ssbauer spectroscopy have revealed that Fe(II) sorbed to iron oxide surfaces is transient and rapidly undergoes electron transfer and atom exchange (ETAE) with the underlying mineral. Fe(II)-Fe(III) ETAE reactions are capable of catalyzing complete iron atom exchange between aqueous Fe(II) and goethite. Here, we utilize a novel double-isotope tracer approach to show, for the first time, that mineral-water oxygen exchange is catalyzed by ETAE reactions between aqueous Fe(II) and goethite. The experimental design is based on reacting isotopically �normal� goethite with 57Fe-enriched Fe(II) solutions that contain 17O-labeled water for 30 days.  Our results show dramatic disparity between the amount of structural oxygen exchange with water relative to the amount of structural Fe(III) exchange with aqueous Fe(II), with only a small fraction of oxygen exchange occurring even when complete iron atom exchange is observed. This surprising result, which is unanticipated because either oxygen or iron isotopic exchange in iron oxides should break Fe-O bonds, suggests that oxygen reincorporates into the structure during recrystallization rather than interacting with the bulk fluid. Similar experiments show that trace elements (e.g., Ni, Zn) incorporated in the goethite structure are primarily retained within the mineral during Fe(II)-catalyzed iron oxide recrystallization instead of partitioning to the surface and fluid. In summary, isotopic tracers are powerful tools for tracking atom exchange across mineral-water interfaces and provide fundamental insight into the �hidden� reactivity of iron oxides during environmentally relevant reactions with aqueous Fe(II).

Probing the reactivity of iron oxide-water interfaces using oxygen and iron isotopic tracers

247th National Meeting (2014)

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