Formation of Actinide(III)-Humate and its Influence on Adsorption on Kaolinite
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Formation of actinide(III)-humate and its influence on adsorption on kaolinite Yoshio TAKAHASHI 1,3 , Yoshitaka MINAI 1 , Takaumi KIMURA 2 , Yoshihiro MEGURO 2 , Takeshi TOMINAGA 1 1 Department of Chemistry, School of Science, University of Tokyo, Hongo, Tokyo 113, Japan. 2 Tokai Research Establishment, Japan Atomic Energy Research Institute, Tokai, Ibaraki 319-11, Japan. 3 Research Fellow of the Japan Society for the Promotion of Science. ABSTRACT Apparent stability constants of Eu(III)- and Am(IIl)-humates determined at various pH (4.8 to 8) and supporting electrolyte concentration (0.02 to 1 mol/l), indicate that the humate complexes may be dominant species in the aquifer. Distribution coefficients of Eu(III) and Am(III) between kaolinite and solution phase were affected strongly by the presence of humic acid in the solution. Adsorption of humic acid may be an important factor controlling fixation of actinides(III) on mineral surfaces. INTRODUCTION Naturally occurring organics like humic substances (humic acid, fulvic acid and humine) have been recognized as one of the most important factors affecting behavior of actinide elements in the environment [1-3]. Complexation of actinide ions with those substances competes with their complexation with other natural ligands like OH- and CQ 3 2- [4]. In particular, trivalent actinides like Am(III) may form stable complexes with humic acid [1,5,6]. In this paper, we have estimated the influence of humate formation on the behavior of actinides(III) in natural aquifer systems based on laboratory experiments to determine apparent stability constants of Eu(III)- and Am(III)-humates [6-8] and distribution coefficients of Eu(III), Am(III), and humic acid. Previous studies on stability constants of actinide(III)-humate have dealt with their pH dependence, influences of origin and nature of humic substances, and determination methodology and complexation models [6,9,10]. Here, we have noted dependences of stability constants on pH and supporting electrolyte concentration (Cs). Those dependences are indispensable because
pH and ionic strength (I) in natural water change widely: in ground water, 6 < pH < 8, I < 0.01; in seawater, pH=8, I = 0.7. Based on the extensive determination of stability constants over wide ranges of pH and Cs, stability of actinide(III)-humate in ground water and in seawater was estimated more precisely. Kaolinite, which is a typical clay mineral and strong ion exchanger, was added to actinide(III)-humate solution to evaluate the influence of humate complex formation on metal adsorption. Distributions of Eu(III) or Am(III) tracers between the aqueous and solid phases were determined by batch experiments. Adsorption behavior of humic acid in this ternary system was also studied. Studies on adsorption on other naturally occurring adsorbent (e.g., montmorillonite, hematite, and silica gel) are in progress. The preliminary results on adsorption on silica gel are also presented in this report. EXPERIMENTAL Extraction, purification, and characterization of the hu
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