Laboratory Studies Of The Facts That Influence The Transformation Of Radionuclides On The Solid-liquid Phases In The Changjiang Estuarine

Estuarine zones between the land and the ocean which have multiple functions, play an important part in the study of geochemistry. Water properties in estuarine, such as concentration of suspended particle and salinity, vary in time and space, and affect the adsorption and desorption of radionuclides between the solid phase and the water phase. As a representative estuarine with abundant water and high concentration of suspended sediments in the world, the Changjiang estuarine is of particularly interest in studying particulate-solution transformation.Radionuclides are little in the nature environment, but play an important part in the study of environment problems as tracers. For example, radioctracers provide a convenient tool to study the behaviour of metal ions and other contaminants in natural waters. Furthermore, the amount of radionuclides is increased in recent years, which could do harm to the health of human, as a result of man-made factors. Therefore, it is necessary to understand the processes and controlling factors that influence the adsorption of radionuclides in estuarine.The use of radiotracers in controlled laboratory experiments provides an ideal approach for investigating, in a systematic manner, the role of individual parameters on adsorption processes. ²³⁴Th,²⁴¹Am,⁷Be,²³⁴Th,²²⁸Ra,¹³⁷Cs,²¹⁰Pb and ¹⁵²Eu were added to the aquatic medium under investigation with samples from the Changjiang estuarine, to determined the kinetics of sorption to marine particulate material and to find out factors that influence the action, by changing concentrations of particle,salinity,pH,temperature and composition of seawater respectively. The results are as follows:1. Adsorption of radionuclides onto solid-phase increase with the increase of concentration of sediment, but it is opposite for the distribution coefficient K_d of radionuclides. The results testify to "particle-concentration effects".2. The adsorption percentage ratios and distribution coefficients of ²²⁸Ra and ¹³⁷Cs both decrease significantly with higher salinity, which may attribute to the competitive adsorption of other cations. With high particle activity, the adsorption behaviors of ¹⁵²Eu and ²¹⁰Pb show no visible change with the increase of salinity. As for ⁷Be, its adsorption percentage ratios and distribution coefficients increase slowly as the salinity enhances. It may have something to do with its smaller radii. ⁷Be,²³⁴Th,¹³⁷Cs and ²²⁸Ra all show "salinity effects", that in to say, all of them have more adsorptions to the solid phase at middle salinity.3. By contrasting the results of the laboratory studies at different temperature levels, it is concluded that the increase of temperature enhances the adsorption of ²⁴¹Am on solid phase and it is contrary for that of ²³⁴Th. So we can infer that ²⁴¹Am and ²³⁴Th differ in adsorption mechanism.4. Simulation results showed that pH variation in the system has significant effect on solid-liquid partitioning of radionuclides. Increasing pH of water samples induces portioning of to the solid phase and enhances the transport of radionuclides into sediment.5. The component of seawater influences the adsorption of radionuclides to a great deal. The effect of Na⁺ on ³⁷Cs adsorption was not observed while the colloids in nature seawater showed significant eliminate activity for ²²⁸Ra.6. According to the data of adsorption-desorption, we can infer that the adsorption of ²⁴¹Am,²²⁸Ra and ¹³⁷Cs, except ¹⁵²Eu, to the solid phase is reversible.7. The results of laboratory simulation and actual measurement show that salinity is the essential fact that controls the adsorption behavior of ²²⁸Ra.8. The distribution coefficients of ²²⁶Ra and ²²⁸Ra in water at several sites along middle and lower reach of Changjiang river were measured in last year(2006), when the discharge of Changjiang river was the lowest in the past 50 years. The results reveal that Ra shows "particle-concentration effects" while it does not for Ra. The reason may be that there is plenty of ²³²Th in the suspended particles in the water.