Sorption of long-lived 94Nb on magnetite: spectroscopic and electrochemical investigation of the associated mechanism

Abstract

94 Nb is a long-lived radioisotope generated upon neutron irradiation of Zr-Nb pressure tube in natural uranium fueled pressurized heavy water reactors (PHWR). The discharged pressure tubes are generally stored in deep tile holes with a concrete cap. Contact of ground water with radioactive waste may lead to leaching and migration of 94 Nb, which may create long-term radiological impact. Therefore, a sorption study of 94 Nb on magnetite was carried out in the pH range of 1-10, both in aerobic and anaerobic conditions to understand its effect on the mobility of the radionuclide. The sorption mechanism of 94 Nb on magnetite and the role of Fe(II) site were investigated using classical (solvent extraction), electrochemical (cyclic voltammetry) and the spectroscopic techniques (X-ray photoelectron, X-ray absorption spectroscopy). Insignificant difference in the sorption pattern and percentage sorption was observed under aerobic and anaerobic conditions, suggesting similar sorption mechanism in both the conditions. The oxidation states of Nb and Fe of magnetite remain unchanged after sorption process. In acidic pH condition, the sorption mainly occurs via ion exchange phenomenon whereas in neutral/ basic medium Nb may be coordinated with magnetite via covalent bond formation. The study is important to understand the sorption mechanism of 94 Nb on magnetite, which is essential to predict the migration behaviour of 94 Nb as well as the retarding effect of magnetite in natural aquatic environment.