The effect of physicochemical properties of activated carbon on adsorption of elemental mercury (Hg-0) was investigated on a series of modified activated carbons. Heat treatment and benzoic acid impregnation were conducted to vary the oxygen functional groups on carbon surface. Hg-0 adsorption experiments were run in a fixed-bed reactor at 140 degrees C. Surface characteristics of carbon samples were studied by N-2 adsorption, Boehm titration, X-ray photoelectron spectroscopy (XPS) and X-ray absorption fine structure (XAFS), respectively. The predominant mechanism of Hg-0 removal was the formation of chemical bonds between Hg and various functional groups. Both XPS and XAFS analysis revealed that mercury bound on carbon surface was mainly in oxidation state. Under N-2 atmosphere, the absorbed Hg was found as Hg2+, and coordinated to O atom. With the existence of HCl in simulated flue gas, Hg-0 was bonded on Cl sites and HgCl2 was assumed to be the dominated form.