Nb solubility and sorption onto clay minerals in the presence of calcium

Ohira, Saki  

The solubility and sorption distribution coefficient of niobium-94 (Nb-94) are one of the important parameters in safety assessment of radioactive waste disposal. However, the solubility and sorption distribution coefficient of Nb under calcium and alkaline conditions are still unknown. In this study, Nb oversaturation solubility experiments in 0.001-0.1 M calcium chloride solutions were systematically carried out and the Nb solubility-limiting solid phase was evaluated: the Nb concentration showed a negative dependence on pH and Ca concentration and the Ca/Nb molar ratio of the precipitated solid phase was 0.66. The pH and Ca concentration dependence was confirmed to be reproducible in the dissolution reaction between the dissolved species of Nb hydroxide ions and the Ca-Nb solid phase showing a Ca/Nb ratio of 0.66. Sorption tests below solubility showed similar values for the Nb sorption distribution coefficient in the presence and absence of Ca, confirming that Nb sorption can be reproduced by the surface complexation model of Nb hydroxide complexes.