Sorption behavior of radionuclides on crystalline synthetic tunnel manganese oxides

Cryptomelane-type (potassium) and todorokite-type (magnesium and calcium) manganese oxides were prepared and studied for the removal of radionuclides from aqueous solutions containing inactive metal ions of interest in radionuclide-bearing nuclear waste effluents. Batch distribution coefficients were measured as a function of pH and sodium, potassium, magnesium, and calcium ion concentrations. Trace strontium (Sr-89) and cesium (Cs-137) ions were taken up by ion-exchange mechanisms. Selectivity coefficients were estimated as K-Cs/K = 0,6 and K-Sr/K = 1.0 for cryptomelane and K-Cs/Mg = 7550, K-Sr/Mg = 50, and K-Sr/Ca = 10 for todorokite. The cryptomelane analogue was particularly effective for the separation of trace silver ions (Ag-110) in low-pH solutions. Todorokite analogues were effective for a wider range of radionuclides, including Co-57 across a wide pH range (1-10) and Cs-137 in acidic solution. The affinity sequence for magnesium and calcium ion-extracted (acid-treated) todorokites in 0.1 HNO3 was Cs-137 > Fe-59 > Cr-51 approximate to Co-57 approximate to Am-241 > Mn-54 > Ni-63 > Zn-65 > Pu-236 > Sr-89.