Structure investigation of U(IV) and Th(IV) silica colloids at near-neutral pH by combining X-ray scattering and X-ray absorption spectroscopy

The solubility and environmental mobility of tetravalent actinides is a widely discussed issue. Already at low pH tetravalent actinides show a strong tendency towards hydrolysis followed by the formation of oligomers and oxyhydroxide colloids. Such colloids may show a high groundwater mobility at certain physicochemical conditions. However, An(IV) oxohydroxide colloids polymerize and precipitate already far below neutral pH values. One of the reasons is that the isoelectric point of oxyhydroxide colloids is at neutral pH.

Is it known that trivalent actinides undergo a complexation with silicic acid resulting in colloidal species which are stable at neutral pH [4]. We found in recent studies that silica is also able to stabilize uranium(IV) colloids at near-neutral pH through modification of the inner structure and by influencing the surface charge [5]. Further studies indicate that thorium(IV) shows a similar behavior. The colloid structure and the formation process was investigated by a combination of synchrotron-based X-ray scattering and spectroscopy experiments supported by TEM, XPS, UV-Vis and 29Si MAS NMR.

The U(IV) and Th(IV) silica colloids are stabilized in water-borne state by the surface charge which seems to be determined by modifications of the particle structure. The presence of silica at the colloid surface is one major reason for the shift of the isoelectric point to lower pH values which results in a long-term stability of such colloidal suspensions at near-neutral pH for several years [5]. The colloid particle size determined by photon correlation spectroscopy, ultrafiltration and ultracentrifugation shows a typical size distribution of ≤ 20 nm. TEM and XRD investigations reveal that the internal structure of U(IV) and Th(IV) silica colloids is highly amorphous. EXAFS measurements indicate a direct bond of U(IV) and Th(IV) with silica, but do not show metal-oxygen-metal bonds. In contrast, HEXS shows clearly such metal-oxygen-metal bonds. The reason of this difference is attributable to different scattering processes of X-rays and photoelectrons which will be discussed in more detail. The internal structure of the U(IV) and Th(IV) silica colloid particles is comprised of An-O(H)-An bonds which are successively replaced by An-O(H)-Si bonds and oxygen atoms from bound aquo ions, oxo and hydroxo groups. The stability of such colloids suggests that the assessment of actinide behaviour in the aquatic environment should take the possible existence of An(IV)-silica colloids into consideration.