11.5 Prediction on Migration Behavior of Radionuclides on the Basis of Their Dissolved Forms in Groundwater


Fig. 11-13 Isolation and purification procedures of humic substances from saline groundwater

Humic substances are weak-acid polyelectrolytes and exist as solutions of humic and fulvic acids in natural groundwater. Trace amounts of humic substances were isolated from 600 l of saline groundwater by nonionic macroporous resins, and then purified by ion-exchange resins. Photographs in Fig. 11-13 show the powdered humic substances after purification and freeze-drying. The light and shade of the color is due to differences in the structure of humic and fulvic acids. The percentage of humic substances with molecular size below 5,000 daltons (ca. 2 nm) is 43% for the dark brownish humic acid and 81% for the brownish fulvic acid. The fulvic acid consists of organic materials with smaller molecular size in comparison with the humic acid.


Fig. 11-14 Molecular size distribution of Pu and Am in the absence and presence of groundwater humic substances

The humic substances (humic and fulvic acids) isolated from the saline groundwater were dissolved into artificial saline groundwater and adjusted to a concentration of 10 mg/l. Pu and Am were spiked into the humus solutions, and were size-fractionated by an ultrafiltration technique. In order to clarify the formation of actinide-humus complexes, similar experiments were carried out for artificial saline groundwater in the absence of humic substances. The percentage of the dissolved forms (molecular size less than 0.45 µm) of Pu and Am in the humus solutions increases in comparison with that in the absence of humic substances. These results indicate that Pu and Am are complexed with humic substances in saline groundwater.




The understanding of radionuclide migration behavior is very important for the assessment of nuclear waste disposal in geological formations. It is necessary to study the dissolved forms of radionuclides in groundwater and then evaluate the mobility of the radionuclides. The role of natural organic materials has often not been explicitly addressed in recent performance assessments, but it is an important point because of the enhancement of the mobility of radionuclides due to complexation. The impact on actinides such as Pu is large because of the long-half life, the toxicity and its strong affinity for organic materials. In order to understand the association of actinides with organic materials, we focused on humic substances - amorphous, polyelectrolytic organic acids. These substances occupy the majority of organic materials in groundwater and have a high complexation ability for actinides. The complexation properties of actinides with humic substances isolated from a large volume of groundwater (Fig. 11-13) were studied on the basis of molecular size distribution.
The percentage of Pu and Am dissolved in the artificial saline groundwater increases in the presence of humic substances due to the complexation of Pu and Am (Fig. 11-14). Size distribution of Pu-humic complex substances is different from that of Am. Therefore, migration of radionuclides in underground layers is affected by type of humic substances in the groundwater. Until now, reports on complexation experiments using humic substances isolated from groundwater are not numerous, and characterization of the actinides-humus complexes on the basis of molecular size distribution, related to the mobility of complexes in aquatic environ-ments, is limited. Therefore, more realistic prediction and evaluation on the dissolved forms of radionuclides and their mobility in underground environments are anticipated
on the basis of these results.


Reference
S. Nagao et al., Molecular Size Distribution of Np, Pu, and Am in Organic Rich, Saline Groundwater, Understanding and Maraging Organic Matter in Soils, Sediments and Waters, 525 (2001).

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