Application of PhreePlot：Drawing the Eh-pH Diagram of Valence-Variable Radionuclides

U, Np, Pu, ⁹⁹Tc, and ⁷⁹Se are redox-sensitive that can exist in several oxidation states. Moreover, actinides like U, Np, and Pu can form various complexes, making their chemical behavior complicated in the biosphere and geosphere. Eh-pH diagram (Pourbaix diagram) is widely used in solving the chemical reactions occurred in aqueous solution. In this study, PhreePlot was used to draw the Eh-pH diagrams of these elements or nuclides in the human serum, saliva, and gastric juice, as well as in Dulbecco’s Modified Eagle’s Medium (DMEM) and Beishan groundwater. The results indicate that in the biosphere the prevalent oxidation states are Ⅵ for U, Ⅳ for Np, Ⅳ and Ⅲ for Pu, Ⅶ and Ⅳ for ⁹⁹Tc, and 0 and Ⅳ for ⁷⁹Se. Particularly, U is more sensitive to the pH condition than other elements or nuclides. In the study of the geological media, according to the Eh-pH diagrams, Beishan repository is in favor of the long-term storage of Np and Pu, whereas U, ⁹⁹Tc, and 79Se are susceptible to mobilization. This study also indicates that the Eh-pH diagrams plotted by PhreePlot are comparable to the one(s) plotted by Geochemist’s Workbench(GWB) program.