Mercury Stable Isotope Fractionation During Coal Combustion in Coal-Fired Boilers: Reconciling Atmospheric Hg Isotope Observations with Hg Isotope Fractionation Theory.

Mercury (Hg) stable isotope is a useful tool to understand the transformation of atmospheric Hg. The observation on the enrichment of heavier isotopes in gaseous elemental Hg (GEM) relative to oxidized Hg species in atmosphere cannot be convincingly explained by isotope fractionation of Hg redox processes. This review shows that the large Hg isotope mass dependent fractionation (MDF) in coal-fired boilers is one of the underlying reasons. The reported Hg isotope data of feed coals and their combustion products are first summarized to give a general overview of how Hg isotopes fractionate before Hg discharge from coal-fired boilers. Then, predictive MDF models are discussed to simulate δHg values of different Hg species in coal combustion flue gases. The discharged GEM is predicted to have the highest δHg followed by gaseous Hg and particulate-bound Hg, which is in consistent with the observed MDF pattern of atmospheric Hg species.