Humate Complexation of Neptunium(V) and its Modeling
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HUMATE COMPLEXATION OF NEPTUNIUM(V) AND ITS MODELING HIROTAKE MORIYAMA*, YASUHIRO NAKATA** AND KUNIO HIGASHI** *Research Reactor Institute, Kyoto University, Kumatori-cho, Sennan-gun, Osaka 590-04, Japan **Department of Nuclear Engineering, Kyoto University, Yoshida, Sakyo-ku, Kyoto 606-01, Japan. ABSTRACT The complexation behavior of Np(V) with humate was investigated by spectrophotometry in the pH range from 5.7 to 10 in 0.01-0.5M NaClO4. The absorption peaks of NpO2+ and Np(V) humate were observed at 979.6nm and 989.9nm, respectively, and the apparent complexation constant of the Np(V) humate was found to be pH-dependent. A two-site complexation model was applied to the interpretation of the observed pH-dependence by considering the participation of the two functional groups of carboxylate and phenolate. The dissociation and complexation constants of both functional groups were determined. INTRODUCTION The humate complexation of radionuclides in the geosphere is recognized to be important for the safety assessment of geologic disposal of radioactive wastes. 1-3 In spite of its importance, however, the humate complexation of Np(V) has not been studied so much and only a few works have been reported. Kim and Sekine 4 investigated the complexation behavior of Np(V) with humic acid, extracted from one of the Gorleben groundwaters, by specrophotometry in the pH range from 6 to 9 in 0.1M NaCIO4. They observed the formation of a 1:1 complex and obtained the complexation constants. The constants were found to be pH-dependent and, by introducing the loading capacities of the humic acid for the Np02+, a new pH-independent constant was evaluated. Recently, a similar complexation behavior of Np(V) of very low concentrations has been reported by Trautmann et al. 5 In the present study, the complexation behavior of Np(V) was investigated by using a commercial reagent of humic acid for comparison. Similarly to the previous studies, the complexation constants were observed to be pH-dependent. A two-site complexation model was applied to the interpretation of the observations. EXPERIMENTAL Preparations 237
Np dissolved in IM HNO 3 was obtained from CEA(Centre Energie Atomique, France). This solution was evaporated and the Np residue was dissolved into 0.1M NaCIO4. The concentration of Np(V) was about 10- 2M. The sodium humate obtained from Aldrich Co. was purified, protonated and freeze dried following the literatures. 6,7 The concentrations of impurities were measured by using an inductively coupled plasma emission spectrometer, Shimadzu ICP-5000, for Al(88ppm), Ba(
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