A Comparative Study of Europium, Thorium and Uranium Binding to an Aquatic Fulvic Acid
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A COMPARATIVE STUDY OF EUROPIUM, THORIUM AND URANIUM BINDING TO AN AQUATIC FULVIC ACID M. NORDtN*, Y. ALBINSSON", J.H. EPHRAIM* AND B. ALLARD* "Department of Water and Environmental Studies, Linkoping University, S-581 83 Linkbping, Sweden "Department of Nuclear Chemistry, Chalmers University of Technology, S-412 96 GtSteborg, Sweden
ABSTRACT Advances in safe management and disposal of radioactive waste have shown that a comprehensive program requires the incorporation of dissolved organics into radwaste and transport effluent models, with respect to their binding of radionuclides. The binding of Eu', Th" and UO22÷ to a well-characterized aquatic fulvic acid has been studied using an ultrafiltration method at a bulk electrolyte concentration of 0.10 M NaClO 4, trace amounts of radionuclides and fulvic acid concentrations of 60 and 120 mg/l. The results expressed as the overall complex formation function, P., versus pH show the following order: Th4 > Eu3 > UO 22÷. The estimated P., values have been discussed by considering the aqueous chemistry of
Eu', Th' and UO 22' vis-A-vis the solution chemistry of the fulvic acid sample. INTRODUCTION
In order to predict the mobility of radionuclides that could escape from a radioactive waste repository it is of importance to consider the binding of radionuclides by humic substances. In this study the interaction between a well-characterized fulvic acid (Table I) and Eu3, Th4 and UO 22÷, chosen to represent three different oxidation states, was investigated. These elements
have been chosen to be representatives of radionuclides that can be found in radioactive waste.
The experimental technique employed was ultrafiltration whose adaptation to metal-humate interaction studies is documented [1,2,3,4]. However investigations of complexation between fulvic acid and the elements thorium and uranium by this method are very scarcely reported in the literature [5].
In this paper the objective has been to undertake a comparative study of the interaction between a well-characterized aquatic fulvic acid and europium, thorium and uranium as a first step towards the contention that natural organics could be important "actors" in the migration of radionuclides that could escape from waste repositories. EXPERIMENTAL For the ultrafiltration an Amicon 8050 ultrafiltration cell was used in conjunction with a YM1 (1000 Dalton cut-off) Diaflo membrane. Prior to ultrafiltration of the samples the membranes were conditioned by filtering 100 ml of the ionic medium, 0.10 M NaC10 4, through them. The ultrafiltration was performed maintaining a N2 gas pressure of about 3.5 bars. The pH measurements were carried out with a Radiometer pHM 82 Standard pH meter and a pHcombination glass electrode, GK 2401B. An LKB (Wallac) 1219 Rackbeta counter (uranium and thorium) and an LKB (Wallac) 1282 Compugamma counter (europium) were employed for the radioactivity measurements. For uranium and thorium a Packard Ultima Gold scintillation cocktail was used. All experiments were performed at a constant ionic streng
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