Mercury emission chemistry: the similarities or are they generalities of mercury and alkali combustion deposition processes?

The recent discovery of mercury's propensity for heterogeneous deposition from flame burned gases shows a behavior that is strikingly similar to those previously seen for the alkali metals. The fact that these elements are located in different parts of the periodic table raises the question that common type behavior may be general among other metals. Both groups show a formation preference in their flame deposits and an insensitivity to the flame. As with the alkalis, if sulfur is present in a chlorine-free system, mercury deposits as mercuric sulfate. As sulfur is decreased, mercury has an option to additionally form its basic sulfate, Schuetteite, HgSO(4)(.)2HgO. If sulfur-free, it produces HgO. These processes all appear to be predominely driven by thermodynamic stabilities and are controlled by the surface and the species available. Interestingly, far more complex molecules can be formed as a surface deposit than can ever exist in the burned gases. The reduced dimensionality of the surface, its possible catalytic influence, and the introduction of condensed phase thermodynamics can induce very fast chemi-deposition with no or minimal activation energies. Unexpectedly, the rates of deposition of mercury and the five alkalis are seen to be extremely similar, indicating an insensitivity to atomic or molecular weights and also showing that these efficient processes are independent of gas phase speciation. Once formed, deposits are very stable within their formation window. They show a reluctance for reversibility but can be converted to a more stable composition by introducing additional species that were not previously present. The comparison of these two very different groups of elements shows such similar behavior that extrapolation and predictions may now be possible for other systems. It also reinforces the fact that the chemistry occurring on surfaces seems to be totally unrelated to that happening in the flame. (c) 2004 The Combustion Institute. Published by Elsevier Inc. All rights reserved.