ASSOCIATIONS OF Pii AND OTHER ACTINIDES IN SOILS 149 



100 

 8 



6.0 



001 



D 

 CD 



^ 0.4 



Q 



Ca2* CONCENTRATION {M) 

 001 01 



1.0 



"1 1 — I I I I 



I I r 



"m 



KjjNp Burhank sand, f(Na) 



KjNp South Carolina subsoil, f(Na 



Sodium 

 Calcium 



J I I I I 



001 



1 10 



Na* CONCENTRATION (M) 



10.0 



Fig. I Distribution coefficients of neptunium in selected soils. (Modified after Routson, 

 Jansen, and Robinson. 1975.) 



Plutonium Sorption. Plutonium in oxidation state IV is very insoluble in water in the 

 absence of soluble complexers. Given a solubility product (Ksp) tbr Pu(0H)4 of ~10^^^ 

 (Coleman, 1965), soluble monomeric Pu(IV) species should be difficult to assay in 

 near-neutral solutions. Considering the various hydrolytic species [Pu(0H)3, Pu(0H)2 , 

 etc.] , concentrations of soluble Pu in equilibrium with crystalline PUO2 might approach 

 those depicted in Fig.-3. This figure was plotted using the hydrolysis constants evaluated 

 by Baes and Mesmer (1976). By analogy to U(IV), a negatively charged pentahydroxy 

 species, Pu(OH)^, was postulated to exist in their analysis of hydrolytic constants. 



Studies on the effect of pH on Pu(IV) sorption by soils have shown that, in the pH 

 range 2 to 8, 99+% of the added Pu is lost from solution (Rhodes, 1957; Rogers, 1975). 

 Rogers (1975) showed that maximum sorption was at about pH 5.5; sorption was less at 

 lower and at higher pH's. Above pH 8, Rliodes (1957), Rogers (1975), Prout (1958), and 

 Nishita (1978) obsei^ved substantial increases in the concentration of Pu in the 

 supernatant (Fig. 4). Rogers (1975) attributed this behavior to dispersed soil colloids that 

 failed to sediment during centrifugation. Rliodes (1957) and Rogers (1975) observed that 

 this decrease in sorption might also have been due to the dispersal of Pu polymer or 

 hydroxy species. 



Prout (1958) observed a decrease in adsorption for three Pu oxidation states (III, IV, 

 and VI) above pH 7 to 8 which might argue against polymer dispersion. In addition, he 

 found that radiostrontium and radiocesium in low-ionic-strength solutions also showed a 

 decrease in adsorption above pH 7 to 8. In higher ionic-strength solutions, the adsorption 

 increased with increasing pH. Figure 5 illustrates this ionic-strength effect using selected 

 adsorption curves reported by Prout (1958). The concentration of strontium used was 



