Influence of Humic Acid on Sorption of Neptunium(V) onto Soil
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INFLUENCE OF HUMIC ACID ON SORPTION OF NEPTUNIUM(V) ONTO SOIL Y.SAKAMOTO*, S.NAGAO*, T.OHNUKI*, M.SENOO*, A.OHASHI**, S.SATO**, AND H.OHASHI** *Department of Environmental Safety Research, Japan Atomic Energy Research Institute, Tokai-mura Naka-gun Ibaraki-ken 319-11, Japan. **Department of Nuclear Engineering, Hokkaido University, Sapporo 060, Japan.
ABSTRACT The pH dependence of the stability constant of Np(V)-humate and the sorption of Np(V) onto soil was studied to clarify the influence of dissolved organic carbon on the migration behavior of Np(V) in soil layer. The stability constant of Np(V)-humate was expressed by logpi1=(0.35 ±-0.03)pH+0.04±0.01 in the pH region from 5.3 to 8.7, and the intrinsic stability constant by logp* 1=3.66±0.05. The sorption of Np(V) increased with pH up to pH 7, and tended to reach maximum, about 95%, at pH 9 in the absence of humic acid. In the presence of humic acid more than 140mg/l, the sorption of Np(V) was lower than that in the absence of humic acid above pH 7, while any influence of humic acid on the sorption of Np(V) onto soil was not observed below pH 7. The sorption behavior of Np(V) onto soil was explained by reversible sorption of NpO2+ and sorption of NpO2CO3- and/or NpO2OH. Further, the influence of the size distribution of Np(V) on the sorption onto soil was examined and discussed. INTRODUCTION 7 Sorption behavior of 23 Np on geologic formation should be clarified for the safety 6 assessment of the radioactive waste disposal because of its long half-life (2.12x10 y) and high toxicity. Dissolved organic carbon(DOC), humic and fulvic acids, in ground water is one of the important factors to affect the sorption behavior of Np. It was reported that the distribution coefficient of Np(V) on basalt decreases with concentrations of humic and fulvic acids[l]. However any relationship between the chemical forms in ground water and the sorbed forms of Np(V) on geologic formation has not yet been clarified in the presence of humic and fulvic acids, and little is known about the detail influence of DOC on the sorption behavior of Np. One of the reason why the influence of DOC on the sorption behavior of Np is not known well is the lack of understanding about complexation of Np with humic and fulvic acids and the influence of chemical forms of Np on the sorption behavior of Np. The stability constants of Np with humic acid have been reported[2][3], but the data is scarce. 7 In the present work, the influence of humic acid on sorption of 23 Np(V) onto soil has been studied. The pH dependence of the stability constant of Np(V)-humate has been determined with a UV-VIS-NIR spectrophotometer to clarify the correlation between the chemical forms of Np(V) and the sorption behavior. The sorption of Np(V) onto soil in the absence and presence of humic acid was examined as a function of pH by a batch method, using the information about speciation of Np(V). The desorption experiment of Np(V) using 1.OM KCI solution was also carried out to examine the sorbed form of Np(V) onto soil.
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