The solubility and colloidal behaviour of neptunium (IV)

The solubility and colloidal particle formation of Np(IV) in 10(-1) M Na2S2O4 solution in the pH range 2-10 were studied by sequential filtration with decreasing filter pore size. With the help of spectrophotometry and an extraction technique, the species remaining in the final filtrate were found to be Np(V) at pH less than 6 and Np(IV) at pH greater than 8; particle species were attributed to Np(IV). The solubility data obtained at pH less than 6 were interpreted using the reaction: Np(OH)4(am)----NpO2+ + e- + 2H2O, log K = -6.7 +/- 0.4 At pH greater than 8, the solubility increased rapidly with increasing pH in the presence of air, and this was interpreted as being due to the formation of the negatively charged species Np(CO3)4x - 2x. The formation constants beta 3 and beta 4 were estimated to be similar to those reported for Pu(IV). The size distribution of colloidal Np(IV) was also obtained. Some simple models used to describe the observed size distributions and to estimate the equilibrium size distributions of colloidal Np(IV) were examined.