Sorption behavior of neptunium onto smectite under reducing conditions in carbonate media

Sorption behavior of neptunium onto smectite was investigated under reducing conditions in carbonate media. Distribution coefficient (Kd) of neptunium on smectite was determined by a batch method as a function of total carbonate concentration (CT) in a weak alkaline solution. The CT value was ranged from 0.09 M (M≡mol·dm-3) to 1.0 M. The obtained Kd value decreased with increasing the total carbonate concentration. It was found that both Np(IV) and Np(V) coexisted in the solution by a TTA (theroyltrifluoroacetone)-extraction method. In the present experimental conditions, contribution of neptunyl ion, NpO2+, carbonate complexes of Np(V), NpO2(CO3)n(2n-1)- (n=1, 2 and 3), and a carbonatohydroxo complex of Np(IV), Np(CO3)2(OH)22-, to the sorption onto smectite was expected. The Kd values of respective species of neptunium were estimated by a least-squares fit to the experimental data. From the obtained Kd values of respective species, sorption behavior of neptunium onto smectite was discussed. (author)