1 TRANSURANIC ELEMENTS IN THE ENVIRONMENT 



oxidizing or reducing agents and complexing ligands. Thus the transuranic elements 

 neptunium and plutonium can exist in more than one oxidation state in the environment, 

 whereas the transplutonium elements will be 3+ cations. 



Because of their very similar electronic structures and ionic radii, transuranic elements 

 of a given oxidation state behave similarly chemically. Thus under most conditions 

 Pu(III) is similar to Am(III) or rare earths, such as La(in); Pu(IV) is similar to Th(IV); 

 and Pu(VI) is similar to U(VI) (Wahlgren et al, 1976). These differences in oxidation 

 states of the transuranic elements and the ability of the elements to form complexes with 

 natural ligands will greatly affect their availability for transfer in the biosphere (Dahlman, 

 Bondietti, and Eyman, 1976). 



Standard oxidation— reduction potentials can be used to predict the possible 

 oxidation states of actinide elements in solution under environmental conditions. This, 

 however, is an equilibrium prediction of the relative thermodynamic stability of various 

 species and does not consider the effect of the kinetics of reaction or other factors, such 

 as complexation, which affect the redox couple. Because measurements have been made 

 of these potentials in near-neutral solution (5 < pH < 9), their magnitude must be 

 calculated from the known hydrolytic behavior of the various ions, their respective 

 formation constants, and standard potentials measured in acid solution (Kraus, 1949; 

 Connick, 1949). The resuhing oxidation-reduction potentials estimated for plutonium in 

 neutral solution are : 



0.94 



Pu3+ ^Ml^ Pu(0H)4 • yH2 0(s) ^^ Pu02 -^^^ Pu02(OH)2 



Since the potential for the oxygen couple in neutral solution 2H2 ->■ Oj + 

 4H''"(10~^M) + 4e is +0.815 volt, the oxidized species in any oxidation— reduction couple 

 with a higher positive potential than this is thermodynamically unstable in water, 

 although in practice a considerable overpotential exists (Pourbaix, 1966) which results in 

 extremely slow reaction rates. The formation of complexes that drive the equilibrium 

 potential to more thermodynamically stable values becomes extremely important. 



With values of the oxidation potential (Eh) relative to the standard hydrogen 

 electrode calculated for the reactions of transuranic elements in solution, it is possible to 

 construct Eh-pH diagrams that delineate the regions of stability of ionic and solid species 

 as a function of pH and soluble actinide concentrations. Earlier efforts at constructing 

 these diagrams and phase relationships between plutonium species have been summarized 

 (Bondietti and Sweeton, 1977). A comparison of Eh— pH diagrams for Pu(III) ^ Pu(IV) 

 and Fe(II)-> Fe(III) suggested that, under environmental conditions where ferric ion is 

 reduced to ferrous ion, Pu(IV) may also be reduced to Pu(III) (Bondietti and Sweeton, 

 1977). 



An Eh-pH diagram that was constructed with published values of E° (Pourbaix, 

 1966) is presented in Fig. 3. The III, IV, and VI oxidation states of plutonium were 

 included. However, recent evidence suggests that Pu(V) can exist in aerobic environments 

 (vide infra). The diagram shows the effect of changes in the concentration of plutonium 

 in solution on the regions of stabiUty of each oxidation state. Because of the tendency to 

 form insoluble hydrolytic species, free Pu**"*" ions can exist principally under strongly 

 oxidizing acid conditions (region I). In the normal range of pH and plutonium 

 concentrations encountered in the environment, plutonium could be present as PuOl''", 

 and this form will slowly come into equilibrium with sohd Pu(0H)4 (regions II and III). 



