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Spectroscopic insights to the Fe(II)-Fe(III) redox system at mineral surfaces:

... Implications for iron mineral formation and contaminant reduction.Speaker: Dr. Philip Larese-Casanova, Geology and Geophysics Department, Yale UniversityAbstract: The attenuation of groundwater contaminants has been closely linked with the chemical reactivity of native iron minerals. Dissolved Fe(II) can provide electrons that reduce contaminants to less toxic products, and this electron transfer process is catalyzed by reactive surface sites on Fe(III) minerals. However, the Fe(II)-Fe(III) redox reactions occurring at mineral surfaces are not fully understood because it is difficult to target and directly observe the physical and chemical activity of surficial iron atoms. Over the past few years, our observations of iron surfaces have become more sensitive by using 57Fe-Mössbauer spectroscopy with selective use of 57Fe and 56Fe isotopes. The process of dissolved 57Fe(II) sorption onto 56Fe(III) oxide surface sites (56hematite) was revealed instead to be a combined process of 57Fe(II) sorption and electron transfer to the bulk oxide, forming a new 57Fe(III) surface layer. 56Hematite has a capacity for this 57Fe(III) layer growth beyond which stable sorbed 57Fe(II) atoms reside on its surface. The transferred electrons within hematite rapidly hop among Fe atoms, a process others predicted to occur using computational methods. These spectroscopic observations are the first of their kind and highlight the need for new metal sorption models to account for redox-active sorbents. Contaminant reduction by dissolved 57Fe(II) and 56Fe(III) oxides results in further 57Fe(III) surface layer growth and the formation of new surface iron minerals, such as nano-sized 57goethite on 56hematite or 57lepidocrocite on 56magnetite, and these surface precipitates can alter the rate of contaminant reduction. Fe surface precipitates can also form via microbial Fe(II)-oxidation, and we have quantified iron phases formed (57lepidocrocite and 57goethite) during microbial oxidation of 57Fe(II) by the Fe(II)-oxidizing bacterium Acidovorax sp. BoFeN1 and have examined their dependence on geochemical solution conditions. Overall, contaminant reduction by the Fe(II)-Fe(III) redox couple is highly dependant on the supporting mineral substrate, and subtle changes to mineral surfaces or geochemical conditions can have profound effects on contaminant reduction rates or Fe(III) mineralogy.

Location: Kaprielian Hall (KAP) - 209 )

Audiences: Everyone Is Invited

Contact: Evangeline Reyes