146 TRANSURANIC ELEMENTS IN THE ENVIRONMENT 

 General 



Chemical Characteristics of the Transuranium Elements (Np, Pu, Am, and Cm) 

 and Related Ac tinides (U and Th) 



The alpha-emitting elements of significance in the uranium and thorium fuel cycles 

 include Th, U, Np, Pu, Am, and Cm. These radioelements can be released to the biosphere 

 by fuel reprocessing, radioactive waste handUng and disposal, and fuel fabrication. Many 

 of these elements have already been dispersed through defense-related activities. 



Although the geochemistries of U and Th are reasonably well understood, the largely 

 man-made elements (Pu, Np, Am, and Cm) are only now under study. Fortunately the 

 chemical characteristics of the transuranic elements are very similar to the naturally 

 occurring rare earths (oxidation state III), Th (oxidation state IV), and U (oxidation 

 states IV and VI). Indeed, early studies on the solution and solid-phase chemistry of the 

 transuranic elements used these analogies (Hindman, 1954; Connick, 1954; Cunningham 

 and Hindman, 1954; Thompson et al., 1949). 



The complexities of the chemistry of any element usually depend on the number of 

 oxidation states that the element can exhibit. For the actinide elements under discussion, 

 these oxidation states are represented by the M^"*", M'*''", MOt(V), and M02^(VI) species. 

 Uranium is a classic example of tlie influence of oxidation state on environmental 

 chemistry. Both U and Th in the tetravalent state are extremely resistant to leaching. 

 However, the oxidation of U(IV) to U(VI) results in much higher U mobilities in the 

 environment (Adams, Osmond, and Rogers, 1959). Likewise, altliough uranyl ion is 

 relatively stable in seawater as the uranyl carbonate complex, ^^^Th formed from the 

 radiodecay of ^^"^U rapidly becomes depleted with respect to ^^'*U (and ^^*U) (Cherry 

 and Shannon, 1974). 



Environmentally Important Oxidation States of the Transuranium Elements 



Curium is trivalent in solution and probably will be present in air-ignited oxides as 

 Cm2 03. Americium is also likely to be trivalent in environmental solutions, although the 

 IV, V, and VI oxidation states are known in the laboratory. The dioxide, Am02, can be 

 formed on ignition. Plutonium can exhibit valences of III, IV, V, and VI in solution and 

 the IV state as the dioxide. Neptunium is probably tetravalent or pentavalent in 

 environmental solutions and tetravalent in tlie oxide. Since limited experimental 

 information is available, these valence assignments for environmental systems are subject 

 to revision. 



Environmental conditions, such as pH and Eh, will control the oxidation-state 

 distribution, although the kinetics of the redox reactions are unknown. Experimental 

 determinations of Pu and Np oxidation states in an environmental context have been 

 undertaken (Bondietti and Reynolds, 1976; Bondietti and Sweeton, 1977: Bondietti, 

 Reynolds, and Shanks, 1976; Bondietti, 1976). A number of speciation diagrams have 

 been constructed (Polzer, 1971; Andelman and Rozzell, 1970; Rai and Serne, 1977) 

 which attempt to evaluate the oxidation-state distribution of Pu in environmental 

 solutions. These investigators have recognized that understanding the environmental 

 speciation of Pu is critical to evaluating its biogeochemistry. The oxidation states of Pu in 

 natural water have actually been determined (Bondietti and Sweeton, 1977). 



