INVESTIGATION OF CADMIUM AND ZINC ISOTOPE SIGNATURES OF COAL COMBUSTION PRODUCTS

Coal combustion is a widely applied industrial process with many potential environmental impacts. Its extensive use for energy production creates large amounts of coal combustion products (CCPs). These CCPs may contain high concentrations of toxic metals, such as Cd and Zn, which can be harmful to environmental and human health. At high temperatures, physical processes such as evaporation and condensation can redistribute and fractionate Cd and Zn isotopes. These processes may allow Cd and Zn isotopic signatures to mark anthropogenic inputs to the environment.

We analyzed Cd and Zn isotopes in various CCPs [bottom ash (BA), economizer fly ash (EFA) and fly ash (FA)], parent feed coal (FC) and pulverized coal (PC) to test this hypothesis. Bulk Cd isotope analysis reveals an unexpected finding of a heavy signature in FA samples (δ¹¹⁴Cd = -0.39 to +0.47‰; NIST 3108) relative to BA samples (-0.75 to -0.52‰). An initial explanation is that isotope fractionation (i.e. condensation) of heavy Cd onto the fine FA begins in the boiler and continues as the FA moves downstream along with Cd vapor. Transport of FA and heavy Cd out of the boiler leaves relatively lighter Cd vapor to precipitate on BA. Fine FA could also provide a larger surface area for condensation to occur. Zn isotope signatures (δ⁶⁶Zn relative to JMC standard) were measured due to Zn and Cd's similar geochemical properties. Heavier Zn isotopes are also enriched in FA samples (0.13 to 1.34‰) relative to BA samples (-0.17 to -0.07‰). To assess potential fractionation processes from the coals and CCPs to the natural environment, Cd isotope signatures in products from sequential leaching experiments (DI water, diluted HNO₃, acetic acid, hydroxyl ammonium chloride and H₂O₂ followed by ammonium acetate) were also investigated. Low temperature ashed FC and PC samples show a narrow range of δ¹¹⁴Cd values after leaching with 5% HNO₃ (0.26 to 1.17‰). A heavy Cd signature is observed in each sequential extraction phase in FA samples (1.1 to 7.05‰). In contrast, δ¹¹⁴Cd values of BA samples are enriched in light Cd (-2.7 to +0.1‰). Finally, electron-microprobe analysis was conducted to map metals such as Zn, Pb on CCPs. Enrichment on the surface of the CCP particles was observed, consistent with the evaporation-condensation process as revealed by the Cd and Zn isotopes.