Mercury Stable Isotope Signatures of World Coal Deposits and Historical Coal Combustion Emissions

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 δ(202)Hg (-3.9 to 0.8‰) and a 1‰ range in Δ(199)Hg (-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 δ(202)Hg, Δ(199)Hg 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 δ(202)Hg 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 δ(202)Hg and Δ(199)Hg values for bulk coal emissions of -1.2 ± 0.5‰ (1SD) and 0.05 ± 0.06‰ (1SD).

Historical (1850–2010) mercury stable isotope inventory from anthropogenic sources to the atmosphere

“…During the thermal processing of source materials, Hg in source materials is transformed into GEM, GOM and PBM in generated flue gases by a series of physicochemical reactions (oxidation, reduction, adsorption, diffusion etc.). These phase and redox transformations of Hg in high-temperature facilities are expected to induce significant Hg isotope MDF, but no photochemical MIF (Sherman et al, 2012;Sonke et al, 2010;Sun et al, 2013; (Biswas et al, 2008;Lefticariu et al, 2011;Sherman et al, 2012;Sun et al, 2013Sun et al, , 2014aSun et al, , 2014bYin et al, 2014a). In contrast, only a limited number of oil sand samples from Athabasca, Canada have been reported .…”

“…We consider that more isotope data on oil are needed before this sector can be included in the isotope emission inventory. Sun et al (2014a) developed a speciated Hg isotope fractionation model for pulverized coal-fired boilers (the worldwide typical boiler type) by treating Hg isotope fractionation associated with Hg transformations in flue gases as two independent and consecutive Rayleigh fractionation systems: homogeneous gaseous oxidation of GEM to GOM, and heterogeneous gas-particle conversion of GOM to PBM. This model assumes Hg in coal is completely thermally reduced into GEM in the combustion zone of a boiler and Hg isotopes are only fractionated between the furnace outlet and inlet of air pollution control devices (APCD).…”

“…The data points either greater than the upper quartile + 1.5 IQR or less than the lower quartile-1.5 IQR are considered to be extreme values. Data sources: coal (Biswas et al, 2008;Lefticariu et al, 2011;Sherman et al, 2012;Sun et al, 2013Sun et al, , 2014aSun et al, , 2014bYin et al, 2014a); oil ; cinnabar (Cooke et al, 2013;Foucher et al, 2009;Gehrke et al, 2011;Smith et al, 2005Smith et al, , 2008Stetson et al, 2009;Yin et al, 2013); Commercial liquid Hg 0 Estrade et al, 2009;Foucher et al, 2009;Laffont et al, 2011;Mead et al, 2013;Sonke et al, 2008); Zn ores (Smith, 2010;Sonke et al, 2010); Au ores (Smith, 2010); silicate rocks (Blum and Anbar, 2010;North, 2011;Sun et al, 2014b;Zhang et al, 2013Zhang et al, , 2014; limestone (sampled from Huainan Basin, China, this study); volcanic gases (this study; Zambardi et al, 2009 144 sector/fuel/technology combinations for combustion sources are designated to coal combustion, and all these combinations have their corresponding Hg species emission characteristics. Here, we improve the previous estimates on isotope composition of speciated Hg from coal combustion as follows: 1) APCDs are divided into five types: cyclone, scrubber, fiber filter (FF), electrostatic precipitator (ESP), flue gas desulfurization (FGD).…”

“…Over one decade of research has generated a large set of Hg isotope data in primary source materials (e.g., coal, cinnabar), and has advanced our understanding of several important Hg isotope fractionation processes during processing/combustion of source materials (e.g., coal combustion, cinnabar roasting) (Blum et al, 2014;Gray et al, 2013;Sun et al, 2014a). Nevertheless, not all Hg emissions sources are covered and Hg isotope MDF during industrial processing is complex.…”

“…We first compile the Hg isotope composition of source materials used by main human activities. Where possible, we estimate Hg isotope MDF during processing/combustion of source materials, following the framework of Sun et al (2014a). A Monte Carlo approach is used to quantify the uncertainties of Hg isotope composition of anthropogenic emissions, following Streets et al (2011).…”