Spectroscopic study of the reaction of aqueous Cr(VI) with Fe3O4 (111) surfaces

We have used soft X-ray core-level photoemission and adsorption spectroscopies to study the reaction of aqueous sodium chromate solutions (50 μM Na2CrO4, pH 6 and 8.5) with clean surfaces of magnetite (111) prepared under UHV conditions. Chromium 2p photoemission and Cr L-edge absorption spectra indicate that tetrahedrally coordinated Cr(VI)aq, which reacts with magnetite (111), is reduced by a heterogeneous redox process to octahedrally coordinated Cr(III) and incorporated in an overlayer on the magnetite. The thickness of the reacted overlayer at pH 6 increases with increasing immersion time in the Na2CrO4 solution for up to ≈10 min but remains unchanged for longer immersion times. The reaction rate is initially fast and follows a logarithmic law. When the passivated magnetite can no longer reduce Cr(VI)aq, the passivating overlayer is 15±5 Å thick and consists of a chromium oxyhydroxide or hydroxide phase with only trace amounts of iron. Evidence for extensive hydroxylation in the overlayer is provided by a chemically shifted component in both the O 1s photoemission and O K-edge absorption spectra. The overlayer appears to lack long-range order based on loss of the first EXAFS-like feature in the O K-edge spectrum with increasing immersion time. Clear evidence for oxidation of Fe(II) to Fe(III) in the magnetite surface during reduction of Cr(VI)aq to Cr(III) is provided by Fe 2p photoemission and Fe L-edge absorption spectra. Strong attenuation of the Fe 2p signal during the first 10 min of the redox reaction indicates that iron does not outdiffuse significantly into the overlayer. At pH 8.5, the reaction follows a similar path, but its rate is lower, and Cr(VI)aq reduction continues for immersion times of up to 1 h. The results of this study are compared with results from an earlier XPS study of the passivation of zero-valent iron by chromate solutions [E. McCafferty, M.K. Bernett, J.S. Murday, Corros. Sci. 28 (1988) 559]. The resulting overlayer compositions, thicknesses, and reaction rates are very similar for the two systems.