The use of smectite-type clay minerals is proposed for capturing radioactive nuclear wastes from sea water. Iodine molecules dissolved in a 3 wt% aqueous NaCl solution were completely taken up by a column packed with Ag-modified synthetic saponite. Cs+ and Sr2+ ions in a 3 wt% aqueous NaCl solution were adsorbed efficiently on a column packed with magnesium vermiculite under appropriate conditions.