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The adsorption of arsenic on an amorphous iron oxy(hydroxides) (ferrihydrite) under reductive conditions is reported. The fabrication of an ATR--IR spectroelectrochemical cell that allows the vibrational characterization of arsenate and arsenite adsorbed on a thin film of ferrihydrite is described. The cell is shown to allow the application of reductive conditions through the introduction of a working electrode that is positioned adjacent to the mineral phase. ATR--IR spectra reveal that increasingly negative solution potentials ([E.sub.h]) leads to the loss of adsorbed arsenate prior to the reductive dissolution of Fe(III) in the ferrihydrite. Under the experimental conditions, there is no evidence of reduction of arsenate to arsenite. Through the use of a miniaturized pH probe, the desorption of arsenate through electrochemical biasing is shown to arise solely from competitive adsorption from hydroxide ions produced by electrolysis of water. The results indicate that, within the time frame accessible in these measurements, only extreme reductive conditions are detrimental to arsenic sequestration in mine tailings facilities. Key words: arsenite, arsenate, ferrihydrite, reductive conditions, electrochemistry, ATR--IR. Nous faisons etat de l'adsorption de l'arsenic sur un (oxy)hydroxyde de fer amorphe (ferrihydrite) en conditions reductrices. Nous decrivons la fabrication d'une cellule spectroelectrochimique infrarouge a reflectance totale attenuee (ATR--IR) permettant la caracterisation vibrationnelle de l'arseniate et de l'arsenite adsorbes sur une couche mince de ferrihydrite. Nous avons montre que la cellule permet d'employer des conditions reductrices en y introduisant une electrode de travail placee de maniere adjacente a la phase minerale. Les spectres ATR--IR revelent qu'une augmentation du potentiel de la solution ([E.sub.h]) vers des valeurs de plus en plus negatives entraine une desorption de l'arseniate avant que la dissolution reductrice du Fe(III) present dans le ferrihydrite ne se produise. En conditions experimentales, nous n'observons aucun signe de reduction de l'arseniate en arsenite. A l'aide d'une sonde de pH miniaturisee, nous avons montre que la desorption de l'arseniate par polarisation electrochimique est le resultat direct de l'adsorption competitive d'ions hydroxyde produits par electrolyse de l'eau. Les resultats indiquent que, dans le cadre temporel d'execution de ces mesures, seules des conditions reductrices extremes peuvent nuire a la sequestration de l'arsenic dans les residus d'installations minieres. [Traduit par la Redaction] Mots-cles : arsenite, arseniate, ferrihydrite, conditions reductrices, electrochimie, ATR--IR., Introduction Understanding the process, mechanism, and stability of arsenic sequestration is of critical importance in the design and oversight of wastewater produced by mining activities. Arsenic is highly polymorphic and [...]

Abstract: The ternary, rare-earth filled (RE) Skutterudites (REM4 Pn 12; M = Fe–Os; Pn = P–Sb) have been proposed for use in high-temperature thermoelectric devices to convert waste heat to useful power. CeFe4Sb12 has been one of the most popular materials proposed for this application; however, it oxidizes at relatively low temperatures. The thermal stability of Skutterudites can be enhanced by selective substitution of the constituent elements and Eu(Fe,Ru)4Sb12 variants have been found to oxidize at temperatures above that of CeFe4Sb12. Unfortunately, these materials have poor thermoelectric properties. In this study, the thermal stability of CeFe4−x Ru x Sb12 was examined depending on the value of x. (These compounds have similar thermoelectric properties to those of CeFe4Sb12.) It has been found by use of TGA and XANES that the temperature at which point CeFe4−x Ru x Sb12 oxidizes increases with greater Ru substitution. XANES was also used to confirm the general charge assignment of Ce3+Fe4−x 2+Ru x 2+Sb12 1−. [Copyright &y& Elsevier]

The Fe K-edge X-ray absorption near-edge (XANES) spectra from Fe(1-x)Ga(x)SbO(4), having a rutile-like structure, have been investigated. Similar to the Ti K-edge XANES spectrum from TiO(2) (rutile), the low-energy pre-edge region observed in the Fe K-edge spectra is too broad to be representative of only a local, quadrupolar 1s → 3d excitation. The broadness of this peak results from the presence of a nonlocal transition, referred to as an intersite hybrid, which involves the excitation of 1s electrons to unoccupied 3d states of a next-nearest-neighbor Fe atom. (These 3d states overlap Fe 4p states of the absorbing atom through O 2p states.) With increasing Ga concentration, the intensity of the intersite hybrid peak decreases because of a deficiency of unoccupied next-nearest-neighbor 3d states. This observation provides important information on how the peak intensities of these nonlocal excitations are affected by substitution of the constituent elements.