a vent gas containing radioactive iodine and methyliodide is effectively decontaminated by chemically adsorbing the iodine contained in the vent gas onto an activated carbon layer at first , then physically adsorbing the methyl iodide contained in the vent gas onto another activated carbon layer separately disposed from the former activated carbon layer , and retaining the radioactive iodine and radioactive methyl iodide on the respective activated carbon layers for a definite period of radioactive decay .