The relevance of speciation to the environmental behaviour of radionuclides

Radionuclides exist in many different chemical forms in the environment, which are initially dependent on source and release conditions, ranging from low molecular mass species such as ions, molecules and complexes to high molecular mass species such as colloids and particles. The physical characteristics of released radionuclides can vary considerably in morphology and structure, size, shape, density, valence and charge. Radionuclide forms can be transformed with time through interaction with various ecosystem components and this may affect their subsequent bioavailability. Low molecular mass species are often more bioavailable than high molecular mass species such as colloids and particles. Various techniques have been developed to estimate bioavailability of radionuclides in soils and sediments, which often act as the major sink for radionuclides, such as estimation of Kd and sequential extraction. Few of these measurements have been directly linked to associated uptake by plants. Source dependent bioavailability has been shown to be an important variable affecting bioavailability, defined as gastrointestinal transfer, for radiocaesium and methods of predicting true absorption for different radiocaesium sources have been developed. For radioiodine, gut absorption is complete and independent of source whereas radiostrontium bioavailability is highly dependent on calcium intake. To reliably predict the environmental impact of radioactive contamination of different ecosystems, we need to link information on radionuclide composition and speciation to an understanding of the influence this might have on environmental transfer. Current information linking radionuclide speciation to bioavailability and environmental transfer is limited and this restricts our capability to adequately integrate the effect of speciation into predictive models of radionuclide behaviour in the environment. Such models must take account not only the time dependent effect of radionuclide speciation, but also that of the interaction with environmental chemistry and biology which together determine environmental transfer rates.