20Congreso Nacional 
Sociedad Mexicana de Ciencia de Superficies y Vacío A.C.

Interaction of Aqueous Chromium Ions with Iron Oxide Surfaces

Tom Kendelewicz
Surface and Aqueous Geochemistry Group, Department of Geological and Environmental Sciences, Stanford University, Stanford, CA 94305-2115, USA

To gain a more fundamental understanding of processes controlling reduction/oxidation reactions of aqueous chromate ions in subsurface environments we carried out molecular-level experimental studies of the interaction of water and aqueous chromate with well characterized single crystal samples of natural and synthetic magnetite and hematite. Clean (0001) and (1-102) surfaces of natural and synthetic hematites and clean (100) and (111) surfaces of natural magnetites were reacted with controlled, sequential doses of water vapor at partial pressures ranging from 10-9 to 10 Torr. These surfaces were characterized following each dose with photoelectron and adsorption spectroscopies. In each case a threshold pressure p(H2O)  of 10-4 to 10-3 Torr was observed, below which water reacted primarily with defect sites and above which water vapor reacted with terrace sites to produce extensive surface hydroxylation. The kinetics of reduction of Cr(VI) to Cr(III) and the nature of the reacted layer on clean (111) magnetite surfaces initially containing Fe(II) were determined by Cr and Fe L-edge and O K-edge x-ray adsorption and Fe 2p and O 1s photoemission spectroscopies. Chromium 2p photoemission and Cr L-edge absorption spectra indicate that tetrahedrally coordinated aqueous Cr(VI) 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. 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) to Cr(III) is provided by Fe 2p photoemission and Fe L-edge absorption spectra. The reduction rate for the 50 micro molar solution of sodium chromate where found to be pH dependent with the fastest rates occurring below the point of zero charge of magnetite (6.6). At the highest rate, the surface redox reaction is ~95 % complete within 10 minutes. The reacted magnetite surfaces were found to consist of a thin (15±5 Å) layer of poorly crystalline Cr(III)-(oxy)hydroxide that passivates the surface and prevents further reduction of aqueous Cr(VI). The reaction rates and products are similar to those previously observed for the reaction of aqueous Cr(VI) with "zero-valent" iron(McCafferty et al., Corrosion Sci. 28 (1998) 559. No redox reaction was observed on nonreduced hematite surfaces.

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