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 ( 89 Sr) and cesium ( 137 Cs) 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 ( 110 Ag) in low-pH solutions. Todorokite analogues were effective for a wider range of radionuclides, including 57 Co across a wide pH range (1−10) and 137 Cs in acidic solution. The affinity sequence for magnesium and calcium ion-extracted (acid-treated) todorokites in 0.1 HNO 3 was 137 Cs > 59 Fe > 51 Cr ≈ 57 Co ≈ 241 Am > 54 Mn > 63 Ni > 65 Zn > 236 Pu > 89 Sr.