Plutonium Isotopes Research in the Marine Environment: A synthesis

Plutonium (Pu), mainly derived from thermonuclear bomb testing, nuclear accidents, nuclear reprocessing facilities and nuclear power plants since the 1950s, entered the ocean by global fallout and by direct release and was then transported by ocean current, exchanged, transformed and eventually buried in the ocean. Pu in the marine environment consists mainly of four isotopes, namely,Pu (T1/2 = 87.7 a), Pu (T1/2 = 24100 a), Pu (T1/2 = 6561 a) and Pu (T1/2 = 14.3 a), in which a very small fraction of Pu originates from uranium mineral and most Pu comes f rom anth ropogenic act iv it y. Additionally, Pu has two isotopes with extremely low concentrations namely, Pu (T1/2 = 376000 a) and Pu (T1/2 = 8.7 ×10 a). The distribution of Pu concentration in the marine environment is influenced by ocean current distribution and biogeochemical cycles, and therefore, Pu isotopes are typically utilized to trace water mass exchange, particle scavenging and biogeochemical cycles. Compared to Sr (Kd: 10-10 L kg) and Cs (Kd: 10-10 L kg), Pu has a much stronger particle affinity in marine environments (Kd: 10-10 L kg) and thus can serve as a better tracer for indicating transport, scavenging and particle deposition. To use Pu as a tracer for environmental process, the geochemical behavior of Pu should be understood and the analytical method for Pu need to be improved. With the development of analytical methods for Pu, the detection limit of Pu in seawater and sediments has continuously decreased, which allows increasing numbers of researchers (Figure 1) to focus on the sources, geochemical behaviors, distribution and environmental implications. Pu fr om different sources or incidents has unique atom (or activity) ratios, e.g., Pu/Pu, Pu/Pu and Pu/Pu, and these ratios can be used to quantitatively evaluate the source of Pu and to study different marine processes along with Pu activity concentration. Therefore, this study aims to synthesize the application of Pu to marine processes based on a summary of its sources, geochemical behaviors, distribution and analytical methods.