Actinides sorption

Studies of Actinide Sorption on Selected Geologic Materials... 

Silva RJ, Benson LV, Apps JA. 1979. Studies of actinide sorption on selected geologic materials. ACS Symp Ser 100 215-240. 

Sorption. In general, actinide sorption will decrease in the presence of ligands that complex with the radionuclide (most commonly humic or fulvic acids, CO3 , SO4 , E ) or cationic solutes that compete with the radionuclide for sorption sites (most commonly Ca, Mg " "). In general, sorption of the (IV) species of actinides (Np, Pu, U) is greater than of the (V) species. 

As discussed previously (Section 9.06.3.1.1), plots of pH sorption edges (see Figure 3) are useful in summarizing the sorption of radionuclide by substrates that have amphoteric sites (i.e., SOH, SO , SOHJ). The pH sorption edges of actinides are similar for different aluminosilicates (quartz, a-alumina, clinoptUolite, montmorillo-nite, and kaolinite). For example, Np(V) and U(VI) exhibit similar pH-dependent sorption edges that are independent of specific aluminosilicate identity (Bertetti et al, 1998 Pabalan et al., 1998). Under similar solution conditions, the amount of radionuclide adsorbed is primarily a function of the surface area. This observation has led several workers to propose that the amount of actinide sorption onto natural materials can be predicted from the surface site density and surface area rather the specific molecular structure of the surface (Davis and Kent, 1990 Turner and Pabalan, 1999). 

Carroll et al. (1992), Stout and Carroll (1993), Van Cappellen et al. (1993), Meece and Benninger (1993), Brady et al. (1999), and Reeder et al. (2001) summarize empirical data and theoretical models of actinide-carbonate mineral interactions. The surface PDI on carbonate minerals may be Ca or Mg " ". Increased solution concentration of Ca " will lead to decreased actinide sorption, which then leads to complex sorption behavior if the carbonate... 

Metal-complexation/SFE using carbon dioxide has been successfully demonstrated for removal of lanthanides, actinides and various other fission products from solids and liquids (8-18), Direct dissolution of recalcitrant uranium oxides using nitric acid and metal-complexing agents in supercritical fluid carbon dioxide has also been reported (79-25). In this paper we explored supercritical fluid extraction of sorbed plutonium and americium from soil using common organophosphorus and beta-diketone complexants. We also qualitatively characterize actinide sorption to various soil fractions via use of sequential chemical extraction techniques. 

The actinide ion can sorb to the surface as a molecular species, V, or it can be embedded in the mineral structure, VI, or it can precipitate to form macroscale secondary phases, VII. Actinide sorption onto mineral surfaces depends on many factors, such as the actinide s speciation, flow rate, and the chemistry and composition of water and surrounding rocks or soil. Generally, batch-sorption experiments are performed to investigate how actinides will be retained. The sorption distribution coefficient, = (ion concentration adsorbed per mass sorbent)/(ion concentration in solution) in (mL g ), has been used to parameterize uptake of contami-... 

In contrast to the relatively pronounced effects of aqueous speciation on actinide sorption, the similarity in the pH-dependence of actinide sorption on a wide variety of minerals such as quartz, a-alumina, clinoptilolite, montmorillonite, amorphous silica, kaolinite, and titanium oxide. suggests a relative insensitivity to surface charge characteri.stics of the sorbent as compared with (he effect of changing the total number of available sites. For example, the data in Fig. 10 3 demon-... 

The pronounced effects of aqueous chemistry on actinide sorption behavior suggest that sorption modeling should account for changing physicochemical conditions. A number of different modeling approaches of varying complexity can be used to incorporate the effects of chemistry on radionuclide sorption. A class of models that has been used with success in modeling pH-dependent sorption for actinides and other metals is the electrostatic surface complexation model (SCM). These models are equilibrium representations of sorption at the mineral-water interface and are discussed in detail elsewhere (Davis Kent, 1990 Dzombak Morel, 1990 Hayes etal., 1991 Seme etal., 1990 Turner, 1995), with only a brief overview presented here. 

Table 3. Actinide sorption onto BVG samples under ADZ conditions...

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