TEM-EDX investigation on Zn- and Pb-contaminated soils

Lead and zinc contaminated soils from a smelter area in the northern part of France have been studied by transmission electron microscopy (TEM). This study was carried out with 4 different soils contaminated by Pb and Zn but with different chemical and physical characteristics. Two soils are tilled and have a neutral or slightly basic pH, one is a wooded soil and the last one is a meadow soil with acidic pH and high total organic content . TEM images of the soil samples have been coupled with focused energy dispersive X-ray (EDX) analyses and chemical mapping on a few micron-sized windows. This study demonstrates that TEM is a particularly efficient method to investigate metal speciation in the fine fractions of the contaminated soils. Zinc could be detected locally in sulphide minerals probably coming from the smelter emissions, but the major phases retaining Zn are Fe-oxyhydroxides and smectites. Lead could be detected in small aggregates which were characterized by EDX and selected area electron diffraction. Their structural formulae correspond to a pyromorphite-like mineral in which Pb is partly substituted by Ca and Na. Pyromorphite is present only in the wooded and meadow soils where it forms partly from amorphous Si-rich phases (slags) coming from the smelter. These results are compared with data previously obtained by spectroscopic methods on the same samples.