INVESTIGATION OF CADMIUM AND ZINC ISOTOPE SIGNATURES OF COAL COMBUSTION PRODUCTS
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.