Mercury stable isotope fractionation during gaseous elemental mercury adsorption onto coal fly ash particles: Experimental and field observations

Abstract

Mercury (Hg) stable isotopes have a great potential to track coal combustion Hg emissions, but mass-dependent fractionation (MDF) during Hg adsorption onto fly ash particles could significantly alter isotope signatures of emitted Hg species. The detailed processes causing this MDF, however, are not well understood. Here, we simulated how isotopes fractionate during gaseous Hg⁰ adsorption onto fly ash at different times and temperatures. Kinetic MDF that preferably transfers light Hg isotopes to fly ash dominated Hg⁰ adsorption processes. The magnitude of MDF during Hg⁰ adsorption was invariable in the time-series experiment but increased significantly with increasing temperature in the temperature-series experiment. The external mass transfer and chemisorption are suggested to be the controlling processes for isotopic fractionation. Relative to diffusion-driven Hg⁰ adsorption, chemisorption is suggested to be a more important Hg⁰ adsorption step causing MDF, especially at high temperatures. The chemisorption involves Hg redox change from Hg⁰ to Hg^II and is likely enhanced with increasing temperature (50-180 °C). The proposed kinetic MDF model reveals that MDF in modern coal-fired power plants is likely driven by temperature-induced redox processes during Hg⁰ adsorption, and has great implications for developing MDF models in coal-fired boilers and tracing coal combustion Hg emissions.

Keywords: Adsorption; Coal-fired Power plants; Fly ash; Mass dependent fractionation; Mercury isotopes.