Halides on Carbonaceous Materials for Enhanced Capture of Hg0: Mechanistic Study

Use of amorphous carbon particles impregnated with halides has been demonstrated to be very effective for the capture of elemental mercury (Hg0) from power plant flue gases. Despite the fact that the halides employed are not oxidants, the Hg0 captured is found to be in an oxidized form. Although this approach has been widely accepted for the control of mercury emissions, the mechanism responsible for enhancing Hg0 capture and subsequent oxidation is not well-understood. We investigated the chemistry of halogenated carbonaceous materials and propose a mechanism that appears to be able to explain both the enhancement of Hg0 capture and its subsequent oxidation. Interaction of microcrystalline graphitic structures in amorphous carbon with halide ions results in the formation of charge-transfer compounds, with halide ions withdrawing electrons from the graphitic lattice, making the carbon particle a stronger Lewis acid and resulting in the enhancement of its ability to attract electrons from the highly polarizable Hg0. Upon contact, the electron transfer produces oxidized mercury, with the graphitic structure as a counterion. This study may have implications in other areas; for example, the combination of sea spray and soot could enhance the oxidation of atmospheric mercury vapor, with potential effects on its residency and bioavailability.