Research Progress in Metal Stable Isotope Fractionation in Coal-Fired Boilers
Coal-fired boilers are significant anthropogenic sources of metal emissions, contributing over half of certain toxic heavy metal releases. Metal stable isotopes have been widely applied to trace metal pollutants from coal-fired power plants and other sources. However, complex physicochemical processes within boilers induce isotopic fractionation between raw coal and combustion products, complicating source tracing. This review outlines the structure and operational principles of coal-fired boiler systems, focusing on recent advancements in understanding the isotopic fractionation behavior of mercury (Hg), zinc (Zn), cadmium (Cd), and lead (Pb) during coal combustion and flue gas emission. These elements exhibit distinct fractionation patterns due to volatility and condensation dynamics. For instance, Hg, being highly volatile, undergoes significant mass-dependent and mass-independent fractionation, while semi-volatile elements like Cd and Zn show enrichment in fine fly ash. The review emphasizes the necessity of characterizing boiler-specific fractionation factors to improve the accuracy of isotopic tracing. It synthesizes field measurements and laboratory studies, highlighting that fractionation magnitudes can exceed analytical uncertainties by tens of times. The paper also discusses the influence of air pollution control devices (APCDs) such as selective catalytic reduction (SCR) and electrostatic precipitators (ESP) on isotope signatures. Ultimately, this work provides a framework for using metal isotopes as robust tracers in environmental forensics, underscoring the need for comprehensive understanding of boiler processes to interpret isotopic data correctly.