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 (89Sr) and cesium (137Cs) 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 (110Ag) in low-pH solutions. Todorokite analogues were effective for a wider range of radionuclides, including 57Co across a wide pH range (1−10) and 137Cs in acidic solution. The affinity sequence for magnesium and calcium ion-extracted (acid-treated) todorokites in 0.1 HNO3 was 137Cs > 59Fe > 51Cr ≈ 57Co ≈ 241Am > 54Mn > 63Ni > 65Zn > 236Pu > 89Sr.