XAFS characterization of mercury captured from combustion gases on sorbents at low temperatures

A review is presented of data obtained by X-ray absorption fine structure (XAFS) spectroscopy on the speciation of mercury captured on a variety of sorbent materials from simulated combustion flue gases at low temperatures (<200 °C). Data for other key elements (S, Cl) are also presented. Implications for bonding mechanisms for the capture of mercury are discussed, including the relative importance of chemisorption and physisorption processes. Systematics in the parameters derived from mercury X-ray absorption near-edge structure (XANES) spectra indicate that mercury can be captured by bonding to I, Cl, S or O anionic species on the surfaces of carbonaceous and other sorbents, but only as ionic Hg2+. None of the observations made by XAFS spectroscopy is consistent with the capture of mercury in the elemental state, i.e. physisorption.