Submicroscopy and chemistry of heavy-metalcontaminated precipitates from column experiments simulating conditions in a soil beneath a landfill

Abstract

The first results of a cooperative research project of IWGSUSM (International Working-Group on Submicroscopy of Undisturbed Soil Materials) are discussed. Wet chemistry of heavy metals which precipitated during column experiments, simulating conditions in a soil beneath a landfill, was studied. To compare these data of bulk chemistry with in situ microchemistry of the same heavy metals, thin sections were made. Using the normal technique of impregnation, problems are encountered in the hardening process. However, if gamma radiation is used, an absorbed dose of 5Mrad (50 kGy) was sufficient to harden the polyester resin. The first thin sections of the upper 30 cm of a sandy column have now been studied with SEM-EDXRA (scanning electron microscope—energy dispersive X-ray analysis) and initial quantification was tried out using EMA (electron microprobe analysis) and SEM-WDXRA (scanning electron microscope—wavelength dispersive X-ray analysis). The first results demonstrate that the heavy-metal-containing cutans (coatings on the walls of voids and on the surfaces of mineral grains) are brown and black when studied with the light microscope. The brown ones contain the smallest amounts of heavy metals and the black ones the largest amounts. Coatings of polluted precipitate are usually only present on part of the larger mineral grains; they can be concentrated in small bands, whereby smaller grains are often completely coated, or occur isolated on the surface of a small number of grains. Brown cutans also often contain fragments of black cutans. The distribution of Fe, Ni, Cu, Zn and Pb is extremely heterogeneous. Often this heterogeneity is already present in measurements of precipitates on the surface of one mineral. This indicates a rather extreme variety in composition of the pollutants, even on a microscale, and emphasises that submicroscopic techniques are certainly necessary to obtain detailed in situ information. The use of a step by step approach and the acquisition of numerous data should eventually allow a good understanding of the processes at work. STEM (scanning transmission electron microscope) — EDXRA measurements at magnifications larger than x 10,000 are of considerable help.