Theoretical, experimental and field studies concerning diffusion of radioisotopes in sediments and suspended particles of the sea: Part B: Methods and experiments

Abstract

Simple laboratory and in situ techniques are developed to determine the diffusion coefficients in sediments. The laboratory results for chloride diffusion agree with the results from the in situ determinations, to water depths of 80 m. For diffusion of radionuclides which are strongly sorbed by the sediment an indirect method is proposed where the distribution coefficient of sorption is used for calculating the diffusion coefficient. Three separate techniques are given to determine the distribution coefficients of sorption. Diffusion in a sediment is dependent on temperature. The increase of the diffusion coefficients is about 2.5% per °C in the range of 0° to 20°C. Diffusion of ions is related to the diffusion of the complete salt. When one ion of the salt diffuses rapidly the oppositely charged ion also diffuses rapidly. Concentration and dissociation only affect the diffusion coefficients by a factor less than 2, whereas the effect of the mean free-path hindrance by the sediment particle reduces the diffusion coefficients of free solution by a factor of 2. Partial immobilization of the diffusion of ions in interstitial water is influenced by the charge of the solid-liquid interfaces of sediment particles, irrespective of the sign of the zeta-potential. Changes in properties of the interstitial water (surface tension, viscosity) are not found in relation to this surface action for sands with particles larger than 100 micron. Drying irreversibly decreases the capacity of re-wetted sediment to adsorb radionuclides. Sediment from the top 10 cm of the sea floor is not necessarily in equilibrium with the trace metals in the sea water. However, for radioisotope sorption this non-equilibrium state is found to be unimportant. Distribution coefficients of sorption are rather independent of the carrier concentrations, and the sediment to sea water ratio within a factor of 10. Different sorption complexes of cobalt may be produced by varying experimental conditions; distribution coefficients of 10 3 and 10 5 were obtained in experiments with respectively, stirred and settled (Mediterranean) sediment. Desorption determined for cobalt is found to be present, although at some lower speed than at the preceding uptake. Speaking about sorption equilibria makes sense because the reactions are reversible. The mathematical models developed in Part A ( Duursma & Hoede, 1967) were experimentally verified. Under the conditions of the models, penetration into a sea sediment, due to concentration gradients, can be regarded as a diffusion process. The model for diffusion into a sediment from a thin supernatant current is applicable only if no sorption by the sediment occurs. The results show that for diffusion of radionuclides in sea sediments the diffusion coefficients are between 10 −11 and 10 −6 cm 2/sec, depending on the sorption of the migrating radionuclides from the interstitial water by the sediment particles. For this sorption the distribution coefficients are between 10 and 10 5. To some extent the problem of adsorption and adsorption by suspended sediment particles can be approached as diffusion processes, with diffusion coefficients of about 10 −11 cm 2/sec for the outer shells and about 10 −15 cm 2/sec for the crystal lattices of the particles.