Allen Pratt

Chemical states in XPS and Raman analysis during removal of Cr(VI) from contaminated water by mixed maghemite-magnetite nanoparticles.

Mixed maghemite-magnetite has been used as adsorbent for Cr(VI) removal in this study. Results show that the adsorption capacity is enhanced with an increase in reaction temperature and decrease in free energy change. Thermodynamic study shows that Cr(VI) adsorption on the mixed maghemite and magnetite is endothermic in nature and is dependent on solution pH between 3 and 6. X-ray photoelectron spectroscopy (XPS) results demonstrate the theoretical multiplet peaks for iron and chromium adsorbed iron at the surface of the γ-Fe(2)O(3) and Fe(3)O(4) mixture. Theoretical multiplet analysis shows that during Cr adsorption, the amount of maghemite increases (from 70 to 89%). In magnetite spectra, the relative content of Fe(II) decreases from 8.2 to 3.6% indicating the reduction of magnetite in the mixture particles. In Raman spectroscopy studies, clear peaks of chromium on iron oxide were generated at 826 cm(-1), which could be attributed to chemical interactions between chromium compound and iron oxide. From the results of Raman and XPS studies, electrostatic attraction and oxidation-reduction between chromium and mixed maghemite-magnetite are postulated as mechanisms for the removal of Cr(VI) from aqueous solutions.

Sulfur species at chalcopyrite (CuFeS2) fracture surfaces

Abstract Pristine fractured surfaces of chalcopyrite (CuFeS2) have been studied using Synchrotron Radiation X-ray Photoelectron Spectroscopy and conventional X-ray Photoelectron Spectroscopy. These high-resolution spectra reveal for the first time three distinct contributions to the S 2p spectrum. The main symmetric peak is located at 161.33 eV and is likely derived from fully coordinated bulk S atoms. A core-level shifted peak was observed at 160.84 eV and is attributed to surface monomeric species (S2-). A second broad contribution at 161.88 eV likely represents surface polymeric species (Sn2-). The data suggest that surface polymers form where S-terminated surfaces such as the (1̅1̅1̅) plane are exposed during fracture.

Minimum XPS core-level line widths of insulators, including silicate minerals

Abstract A monochromatic AlKα source and magnetic confinement charge compensation yield an As 3d line width of 0.52 eV for the bulk insulator As2S3 (orpiment) which is as narrow as the As 3d line width obtained for the semiconductor FeAsS (arsenopyrite). These results, and the XPS results for silicates, demonstrate that charge broadening of XPS spectra has been overcome for most insulators provided care is taken with sample preparation. XPS core level spectral investigations of these minerals now may be conducted at the same level of detail as have been conducted on semi-conductors and metals. The Si 2p spectrum of a non-conducting orthosilicate (Mg-rich olivine) displays a total line width of 1.36 eV compared to very high resolution synchrotron XPS line widths of 1.1 eV for an Si 2p spectrum of gaseous Si(OCH3)4, and 1.42 eV for the Si 2p peak of non-charging SiO2 thin-films grown on Si metal. These results demonstrate that Si 2p peak broadening of these silicates does not result from differential charging. Comparison of the Si 2p spectrum of Si in olivine which contains non-polymerized, independent SiO4 tetrahedra, with that of a gaseous analogue, Si(OCH3)4, also composed of non-polymerized Si atoms tetrahedrally bonded to O, provides insight into the cause of Si 2p peak broadening for olivine (and probably for all other silicates). The vibrational contributions to the Si 2p spectrum of Si(OCH3)4 were used to fit the Si 2p peak of olivine, with each vibrational contribution broadened to reflect the minimum line width of the XPS instrument. Although the Si 2p envelope resulting from this procedure yields a reasonable fit to the olivine Si 2p spectrum, additional minor vibrational contributions may be required to reproduce accurately the Si 2p spectrum of olivine. Continued development of the technique should allow determination of transition metal oxidation states and other chemical state properties of non-conductor Al-silicates

Acid gas injection in the Brazeau Nisku Q carbonate reservoir: Geochemical reactions as a result of the injection of an H2S-CO2 waste stream

Publisher Summary Storage of greenhouse gases in geological media is regarded as a potential option for reducing release of CO2 emissions to the atmosphere. Industrial analogues are found in acid gas injection which injects mixtures of waste streams of H2S and CO2 (derived from gas processing of sour gas) into spent oil and gas reservoirs and aquifers for disposal. This is currently occurring at over forty sites in western Canada. To date no leakage has been reported. A review of these acid gas operations has been undertaken in the chapter with a view to evaluating their storage security. The reservoir temperature and pressure, before injection of an acid gas stream of 85% H2S and 15% CO2, was 110°C and 11 MPa. Geochemical mass transfer modeling using GAMSPATH.99 under these conditions predicts that the iron containing minerals in the reservoir will break down rapidly to form iron sulphide minerals. This was verified experimentally in the laboratory at much lower pressures through observation of significant reaction of siderite to iron sulphide at 54°C and 0.5 MPa in a CO2-H2S atmosphere in two weeks. Solubility trapping and ionic trapping are significantly larger, by approximately 10%, if the acid gas-charged fluids react to equilibrium with the reservoir rock, compared to no reaction with the reservoir minerals. In Brazeau, mineral trapping of H2S by iron sulphide minerals dominates due to the high H2S content of the injected gas, reaching trapping levels exceeding 20 moles of H2S per 1000 cc of pore space. In the absence of H2S, mineral trapping of CO2 by carbonate minerals would have dominated.

Core level electron binding energies of realgar (As4S4)

Abstract XPS broad scans and high-resolution narrow-region spectra were collected from fresh realgar (As4S4) surfaces to measure core level S and As binding energies. Reasonably accurate As and S concentrations were determined from XPS broad scans using peak areas and manufacturer supplied sensitivity factors. High resolution S(2p) and As(3d) narrow region spectra were comprised of photoelectron emissions indicative of As and S in intermediate oxidation states akin to binding energies of As and S polymeric species. S(2p) spectra were interpreted using only S contributions expected from the bulk mineral matrix and showed that S was not greatly affected by surface state phenomena. This was attributed to breakage of intermolecular van der Waals bonds rather than covalent interatomic bonds. As(3d) spectra were found to contain two contributions one from As atoms in As4S4 molecules in the bulk mineral matrix and another possibly from As atoms in molecules situated at the surface.

An assessment of the potential benefits of ion implants as trace-element reference material for electron probe X-ray microanalysis: The case of invisible gold

Abstract The reliability of trace element concentrations obtained by EPMA can be significantly improved with the use of high-quality secondary standards. In the case of Au residing in sulfides, such standards are lacking. Natural materials have heterogeneous Au distribution, whereas synthesis is very difficult. The benefits of using ion implants as trace-element reference material for EPMA were assessed by characterizing grains of magnetite, pyrite and galena implanted with 1 × 1014 to 5 × 1014 Au atoms/cm2 at energies from 1 to 3 MeV. The first interesting observation is the excellent lateral micrometer-scale homogeneity of the Au levels across the implants. The ratio of analytical to statistical standard deviations never exceeds 1.7. Additionally, the Au X-ray intensities measured by EPMA show excellent correlation with those predicted for multilayered structures used to model the continuous Au concentration profile for the three implants investigated. Small discrepancies arise only at low accelerating voltage. In these situations, the predicted Au X-ray intensities become sensitive to uncertainties in the determination of the location of the Au concentration profile because of insufficient excitation of the bottom of the Au layer. Fortunately, by varying the implantation energy, optimal implants yielding X-ray intensities that are insensitive to uncertainties on the Au depth profile can be obtained for a wide range of accelerating voltages. These results suggest that ion implants may represent excellent EPMA reference material, especially in cases where natural and synthetic standards are unavailable. Interesting materials presenting specific analytical challenges can be engineered due to the excellent control of the implantation parameters.