Sorption of 60Co, 85Sr, 137Cs, 237Np and 241Am on soil under coexistence of humic acid: Effects of molecular size of humic acid

Sorption experiments have been performed by a batch method, to study the effects of humic acid of different molecular size on the complexing stability with 60Co, 85Sr, 137Cs, 237Np and 241Am, and on the sorption behavior of these radionuclides on a sandy soil. Equilibrium constants K in the sorption of 137Cs and 237Np onto the soil were not changed at different concentrations of humic acid since 137Cs and 237Np do not interact with humic acid, while those of 60Co and 241Am decreased with increasing humic acid concentration due to forming humic complexes. However, the K of 85Sr was not changed at different humic acid concentrations, despite 85Sr interacts with humic acid. This contradiction was probably caused from that a main binding of 85Sr with humic acid is not based on coordination bond but electrostatic force, due to relatively high concentration of non-radioactive strontium. The theoretical sorption model taking account of the interaction of 60Co and 241Am with humic acid could well reproduce the values of K for each radionuclide at different concentrations of humic acid. Concentration profiles of the radionuclides in each size fraction of the solution before and after the sorption experiments were examined by ultrafiltration technique. The reduction of concentration of 60Co in the fraction less than 300,000 of cutoff molecular weight (MW) and that of concentration of 241Am in the fraction larger than 100,000MW, respectively, by the sorption onto the soil decreased with increasing humic acid concentration. This decrease resulted in the decrease in the K of 60Co and 241Am with increasing humic acid concentration