Article pubs.acs.org/est
Mercury Stable Isotope Signatures of World Coal Deposits and Historical Coal Combustion Emissions Ruoyu Sun,*,†,‡ Jeroen E. Sonke,*,† Lars-Eric Heimbürger,† Harvey E. Belkin,§ Guijian Liu,‡ Debasish Shome,∥ Ewa Cukrowska,⊥ Catherine Liousse,# Oleg S. Pokrovsky,†,∇ and David G. Streets○ †
Observatoire Midi-Pyrénées, Laboratoire Géosciences Environnement Toulouse, CNRS/IRD/Université de Toulouse, 14 avenue Edouard Belin, Toulouse, 31400 France ‡ CAS Key Laboratory of Crust-Mantle Materials and Environment, School of Earth and Space Sciences, University of Science and Technology of China, Hefei, Anhui 230026, China § U.S. Geological Survey, 956 National Center, Reston, Virginia 20192, United States ∥ Department of Geology, Jadavpur University, 188 Raja S. C. Mullik Road, Kolkata, 700032, India ⊥ Molecular Sciences Institute, School of Chemistry, University of the Witwatersrand, P.Bag X3, WITS 2050, Johannesburg, South Africa # Observatoire Midi-Pyrénées, Laboratoire d’Aérologie Toulouse, CNRS/Université de Toulouse, 14 avenue Edouard Belin, Toulouse, 31400, France ∇ BIO-GEO-CLIM Laboratory, Tomsk State University, 36 Lenin Prospekt, Tomsk, 634050, Russia ○ Decision and Information Sciences Division, Argonne National Laboratory, Argonne, Illinois 60439, United States S Supporting Information *
ABSTRACT: Mercury (Hg) emissions from coal combustion contribute approximately half of anthropogenic Hg emissions to the atmosphere. With the implementation of the first legally binding UNEP treaty aimed at reducing anthropogenic Hg emissions, the identification and traceability of Hg emissions from different countries/ regions are critically important. Here, we present a comprehensive world coal Hg stable isotope database including 108 new coal samples from major coal-producing deposits in South Africa, China, Europe, India, Indonesia, Mongolia, former USSR, and the U.S. A 4.7‰ range in δ202Hg (−3.9 to 0.8‰) and a 1‰ range in Δ199Hg (−0.6 to 0.4‰) are observed. Fourteen (p < 0.05) to 17 (p < 0.1) of the 28 pairwise comparisons between eight global regions are statistically distinguishable on the basis of δ202Hg, Δ199Hg or both, highlighting the potential application of Hg isotope signatures to coal Hg emissions tracing. A revised coal combustion Hg isotope fractionation model is presented, and suggests that gaseous elemental coal Hg emissions are enriched in the heavier Hg isotopes relative to oxidized forms of emitted Hg. The model explains to first order the published δ202Hg observations on near-field Hg deposition from a power plant and global scale atmospheric gaseous Hg. Yet, model uncertainties appear too large at present to permit straightforward Hg isotope source identification of atmospheric forms of Hg. Finally, global historical (1850−2008) coal Hg isotope emission curves were modeled and indicate modern-day mean δ202Hg and Δ199Hg values for bulk coal emissions of −1.2 ± 0.5‰ (1SD) and 0.05 ± 0.06‰ (1SD).
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INTRODUCTION
Mercury is commonly present at trace levels of 10−1000 ng/ g in coal.5,6 However, the large volumes of coal used in the energy and industrial sectors have made coal combustion the dominant Hg emission source. On a global scale, coal contributed more than a quarter of cumulative anthropogenic Hg emissions (21 500 tons) between 1850 and 2008.7 Mercury
Mercury (Hg) is a persistent toxic element in our environment. Once Hg is emitted to air from natural and anthropogenic sources, it can be transported globally by atmospheric circulation before being oxidized and deposited.1,2 Following deposition to Earth surface reservoirs, a portion of Hg will be transformed by microorganisms into neurotoxic methylmercury (MeHg). MeHg can be bioaccumulated and biomagnified stepwise along food chains and poses great threat to the health of humans and wild life.3,4 © 2014 American Chemical Society
Received: Revised: Accepted: Published: 7660
March 11, 2014 June 6, 2014 June 6, 2014 June 6, 2014 dx.doi.org/10.1021/es501208a | Environ. Sci. Technol. 2014, 48, 7660−7668
Environmental Science & Technology
Article
possible variation in δ202Hg of different Hg species in coal flue gas emissions, and (iii) to model historical coal Hg isotope emissions.
emissions from stationary coal combustion facilities, primarily coal-fired utility boilers (CFUB), industrial boilers and residential boilers, have soared over the recent two decades, especially in developing countries (e.g., China, India). At present, coal combustion Hg emissions have reached 700−900 tons/year, representing approximate half of anthropogenic Hg emissions to the atmosphere.7−9 Quantitative assessment of the impact of coal Hg emissions on local, regional, and global ecosystems is of substantial interest to both environmental scientists and decision-makers. Moreover, with the implementation of the first global, legally binding UNEP treaty, aimed at reducing anthropogenic Hg emissions, the traceability of coal Hg emissions from different counties or regions is critically important. Global Hg chemistry and transport models have been used to estimate modern transboundary impacts of Hg emissions.10,11 Atmospheric gaseous Hg/CO ratios have also been used to tracing long-range Asian Hg emissions.12 Yet, substantial debate exists on the importance of natural volcanic Hg emissions, on 19th century silver/gold mining Hg emissions, and on the timing and magnitude of Anthropocene perturbations to the natural Hg cycle.1,7,13,14 A widely applicable, source tracing method for Hg emissions has not been developed thus far. Over one decade of Hg stable isotope research has shown that Hg isotope signatures carry information on Hg sources and environmental transformations.15 Incomplete Hg transformations, such as partial oxidation, reduction, methylation, volatilization in natural and man-made environments separate the seven stable Hg isotopes as a function of isotope mass, nuclear volume or nuclear magnetic moment.16,17 Additional isotope fractionation mechanisms, including nuclear selfshielding, are suspected to exist.18 Most natural samples, including the coals considered here, show Hg isotope variations that are controlled by mass dependent fractionation (MDF, denoted by δ202Hg) and mass independent fractionation (MIF) related to the magnetic isotope effect (denoted by Δ199Hg or Δ201Hg). Published Hg isotope data in coals have revealed a ∼3‰ variation in δ202Hg and a ∼1‰ variation in Δ199Hg.19−22 Two important observations have strengthened the potential application of Hg isotope signatures as coal Hg emissions tracers. First, different coal deposits can be distinguished using the combined δ202Hg and Δ199Hg isotope signatures.19,20 Second, the Hg isotopic shift between feed coal and bulk flue gases emissions has been suggested to be relatively small (
