Determination of Chemical States of Mercury on Activated Carbon Using XANES

Although the adsorption of mercury vapor onto activated carbon is a widely used technology to prevent environmental release, the adsorption mechanism is not clearly understood. In this study, we determined the chemical states of mercury on two kinds of activated carbon using X-ray absorption near-edge spectroscopy (XANES) to elucidate the adsorption mechanism. The adsorption experiments of elemental mercury onto activated carbon were conducted under air and nitrogen atmospheres at temperatures of 20 and 160°C. Two types of activated carbon were prepared. X-ray absorption fine structure (XAFS) measurements were carried out on beamline BL01B1 at SPring-8. HgLIII edge XANES spectra suggested that chemical adsorption of elemental mercury on the activated carbon occurred in the 20–160°C temperature range. According to the XANES spectra, a difference occurred in the chemical states of mercury between AC#1 and AC#2. The Hg XANES spectra on AC#1 were similar to those of Hg2Cl2 and HgS, and the Hg XANES spectra on AC#2 were similar to that of HgO, which suggested that nitric acid treatment removed sulfur from AC#1 and functional groups that were strong oxidizers on the surface of AC#2 created HgO. According to the EXAFS oscillation, a difference occurred in the chemical states of mercury on AC#1 between 20 and 160°C. We found that impurities and oxidant functional groups on activated carbon play key roles in mercury adsorption.