This materLlal is obviously not particulate, but is present in insufficient concentration for characterization using current methods, The question remains--what is the form of the small quantity of Pu available to plants? This information is essential to understanding the mechanisms whereby Pu may be resupplied to solution from the solid phase in a range of soils and to predictions of the long-term availability of Pu to plants, From investigations of Pu valence state in a neutral, 0.0004 M NH,HCO3; solution equilibrated with PuO2 microspheres and in burial groundleachates, Bondietti and Reynolds (1976) concluded that Pu(VL) may be stable in significant quantities in solution and suggested that monomeric Pu(VI) and its complexes may be important in Pu mobilization. In the present studies, several lines of evidence were presented suggesting that that Pu ions are stabilized in soil solution by inorganic or organic ligands for subsequent uptake by the plant. Furthermore, equilibration of weathered Pu-contami- hated soil with chelating resins has been shown (Bondietti et al., 1975) to result in significant desorption of Pu from the solid phase. It is known that organic ligands result in the most stable Pu complexes. Soluble organic ligands in soil are generally derived from microbial processes. The organic complexation reactions and the microbiclogical factors potentially influencing Pu behavior in soil will be discussed in subsequent sections. Other Transuranic Elements Principal tsotopes of other transuranics of concern in the nuclear fuel cycle include 2" Tam, 24 3am, 242om, 7"3 cm, 244en, and 737Np, Although detailed studies of the interaction of these elements with soils are lacking, some information has become available in recent years, Further~ More, the aqueous chemistries of these elements have been fairly well eatablished (Katz and Seaborg, 1957). The most stable ions of Am and Cm in aqueous solutions are the cations (IIE); Np is most stable as the oxyion (NpO2.+). Disproportionation is not common with these elements. Thus, major differences in their environmental behavior as compared to Pu, would be expected. Hydrolysis reactions may still be a primary factor governing the environmental behavior of Am and Cm but greater mobility and plant availability in soils might be predicted on the basis of greater solubility of the hydroxides in comparison to Pu(OH),. The Np oxycarion Should not be subject to significant hydrolysis at environmental pH values (Burney and Harbour, 1974). The environmental behavior of Np has been least Studied of the transuranics but because of its chemical characteristics it may be the most available to the biota, A comparison of Pu, Am, and Np Sorption in several soils (Routson et al., 1975) indicated sorption in the order Pu > Am > Np. The chemistry of Cm should be very similar to Am if present at equal mass concentrations. The petential for complexation of these elements will be discussed in a subsequent section. Organic Complexation Reactions Research to date on the chemistry of the transuranic elements in soil has pointed to the importance of understanding transuranic organic complexation reactions in soil; particularly in surface soils and aquatic sediments where organic matter content is generally highest or in subsoils where the 140 transuranics may be dispersed in conjunction with synthetic complexing agents. Very little information 1s available concerning the interaction of the transuranic elements with the soil organic fraction. However, despite the difficulties in characterization of soil organic complexes, much is known both theoretically and experimentally regarding the interactions of metals with functional groups of soil organic matter (Keeney and Wildung, 1976). Much of this information concerns micronutrients of greatest agronomic importance (B, Co, Cu, Fe, Mn, Mo, Se, Zn} and this research has been the subject of a number of excellent reviews over the 1963; last two decades (Mitchell, 1964, 1972; Mortensen, 1963; Hodgson, In general, earlier studies emphasized metal Stevenson & Ardakani, 1972). humic interactions with intact soil or with the higher molecular weight the more components of soil whereas more recent studies have emphasized soluble components of soil. It ts practical to categorize metal complexes in soil in terms of their solubility since, in general, it is this factor, as previously noted, Three principal which most influences their mobility and plant availability. of categories have been proposed (Hodgson, 1963) although the complexity These the soil system results in considerable overlap between categories. containsubstances inelude (1) the relatively high molecular-weight humic secondary ing condensed aromatic nuclei in complex polymers derived from insoluble syntheses which have a high affinity for metalg but are largely as in soil, (2) low molecular weight organic acids and bases, classified and metabolism nonhumic substances, and derived largely from microbial cells with metals, which demonstrate relatively high solubility in assoctation and (3) soluble ligands which are precipitated on reaction with metals. into three Humic Substances. Humic substances are generallydivided The humin (alkali categories baged on their solubilities (Felbeck, 1965). conditions and drastic and acid ingoluble) fraction is soluble only under The humate (alkali soluble, 1s apparently of the highest molecular weight. of soil acid insoluble) and fulvate (alkali and acid soluble) fractions 1966). may constitute up to 90% of the soil organic fraction (Kononova, a high charge The humates and fulvates are characterized, in part, by density due to acidic functional groups (Stevenson & Ardakani, 1972; This property leads to a high degree of reactivity and Felbeck, 1965). cations in these materials exhibit a strong pH-dependent affinity for organic solution and are likely strongly bound to soil minerals and other groups constituents in soil (Greenland, 1965). The acidic functional carboxyl, hydroxyl consist principally (in genera! order of acidity) of (Broadbent and Bradford, (phenolic, alcoholic), enolic, and carbonyl groups Total acidity has been 1952: Felbeck, 1965; Schnitzer et al., 1959), meq/100 g for humic estimated to range between 500 to 900 and 900 to 1,400 The 1969). acids and fulvic acids, respectively (Stevenson & Butler, (1965) into three acidic H of humic acids was differentiated by Thompson ; using g meq/100 groups at 100 ta 200, 500 to 700, and 1,000 to 1,200 Basic functional groups, likely amides an nonaqueous titration methods. heterocyclic nitrogen compounds (Bremner, 1965), probably also contribute than acidic groups to retention of metals but are of much less importance at most soil pH values. 141