Actinide colloid

Kim, J. I. 1991. Actinide colloid generation in ground-water. Radiochimica Acta, 52/53, 71-81. 

Keywords Actinides Colloidal semiconductor Nuclear fuel processing Photocatalysis Valence control... 

Four types of colloids were considered in the WIPP program intrinsic actinide colloids, mineral colloids, microbes, and humic acid colloids (US DOE, 1996). Intrinsic actinide colloids, consisting of polymerized hydrated actinide hydroxides, are not stable in the neutral to moderately basic pH conditions expected in the WIPP, and were assumed not to contribute to the total actinide concentrations in solution. Mineral colloids are destabilized and tend to flocculate in the high-ionic-strength WIPP brines (Kelly et al., 1999). In the performance assessment calculations for the WIPP, a highly conservative value of 2.6 X 10 mol actinide per liter, for each actinide, was assumed to be bound to mineral colloids and to contribute to the mobile fraction. Actinides sorbed onto microbes and humic acids were estimated to contribute significantly to the concentration of mobile actinides in WIPP brines as discussed above (Section 9.06.3.2.2). 

Actinide Colloids Actinide cations undergo hydrolysis in water. Hydrolysis is a step to polynucleation and thus to the generation of actinide colloids the polynuclear hydrolysis species become readily adsorbed to the surface of natural colloids. This also applies to Th, daughter of the primordial radionuclide... 

PWR wastes (wastes containing actinides— colloidal forms of plutonium and americium—in alkaline solutions, laundry wastes)... 

Grolimund et al., 1996). Evidence from field experiments is essentially nonexistent and many field examples of colloid-facilitated contaminant transport remain deductions from actinide-colloid associations (e.g, Kersting et al., 1999). 

A general review of actinide transport on colloids, and how this may relate to radionuclide transport studies, is provided by Ivanovich (1991). It has been found that colloids can carry a large fraction of U (Dearlove et al. 1991). Due to the greater reactivity. 

Orlandini KA, Penrose WR, Harvey BR, et al. 1990. Colloidal behavior of actinides in an oligotrophic lake. Environ Sci Technol 24(5) 706-712. 

The potentially important role of colloidal species in the geochemical behaviour of the polyvalent actinides has nevertheless been stated by various authors (e.g., Kim 1991 Kersting et al. 1999). The present paper discusses the role of colloids on the release of radionuclides from a nuclear waste repositoiy with regard to the processes leading to (1) colloid generation and stability (2) radionuclide interaction with aquatic colloids and (3) colloid-borne radionuclide migration. 

Th-oxyhydroxide species readily dissolve upon dilution below the solubility limit, it is not veiy likely that such actinide(IV) colloids play a role away from the source in the far field of a repository. In the near field of a repository, however, they may be predominant species controlling the solubility of tetravalent actinide species such as U(IV) and Pu(IV) and thus the source term. Unusual stability at high ionic strength has been also reported for amorphous SiOz colloids (Iler 1979 Healy 1994) which also cannot be explained solely by electrostatic repulsion. Formation of oligomeric or polymeric silicate species at the colloid-water interface are thought to exert additional steric stabilization by preventing close approach of those particles. 

Much stronger kinetic stabilization can be expected for processes leading to the inclusion of radionuclide ions into the colloid structure (Fig. 7, lower part). Spectroscopic indications for such processes have indeed been found again by TRLFS for the Cm(III) interaction with colloidal and particulate amorphous silica, calcite and CSH phases (Chung et al. 1998 Stumpf Fanghanel 2002 Tits et al. 2003). The incorporation of actinide ions into colloidal precursor clay phases has been recently investigated as a possible mechanism in natural... 

Fig. 7. Potential mechanisms of actinide (represented by Cm(ni)) interaction with colloids as interpreted from laser fluorescence spectroscopy (TRLFS) experiments. Spectra are taken from Stumpf et al. (2001o, b) and Chung et al. (1998).

The mobility of humic/fulvic colloid-borne tri-, tetra-, penta- and hexavalent actinide ions has been clearly observed in sand column experiments (Kim et al. 1994 Artinger et al. 

In those studies, the colloid-containing groundwater had been previously equilibrated with the sandy sediment until the humic/fulvic acid concentrations of inflowing and outflowing water were identical. Under these conditions, the actinide dissociation rate from the colloids plays the dominant role in controlling the fraction of mobile colloid-borne actinides. 

---