A comparison of devices using thermal desorption for mercury speciation in solids

Abstract

The goal of this work is to emphasize the reliability of the thermal desorption technique in identifying mercury species. The analysis of mercury species in solids is essential for assessing the risk of disposal or re-use of mercury-contaminated materials. This study evaluates the accuracy and reliability of thermal desorption as a technique for identifying mercury species by means of different thermo-desorption devices. For this purpose, mercury species present in samples related with coal utilization processes were identified. Three devices were compared for analyzing samples free of carbon or with a low carbon content (fly ashes, gypsums and soils), and a new equipment was developed to analyze samples with a high carbon content (coal). In spite of the fact that the first three devices employ different experimental conditions (i.e., heating rate, gas flow and carrier gas), the mercury species identified in the samples were comparable in all cases.The need for new equipment for mercury speciation in materials containing carbon was a consequence of interferences produced from the pyrolysis products of the organic matter. The new device consists of two furnaces and two gas inlets to allow thermal oxidation of organic pyrolysis products and the identification of mercury species in carbonaceous samples. This new approach offers the application of thermal desorption to mercury speciation in all types of materials contaminated with mercury.

Application of thermal desorption for the identification of mercury species in solids derived from coal utilization

“…The temperature rate was kept at 40 ºC min −1 for 575 s. Then the heating velocity was increased up to 50 ºC min −1 where it was held for 200 s and then up to 80 ºC min −1 where it was held for a further 125 s. The temperature was controlled by a thermocouple placed in the boat sample. The parameters for the selective desorption were previously optimised (Rumayor et al, 2013), additionally, Hg(II) complexed by humid acids (Hg-HA) has been prepared and used as reference sample in this work. Hg-HA was prepared according to the method described by Terzano et al, 2010.…”

“…The main temperature points used to identify the species were i) the temperature at which thermal release started, ii) the maximum temperature of release and iii) the temperature at which desorption returned to the baseline. The thermograms of the samples were compared with the reference thermograms of fifteen pure mercury compounds (Rumayor et al, 2013) (Table 1). About 100 mg of sample was used for each analysis.…”

“…In a previous work (Rumayor et al, 2013) a database was established for the identification of mercury species by means of thermal desorption by recording the thermograms of several pure commercial compounds. Table 1 summarises their characteristic peak temperatures that serve as footprints for the identification of the mercury species, additionally Hg-HA was prepared in this study and added to the database.…”

“…However, mercury speciation in solid samples is still a difficult task. Various methods of analysis have been tested, one of the most promising of which is desorption (Bloom et al, 2003;Reis et al, 2010;Biester et al, 1997;Shuvaevaa et al, 2008;Reis et al, 2012;Rumayor et al, 2013;Kim et al, 2003. Nevertheless, this method still needs to be validated and the scientific community involved in the analysis of mercury has yet to demonstrate that the method is reproducible for any solid matrix.…”

“…Although selective extraction is handicapped by problems associated with readsorption, background contamination or losses of volatile mercury provide useful information on the mobility of mercury in solids. Other methods are being evaluated to determine the speciation of mercury in solid samples, including thermo-desorption (TD) (Biester et al, 1997;Shuvaevaa et al, 2008;Reis et al, 2012;Rumayor et al, 2013), and X-ray adsorption fine structure spectroscopy (EXAFS) (Kim et al, 2003), although both of these methods need to be further optimized and validated. Because these methods are based on the volatility of mercury compounds (TD) and the properties related to the atomic structure (EXAFS), the results are less likely to be affected by the substrate matrix than those obtained by the sequential and selective extraction method which is based on chemical interactions in the complex substrate.…”