The contamination of environmental materials (soil and dust) by heavy elements is of concern in assessing environmental pollution. Characterization of samples is performed in order to provide information that can be used to assess the potential hazard of the material, to assess remediation strategies, and to determine the source of the heavy element-bearing material. This is difficult because the particles in which the element of interest occur may consist of multiple phases, some potentially hazardous and some benign. Bulk properties of samples are often determined to assess the potential risk related to such a sample. Such an assay may reveal the total content of a contaminant, and so document the maximum hazard potential, but cannot directly address important mitigating factors, including the specific phases present, their size distribution and internal particle structure.
Microbeam analysis using a scanning electron microscope (SEM) or electron microprobe on polished particle mounts allows the size, internal morphology and chemical composition to be determined on a particle-by-particle basis. Computer controlled SEM allows all factors except internal structure to be acquired in an automated fashion.
We present examples of two studies using microbeam techniques. In the first example, we illustrate how cadmium associated with sphalerite (zinc sulfide) in soils can be quantified at levels down to a few parts per million (ppm) using computer controlled SEM techniques. In the second example, we illustrate how lead in soils can be assigned to phases, phase sizes and to internal structure types using manual electron microprobe techniques.