Sorption of 60 Co, 85 Sr, 137 Cs, 237 Np and 241 Am on Soil Under Coexistence of Humic Acid: Effects of Molecular Size o
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SORPTION OF
,Sr, 2Np NpCs, AND "'Am ON SOIL UNDER COEXISTENCE OF HUMIC ACID; EFFECTS OF MOLECULAR SIZE OF HUMIC ACID
TADAO TANAKA AND MUNEAKI SENOO Department of Environmental Safety Research, Japan Atomic Energy Research Institute, Tokai, Ibaraki, Japan, 319-11. ABSTRACT 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 2- 7Np 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 6°Co and 2A1Am 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 'Sr interacts with humic acid. This contradiction was probably caused from that a main binding of '5Sr 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 6°Co and 24Am 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 6WCoin the fraction less than 300,000 of cutoff molecular weight (MW) and that of concentration of 24'Am 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 6WCo and "U'Amwith increasing humic acid concentration. INTRODUCTION
The involvement of dissolved organic substance in the sorption ability and mobility of radionuclides in soil and water has been investigated"-3 . Much of the dissolved organic substance in natural water consists of humic substance, which occurs during microbial degradation of biomass in soil and water. The resulting product "humic acid" is very stable to further degradation and is of indeterminate structure, with carboxyl, methoxy, phenolic, quinone, and other functional groups, and molecular weights of under 1,000 to over 200,0003-5. As such humic acid has substantial chelation properties for metals, especially transition metals, it considerably affects the sorption and migration behavior of the radionuclides in soil layer. From a geochemical point of view, it is important to know reactivity of humic acid with radionuclides released from radioactive-waste repository 6. Davis et aL has found that humic acid of different molecular sizes affects its sorption ability onto soils 3. It has been also reported that the interaction affinity between humic acid and metal ions is affected by the molecular size2 . Since humic acid gi
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