Geochemical Behavior of Neptunium

This is a revised version of the invited talk presented at “Migration ‘91’ which was held at Jerez de la Frontera, Spain on October 21–25, 1991. Neptunium is one of the most important radionuclides in the safety assessment of radioactive waste disposal because of its long half-life and radiological toxicity. In the present paper, recent progress of the studies is reviewed on the geochemical behavior of Np. Neptunium shows complicated radiochemical features of multivalence, high stability constants for hydrolysis and complexation, and colloidal behavior. Although our knowledge has been much improved through extensive studies, more experimental and modeling efforts are still required especially for understanding the retardation mechanism of complexation products, undissolved species and pseudocolloids of Np.