Diffusion of Neptunyl(V)- and Pertechnetate Ions in Marine Sediments
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DIFFUSION OF NEPTUIYL(V)-
AND PERTECHNETATE IONS IN MARINE SEDIMENTS*
F. SCHREINER, S. FRIED AND A. FRIEDMAN Chemistry Division, Argonne National Laboratory, Argonne, Illinois 60439
9700 South Cass Avenue,
ABSTRACT The diffusion of the 235Np0 2 + and 9 5 mTCo4- -ions has been measured directly in sample cylinders of two different sediments from the floor of the deep sea. In smectiterich sediment not shielded from contact with atmospheric oxygen the following values were obtained for the effective diffusion coefficients of neptunyl- and pertechnetate2 ions, respectively: Deff(Np02+) = 1.5 x 10-12 m s-' and 2 Deff(TcO4-) = 3.2 x 10- 0 m s-. Under anoxic conditions in sediment with known reducing properties, the pertechnetate ion appears to undergo slow reduction and the effective diffusion coefficient of the reduced species of 2 Deff(Tc,red) = 1.1 x 1014 m s- 1 reflects a substantial decrease of the mobility of the lower-valent technetium.
INTRODUCTION The disposal of nuclear waste in the sediment of the deep sea relies on the sediment to provide a natural barrier preventing leached radionuclides from reaching the biosphere. It is therefore essential to verify the mobility behavior of those radionuclides that represent a hazard to the environment. A method for the direct measurement of ionic migration rates in sea-floor sediments has been reported previously along with effective diffusion coefficents for the transuranium elements plutonium and americium [1]. The same method has now been applied to determine the mobility behavior of the pertechnetate and neptunyl(V)-ions in ocean floor sediments of different origin. The experimental procedure for the determination of effective diffusion coefficients involves the following steps: First, samples of the sediments are made up in the shape of right cylinders, 12.5 mm high and 14.4 mm in diameter. At the time of sample preparation a suitable quantity of tracer nuclide is added to the top half of the cylinder and placed in intimate contact with the bottom half at a cross-sectional plane. During preparation and storage these samples are contained in polyethylene tubes and prevented from drying out by keeping them in an atmosphere of saturated vapor of sea water. After an appropriate time span during which the radionuclide tracer has the opportunity to diffuse, the concentration profile of the tracer in the sample cylinder is measured, and an effective diffusion coefficient is determined by matching the observed concentration distribution to the expected calculated curve. In order to measure the activity as a function of cross-sectional level in the sample the sediment is rubbed off on long strips of filter paper, which are later divided into small squares 2 x 2 cm in size for assay of the activity. 9 In the case of 5mTc the 204 keV y-radiation was recorded with a germaniumlithium detector, while the low-energy radiation of 2 3 5 Np (approximately 20 keV) was measured with a silicon-lithium X-ray detector. Based on work performed under the auspice
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