Adsorption of heavy metal ions onto activated carbon is explained as a consequence of ion exchange with acidic functional groups such as the carboxyl group. In this study, bead-shaped activated carbon (BAC) was oxidized with nitric acid or ammonium persulfate to introduce surface acidic functional groups. Cadmium ions (Cd(II)) were employed to evaluate the heavy metal adsorption capacity of the oxidized BAC. The amount of surface functional groups was determined by Boehm titration. Surface area and pore volume were measured from N-2 adsorption/desorption isotherms. The results showed that, although similar amounts of surface carboxyl groups were introduced by nitric acid and ammonium persulfate oxidation, BAC oxidized by ammonium persulfate adsorbed a larger amount of Cd(II) (0.76 mmol/g) than that oxidized by nitric acid (0.61 mmol/g) at an initial pH of around 5.0. The amount of Cd(II) adsorbed increased with increasing equilibrium solution pH, and the maximum Cd(II) adsorption for BAC oxidized by ammonium persulfate reached as much as 1.8 mmol/g in the solution equilibrium pH range of 7.1-7.6. The surface area and pore volume were decreased in each oxidized BAC. Since N-2 is not adsorbed onto macropores, this implies that the oxidation process increases pore radius, causing the expansion of micro and mesopores into macropores.