A preliminary model for isotopic fractionation

20 The aim of this paper is to assess the Zn isotopic variabilty in feed materials and in 21 combustion by-products collected from three different coal-fired power plants and to 22 develop a generalized model that accounts for the Zn isotopic fractionation occuring 23 during coal combustion processes. Partitioning of Zn between combustion residues and 24 the isotopic composition of feed materials and by-products were determined to 25 furthermore calculate the isotopic signature of gaseous stack emissions using mass 26 balances. We show that the  66 Zn signature of the fly ash samples produced from coal 27 combustion plants can be explained by a Rayleigh fractionation model and propose that 28 the enrichment of heavy isotopes in fly ash is due to condensation in an open system. 29 Rayleigh isotope fractionation models using α coal-fly ash of 1.0003 and 1.0007 account for 30 the observed isotope signatures in fly ash. The isotopic signatures of Zn in the coals 31 (δ 66 Zn IRMM ranging between +0.73 and +1.18‰) fall within the previously determined 32 isotope range for peat (+0.6 – +2.0‰). The Zn isotopes in the by-products (bottom ash 33 and fly ash) are highly fractionated relatively to the Zn ratios found in coals, reaching 34 δ 66 Zn IRMM values up to +1.66‰ in the fly ash due to the preferential condensation of the 35 heavy isotopes in the electrostatic precipitators. The bottom ash collected in these power 36 plants, however, display lower δ 66 Zn IRMM (ranging between +0.26‰ and +0.64‰). The 37 observed direction and magnitude of isotope fractionation in this study suggest that Zn 38 fractionation patterns follow the same pattern in coal-fired power plants but the isotope 39 signatures of the by-products depend on those of the fuels. Our model to predict the 40 δ 66 Zn of the flue gas suggests that the isotopic composition of the flue gas becomes 41 more enriched in the lighter isotopes when more Zn is retained in the fly ash and the 42 fraction of Zn remaining in the bottom ash decreases. Since the percentages of Zn in the 43 gas stack of the coal-fired power plants studied here are lower than 10% of the total Zn 44 3 that is evaporated, we predict that 66 δZn IRMM of the flue gas emitted through the stack 45 will be isotopically light and below-0.5‰. The relation between Zn …