The role of colloids in uranium transport: Facilitating and impeding effects

Tetravalent uranium is generally regarded as sparingly soluble and hence immobile in the aquatic environment. The solubility product of UO2·xH2O was determined by laser-induced breakdown detection (LIBD). For the given reaction, UO2·xH2O + 4 H+ = U4+ + (2+x) H2O, a value of log Ksp0 = -54.30 ± 1.0 was found (cf. [1]). It results in U(IV) solubilities of about 10-8 M in the near-neutral pH region. On the other hand, hexavalent uranium is much more soluble, in particular if carbonate is present in a water. Calculations with the EQ3/6 geochemical speciation software (thermodynamic data from the NEA data base) were performed for a typical oxic groundwater in equilibrium with air CO2 and varying pH. They yielded uranium solubilities of as much as 10-5 M in the near-neutral pH region. Normally, uranium is regarded as mobile under typical groundwater conditions in geochemistry.
However, this apparently clear picture can be counteracted if colloids come into the play. By our laboratory experiments (cf. [2]) the formation of intrinsic U(IV) colloids in the presence of silicate was demonstrated. The possibility that U(IV) forms colloids can not be ruled out also in the aquatic environment. In the colloidal form U(IV) is “undissolved” in the thermodynamic sense of the word but nevertheless mobile. Second, the high mobility of U(VI) in groundwaters or mine waters can be significantly reduced by scavenging and co-precipitation of U(VI) due to the formation of pseudocolloids with unstable (coagulating) ferrihydrite colloids. Laboratory mine flooding simulation experiments were carried out and it was found that more than 98 % of the U(VI) present was scavenged by such colloids ([3, 4]). It follows that both transport-facilitating effects of colloids on “immobile” contaminants such as U(IV) and transport-impeding effects on “mobile” contaminants such as U(VI) may play a part. It is the geochemical ambiance that decides which effect prevails. A comparison of geochemical cases (uranium mine / nuclear waste depository, oxidizing/reducing, acidic/near-neutral) is given.

[1] Opel K., Weiß S., Hübener S., Zänker H., Bernhard G., Radiochim. Acta 95 (2007) 143-149.
[2] Zänker, H., Opel, K., Weiß, S., Hübener, S., Bernhard, G., 2nd Annual Workshop of the IP FUNMIG, 21.-23.11.2006, Stockholm.
[3] Zänker, H., Richter, W., Hüttig, G., Coll. Surf. A: Physicochem. Eng. Aspects 217 (2003) 21-31.
[4] Ulrich, K.-U., Rossberg, A., Foerstendorf, H., Zänker, H., Scheinost, A., Geochim. Cosmochim. Acta 70 (2006) 5469-5487.