Efforts to discern the role of an activated carbon's surface functional groups on the adsorption of elemental mercury (Hg⁰) and mercuric chloride demonstrated that chlorine (Cl) impregnation of a virgin activated carbon using dilute solutions of hydrogen chloride leads to increases (by a factor of 2−3) in fixed-bed capture of these mercury species. A commercially available activated carbon (DARCO FGD, NORIT Americas Inc. [FGD]) was Cl-impregnated (Cl-FGD) [5 lb (2.3 kg) per batch] and tested for entrained-flow, short-time-scale capture of Hg⁰. In an entrained flow reactor, the Cl-FGD was introduced in Hg⁰-laden flue gases (86 ppb of Hg⁰) of varied compositions with gas/solid contact times of about 3−4 s, resulting in significant Hg⁰ removal (80−90%), compared to virgin FGD (10-15%). These levels of Hg⁰ removal were observed across a wide range of very low carbon-to-mercury weight ratios (1000−5000). Variation of the natural gas combustion flue gas composition, by doping with nitrogen oxides and sulfur dioxide, and the flow reactor temperature (100−200 °C) had minimal effects on Hg⁰ removal by the Cl-FGD in these carbon-to-mercury weight ratios. These results demonstrate significant enhancement of activated carbon reactivity with minimal treatment and are applicable to combustion facilities equipped with downstream particulate matter removal such as an electrostatic precipitator.