Importance of activated carbonʹs oxygen surface functional groups on elemental mercury adsorption☆

The effect of varying physical and chemical properties of activated carbons on adsorption of elemental mercury (Hg0) was studied by treating two activated carbons to modify their surface functional groups and pore structures. Heat treatment (1200 K) in nitrogen (N2), air oxidation (693 K), and nitric acid (6N HNO3) treatment of two activated carbons (BPL, WPL) were conducted to vary their surface oxygen functional groups. Adsorption experiments of Hg0 by the activated carbons were conducted using a fixed-bed reactor at a temperature of 398 K and under N2 atmosphere. The pore structures of the samples were characterized by N2 and carbon dioxide (CO2) adsorption. Temperature-programmed desorption (TPD) and base–acid titration experiments were conducted to determine the chemical characteristics of the carbon samples. Characterization of the physical and chemical properties of activated carbons in relation to their Hg0 adsorption capacity provides important mechanistic information on Hg0 adsorption. Results suggest that oxygen surface complexes, possibly lactone and carbonyl groups, are the active sites for Hg0 capture. The carbons that have a lower carbon monoxide (CO)/CO2 ratio and a low phenol group concentration tend to have a higher Hg0 adsorption capacity, suggesting that phenol groups may inhibit Hg0 adsorption. The high Hg0 adsorption capacity of a carbon sample is also found to be associated with a low ratio of the phenol/carbonyl groups. A possible Hg0 adsorption mechanism, which is likely to involve an electron transfer process during Hg0 adsorption in which the carbon surfaces may act as an electrode for Hg0 oxidation, is also discussed.