Intrinsic Formation of Neptunium Nanoparticles in Presence and Absence of Silica: Formation of Np(IV)-silica Colloids and NpO2 Nanocrystals

The chemistry of tetravalent actinides An(IV) in aqueous solution is typical of a small highly-charged metal ion with a strong tendency to hydrolize and therefore a low solubility. Disregarding the formation of clusters, nanoparticles and colloids lead to underestimation of the migration behavior. The reaction mechanisms of formation and growth, especially under alkaline and near-neutral environmental conditions are still unexplored. On the way from aqueous species to nanoparticles two fundamental questions in An(IV) chemistry provoke discussion in literature: 1) are hydrolysis and condensation the driving forces toward nano-scaled solids or 2) are formed aggregates ill-defined complex hydroxides, hydrous oxides or highly structured clusters/nanoparticles? Excluding the presence of other oxidation states then An(IV), we performed in situ investigation of the self organised formation of neptunium(IV) aggregates and nanoparticles from aqueous complex precursor under alkaline conditions. The kinetics of the self-assembling of the nanoparticles, their morphology and internal structures were determined. The influence of silica on the formation of highly coordinated NpO2 structure was proved. In particular, former studies confirmed the formation of amorphous silica-containing U(IV) and Th(IV) colloids. Starting from aqueous neptunium(IV) carbonate complexes, we investigated the behavior after dilution in presence and absence of silica by TEM, EXAFS, UV vis spectroscopy, Ultrafiltration and DLS (dynamic light scattering). The formation of nanoparticles was observed. TEM and diffraction pattern show different morphologies and internal structures in dependence of presence or absence of silica.