Spectroscopic evidence for interfacial iron(II)-iron(III) electron transfer in clay minerals

Although interfacial electron transfer has been shown to occur for sorbed Fe(II) and Fe(III)-oxides, it is unclear if a similar reaction occurs between sorbed Fe(II) and Fe(III)-bearing clay minerals. Here, we use the isotope specificity of 57Fe Mossbauer spectroscopy to demonstrate electron transfer between sorbed Fe(II) and structural Fe(III) in a nontronite clay mineral (NAu-2). Appearance of an Fe(II) doublet in the NAu--2 spectra after reaction with aqueous 56Fe(II) ( 56Fe is transparent in Mossbauer spectra) provided evidence for reduction of structural Fe(III). Mossbauer spectra using enriched 57Fe(II) reveal that Fe(II) is oxidized upon sorption to NAu-2, and the oxidation product of this reaction is a ferric oxide with spectral parameters similar to lepidocrocite. The reduction of structural Fe(III) by Fe(II) induces electron delocalization in the clay structure, which we observe by variable-temperature Mossbauer spectra and macroscopic color change indicative of Fe(II)-Fe(III) pairs. The extent of structural Fe(III) reduction in NAu-2 is equal to the amount of Fe(II) sorbed until approximately 15% reduction, after which point reduction is no longer concomitant with the amount of sorbed Fe(II).