Inhibition of the reduction of Cr(VI) at the magnetite–water interface by calcium carbonate coatings

The effect of calcium carbonate coatings on the reduction of aqueous chromate on the magnetite(1 1 1) surface has been investigated using a combination of synchrotron based X-ray photoemission spectroscopy (PES) and X-ray absorption near edge structure (XANES) spectroscopy, along with laboratory-based powder X-ray diffraction (XRD) and scanning electron microscopy (SEM). CaCO₃ coatings (dominantly calcite with minor quantities of aragonite and vaterite) of thicknesses ranging from 10 Å to 20 m were grown on magnetite(1 1 1) surfaces by exposure to supersaturated aqueous solutions followed by evaporation of the solution—a process that mimics pore-water evaporation in vadose zones leading to the formation of caliche and calcium carbonate coatings on mineral grains. Coating thicknesses were determined from attenuation of the Fe 2p photoemission signal by the carbonate coating. For coatings less than 15 Å thick, Cr 2p photoemission and Cr L_II, L_III-edge XANES spectra show that chromate is reduced by the underlying magnetite surface; however, as the minimum coating thickness increases beyond 15 Å, the magnetite surface becomes passivated and further chromate reduction ceases. Our findings suggest that carbonate coatings on natural magnetite grains can significantly reduce or eliminate their ability to reduce Cr(VI), which is a toxic and highly mobile environmental contaminant.     

Fundamental reflectivity spectra of Hg1−xMnxTe mixed crystals in the 1.7 to 3.5 eV energy range

The room temperature reflectivity coefficient R(E) for the mixed crystals Hg_1?xMn_xTe (x up to 0.57) in the energy range 1.7 to 3.5 eV was investigated. Two distinct maxima E₁ and E₁ + Δ₁ connected with the transitions in the critical points on the [111] direction of the Brillouin zone for the samples with x up to 0.3 and the more diffused structure of R(E) for the samples with x > 0.3 was observed. A quadratic dependence of E₁ and E₁ + Δ₁ transition energy vs alloy composition with x up to 0.3 was found, with bowing coefficient c = 1.21 ± 0.02 and 1.06 ± 0.02 respectively. The energy variation of an additional shoulder probably connected with the e₁ transitions at L point of the Brillouin zone is also reported.     

New surface phases for potassium adatoms on cleaved Si(111)

In a previous study (B. Reihl and K.O. Magnusson, Phys. Rev. B 42 (1990) 11 839) no change was observed in the LEED pattern from the 2 × 1 pattern of the clean Si(111) surface upon potassium adsorption. In contrast to this paper, we observe six surface phases during room temperature dosing of K on the cleaved Si(111) surface. In addition, we observe a 3 × 1 pattern on the cleaved K/Si(111) surface upon annealing. This paper will provide photographs of the new phases observed with LEED. It also will set limits on the K coverage required for these phases using results from photoemission and secondary electron cutoff measurements. Tentative models for these surfaces will be proposed. These new LEED results show a more complex interaction between K and the cleaved Si(111) surface than previously thought and hopefully will encourage further exploration of adsorption on the cleaved Si(111) surface.     

Formation of hydroxyl and water layers on MgO films studied with ambient pressure XPS

To understand the interaction of water with MgO(100), a detailed quantitative assessment of the interfacial chemistry is necessary. We have used ambient pressure X-ray photoelectron spectroscopy (XPS) to measure molecular (H₂O) and dissociative (OH) water adsorption on a 4 monolayer (ML) thick MgO(100)/Ag(100) film under ambient conditions. Since the entire 4 ML metal oxide (Ox) film is probed by XPS, the reaction of the MgO film with water can be quantitatively studied. Using a multilayer model (Model 1) that measures changes in Ox thickness from O 1s (film) and Ag 3d (substrate) spectra, it is shown that the oxide portion of the MgO film becomes thinner upon hydroxylation. A reaction mechanism is postulated in which the top-most layer of MgO converts to Mg(OH)₂ upon dissociation of water. Based on this mechanism a second model (Model 2) is developed to calculate Ox and OH thickness changes based on OH/Ox intensity ratios from O 1s spectra measured in situ, with the known initial Ox thickness prior to hydroxylation. Models 1 and 2 are applied to a 0.15 Torr isobar experiment, yielding similar results for H₂O, OH and Ox thickness changes as a function of relative humidity.