Application of simplified desorption method to study on sorption of neptunium(V) on montmorillonite-based mixtures

To elucidate the sorption behaviors of radionuclides in multi-mineral systems and the mutual effects of minerals on this sorption, we carried out the sorption and desorption experiments of neptunium(V) on Na-montmorillonite-based two-mineral phasic systems (montmorillonite–calcite, montmorillonite–apatite). In the montmorillonite–calcite system, the sorption on the montmorillonite moiety decreased with increasing calcite content due to interference by the calcium ions dissolved from the calcite moiety, while no accumulation of Np to the calcite was observed. Total sorption of Np on the montmorillonite–apatite system was larger than that on apatite-free Na-montmorillonite, but the sorption on the montmorillonite moiety in this system was less than that on apatite-free Na-montmorillonite. Under weakly acid and neutral pH conditions, Np accumulated on the apatite moiety in a short period. At final pH 4 or less, though the pH condition was sufficient to dissolve the apatite moiety completely, the sorption very slowly increased with time and the increased Np was unexchangeable with 1 M KCl solution. This increase of the unexchangeable sorption cannot be explained by the knowledge accumulated so far.