Actinides Adsorption

As detailed above, the adsorption behavior of most actinides varies widely with solution pH, Eh, complexation, competitive adsorption and ionic strength, and the surface properties of sorbent phases. For this reason, many researchers have modeled actinide adsorption using surface complex-ation (SC) models that can quantitatively account for such variables. These models include the constant capacitance (CC), diffuse-layer (DL), and triple-layer (TL) models (Chap. 10). Much of the ra-... 

An advantage of the SC models is that they provide a mechanistic and scientifically defensible approach for predicting adsorption behavior outside the range of laboratory conditions used to parameterize the models. The models are capable of relatively accurate predictions of actinide adsorption behavior in complex systems. By comparison, Kj values are largely restricted in application to systems similar to those used for the measurement. Alternatively, highly conservative (often... 

Fig. 13.39). The accumulation of carefully measured empirical adsorption data and K values for actinide adsorption, have encouraged a number of researchers to estimate SC parameters for... 

Kosyakov, V.N., Yakovlev, N.G. and Veleshko, I.E. (2002) Application of chitin-containing fiber material mycoton for actinide adsorption. Journal cf Nuclear Science and Technology, 39 (suppl. 3), 508-511. 

The highly fractionated nature of the and Th series nuclides is illustrated by the measured activities in some representative waters in Figure 1. The highest activities are typically observed for Rn, reflecting the lack of reactivity of this noble gas. Groundwater Rn activities are controlled only by rapid in situ decay (Table 1) and supply from host rocks, without the complications of removal by adsorption or precipitation. The actinide U, which is soluble in oxidizing waters, is present in intermediate activities that are moderated by removal onto aquifer rocks. The long-lived... 

The overall distribution of lanthanides in bone may be influenced by the reactions between trivalent cations and bone surfaces. Bone surfaces accumulate many poorly utilized or excreted cations present in the circulation. The mechanisms of accumulation in bone may include reactions with bone mineral such as adsorption, ion exchange, and ionic bond formation (Neuman and Neuman, 1958) as well as the formation of complexes with proteins or other organic bone constituents (Taylor, 1972). The uptake of lanthanides and actinides by bone mineral appears to be independent of the ionic radius. Taylor et al. (1971) have shown that the in vitro uptakes on powdered bone ash of 241Am(III) (ionic radius 0.98 A) and of 239Pu(IV) (ionic radius 0.90 A) were 0.97 0.016 and 0.98 0.007, respectively. In vitro experiments by Foreman (1962) suggested that Pu(IV) accumulated on powdered bone or bone ash by adsorption, a relatively nonspecific reaction. On the other hand, reactions with organic bone constituents appear to depend on ionic radius. The complexes of the smaller Pu(IV) ion and any of the organic bone constituents tested thus far were more stable (as determined by gel filtration) than the complexes with Am(III) or Cm(III) (Taylor, 1972). 

Means JE, Crerar DA, Borcsik MP, et al. 1978. Adsorption of Co and selected actinides by Mn and Ee oxides in soils and sediments. Geochim Cosmochim Acta 42 1763. 

Higgo J. J. W. and Rees L. V. C. (1986) Adsorption of actinides by marine sediments effect of the sediment/seawater ratio on the measured distribution ratio. Environ. Sci. Technol. 20, 483-490. 

The purpose of this study was to investigate the anionic exchange behavior of neptunium(V) in sulfate-sulfuric acid, because neptunium is often present as a contaminant during the separation of other actinides (l ). Sulfuric acid systems are seldom utilized in industrial processes, but are often used as part of a laboratory analytical procedure. Literature on neptunium in HC101, HC1, HC1-HF, and HNO3 is quite complete, but the information on the H2S0l system is sketchy at best. There is one report 2) that neptunium(V) is adsorbed strongly on Dowex 2 resin from 0.1 IT to 1 IT H SOip Our measurements indicate that there is very little adsorption of Np(V) on Dowex 1 resin even at low concentrations of sulfate-sulfuric acid. We believe the differences in chemical structure of the two resins are not sufficient to explain the disparity in adsorption. 

Silva and Nitsche (1996) note that distribution coefficient (K i) values for adsorption of the actinides are roughly equal for actinide cations of similar charge and structure. Based on earlier work, they report average actinide (An) cation KjS for 12 different minerals and 4 rock types of 500, 50, 5, and 1 respectively, for An", An ", AnO +, and AnOt ions. The sequence of adsorption edges with increasing pH forTh(IV), Am(III), Np(V), and Pu(V) adsorption by y-Al203 in Fig. 13.34 (Bidoglio et al. 1989) is consistent with this order, with pH values of 2.5,5.8, and 7.3, respectively, at 50% ad-... 

Figure 13.34 Comparison of the adsorption of actinide cations of different oxidation states onto y-AljOj. The solid lines refer to Th(IV), Am(III), and Np(V). Open circles are adsorption data for Pu(V), with 2Pu(V) = 2 x lO" M. y-AliOj concentrations are 10 mg/L for Th and Am and 200 mg/L for Np and Pu. Modified after Bidoglio et al.. Interactions and transport of plutonium-humic acid particles in groundwater environments, Mat. Res, Soc. Symp. Proc. 1989, 127 823-30. Used by permission.

The adsorption edge plots in Fig. 13.34 show that adsorption of actinide cations increases with pH and has a strong pH dependence. This reflects that the actinide cations must compete with protons for adsorption sites. Cations whose adsorption edges occur at lowest pH values must, therefore, form the strongest bonds with sorbent surface sites. The adsorption edges of weaker-bonding cations then occur at successively higher pH values. 

The oxidation state of an actinide is obviously key to its adsorption behaviour. Lieser and Mohlenweg (1988) measured the apparent for Np adsorption at pH = 7.0 as a function of Eh. Their... 

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