148 TRANS URANIC ELEMENTS IN THE ENVIRONMENT 



and the equilibrium expression as 



j^ ^ (NpO^CHNa) 

 (NpOt)(NaC) 



where K is the equilibrium constant, C is clay, and the parentheses denote activity. 



In a system in which neptunium is present in trace quantities and the clay is 

 essentially sodium saturated, the activity coefficients of tlie sodium clay, neptunium clay, 

 and neptunium and sodium ions can be considered to be a constant. Equation 2 can be 

 rewritten in the form 



_ [NpO.C] [Na] 

 [NpOt] [NaC] ^ ^ 



where K' incorporates the constant activity coefficients and the brackets denote 

 concentrations. By definition, the distribution coefficient (Kj) for neptunium is 



[Np02 J 



and Eqs. 3 and 4 can be combined to form 



> [NaC] 



Since the clay remains essentially sodium saturated, NaC is a constant, and a log Kj vs. 

 log [Na] plot should yield a straight line. The slope of the line is a function of the 

 exponent of Na ; in this case the slope is --1. If the clay is calcium saturated, it can be 

 written as Cao.sC in Eq. 1. The final expression of Eq. 5 would contain the exponent of 

 0.5, and thus the slope would be -0.5. 



Data reported by Routson, Jansen, and Robinson (1975) on neptunium sorption by 

 two soils at different sodium and calcium ion concentrations are shown in Fig. 1. The 

 notable feature in the plot is the absence of any effect of sodium on neptunium sorption. 

 Thus the slope of the Np02 Kj vs. Na concentration plot approaches rather than —1. 

 Calcium exerted a more pronounced effect on Np sorption, but even here the Np Kj vs. 

 Ca^ concentration plot had a slope of about —0.3 rather than -0.5. 



The data of Routson, Jansen, and Robinson (1975) and the effect of clay surface 

 treatment on Np02 sorption (Fig. 1) indicate that electrostatic interactions alone do not 

 explain NpOa sorption. One surface component, organic matter, appears to have an 

 influence. In general, the stability of Np(V) chelates is comparable with divalent cation 

 chelates (Zn^"^, Ca^"*", etc.). The interactions of Np(V) with soil humic acids reflect this. 

 Figure 2 represents the observed distribution of Zn^"^, Cd^"^, Ca^"^, Sr^"*", and NpOt 

 between complexed and free forms in the presence of soil humic acids. As is illustrated in 

 the figure, Zn and Cd form stronger complexes than Ca or Sr, which is expected. The 

 Np02 cation forms complexes that are slightly stronger than Ca. It should be apparent 

 from the above samples that even the least hydrolytic actinide oxidation state interacts 

 with soil constituents in complicated ways. 



