Interaction of Neptunium with Humic Acids　　

• Neptunium : Np-237 is a long-lived, alpha-emitting nuclide and important in the safety assessment of geological disposal of radioactive wastes.
• Since neptunium takes various oxidation states, it is necessary to control and check its oxidation state in the experiment.
• The interaction of Np(V) with humic acids (the most important ligand in natural waters) was studied.

Redox speciation of actinide

：Radiochim. Acta, 91, 1-6 (2003).
A. Kirishima, O. Tochiyama, K. Tanaka, Y. Niibori and T. Mitsugashira
　The use of solvent impregnated resin and bismuth phosphate precipitate in microporous centrifugal filter unit enabled the compact and rapid determination of the oxidation state of actinide.

Humic acid as a complexant

　Heterogeneous composition: various functional groups fixed in various matrix environments.
　Polyelectrolyte nature: Many charged are fixed to the matrix of a macromolecule

Mechanism of the interaction of humic acids with metal ions

　Radiochim.Acta, 88, 547-552 (2000), 90, 569-574 (2002), 90, 555-561 (2002), 原子力バックエンド研究, 9, 21-28 (2002)
T. Kubota, A. Kirishima, B. Setiawan, O. Tochiyama, K. Tanaka, Y. Niibori, T. Mitsugashira

　The interaction coefficients of Np(V), Eu(III) (= Am(III)), Ca(II), Fe(II)(III) were quantitatively described as a function of pH, ionic strength and metal-ion concentrations. The effect of heterogeneous composition and polyelectrolyte nature of humic acids was clarified.