X-ray absorption and photoemission study of the adsorption of aqueous Cr(VI) on single crystal hematite and magnetite surfaces

The speciation of chromium in overlayers on atomically clean surfaces of si ngle crystal magnetite (Fe3O4) and hematite (alpha-Fe2O3) were studied usin g Cr L-edge, Fe L-edge, and O K-edge X-ray absorption spectra collected wit h synchrotron radiation. The overlayers were produced by reaction of the su rfaces with 5 mM or 50 mu M Na2CrO4 aqueous solutions in a N-2-filled glove bag in opaque containers at exposure times ranging from 1 min to over 60 m in. X-ray absorption data were acquired using total electron yield and more surface-sensitive Auger yield detection. O 1s, Cr 2p, and Fe 2p photoemiss ion spectra taken on the same samples corroborate findings from the Cr and Fe L-edge and O K-edge spectroscopy, and show that the outermost portion of the overlayers contains mostly Cr with little if any Fe, and are similar t o 15 Angstrom thick for the high concentration exposures. The thickness of the overlayer increases with Cr(VI) solution concentration and exposure tim e up to this maximum thickness. The presence of chemically shifted componen ts in the O Is photoemission spectra indicates OH- ligands around Cr and Fe . Comparison of the Cr L-edge X-ray absorption spectra of crystalline PbCrO 4, which contains tetrahedrally coordinated Cr(VI), and crystalline FeCr2O4 , which contains octahedrally coordinated Cr(III), with Cr L-edge spectra o f the aqueous Cr(VI)-exposed iron oxide surfaces shows that Cr(VI) is reduc ed to Cr(III) on magnetite (111). For magnetite(111) surfaces reacted with 5 mM solutions for 10 min or longer, a significant amount of Cr(VI) is pres ent in the outermost portion of the overlayer, indicating that Cr(VI) reduc tion is incomplete. Complete reduction of Cr(VI) was found on magnetite (11 1) exposed to the 50 mu M Na2CrO4 solution for 1 min or longer, indicating rapid reaction kinetics. We find that while Cr adsorbed on the (1 (1) over bar 02) surface of hematite (Fe2O3) remains in the original Cr(VI) form, sm all amounts are reduced to Cr(III) on (0001) hematite surfaces. This differ ence is due to the presence of Fe(II) on the (0001) hematite surfaces produ ced during annealing in a vacuum environment. These results corroborate ear lier studies which have demonstrated that oxides containing Fe(II) in the n ear-surface region can reduce Cr(VI) to Cr(III), and they show that the ove rlayer consists of a mixture of FeOOH- and CrOOH-like phases, with the latt er concentrated in the outermost 7-15 Angstrom and the former concentrated in the innermost portion of the passivating overlayer. At a thickness of >1 5 Angstrom, the passivating overlayer causes a reduction in the kinetics of the Cr(VI) + 3Fe(II) = Cr(III) + 3Fe(III) redox reaction. (C) 1999 Publish ed by Elsevier Science B.V. All rights reserved.