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

Abstract

The speciation of chromium in overlayers on atomically clean surfaces of single crystal magnetite (Fe 3O 4) and hematite (α-Fe 2O 3) were studied using Cr L-edge, Fe L-edge, and O K-edge X-ray absorption spectra collected with synchrotron radiation. The overlayers were produced by reaction of the surfaces with 5 mM or 50 μM Na 2CrO 4 aqueous solutions in a N 2-filled glove bag in opaque containers at exposure times ranging from 1 min to over 60 min. X-ray absorption data were acquired using total electron yield and more surface-sensitive Auger yield detection. O 1s, Cr 2p, and Fe 2p photoemission 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 ∼15 Å thick for the high concentration exposures. The thickness of the overlayer increases with Cr(VI) solution concentration and exposure time up to this maximum thickness. The presence of chemically shifted components in the O 1s 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 FeCr 2O 4, which contains octahedrally coordinated Cr(III), with Cr L-edge spectra of the aqueous Cr(VI)-exposed iron oxide surfaces shows that Cr(VI) is reduced 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 present in the outermost portion of the overlayer, indicating that Cr(VI) reduction is incomplete. Complete reduction of Cr(VI) was found on magnetite (111) exposed to the 50 μM Na 2CrO 4 solution for 1 min or longer, indicating rapid reaction kinetics. We find that while Cr adsorbed on the (11̄02) surface of hematite (Fe 2O 3) remains in the original Cr(VI) form, small amounts are reduced to Cr(III) on (0001) hematite surfaces. This difference is due to the presence of Fe(II) on the (0001) hematite surfaces produced during annealing in a vacuum environment. These results corroborate earlier studies which have demonstrated that oxides containing Fe(II) in the near-surface region can reduce Cr(VI) to Cr(III), and they show that the overlayer consists of a mixture of FeOOH- and CrOOH-like phases, with the latter concentrated in the outermost 7–15 Å and the former concentrated in the innermost portion of the passivating overlayer. At a thickness of >15 Å, the passivating overlayer causes a reduction in the kinetics of the Cr(VI)+3Fe(II)=Cr(III)+3Fe(III) redox reaction.