ASSOCIATIONS OF Pu AND OTHER ACTINIDES IN SOILS 147 



TABLE 1 Effect of Clay Treatment on Adsorption of 

 Actinide Elements to Miami Silt Loam Clay 



*Organic matter removed with NaOCl; Fe removed with sodium dithionite. 



tpH 6.5; 5mM CaCNOj)^; solution/clay ratio of 400/1; 48 hr equilibration; U and Np at 

 <microgram per gram levels; Th and Cm at <nanogram per gram levels. 

 jMean ± standard deviation. 



Interactions with Environmental Colloids 



Effect of Oxidation State on Sorption The partitioning of the actinides between solid 

 and solution phases may be dependent on the charge characteristics of the element, the 

 physicochemical characteristics of the solid, and the composition of the solution. 

 Complexation by OH (hydrolysis) and other ligands affects sorption because all four 

 common oxidation states (III, IV, V, and VI) form complexes of varying stabilities. For 

 example, the competition between hydrolysis and complexation by carbonate dominates 

 the sorption behavior of uranyl ion in natural solutions. Above pH 7.5 (and in 

 equilibrium with atmospheric CO2) soluble uranyl carbonate complexes can predominate; 

 below this pH sorption to particulates readily occurs (Starik and Kolyadin, 1957). 

 Another example is Pu(IV), which is extensively hydrolyzed in near-neutral solutions and 

 is probably not adsorbed by normal ion exchange mechanisms (Tamura, 1972). 

 Oxidation— reduction reactions, since they affect oxidation state, also influence sorption. 

 Thus NpOt shows poor adsorption to soil, but reduction to Np(IV) increases sorption 

 (Bondietti, 1976). 



The different actinide oxidation states with respect to sorption are compared in 

 Table 1. The distribution of Th(IV), U(VI), Cm(III), and Np(V) between a soil-clay 

 fraction and a 5mAf Ca(N03)2 solution showed that relative sorption followed the 

 oxidation-state order IV > III > VI > V under the specified conditions. The organic 

 matter and free iron oxide (Fe) coatings were removed to evaluate the effects of colloid 

 surface constituents [and thus cation exchange capacity (CEC)] . Only Np(V) sorption 

 was strongly influenced by these treatments. The removal of organic matter decreased the 

 CEC by 35% and Np sorption by 20%. 



Removal of organic matter and Fe did not further affect the CEC, but the Np(V) 

 sorption value decreased to 29% of the intact clay value. This observation of an apparent 

 surface-dependent sorption mechanism suggests that, even for the MO2 oxidation-state 

 species, which is largely unhydrolyzed at environmental pH's, mass-action relationships 

 may not describe adsorption equilibria. 



Mass-action expressions have been used to describe ion-exchange equilibria. The 

 exchange of Np02 on a sodium-saturated clay can be expressed as 



NpOt + NaC ^ NPO2C + Na"" 



(1) 



