Sorption/desorption of radiorutheniun on the surface sediments of Daya Bay, China

Sorption process on the mineral surface is a basic and important interaction for the retention and migration of poison chemicals such as heavy metal ions and radioactive nuclides in environment. The surface sediment is the significant materials for the migration and fate of pollutant such as radioactive nuclides in the coastal region. In the present paper, sorption/desorption of radiorutheniun on the surface sediments (0-2 cm) was investigated by batch method in sea water system at ambient temperature. The surface sediments were obtained from four stations (W0, W2, W6, W9) around the Daya Bay, where the first nuclear power station of China has been running from A 30 mg surface sediments powder, which was already naturally dried and grinded by 100 mesh, was introduced into the tubes with 30.0 ml sea water (pH 8.20, 35% salinity, filtered by 0.45 μm). After interval contact time, the separation of solid from suspension solution was performed by 0.45 μm micro-filter. The radioactivity of radio ruthenium on filter paper was determined by a γ-ray spectrometer (EG and G ORTEC ADCAM-2000, P-type, relative efficiency 35%) with 497.08 kev. The plots of percentage of sorption versus the contact time are showed in Fig. 2. It is seen in Fig.2 that the sorption percentage can be obtained to be around 30% for all the surface sediment in one hour, then, the tendency of the increase of sorption percentage becomes slow. The sorption percentage of radio ruthenium reached only not more than 80% even if the contact time reaches to be 113 days (2713 hours). Similar results were observed in the study of the sorption kinetics in sediments/radionuclides of Stepovogo bay and Abrosimov bay. In fact, a variety of binding mechanisms are responsible for controlling the exchange of radionuclides between sediment and sea water, including of precipitation, co-precipitation, adsorption, diffusion into sediment pore spaces. Of these, adsorption onto sediments is believed to be the most important removal mechanism for elements in the ocean. In this case, sorption kinetics is an important consideration in coastal bays where sediment-water contact times occur over time scales comparable to the reaction rates of radionuclide. As a result, dispersion of some released radionuclide in the soluble fraction away from nuclear power station may be greater than predicted from equilibrium models of particle scavenging. Conversely, exposures involving local transport of sediment would be overestimated by equilibrium assumptions. The result of desorption experiment suggest that the sorption of radio ruthenium is irreversible. The values of distribution coefficient decrease with increase of the concentration of surface sediment in this system. It is noted in the Fig.2 that the sorption percentage of initial step decreases in the increasing of contact time. More details of this result will need to be elucidated.