Some physical and chemical features of the variability of kd distribution coefficients for radionuclides

k_d distribution coefficients are widely used as a first approach to the understanding and determination of the eventual fate of metals and radionuclides released into the aquatic environment. For some radionuclides, k_d varies within a range of more than two orders of magnitude. Based on some physical and chemical aspects of the interactions between the dissolved and the particulate phases, some mathematical equations have been developed to explain some features of the observed k_d variability quantitatively. Thus, we have studied the decrease in particle size of k_d in the case of man-made radionuclides interacting with natural particles, and its increase in the case of naturally occurring radionuclides. For the latter, the dependence of k_d on the instantaneous water state has also been investigated. The study also shows the competition effects, particularly the dependence of k_d on pH and salinity. Finally, the effects produced by the size distribution of a given population of particles or the mixture of different populations, are also included. The present work provides for each situation, experimental support from literature data and/or modelling exercises.