The removal of Hg from aquatic environments using activated carbon (AC) impregnated with humic acid was investigated. The Hg adsorption capacity of the AC was controlled by the specific surface area and by surface functional groups, and the Hg adsorption process was exothermic and more effective at low pH. The Hg adsorption process in this study corresponds to a Lagergren pseudo-2nd-order model, indicating that it is controlled by the chemical rather than the physical properties of the AC. In addition, the equilibrium isotherm data fit the Langmuir isotherm better than the Freundlich model, indicating that the AC surfaces were uniform and that a Hg adsorbate monolayer formed at equilibrium.