A granulated type of commercial activated carbon (GAC) with surface area of 828 m$^{2}$/g was treated with a strong solution of sulfuric acid (98% wt.) at a temperature of 30 $^{\circ}$C . The physicochemical and porous properties and the surface chemistry of the sorbents were investigated and compared in detail. It was established that the lower temperatures of impregnation and the higher concentration of H$_{2}$SO$_{4}$ solution resulted in the introduction of higher percentages of sulfur-containing groups and smaller porosity loss. The results of EDS, FTIR, and XPS tests confirmed the introduction of sulfone groups and acidic oxygenated ones, which increased the adsorbent affinity towards mercury species available in the aqueous phase (pH 7) by 20%. It was found that acid-washing treatment helped to reduce the ash content of GAC and cleaned its internal space; however, with the introduction of bulky H$_{2}$SO$_{4}$ molecules into micropores and narrow mesopores of GAC, the surface area and pore volume were reduced. The increase of mercury adsorption capacity in spite of decreasing porosity after acid treatment shows that trapping in pores is not the only mechanism involved in mercury adsorption.