12 TRANS URANIC ELEMENTS IN THE ENVIRONMENT 



Similar Eh-pH diagrams can be constructed for americium (Pourbaix, 1966) which 

 indicate that the standard potential for Am(III) -^ Am(IV) is much greater than that for 

 Pu(III)^ Pu(IV). Hence the range of stability of Am(0H)4 moves to higher values of pH, 

 and it appears unlikely that Am(IV) can exist in solution under normal environmental 

 conditions. 



The formation of complexes can also strongly affect oxidation-reduction potentials 

 in solution and, depending on the relative values of stability constants, can stabilize 

 different oxidation states in solution. Complex-ion formation in solution has been 

 extensively studied because of the need to understand the behavior of transuranic 

 elements in ion exchange, solvent extraction, and precipitation reactions. The general 

 tendency for complex formation depends on such factors as ionic radius and charge. The 

 order of stability constants is M^^ > MOa^ > M'^^ > MO2 , and for anions it is generally 

 COa" > oxalate^" > SOl^ for divalent ions and F~ > NO^ > Cr > ClO^ for mono- 

 valent ions. At relatively high concentrations of metal ions, hydrolysis reactions as acid 

 solutions are neutraHzed lead to the formation of low-molecular-weight hydrolytic 

 polymers, which can be described in some cases by simple equilibria, and higli-molecular- 

 weight polymers, which are not in equilibrium with the ions in solution. 



Stability constants for some ligands present in natural waters have been summarized 

 (Rai and Seme, 1977). For oxidizing conditions at pH8, tlie conclusion was that the 

 dominant species are Pu02C03 0H^ and PuOj in the solution and solid phases, 

 respectively. Unfortunately, the values of the stability constants for many of the 

 hydroxo- and carbonato- complexes of plutonium, particularly Pu(IV), are not known, 

 and the values given in the Hterature are suspect (Cleveland, 1970; 1978). 



Since the effective charges of the metal ions in UOl^ and Pu02^ ions are so similar, 

 the formation constants of complexes would be expected to be essentially the same for 

 each ligand. Woods, Mitchell, and Sullivan (1978) measured the stability constants of 

 complexes of PuO^^ with carbonate ions and found that at pH 8, where the HCO^ ion 

 predominates, there is a 1 : 1 complex (Pu : HCOf ) with a formation constant of about 

 4x 10^. At pH 11, where the CO^" predominates, there is a 1:3 complex, i.e., 

 Pu02(C03)3". In contrast, Langmuir (1978) indicated that, for UOJ'^ in waters at pH 8 

 in equilibrium with partial pressures of CO2 much higher than atmospheric concentra- 

 tion, the predominant complex is U02(C03)3~. He also suggested that at pH <7.5 

 U02(HP04)2 ^ is the dominant species in natural waters with a phosphate concentration 

 of lO'^M. 



Ahhough the effect of complexing in solution is to increase the total concentration 

 of metal, it is not clear if such reactions will make them more or less available for 

 bioaccumulation in the water column. Some of the smaller complexes may be readily 

 assimilated; larger ones may not. The known distribution of the transuranic elements in 

 the environment and their expected solubilities in the presence of particle surfaces 

 indicate that their biological availability also will be markedly influenced by their 

 chemistry/biochemistry in soils and sediments. 



Terrestrial Ecosystems 



Most studies indicate that plutonium is associated primarily with the solid phase in soils 

 and sediments (Tamura, 1976; Garland and Wildung, 1977; Edgington, Wahlgren, and 

 Marshall, 1976). Even in experiments where micromolar concentrations of Pu(N03)4 are 

 added to soil, the water-extractable and nonfilterable (<0.01 membrane filter) portion 

 exists principally as hydrous oxide particles with a diffusion coefficient of approximately 



