Scanning Electron Microscope (SEM) Analysis of Contaminated Soils at the Anaconda Smelter Site, Montana, USA

The Anaconda Copper Smelter site in Montana, USA, is an Environmental Protection Agency (EPA) superfund site. Smelter waste, comprising high metal concentrations, was disposed of at the site over a nine decade period. Copper porphyry ore was the primary material processed at the smelter, and included chalcocite (Cu₂S), bornite (Cu₅FeS₄), and enargite (Cu₃AsS₄). The primary metal phases at the Anaconda Smelting site are metal-sulfides.

Two sequential extraction leaching procedures, the Tessier method (Analytical Chemistry, 1979) and EPA Method 3050B, were performed to determine the distribution of metals in a suite of soil samples collected from the smelter site. Five metals, including As, Cd, Cu, Pb and Zn, were of particular interest. In addition, soil samples and residues from the extraction analyses were characterized by thin-section microscopy and Scanning Electron Microscopy (SEM). This presentation will focus on the SEM imaging results.

The SEM imaging showed clear variations in grain morphology between sieved un-reacted soil samples and the soil residual that resulted from each sequential extraction step. The sieved soil samples (grain size of ≤ 0.250 mm) comprised large framework grains, which were fully or partially coated with micro-crystalline particles (possibly iron-oxides, sulfides or clay phases). The particles coating framework grains range from micrometer to nanometer in diameter. The Tessier procedure defines one extraction step as the "iron- & manganese-oxide bound fraction"; following this extraction step all sub-micron particles were absent from the surfaces of the framework grains. Further elemental analyses are being performed utilizing HR-SEM, with energy dispersive X-ray (EDS) spectrometers and electron backscattered diffraction analysis (EBSD)

Acknowledgements: Dr. M. Grimson for assistance at the Imaging Center, Department of Biological Sciences, Texas Tech University.