Diffusion and fixation of a mobile radionuclide in deep sea sediments

Diffusion experiments of pertechnetate anion (95mTcO4-) were conducted at room temperature on some deep sea sediments and bentonite, in order to evaluate diffusion rates of mobile hazardous long-lived radionuclides in seabed sediments. Measured apparent diffusion coefficients in both deep sea sediments (6-8 x 10-10m2/s) and bentonite (1-3 x 10-10m2/s) have linear correlations on logarithmic scales with water contents and porosities of sediments. These correlations are useful to estimate diffusion coefficients of mobile radionuclides in sediments. Retardation of pertechnetate was observed during the diffusion experiments on only one calcareous deep sea sediment sample containing black spots rich in pyrite. Local accumulation of pyrite replacing diatoms can be an indicator of a micro-reducing-environment in deep sea sediments at least at the time of pyrite deposition. Technetium fixation at these black spots, even after air oxidation of sediments and even under oxic experimental conditions, suggests the reduction of Tc oxides by some organic reducing agents still active. The importance of these organic reducing agents in the radionuclide immobilization should be noted. (author)