The complexation of some radionuclides with natural organics : Implications for radioactive waste disposal

Abstract: Natural organics like humic substances are important for the speciation, mobility and bioavailability of metals in the environment. This ability necessitates the investigation of what impact humic substances may have on radionuclides that accidentally can escape from radioactive waste repositories. The objective of the present thesis is to contribute to the understanding of how radionuclides are complexed by humic substances as well as cellulose degradation products and how this may influence their adsorption to particularly geological materials.A comparison of characteristics of humic substances from different origins has been carried out in an attempt to give a general picture of the properties of these compounds. Evidence was found that supports the statement that the conformation and the molecular weight of the humic substance molecule are important factors in detennining ionic strength effects. A large fraction of the investigated fulvic acids (FA) had pK,'PP-values around 4.0 which suggestes -COOH functionality.A well-characterized aquatic FA, and its complexation with four different radionuclides (Sr, Eu, Th, U) ha;e been studied. The effect of pH, ionic strength, competing metal ions and different methods of investigation (IEDS and UF) on the complexation was examined. It was found that the influence of pH and ionic strength varied with the method of investigation, but that the ionic strength effect was greatest when applying JEDS. Both methods showed that Eu formed stronger complexes than Sr. Thorium formed the strongest complexes. The hexavalent U seemed to form weaker complexes than Eu, especially at pH values below 4. No effect of competing ions (AI, Fe, Sr) on the Eu-FA complex was noticed and it was suggested that this was due to different binding sites for Eu and the competing ions.The investigation of how FA influences the adsorption of Sr and Eu onto alumina and quartz showed that FA has the ability to both increase and decrease the adsorption of Sr and Eu depending on pH and ionic strength.Cellulose degradation products as well as some organic hexose derivatives were found to decrease the adsorption of Eu onto cement at high pH (> 12). However, this decrease disappeared with time probably due to continued degradation.A general conclusion is that aquatic humic substances maY have a significant impact on speciation and transport properties of tri- and tetravalent elements like Eu and Th as well as hexavalent U, in the presence of high concentrations, of humics (surface waters), butinsignificant effect under most groundwater conditions.