USING A NOVEL CADMIUM ISOTOPE TOOL TO UNDERSTAND THE BEHAVIOR OF TRACE ELEMENTS DURING COAL COMBUSTION IN TWO COAL-BURNING POWER PLANTS IN THE UNITED STATES

More than 40% of the electricity in the United States is produced by coal combustion. The burning of coal produces over 100 million tons of coal combustion products (CCPs) every year. Because some CCPs are known to contain elevated levels of toxic trace elements such as As, Cd, Hg, Se, and Zn, their reuse and disposal can be problematic to the environment. To evaluate potential impacts from CCPs on the environment and human health, it is important to understand the behavior and partitioning of these metals during coal combustion. Cd has a low boiling point (~760 °C) and easily evaporates and condenses in coal-fired blast furnaces. Evaporation and condensation processes could generate mass-dependent isotope fractionation between Cd in the solid CCPs and naturally occurring Cd in sulfide minerals of the parent feed coals (FC). This fractionation may allow Cd isotopes to be used as a novel tracer of materials that have been affected by industrial processes.

Here, for the first time, we utilize Cd isotopes in FCs and CCPs to understand the behavior and partitioning of Cd and other volatile trace metals during coal combustion. The FC and CCP samples, including bottom ash (BA), economizer fly ash (EFA), and fly ash (FA), were collected from two power plants in the Central Appalachian Basin and Colorado Plateau as part of a larger USGS study on geochemistry of coal and coal combustion products. In our preliminary study, measured Cd isotope compositions (δ¹¹⁴Cd/¹¹⁰Cd; relative to NIST Cd 3108) show significant isotopic variations. δ¹¹⁴Cd/¹¹⁰Cd values of BA samples are systematically 0.5 - 1.0 ‰ lower than those of FAs and EFAs. This observation is the opposite of expected Cd isotope fractionation if Cd evaporation is the major mechanism controlling Cd redistribution during coal burning. Our results suggest that it is likely that Cd isotopes fractionate and redistribute in CCPs as the CCPs cool after combustion. For example, heavy Cd isotopes condense first onto EFAs and FAs and the residual lighter Cd isotopes condense later onto BAs. Our ongoing Cd isotope study will further use the Cd isotopic ratios of FCs and CCPs to construct a mass balance and evaluate the Cd emission from coal burning power plants. Cd isotope measurements thus hold significant promise for tracing anthropogenic sources of this highly toxic metal in the environment.