304 TRANSURANIC ELEMENTS IN THE ENVIRONMENT 



Plutonium ions may commonly exist in aqueous solution in valence states III, IV, 



V(Pu02), and VKPuOf*). Other valence states are known (II and VII) and predicted 



(VIII), but these occur under unique conditions (Cleveland, 1970). Disproportionation 



reactions are common, and, due to kinetic factors, plutonium is unique among the 



chemical elements in that it may simultaneously exist in all the common valence states. 



The tendency of plutonium to hydrolyze in aqueous solutions of low acidity follows the 



order Pu'*'' > PuOi^ > Pu^"" > ?\xO\ (Cleveland, 1970). Hydrolysis, which occurs in a 



stepwise fashion, is likely the major mechanism whereby plutonium is insolubilized in the 



environment. At high (grams per liter) plutonium concentrations, hydrolysis of Pu"*^ may 



lead to the formation of a colloidal plutonium polymer. At these concentrations the 



polymer is characterized by a distinct absorption spectrum. Although the polymer has 



not been fully characterized, it is generally thought to be an intermediate hydrolysis 



product of Pu^"^ containing oxide or hydroxide bridges with an absorption spectrum 



different from that of Pu(0H)4. However, studies by Lloyd and Haire (1973) indicated 



that the polymer may be aggregates of small, discrete, amorphous or crystalline, primary 



particles of 5 to 20 A in diameter. It is of interest that X-ray-diffraction patterns of the 



polymeric plutonium and that of Pu(0H)4 (Ockenden and Welch, 1956) showed a 



pattern characteristic of the cubic Pu02 lattice, which suggests that the polymer and the 



hydroxide of Pu'*'^ may be hydrated Pu02 with differences occurring in primary particle 



size and crystallinity (Lloyd and Haire, 1973). The formation of the hydrated PuOo is 



likely directly related to Pu'*" concentration and inversely related to the acid 



concentration. 



Plutonium also tends to form many complexes with a range of stabilities. The 



strongest complexes are generally formed by reaction of organic ligands with Pu^*. 



However, many inorganic complexes and organic complexes of all valences may be stable 



under appropriate conditions. The presence of organic ligands in soils likely influences the 



equilibrium and concentration form of plutonium in solution through complexation and 



subsequent inhibition of hydrolysis, polymerization, or disproportionation. These 



reactions, in various highly complex combinations resulting from differences in source 



term, soil properties, and processes, govern plutonium solubility in soil and availability to 



plants. 



Soil chernical reactions are important in governing the behavior of the various torms 



of plutonium entering soil. Initially, soluble forms entering soil have the potential for 



undergoing a range of chemical transformations. Insoluble plutonium, such as high-fired 



oxide, entering soil likely will be solubilized with time, provided that soluble, stable 



complexes are formed. However, regardless of the form of plutonium entering soil, its 



ultimate solubility will be controlled by its aqueous chemistry and by soil factors. Soil 



physicochemical properties can be expected to have complex, interdependent effects on 



plutonium solubility. The long-term behavior of plutonium in soil will be a function of 



the kinetics of these reactions. 



On the basis of research with other trace metals, recently summarized by Keeney and 



Wildung( 1977), and limited information on the transuranic elements, it can be concluded 



that the soil physicochemical parameters most important in influencing the solubility of 



the transuranic elements include (1) solution composition. Eh and pH; (2) type and 



density of charge on soil colloids; and (3) reactive surface area. These phenomena will, in 



turn, be dependent on soil properties, including particle size distribution, organic-matter 



content, particle mineralogy, degree of aeration, and microbial activity. The delineation 



ot the influence of these factors on plutonium solubility is difficult owing to the complex 



chemistry of plutonium. 



