Environ. Sci. Technol. 1997, 31, 233-239
Determination of Mercury Binding Forms in Contaminated Soils: Mercury Pyrolysis versus Sequential Extractions HARALD BIESTER* AND CHRISTIAN SCHOLZ Institute of Environmental Geochemistry, P.O. Box 10 30 20, 69020 Heidelberg, Germany
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 leaching 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, matrixbound 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.
Introduction Besides the determination of the total Hg content in Hgcontaminated soils, investigations on Hg-binding forms are essential for evaluating environmental risks. Except methods to determine organic Hg compounds, where defined Hg species such as methylmercury or ethylmercury are determined (1, 2), most methods to investigate inorganic Hg compounds such as metallic Hg (Hg0) or mercury sulfides (HgS) are only operationally defined (3). These methods are usually based on sequential leaching procedures (SEP), which determine the solubility of the present Hg compounds in different leachants. Some authors coupled the SEP with a thermal desorption step prior to the leaching procedure to determine the proportions of volatile Hg0 (4, 5). In many investigations where SEPs were used, Hg in soils and sediments is described as occurring predominantly in the so-called humic, organic/sulfide, or residual fractions (6-8). However, some authors argue that the leachants may not be selective enough or the duration of the extractions may be too short or too long to distinguish specific Hg compounds (6, 7). Moreover, it could be shown for metals others than * Corresponding author telephone +49-6221-544819; fax: +496221-545228; e-mail:
[email protected].
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1996 American Chemical Society
Hg that there might be changes of the metal bonding during SEP caused by the leachants themselves (9, 10). The large number of various procedures used by different authors results in a large quantity of non-comparable data. As an alternative method, we used a Hg pyrolysis technique that is based on the determination of Hg-binding forms in soils by their thermally induced Hg desorption behavior. Mercury pyrolysis techniques were mostly applied in the field of metal prospecting and were developed to determine the bonding of Hg in rocks and soils overlying sulfide ores (11). In recent years, these methods have been sometimes applied for investigations of Hg compounds in polluted soils and sediments (8, 12-14). It could be shown by various investigations of different Hg-contaminated sites that it is possible to distinguish different Hg-binding forms in soils and sediments by their thermal Hg release characteristics. Moreover, specific Hg compounds such as Hg0 and HgS could be identified by the comparison with standard Hg compounds and distinguished from nonspecific matrix-bound Hg compounds (15). The emphasis of the present work was on the investigation of possible changes of the Hg bonding during the extraction procedure. For that purpose, we investigated Hg phases in different contaminated soils by means of the pyrolysis technique and a sequential leaching procedure and compared the results of both methods. After each leaching step, the sample was checked for changes of the Hg bonding induced by the extraction procedures by the determination of the Hg release behavior.
Methods and Materials Samples. Soil samples were chosen from four Hg-contaminated sites differing in the Hg compounds that were released to the soil. Additionally, samples were selected with respect to differences in soil types to represent a broader range of parameters that might influence the results of the used methods. All samples were taken from the upper soil layer (
