Application of three methods for determining mercury speciation in mine waste Chris Sladek 1 , Mae Sexauer Gustin 2 , Christopher S. Kim 3 & Harald Biester 4 1 Department of Geological Sciences University of Nevada, Reno, Nevada, 89557, USA 2 Department of Environmental and Resource Sciences, University of Nevada, Reno, Nevada, 89557, USA (e-mail: msg@unr.nevada.edu) 3 Department of Geological and Environmental Sciences, Stanford University, Stanford, California 94305, USA 4 Institute of Environmental Chemistry, University of Heidelberg, Heidelberg, 69120 Germany ABSTRACT: Three methods, pyrolytic and chemical extractions (PCE), extended X-ray adsorption fine structure spectroscopy (EXAFS) and solid-phase-Hg-thermo- desorption (SPTD) were applied to determine mercury speciation in amended substrates and mine waste samples. Although these three methods determine Hg speciation by fundamentally different processes, comparison of the results are useful for validation of the three methods. PCE uses pyrolysis and weak leaches to determine relative percentages of volatile, ‘soluble’ and residual Hg in substrate. The results are operationally defined and specific species cannot be determined with this method. EXAFS is a nondestructive method which uses high energy synchrotron- sourced X-ray radiation to identify specific species based on scattering patterns. Least squares data analysis is done to link patterns to a database of model compounds. This method is most useful for identification of specific species, given that they are included in the model database. Identification of Hg 0 is difficult using EXAFS. SPTD identifies Hg species by incremental heating and comparison of thermal release patterns to a database of compounds. SPTD allows the identification of a more limited number of specific species than EXAFS, but is the best of the three methods for the identification of Hg 0 . Overlapping release patterns make the identification of species, such as HgS and some forms of matrix-bound Hg, difficult. Results of PCE analyses indicate that volatile and leachable forms of Hg in mine waste are low relative to the total Hg concentration. This was supported by EXAFS and SPTD analysis which identified HgS as the primary component of mine waste. In contrast, analysis of tailings from mills that utilized Hg to amalgamate Au and Ag from ores yielded conflicting results. The results of this study illustrate the importance of using multiple analytical methods for the evaluation of Hg in the substrate. KEYWORDS: mercury, mine waste, speciation, EXAFS, pyrolysis, selective extraction INTRODUCTION Published methods for Hg speciation in a substrate include sequential extractions (Revis et al. 1990; Lechler et al. 1997; Wallschlager et al. 1998), solid-phase-Hg-thermo-desorption (SPTD) (Biester & Scholz 1997) and extended X-ray adsorption fine structure (EXAFS) spectroscopy (Kim et al. 2000). Sequen- tial extraction methods, unless derived for specific substrates and carefully tested, may not be appropriate for determining Hg forms or binding sites in substrates (Sladek & Gustin 2002). Nirel & Morel (1990) stressed that results of sequential extrac- tions for most trace elements have not been validated, and indiscriminate application of a method will produce meaning- less results. Discrepancies of greater than 50% were noted in Hg analyses performed on samples from the Carson River Superfund Site, NV, by separate labs (Hogan & Smucker 1994). This degree of uncertainty indicates the need for critical evaluation of Hg speciation methods and their uses. Sladek & Gustin (2002) suggested that pyrolytic and chemical extractions (PCE) are best applied for determining the potential for Hg to be released from a substrate by volatilization or leaching. They found that pyrolysis at 80C for 8h efficiently removed elemental Hg (Hg 0 ) but small amounts (1–3%) of organic matter significantly reduced extraction efficiency. Pyrolysis was also found to remove a significant amount of amended HgCl 2 . The EXAFS spectroscopy method, which identifies Hg compounds by X-ray scattering patterns, represents the most recent advance in identification of specific Hg species. This method entails the bombardment of a sample with high energy X-rays generating photoelectrons from a specific element. The generation of photelectrons induces specific electronic scattering interactions between a central absorbing atom and Geochemistry: Exploration, Environment, Analysis, Vol. 2 2002, pp. 369–376 1467-7873/02/$15.00  2002 AEG/Geological Society, London