Speciation and Characterization of Heavy Metal- Contaminated Soils Using Computer-Controlled Scanning Electron Microscopy

摘要

The total heavy metal content of soils can be determined quickly and at low cost by various chemical analysis methods. However, the total concentration of a heavy metal cannot, in general, assess the degree of bio availability or the success of a remediation strategy or implicate a specific source of the contamination. Particle-by-particle determination of species (i.e., phase or mineral) as well as the characterization of particle size, external and internal morphology, can be used to assess bio availability, remediation potential, and source. The imaging capabilities of the scanning electron microscope (SEM) coupled with the elemental analysis provided by energy dispersive spectroscopy (EDS) can be used to acquire both chemical and physical information. However, at the generally low concentrations of interest, the manual SEM analysis of samples is rather time-consuming (and expensive). Fortunately, heavy metal particles can be recognized, measured, and analyzed automatically using computer-controlled scanning electron microscopy (CCSEM). Two examples illustrate the use of this technique in source identification. In one example, CCSEM recognized approximately 85% of the arsenic in suspect soils as discrete particles of arsenic oxide with a mean diameter of 18 um and an additional 8% occurred as leaded arsenic oxide. Occurrences of these types were not observed in samples of the alleged source, and are attributed to a manufactured rather than a fugitive source. In a second example, the alleged source was an industrial operation dealing with a leaded sulfur phase. CCSEM observed no lead consistent with the proposed source. Instead, the lead phases observed as well as an association with a vuggy aluminosilicate phase and unburned coal suggest coal ash as the probable source.