Redox potential measurements and Mössbauer spectrometry of Fe II adsorbed onto Fe III (oxyhydr)oxides

Abstract

The redox properties of Fe II adsorbed onto a series of Fe III (oxyhydr)oxides (goethite, lepidocrocite, nano-sized ferric oxide hydrate (nano-FOH), and hydrous ferric oxide (HFO)) have been investigated by rest potential measurements at a platinum electrode, as a function of pH (−log 10[H +]) and surface coverage. Using the constant capacitance surface complexation model to describe Fe II adsorption onto these substrates, theoretical values of the suspension redox potential ( E H) have been computed, under the assumption that Fe II adsorption occurs at crystal growth sites of the substrate surface. Good agreement between calculated and experimental E H values is observed for nano-FOH and HFO, however the redox potentials measured for lepidocrocite and goethite are significantly more oxidizing than predicted. Mössbauer spectroscopic analysis of 57Fe II adsorbed onto HFO and goethite shows that in both cases the adsorbed 57Fe II is incorporated into the crystal structure of the substrate, in broad agreement with the thermodynamic model, but is almost completely oxidized to 57Fe III. The mechanism by which the adsorbed 57Fe II is oxidized is not resolved in this work, but is thought to be due to electron transfer to the substrate, rather than a net oxidation of the suspension. The disagreement between experimental and calculated rest potential measurements in the goethite and lepidocrocite systems is thought to be due to the poor electrochemical equilibration of these suspensions with the platinum electrode, rather than a failure of the thermodynamic model. The model developed for the redox potential of adsorbed Fe II allows direct assessment of the reactivity of this species towards oxidized pollutants.