A carbon-based catalyst for flue gas desulfurization is brought into contact with a flue gas containing at least SO2 gas, oxygen and water vapor so that the SO2 gas can react with the oxygen and the water vapor to form sulfuric acid which is to be recovered. On a surface of the carbon-based catalyst, iodine, bromine or a compound thereof is added, ion exchanged or supported and a water-repellent treatment is applied. The carbon-based catalyst can also be used as a mercury adsorbent for flue gas treatment for adsorbing and removing metallic mercury from a flue gas containing metallic mercury, SO2 gas, oxygen and water vapor.