Diana Loomer

Manganese valence imaging in Mn minerals at the nanoscale using STEM-EELS

Abstract Electron energy loss spectroscopy (EELS) was used with scanning transmission electron microscopy (STEM) to quantify the average Mn valence in natural minerals at the nanometer scale. A method was developed to calibrate the energy-loss scale accurately, providing a comparison between STEMEELS and the X-ray absorption spectroscopy methods that investigate the L-edge chemical shift as Mn valence changes. The chemical-shift measurements were consistent with data reported by previous researchers from both X-ray and electron energy-loss spectroscopy. The L3/L2 white-line intensity ratios also were consistent with previous work. A calibration curve for Mn valence was produced using the L3/L2 white-line intensity ratios from measurements of synthetic standards. The average Mn valence was determined because it is not possible to distinguish Mn3+ from mixtures of Mn2+ and Mn4+ using either method. The white-line intensity method was implemented in automated software that allows for rapid processing of point spectra, and 1-D and 2-D spectrum images. Point analyses of two natural pyrolusite samples indicated a Mn valence of 4.0, and point analyses of romanechite and manganite gave values of 3.8 and 3.4, respectively. An interface between braunite and bementite was used to illustrate 1-D and 2-D spectrum-imaging capabilities. The measured valence of Mn in the braunite and bementite was 2.9 and 2.0, respectively; both consistent with theoretical values. The braunitebementite sample demonstrated the heterogeneity of Mn valence common to natural minerals and the advantages of acquiring quantitative valence information in a known spatial context.

Manganese valence in oxides formed from in situ chemical oxidation of TCE by KMnO4.

Batch and column experiments designed to simulate in situ chemical oxidation (ISCO) in a sand aquifer were conducted to create Mn-oxides (MnOx) by oxidation of trichloroethylene (TCE) with permanganate (MnO4-). Electron energy-loss spectroscopy (EELS) and X-ray photoelectron spectroscopy (XPS) were used to quantify Mn valence in the oxides. The valence of Mn in the MnOx generated in near-source ISCO conditions was 2.2 and 2.3 when formed at low (<3) and neutral (6-7) pH conditions, respectively. There is no significant difference between these values. Valence was found to be sensitive to the preparation method and to aging. When formed in the presence of excess MnO4-, or aged for 3 months, Mn valence ranged from 2.5 to 3.6. Aging in a lower pH environment inhibited Mn oxidation. The EELS and XPS methods provided similar results, but there was a slight bias to higher values for XPS. This work demonstrates that MnO2(s) may not be the main product of MnO4- reaction with chlorinated solvents as is commonly assumed and that the efficiency of ISCO treatment may be greater than previously known.

Diffusive anisotropy in low-permeability Ordovician sedimentary rocks from the Michigan Basin in southwest Ontario.

Diffusive anisotropy was investigated using samples from Upper Ordovician shale and argillaceous limestone from the Michigan Basin of southwest Ontario, Canada. Effective diffusion coefficients (De) were determined for iodide (I(-)) and tritiated water (HTO) tracers on paired cm-scale subsamples oriented normal (NB) and parallel to bedding (PB) prepared from preserved drill cores within one year from the date of drilling. For samples with porosity >3%, an X-ray radiography method was used with I(-) tracer for determination of De and porosity accessible to I(-) ions. A through-diffusion method with I(-) and HTO tracers was used for most siltstone and limestone samples with low-porosity (<3%). The De values range from 7.0×10(-13) to 7.7×10(-12) m(2)·s(-1) for shale, 2.1×10(-13) to 1.3×10(-12) m(2)·s(-1) for limestone, and 5.3×10(-14) to 5.6×10(-13) m(2)·s(-1) for siltstone and limestone interbeds within the Georgian Bay Formation shale. The sample-scale anisotropy ratios (De-PB:De-NB) for De values obtained using the I(-) tracer are 0.9 to 4.9, and the anisotropy ratios for the HTO tracer are in the range of 1.1 to 7.0. The influence of porosity distribution on diffusive anisotropy has been investigated using one-dimensional spatially-resolved profiles of I(-)-accessible porosity (shale only) and the use of AgNO3 for fixation of I(-) tracer in the pores, allowing for SEM visualization of I(-)-accessible pore networks. The porosity profiles at the sample scale display greatest variability in the direction normal to bedding which likely reflects sedimentary depositional processes. The SEM imaging suggests that diffusion pathways are preferentially oriented parallel to bedding in the shale and that diffusion occurs dominantly within the argillaceous component of the limestone. However, the fine clay-filled intergranular voids in the dolomitic domains of the limestone are also accessible for diffusive transport.

Manganese and trace-metal mobility under reducing conditions following in situ oxidation of TCE by KMnO4: A laboratory column experiment

The stability of Mn oxides, and the potential for mobilization of associated trace metals, were assessed by simulating the onset of microbially-mediated reducing conditions in a continuous-flow column experiment. The column had previously been used for an in situ chemical oxidation (ISCO) experiment in which trichloroethylene was reacted with permanganate in the presence of aqueous trace metals, which produced Mn oxyhydroxides (MnO(x)) that sequestered the trace metals and coated the column sand. The column influent solution represented the incursion of ambient groundwater containing dissolved organic carbon (DOC) into an ISCO treatment zone. The influx of DOC-containing groundwater initiated a series of cation-exchange, surface-complexation and reductive-dissolution reactions that controlled the release of aqueous metals from the system. Peak concentrations in the effluent occurred in the order Na, Mo, Cr, Zn, K, Mn, Fe, Pb, Mg, Ni, Cu and Ca. Manganese release from the column was controlled by a combination of cation exchange, reductive dissolution and precipitation of rhodochrosite. The trend in Fe concentrations was similar to that of Mn, and also resulted from a combination of reductive dissolution and cation exchange. Cation exchange and/or surface-complexation were the primary mechanisms controlling Cu, Ni, Mo and Pb release to solution, while Zn and Cr concentrations did not display coherent trends. Although metal release from the treatment zone was evident in the data, concentrations of trace metals remained below 0.05 mg L(-1) with the exception of Mo which reached concentrations on the order of 1 mg L(-1). The establishment of anaerobic conditions in ISCO-treated aquifers may result in a prolonged flux of aqueous Mn(II), but with the exception of MoO(4)(2-), it is unlikely that trace metals sequestered with MnO(x) during ISCO will be released to the groundwater in elevated concentrations.

Application of High-Resolution Microscopy Methods to the Analysis of Fine-Grained and Amorphous Treatment Sludges

Lime addition is a common method for the treatment of acid mine drainage (AMD) whereby neutralization promotes a reduction in acidity and the precipitati on of metals as voluminous sludges that may contain gypsum, calcite and a spectrum of other phases. Due to the extremely fine-grained and often amorphous ( i.e ., non crystalline) character of sludge solids, the compos ition of these materials has been difficult to eluc idate. Traditional methods, such as X-Ray Diffraction (XRD) and optical microscopy, have proved largely ineff ective. In order to provide further insight into the solid- phase characterization of neutralization sludges, s amples from mine sites across Canada were examined by high resolution microscopy techniques, including Scanning Electron Microscopy (SEM), Scanning Transmission Electron Microscopy (STEM), and X-ray Absorption Spectroscopy (XAS). The results revealed sludge-specific host phases, including relatively pure Feoxyhydroxide, amorphous Mg-Al-(Fe) hydroxysulphate and amorphous metal hydroxides. The data indicate that the nature of metal phase associations is stro ngly dependent on AMD influent composition.