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 (Hg ⁰) 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.