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

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 HgII 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.

Determination of Chemical Speciation of Arsenic and Selenium in High-As Coal Combustion Ash by X-ray Photoelectron Spectroscopy: Examples from a Kentucky Stoker Ash.

Knowledge of the chemical speciation of arsenic and selenium in coal fly ash is essential in the evaluation of the environmental behavior of fly ash disposed in a landfill in a natural environment. In this study, a series of high-As coal fly ash from stoker boilers were collected to determine the chemical forms of arsenic and selenium. The ash surface chemical characteristics and the speciation of arsenic and selenium were characterized by X-ray photoelectron spectroscopy and X-ray-induced Auger electron spectroscopy. The results indicate that the surface enrichment ratio for selenium (63.3-309.5) is higher than that of arsenic (1.2-21.2). The Wagner chemical-state plot of arsenic indicates that As is mainly present as As(V) bonded to oxygen ligands, that is, the [AsO4]3- anion; Se is found predominantly as elemental Se (62.0-83.3%), followed by selenite (16.7-38%). The extreme enrichment of both arsenic and selenium arsenic is controlled by iron oxides in the coal fly ash.