HR: 0800h
AN: V41F-1523    [Abstracts]
TI: Kinetic Isotopic Fractionation of Cd and Zn During Condensation
AU: * Cloquet, C
EM: cloquet.christophe@courrier.uqam.ca
AF: UniversitAc du QuAcbec Aÿ MontrAcal Centre GEOTOP-UQAM-McGill, C.P. 8888, Succ. centre ville, MontrAcal, H3C 3P8 Canada
AU: Carignan, J
EM: carignan@crpg.cnrs-nancy.fr
AF: CRPG-CNRS, 15 rue notre dame des Pauvres, Vandoeuvre les nancy, 54501 France
AU: Libourel, G
EM: libou@crpg.cnrs-nancy.fr
AF: CRPG-CNRS, 15 rue notre dame des Pauvres, Vandoeuvre les nancy, 54501 France
AB: The development of MC-ICPMS allowed to measure precisely isotopic compositions of transitional metals with a resolution generally better than +/- 0.15 per mil per atomic mass unit. The isotopic composition of such metals (Zn, Cd, Cu) varies in terrestrial and extra-terrestrial samples resulting most probably from phase transformations (solid-gas-solid) and/or biogenic fractionation. Up to now, very few experiments were conducted in order to document isotope fractionation during evaporation and condensation of metals. In this study, we report Cd and Zn elemental and isotopic variations measured in fly ashes collected from an urban waste combustor (UWC) equipped with various ash and flue gas filtration devices, including a heat transfer system. In the evacuation system, temperature drop from ca. 900 to 250 degree Celsius. This temperature range allows the evaporation and then condensation of Cd and Zn and probably fusion and oxido-reduction reactions. Indeed, the composition of combustion residues clearly indicates that most of the cadmium and part of the zinc were evacuated in the flue gases. Chemical analysis of the fly ashes collected along the thermal gradient suggest a massive condensation of semi-volatile elements such as Cd and Zn just above the heat transfer plate, which behave as a cold point in the evacuation system. As a first approximation, Cd and Zn contents are directly related to the amount metal condensates on more refractory particles. Leaching experiments on different fly ashes thermodynamic calculations confirmed the occurrence of soluble Cd and Zn salt condensates. The cadmium and Zn isotopic composition of bulk, leachate and residue samples provided variations as large as 1 per mil for delta ¹¹⁴Cd (¹¹⁴Cd/¹¹⁰Cd ratio) and up to 0.5 per mil for delta ⁶⁶Zn (⁶⁶Zn/⁶⁴Zn ratio). Both Cd and Zn delta values are positively correlated to concentrations. According to the position of samples in the thermal gradient of the evacuation system, we suggest that the first gas fraction to condense is depleted relative to the bulk composition of the gas. This is only compatible with kinetic isotopic fractionation which is mass dependant only. As the temperature dropped, most of the Cd and Zn gas condensed and mixed so that the isotopic composition of condensates is very similar to the bulk gas. The very last gas fraction had an isotopic composition enriched in heavy isotopes as a result of progressive condensation and isolation of condensates from the residual gas. It condensed on the cold plate of the heat transfer system and was included in the ash fraction having high metal concentrations. Considering the information on the behaviour observed during the gas cooling and the short time of the reactions, the Cd and Zn isotopic fractionation observed in the fly ashes are interpreted as the result of kinetic isotopic fractionation during condensation.
DE: 1009 Geochemical modeling (3610, 8410)
DE: 1012 Reactions and phase equilibria (3612, 8412)
DE: 1041 Stable isotope geochemistry (0454, 4870)
DE: 1065 Major and trace element geochemistry
DE: 1094 Instruments and techniques
SC: Volcanology, Geochemistry, Petrology [V]
MN: Fall Meeting 2005