Actinide(IV)-silica colloids and their potential geochemical implications


Actinide(IV)-silica colloids and their potential geochemical implications

Zänker, H.; Weiss, S.; Hennig, C.; Dreissig, I.

Due to their low solubility, tetravalent actinides, An(IV), are usually assumed to be immobile in natural waters. However, it is also well known that insoluble precipitation products can be mobile if they occur as colloids. For An(IV) oxyhydroxides this phenomenon has thoroughly been studied [1-3]. Here (see also [4]) we describe the formation of a new type of An(IV) colloids.
Evidence is provided by photon correlation spectroscopy (PCS), ultrafiltration and ultracentrifugation that uranium(IV) and Th(IV) can form silicate-containing colloids. The An(IV)-silica particles are generated in near-neutral to slightly alkaline solutions containing background chemicals of geogenic nature (carbonate, silicate, sodium ions). They remain stable in aqueous suspension over years. A concentration of up to 10-3 M of colloid-borne An(IV) was observed which is a concentration significantly higher than the concentrations of truly dissolved or colloidally suspended waterborne An(IV) species hitherto reported for the near-neutral pH range. The prevailing size of the particles is below 20 nm. The size of the < 20 nm particles depends on silicate concentration and pH. The higher the silicate concentration and the pH, the smaller (and obviously the more stable) are the particles that are formed (however, silicate at the concentrations tested does not form particles in the absence of the actinides).
Laser Doppler velocimetry reveals that the nanoparticles are stabilized in solution by electrostatic repulsion due to a negative zeta potential caused by the silicate. The isoelectric point of the nanoparticles is shifted toward lower pH values by the silicate.
The mechanism of colloidal stabilization can be regarded as “sequestration” by silicate, a phenomenon well known from trivalent heavy metal ions of high ion potential such as iron(III) [5, 6) or curium(III) [7], but never reported for tetravalent actinides so far. Extended X-ray absorption fine structure (EXAFS) spectroscopy on the U(IV)-silica nanoparticles showed that U-O-Si bonds, which increasingly replace the U-O-U bonds of the amorphous uranium(IV) oxyhydroxide with increasing silicate concentrations, make up the internal structure of the colloids. The next-neighbor coordination of U(IV) in the U(IV)-silica colloids is comparable with that of coffinite, USiO4.
The assessment of actinide behavior in the aquatic environment should take the possible existence of An(IV)-silica colloids into consideration. Their occurrence might influence actinide migration in anoxic waters.
[1] Neck, V. et al., Radiochim. Acta 90, 485 (2002).
[2] Bitea, C. et al., Colloids Surf., A 217, 63 (2003).
[3] Altmaier, M. et al., Radiochim. Acta 92, 537 (2004).
[4] Dreissig, I. et al., Geochim. Cosmochim. Acta 75, 352 (2011).
[5] Browman, M. G. et al., Environ. Sci. Technol. 23, 566 (1989).
[6] Robinson, R. B. et al., J. Am. Water Works Assn. 84, 77 (1992).
[7] Panak, P. J. et al., Radiochim. Acta 93, 133 (2005).

Keywords: Colloids; nanoparticles; uranium; thorium; silica; anoxic groundwater

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