Desorption kinetics and mobility of some radionuclides in sediments

The desorption kinetics of five radionuclides, 85Sr, 137Cs, 60Co, 54Mn, and 65Zn, 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, greater than 90% of most radionuclides sorbed were extracted by 6N-HCl, whereas in 1N-HCl approximately 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 greater than 70% of 65Zn, 54Mn, and 60Co, and the ammonium oxalate extracted greater than 60% of 54Mn and 60Co. Readily exchangeable cations extracted by ammonium acetate were greater than 80% for 85Sr, approximately 50% for 137Cs, and 40% for 60Co and 54Mn. 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 distribution 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 60Co and 54Mn 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.