Determination of Mercury Binding Forms in Contaminated Soils:  Mercury Pyrolysis versus Sequential Extractions

Binding forms of mercury (Hg) in contaminated soils were studied by means of a pyrolysis method and a sequential leaching procedure (SEP). Changes of the Hg-binding forms occurring during the sequential extraction procedure were characterized by determination of the thermal mercury release characteristics after each extraction step. Soil samples were selected from the site of a former chloralkali plant, a wood preservation site, and two mining areas contaminated by metallic mercury (Hg0), mercury(II) chloride (HgCl2), or mercury sulfide (HgS), respectively. The results show that Hg0, matrix-bound Hg, and HgS occurring in the studied soils could be identified by their thermal release behavior. In contrast, results obtained from sequential leach ing did not allow the identification of any specific Hg compounds as in all samples most of the Hg is extracted in the nonspecific residual Hg fraction. Results of pyrolitic measurements after each extraction step demonstrated the insufficient selectivity of the used leachants, indicating that Hg found in the residual step can consist of Hg0, matrix-bound Hg, or mercury sulfide as well. Moreover, the effectiveness of the leachants is strongly influenced by the amount of soil organic matter or buffering carbonates, whereas the dissolution of the organic matter during SEP was concluded to be the critical step.