Mordenites were synthesized using the natural resource such as coal fly ash and rice husk ash. The Cs + adsorption rate (100 ppm Cs + , 100 mL) in seawater for the prepared mordenite (1.0 g) was 83.2%. The Cs + adsorption rate for these mordenites almost depended on the calculated Cs + exchangeable value (mmol) using the cation exchange capacity (CEC) values. Composite powder materials consisting of mordenite and nanosized magnetite (Fe 3 O 4 ) (10, 20, 30 wt%) were also synthesized from coal fly ash and rice husk ash. The total Cs + decontamination rates using the magnetic collection after the Cs + adsorption in water and seawater were ca. 90% and ca. 70% for the examined 20 wt% magnetite-containing composite material, respectively. Although the elution ratio of the Cs + ion was ca. 8% from all of the adsorbed Cs + by the dissolution test in slowly shaken deionized water for 14 days in the case of the Cs + adsorbed composite material (20wt% magnetite), the Cs + ion did not elute when the sample was heated at 1000°C and higher temperature due to the Cs + containment in the glassy phase.