Spectroscopic Evidence for Interfacial Fe-Fe Electron Transfer in a Clay Mineral

摘要

Interfacial electron transfer has been shown to occur between sorted Fe(II) and structural Fe(III) in Fe oxides, but it is unknown whether a similar reaction occurs between sorbed Fe(II) and Fe(III)-bearing clay minerals. Here, we used the isotopic specificity of ~(57)Fe Moessbauer spectroscopy to demonstrate electron transfer between sorbed Fe(II) and structural Fe(III) in an Fe-bearing smectite clay mineral (NAu-2, nontronite). Mossbauer spectra of NAu-2 reacted with aqueous ~(56)Fe(II) (which is invisible to ~(57)Fe Moessbauer spectroscopy) showed direct evidence for reduction of NAu-2 by sorbed Fe(II). Mossbauer spectra using aqueous ~(57)Fe(II) showed that sorbed Fe(III) is oxidized uponsorption to the clayand pXRD patterns indicate that the oxidation product is lepidocrocite. Spectra collected at different temperatures indicate that reduction of structural Fe(III) by sorbed Fe(II) induces electron delocalizarjon in the clay structure. Our results also imply that interpretation of room temperature and 77 K Mossbauer spectra may significantly underestimate the amount of Fe(II) in Fe-bearing clays. These findings provide compelling evidence for abiotic reduction of Fe-bearing clay minerals by sorbed Fe(II), and require us to reframe our conceptual model for interpreting biological reduction of clay minerals, as well as contaminant reduction by reduced clays.