Roles of aqueous Fe(III) in oxidation of partially reduced nontronite under sub-acidic conditions

The interfacial Fe(II)-Fe(III) electron transfer in iron-rich clay minerals is a critical process for redox transfor-mation of many environmental contaminants including As, Sb, Tc, CCl4 and nitroaromatic compounds. Present findings of electron transfer from aqueous Fe(II) to structural Fe(III) in clay minerals raise question whether electron transfer occurs reversely from structural Fe(II) to aqueous Fe(III). To test this hypothesis, pretreated nontrontite NAu-1 was partially reduced by dithionite, and this was then mixed with FeCl3 in an acetic acid/ acetate buffer (pH 4.0). Structural Fe(II) was rapidly oxidized to Fe(III) whilst aqueous Fe(III) was corre-spondingly reduced to Fe(II). Low-temperature (13K) Mossbauer, accompanied with FTIR, spectroscopy pro-vided evidence for the oxidation of structural Fe(II) based on the almost complete disappearance of the Fe(II) doublet, and the re-appearance of Fe(III)Fe(III)-OH absorption bands. The precipitation of aqueous Fe(III) in acetate buffer with elevated pH value might inhibit interfacial Fe(II)-Fe(III) electron transfer, as solid phase Fe (III) was redox-inactive to structural Fe(II) over a time scale of less than 24 h. Given that both the XRD and chemical extraction results rule out the possibility of electron transfer through basal planes of NAu-1, we hy-pothesize that the edges may be responsible for the Fe(II)str-Fe(III)aq electron transfer. The overall results are important for assessing iron cycling in subsurface environments, as well as the fate and transport of con-taminants.