Hg exists seven stable isotopes (196, 198, 199, 200, 201, 202, and 204 amu), exhibiting a total relative mass difference range of 4%. In recent years, promoted by the development of multiple-collector inductively coupled plasma mass spectrometry (MC-ICP-MS), exploration of the isotopic dimension of Hg has been demonstrated to be a powerful tool in studying the sources and fates of this metal. Since the first precisely measurement of Hg isotope at the beginning of this century (Klaue et al., 2000), to the first ever session on Hg isotope geochemistry on the 9th International Conference on Mercury as a Global Pollutant in 2009, great progress has been made to study the isotope geochemistry of Hg in the past decade. Hg Isotope geochemistry is thus becoming a new frontier subject to earth and environmental science (Yin et al., 2010a).
In 2009, Professor FENG Xinbin’s Hg research group from the State Key Laboratory of Environmental Geochemistry (SKLEG) at Institute of Geochemistry, Chinese Academy of Sciences (IGCAS) developed a method for high-precision measurement of Hg isotope ratios using Nu-Plasma MC-ICP-MS. Based on that method, successful case studies have been carried out to track the sources of Hg in different landscapes, such as lakes (Feng et al., 2010), watersheds (Liu et al., 2011) and peat lands (Shi et al., 2011). Recently, FENG and his coworkers have made the following important progress on Hg isotope geochemistry in Hg mining areas of Wanshan Hg Mine (WSMM) (Guizhou Province, SW China):
The researchers investigated the Hg isotope compositions of surface soil samples in the Hg mining area, zinc smelting area, coal combustion area, which are known to be the typical pollution regions of Hg in this province. Their results showed that Hg stored in surface soils in different pollution regions displayed a wide range of both mass dependent fractionation (MDF, δ202Hg) and mass independent fractionation (MIF, Δ199Hg). δ202Hg varied in surface soils by 3‰ and Δ199Hg varied by 0.9‰. Their study demonstrated that using a “MDF-MIF” two dimensional Hg isotope system could be particularly useful in discriminating Hg sources in soils. The study was published in Chinese Science Bulletinand highlighted as Cover Article (Feng et al. (2013)).
Their other study revealed that Hg isotopes could represent an important contribution both to the study of Hg transportation in plants and to the understanding of sources of Hg contamination to critical food crops. By studying the stable Hg isotope variations in different tissues (foliage, root, stem, and seed) of rice collected from WSMM, the researchers observed that MDF of Hg differed by up to 3.0‰ in δ202Hg values and MIF of Hg isotopes affected the odd Hg isotopes to produce a 0.40‰ range in Δ199Hg values in tissues of rice plant. The other data supported the hypothesis that a fraction of Hg in tissues of rice plants has undergone a photo-reduction process prior to being accumulated by rice plants. Hence, they suggested that the variation of MIF represented a mixing between soil Hg and atmospheric Hg in rice plants. The estimated fraction of atmospheric Hg (f) in tissues of rice were found to follow the trend of fleaf> fstem > fseed > froot. (Yin et al. (2013a) published in Environmental Science and Technology)
Using extended X-ray absorption fine structure (EXAFS) spectroscopy, the researchers determined the Hg species and estimated relative proportions of the multiple sources of Hg contamination in Hg-bearing wastes from WSMM. According to their study, cinnabar was the dominant Hg species in the unroasted ore samples, while the most prevalent Hg compounds in mine waste calcine was in the following order: meta-cinnabar, cinnabar and mercuric chloride. Meanwhile, Hg isotope analysis indicated that MDF of Hg isotopes may occur during transformation of cinnabar to by-products (such as meta-cinnabar and mercuric chloride) by the roasting process. The variation of δ202Hg appeared to be large enough to distinguish between different contamination sources in the downstream sediment in WSMM. Finally, using a combined triple mixing model, the source attribution of the downstream sediment in WSMM was estimated, suggesting that the Hg isotope could be a useful tool to trace and quantify the source of Hg in the environment. (Yin et al. (2013b))
Furthermore, the researchers investigated the Hg isotope composition and the mobility of soil Hg with two different extraction treatments (application of water and (NH4)2S2O3-solution respectively) in contaminated soil collected from WSMM. The experimental data revealed that bioavailable Hg species were enriched in heavier Hg isotopes compared to total Hg in soil and the addition of (NH4)2S2O3 solution in soil increased the solubility of Hg in soil. Their study demonstrated that Hg isotope may provide a new tool to the study of solubility, toxicity and bioavailability of Hg in soils. (Yin et al. (2013c)) ()
The last two of the above case studies (Yin et al., 2013b; Yin et al., 2013c) have been collected by Chemical Geology, an international journal, in its special issue titled “Advances in Mercury Stable Isotope Biogeochemistry”.
The series of studies are financially supported by Natural Science Foundation of China (40825011).
References:
Feng X., Yin R., Yu B., et al. Mercury isotope variations in surface soils in different contaminated areas in Guizhou Province, China. Chinese Sci. Bull., 2012, 58(2): 249-255.
Feng X., Foucher D., Hintelmann H.,et al. Tracing mercury Contamination Sources in Sediments Using Mercury Isotope Compositions. Environmental Science & Technology, 2010, 44:3363–3368
Liu J., Feng X., Yin R., et al. Mercury distributions and mercury isotope signatures in sediments of Dongjiang River, the Pearl River Delta, China. Chemical Geology, 2011,287, 81-89.
Shi W., Feng X., Zhang G., et al. High precision measurement of mercury isotopes deposit record over the past 150 years in a peat core from Hong Yuan. Chinese Sci. Bull,2011, 56(9): 877-882.
Yin R., Feng X., Meng B. Stable Hg Isotope Variation in Rice Plants (Oryza sativa L.) from the Wanshan Hg Mining District, SW China. Environmental Science & Technology. 2013a, doi.org/10.1021/es304302a.
Yin R., Feng X., Wang J., et al., Mercury speciation and mercury isotope fractionation during ore roasting process and their implication to source identification of downstream sediment in Wanshan mercury mining area, SW China. Chemical Geology. 2013b. doi:10.1016/j.chemgeo.2012.04.030.
Yin R., Feng X., Wang J., et al., Mercury isotope variations between bioavailable mercury fractions and total mercury in mercury contaminated soil in Wanshan Mercury Mine, SW China. Chemical Geology. 2013c. doi:10.1016/j.chemgeo.2012.04.017.
Yin R., Feng X., Shi W., Application of the stable-isotope system to the study of sources and fate of Hg in the environment: A review. Appl. Geochem., 2010a, 25, 1467–1477.
Yin R., Feng X., Foucher D., et al., High Precision Determination of Hg Isotope Ratios Using Online Mercury Vapor Generation System Coupled with Multi-collector Inductively Coupled Plasma-Mass Spectrometer. Chinese J. Anal. Chem. 2010b, 38(7), 929–934.
(By YIN Runsheng)
