Spectroscopic Evidence for Interfacial Fe(II)-Fe(III) Electron Transfer in a Clay Mineral

Interfacial electron transfer has been shown to occur between sorbed 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 Fe-57 Mossbauer spectroscopy to demonstrate electron transfer between sorbed Fe(II) and structural Fe(Ill) in an Fe-bearing smectite clay mineral (NAu-2, nontronite). Mossbauer spectra of NAu-2 reacted with aqueous Fe(II) (which is invisible to Fe-57 Mossbauer spectroscopy) showed direct evidence for reduction of NAu-2 by sorbed Fe(II). Mossbauer spectra using aqueous Fe-57(II) showed that sorbed Fe(II) is oxidized upon sorption to the clay and 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 delocalization 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.