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Mercury 'speciation analysis' by using thermal desorption analysis

(17.04.2017)


Background:
Thermal desorption atomic absorption analysis is a technique for the direct analysis of solid samples for their total mercury content. The sensitive technique requires little if any sample preparation and can be calibrated with simple standards. Mercury is liberated from the solid sample material by heating the materials placed in a quartz furnace, transported by a carrier gas towards a gold trap, concentrated on the gold trap and finally released by heating the gold trap and determining by flameless atomic absorption spectrometry. By using the gold amalgamation step for collecting the mercury, its later release from the trap makes the signal generation independent from the desorption  kinetics of the mercury from the original sample material. While in this way a reliable determination of total mercury from different solid sample materials is possible, no information on the presence of different mercury species is available.   


Mercury "speciation analysis":
The concept of using thermal desorption analysis for speciation analysis is based on the observation that mercury is released from the sample material over a wide range of temperatures with showing different peaks at different temperatures when raising the temperature slowly. The hope is that such peaks within the thermogram can be assigned to different mercury species. Temperature ranges for the release of different mercury species obtained by different authors are shown in Table I.

Table 1: Temperature ranges for the release of different mercury species from soils and ores

Mercury species
Mercury release
temperature
Reference
Hg° 
< 80 
Watling, Davis et al, 1973
 < 150 
Bombach et al, 1994
Hg2Cl2 170   Watling, Davis et al. 1973
HgCl2 <250
Koksoy, Bradshaw and Tooms, 1967

220   Watling, Davis et al., 1973
HgO270-535
Koksoy, Bradshaw and Tooms, 1967

160-495 
Watling, Davis et al. 1973
HgS 
300 Watling, Davis et al. 1973   

280-400 
Biester, 1994

210-340Koksoy, Bradshaw and Tooms, 1967

400 Bombach et al, 1994
Hg in Pyrite450Watling, Davis et al, 1973
Hg in Sphalerite600Watling, Davis et al, 1973
Hg-humic substance   200-300
Biester, 1994

    
                     
It can be seen that species with low oxidation states, i.e., Hg ° and Hg1+ are released at lower temperature. It can also be observed, that the release temperature of different species overlap with each other. Even further, when looking at the thermograms of different species, it can be observed that several species have more than one peak in the thermogram. It can be seen that there is no direct correlation between the physicochemical properties of the pure mercury compounds, i.e., melting and sublimation point and vapour pressure, with the temperature range in which they are released from the soil matrices.

Unfortunately the release kinetics of mercury from solid sample materials is dependent on many factors:
a) the mercury species
b) its interaction with the solid sample material
c) the particle size of the solid material
d) the operating parameters of the system (e.g. heating rate, carrier flow rate, carrier gas)

The release behavior of the Hg-species, as demonstrated by the thermograms, can be in part a reflection of the kinetics, the thermodynamics and mechanisms of the vaporisation processes. These characteristics under non-equilibrium conditions are different from those in equilibrium (Somorjai and Lester, 1967), resulting in different volatilisation rates. Therefore, vaporisation behaviour, in non-equilibrium conditions, cannot be predicted from equilibrium measurements.

Also, since the sample mass that can be accepted by the furnace is rather limited, reproducibility is dictated by the homogeneity of the sample. Species transformation during heating can be observed, depending on the composition of the matrix. For example, the oxidation of elemental mercury to Hg(II) can be observed, causing double peaks in the mercury release curve. Wallschläger et al. (1998) observed that upon addition of Hg model compounds to a sediment matrix, all species were transformed to the same new speciation pattern, regardless of their original speciation.

While most authors claim that the thermal release analysis of soil and ore samples was suitable to differentiate mercury species, quantitative results for different species are seldom reported. Observed thermograms of real samples are most often qualitatively discussed and compared and related species are suggested rather than identified. Different fractions are often qualitatively reported as "main" and "secondary" fraction and characterized by terms such as "mobile", "semi-mobile" and "non-mobile" rather than true species identification. In cases where quantitative results are assigned to different species, authors are very vague in describing which part of the thermogram was assigned to which species and how signal integration was performed. For example Tersic et al. (2011) "quantified" non-cinnabar Hg by doubling the first half of the peak occurring at 150–250 °C and then calculated the content of cinnabar as the difference between non-cinnabar Hg and total Hg.

The thermal release of Hg(II) is quite different for each mercury compound. Reported temperatures for HgCl2 are widely dispersed. Thus, the assignment of the peak at 250°C is not clear, there may be an overlap with the peak of Hg0 and it can theoretically be confused with the peak of Hg(I). Also the desorption range of mercury bound to humic substances is partially overlapped with the temperatures of synthetic and natural cinnabar.Validation of the methods is often performed by comparing the total mercury content indicating accuracy in the range of +- 20%.

While the technique has been used now for more than 20 years, there is hardly any development observable with respect to instrumentation and methodology.  According to the definition of "speciation analysis" by IUPAC, thermal desorption analysis is not a method for speciation analysis but a fractionation technique. The separation power of the technique is simply not sufficient to differentiate between distinct species but can only separate more volatile fractions from less volatile fractions. Anyhow, in absence of more powerful methodology for the true speciation analysis for mercury in solid sample materials, the information provided by thermal desorption analysis might still be highly welcomed.  



Related studies

M. Koksoy, P.M.D. Bradshaw, J.S. Tooms, Notes on the determination of mercury in geological samples, Trans. Inst. Min. Metall., B, 76 (1967) 121-124.

Joseph A. Goleb, The Determination of Mercury in Small Terrestrial and Nonterrestrial Rock Samples by Atomic-Absorption Spectroscopy, and the Study of Mercury Release at Elevated Temperatures, Appl. Spectrosc., 25/1 (1971) 522-525. DOI: 10.1366/000370271779950526

R.J. Watling, G.R. Davis, W.J. Meyer, Trace identification of mercury compounds as a guide to sulphide mineralization at Keel, Erie. In: Proceedings of the 4th International Geochemical Exploration Symposium, April 17-20, London, 1972, p. 59-69

Louis M. Azzaria, Alijan Aftabi, Stepwise Thermal Analysis Technique for Estimating Mercury Phases in Soils and Sediments, Water, Air, and Soil Pollution, 56 (1991) 203-217. DOI: 10.1007/BF00342272

V.L. Tauson, V.I. Men'shikov, V.S. Zubkov, Use of thermal atomic absorption analyses of synthetic crystals for identifying the form of mercury in minerals, Geokhimiya, 8 (1992) 1203.

G. Bombach, K. Bombach, W. Klemm, Speciation of mercury in soils and sediments by thermal evaporation and cold vapor atomic absorption, Fresenius' J. Anal. Chem., 350/1 (1994) 18–20. DOI: 10.1007/BF00326246.

Claudia Carvalhinho Windmöller, Rolf-Dieter Wilken, Wilson De Figueiredo Jardim, Mercury Speciation in Contaminated Soils by Thermal Release Analysis. Water, Air, and Soil Pollution 89 (1996) 399-416. DOI: 10.1007/BF00171644.

V.L. Tauson, V.F. Gelety, V.I. Men’shikov, Mercury Speciation in Mineral Matter as an Indicator of Sources of Contamination, in: Global and regioional mercury cycles: Sources, fluxes and mass balances. Dordrecht, Kluwer, Nato A. S. I. 2, 1996, vol. 21, p. 441-452. DOI: 10.1007/978-94-009-1780-4_23.

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Dirk Wallschläger, Madhukar V. M. Desai, Markus Spengler, Cláudia Carvalhinho Windmöller, Rolf-Dieter Wilken, How Humic Substances Dominate Mercury Geochemistry in Contaminated Floodplain Soils and Sediments, J. Environ. Qual., 27 (1998) 1044-1054. DOI: 10.2134/jeq1998.00472425002700050009x 

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Claudio Raposo, Claudia Carvalhinho Windmöller, Walter Alves Durao Junior, Mercury speciation in fluorescent lamps by thermal release analysis, Waste Management, 23 (2003) 879–886. DOI: 10.1016/S0956-053X(03)00089-8.

P. Higueras, R. Oyarzun, H. Biester, J. Lillo, S. Lorenzo, A first insight into mercury distribution and speciation in soils from the Almadén mining district, Spain, J. Geochem. Exploration, 80/1 (2003) 95–104. DOI: 10.1016/S0375-6742(03)00185-7

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Vladimir L. Tauson, Irina Yu. Parkhomenko, Dmitriy N. Babkin, Vitaliy I. Men’shikov, Esfir E. Lustenberg, Cadmium and mercury uptake by galena crystals under hydrothermal growth: A spectroscopic and element thermo-release atomic absorption study, Eur. J. Mineral. 17 (2005) 599–610. DOI: 10.1127/0935-1221/2005/0017-0599

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Further chapters on techniques and methodology for speciation analysis:

Chapter 1: Tools for elemental speciation
Chapter 2: ICP-MS - A versatile detection system for speciation analysis
Chapter 3: LC-ICP-MS - The most often used hyphenated system for speciation analysis
Chapter 4: GC-ICP-MS
Chapter 5: CE-ICP-MS for speciation analysis
Chapter 6: ESI-MS: The tool for the identification of species
Chapter 7: Speciation Analysis - Striving for Quality
Chapter 8: Atomic Fluorescence Spectrometry as a Detection System for Speciation Analysis
Chapter 9: Gas chromatography for the separation of elemental species
Chapter 10: Plasma source detection techniques for gas chromatography
Chapter 11: Fractionation as a first step towards speciation analysis
Chapter 12: Flow-injection inductively coupled plasma mass spectrometry for speciation analysis
Chapter 13: Gel electrophoresis combined with laser ablation inductively coupled plasma mass spectrometry for speciation analysis
Chapter 14: Non-chromatographic separation techniques for speciation analysis


last time modified: April 17, 2017









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