Investigation into the formation of Np(IV) silica colloids

In the near and far field of nuclear waste repositories lower oxidations states of the actinides (An) are expected to become predominant because of the prevalent reducing conditions. Due to the low solubility at neutral pH, tetravalent actinides (An(IV)) are assumed to reveal immobile migration behaviour. Nevertheless, a high environmental mobility has been found, e.g. for Pu(IV), which is obviously related to the formation of An(IV) eigencolloids or to the sorption of the heavy metal ions onto other colloids. In the laboratory, An(IV)O2×H2O colloids for Th(IV) and Np(IV) have been synthesized. Regarding to erosion processes in repository sites, e.g. on glasmolds and ubiquitous occurring matter, the interaction of An(IV) with carbonate and silicate compounds and the potential formation of colloids has to be considered carefully. In particular, former studies evidenced the formation of U(IV) and Th(IV) silica-containing colloids.
We developed a method to generate aqueous Np(IV) solutions, explicitly excluding the presence of other oxidation states, for the synthesis of silica-containing colloids. Under anaerobic conditions, Np(IV) carbonate solutions in presence and absence of silicate were investigated by TEM, EXAFS, UV-vis spectroscopy, ultrafiltration, LSC and DLS (dynamic light scattering). When silicate was admixed to such a solution Np(IV) is prevented from precipitation and the spectrum obtained differs from that of dissolved Np(IV) in 1.0 M carbonate. The absorption band at 741 nm is significantly increased and in dependence of the concentration of silica shifted to 747 nm. Thus, the formation of colloidal Np(IV) silicate is strongly suggested. Furthermore, the colloid-disperse Np(IV) silicate solution exhibits an increased scattering light intensity and diameters of the particles were determined in the range from 1 to 20 nm. Ultrafiltration removes these particles and the respective UV-vis spectrum shows considerably reduced absorption bands. In addition to the disappearance of the previously observed absorption bands around 745 nm, the concentration of Np was reduced from 1.0 × 10−3 to 0.1 × 10−3 M and 1.8 × 10−3 to 0.3 × 10−3 M. All these findings can be explained by a colloidal behaviour. Moreover it is ascertained that these colloidal systems are stabil over a period of more than 120 days. In presence of silicate we observed a stabilized dispersion of Np(IV) silica colloids. Hence, Np(IV) may become waterborne even if the limit of solubility is exceeded. The existence of such colloids has never been reported so far.