DESORPTION-KINETICS AND MOBILITY OF SOME RADIONUCLIDES IN SEDIMENTS

The desorption kinetics of five radionuclides, 85Sr, I37Cs, ®°Co, MMn, and #5Zn, were studied for several coastal sediments, a few limnetic sediments, and one subsoil, using various aqueous solutions for extraction in batch systems. In a 2-wk desorption experiment, >90% of most radionuclides sorbed were extracted by 6N-HC1, whereas in 1N-HC1 ∼30% of the 137Cs was retained within the sediments and a soil due to its fixation following intraparticle diffusion. For longer desorption periods (6 wk), reversible sorption with sediments could be seen for the 137Cs compared with the other four radionuclides in seawater. The EDTA-2Na solution extracted >70% of #5Zn, MMn, and “Co, and the ammonium oxalate extracted >60% of S4Mn and “Co. Readily exchangeable cations extracted by ammonium acetate were >80% for 85Sr, ∼50% for 137Cs, and 40% for “Co and MMn. Many of the distribution coefficients for these radionuclides in coastal sediments obtained by desorption experiments were one order of magnitude or more larger than those obtained by the adsorption experiments. For 85Sr, larger-than-expected dis-tribution coefficients were obtained in the desorption experiments, an indication of the irreversible formation of metal-oxyhydroxides during a slow reaction. Using batch and column diffusion experiments, it was recognized that “Co and HMn are less strongly associated with coastal sediments under suboxic conditions, and their migration rates are greater in deeper layers of sediment than in the top layer, an indication that a single Kd model is inappropriate to forecast the behavior of radionuclides near sediment surface layers. © 1990 Health Physics Society Pergamon Press plc.