In batch equilibration studies (Bondie tti, 1974}, Ca-saturated humates removed greater than 94% of the Pu(iV) from pH 6.5 aqueous solutions | (compositions not given). It is unclear whecher the humates repres ented surface for precipitation of hydrol yzed species or were Iinvelved in ‘ complexation of Pu. However, in studies of Pu desorption from humates and reference clays, cttrate removed 10-302 of sorbed Pu from the clays but less than 1% from the humic acids. Ligands forming stronger complexes with Pu (DTPA and EDTA) were required to remove significant quantities ( to 30%) of the Pu from the humate complex . mp Although humic and fulvic acids likely account for most of the metal immobilization attributed to the soil organic matter, (e.g., Hodgson 1963; Stevenson & Ardakani, 1972), they have the potential for Formation of soluble complexes with metals, particu larly in dilute solutions. Smatlh quantities of metal fulvates, thought to be of lower molecular weight than the humates, may be present in soil solutio n. A nondialyzable material with infrared absorption spectra and element al analyses similar to fulvie acids was isolated from a dilute salt (0.01 M KBr) extract of a mineral soil by Geering and Hodgson (1969). The material exhibited a concentration equivalent to 2.5% of a dialyzable fractio n but was more effective in complexing Cu and 2n, Nonhumic Substances With Potential For Metal Complexation. Lower molecular weight biochemicals of recent origin have been implicated in metal complexation and solubilization in soil. These materials represent (1) components of living cells of microorganisms and plant roots and their exudates and : (2) the entire Spectrum of degradation products which ultimately serve as the building units of the soil humic fraction . The quantity and composition of these materials will vary with soil, vegetation, and environmental conditions (Alexander, 1961, 1971). Readily decomposable wastes disposed to soil under conditions appropriate for microbta l growth may, for example result in immediate and marked increases in organic materials identified in category (1) and longer term increases of materials in category (2). Conversely, toxic materials may have the opposite effects. The specific compounds produced will be dependent upon the properties of the waste and soil environmental conditions after disposal (Routson and Wildung, 1969). Although the concentration of the transuranics and other metals soluble in the soil solution or in mild extractants is low, often near minimum detect—able levels, the major portion of Cu and Zn have been shown to be associared with low molecular weight components. Most of the titratable acidity of this fraction has been attributed (Geering & Hodgson, 1969) to aliphatic acids (< pH 7.0) and amino acids (> pH 7.0). The production, distribution, and action of organic acids in soil has been reviewed by Stevenson (1967). A wide range of organic acids are produced by microorganisms known to be present in soil. These include (1) simple acids such as acetic, propionic, and butyric, produced in largest quantities by bacteria under anaerobic conditions, (2) carboxyli c acids derived from monosaccharides, such as gluconic, slucuronic, and a-ketoglucenic acids produced by both bacteria and fungi, (3) products of the citric acid cycle such as succinic, fumaric, malic, and citric acid, which are common 142 metabolic excretory products of fungi, and (4) aromatic acids such as p-hydroxybenzoic, vanillic, and syringic acids thought to be fungal decomposition products of plant lignins. A variety of organic acids have also been reported in root exudates. The other important group of compounds identified in significant quantities in the soil solution by Geering and Hodgson (1969) which may be expected to exhibit strong affinity for metals are the amino acids. The qualitative and quantitative aspects of amino acida and other nitrogenous components in soils have been reviewed by Bremmer (1967). It was concluded that soil acid hydrolysates do not differ greatly in amino acid composition but quantitative differences may occur with differences in soil, climatic, and cultural practices. A number of acidic and basic amino acids have been reported in soil. However, it appears that the major portion of amino acid-N that is present in hydrolysates is in (1) the neutral amino acids glycine, alanine, serine, threonine, valine, leucine, isoleucine and proline, (2) the acidic amino acids, aspartic acid and glutamic acid, and (3) the basic amino acids, lysine and arginine. Most of the amino acids detected in soil hydrolysates have also been shown to exist free in smail quantities in scils with levels seldom exceeding 2 ywe/g. In the soil solution (Geering & Hodgson, 1969), neutral amino acids also appeared to predominate, Basic amino acids were not detected although two acidic amino acids (aspartic and glutamic acids) were present. Stevenson and Ardakani (1972) concluded that organic acids and amino acids, while present only in small quantities in soil, were present in sufficient quantities in water-soluble forms to play a significant role in solubilization of mineral matter in soil. Small quantities of a number of other complexing agents, such as nucleotide phosphates, polyphenols, phytic acid, porphyrins, and auxina, also exist in soil (pertinent references have been summarized by Mortensen, 1963). However, it is unclear at present, whether these materials would be present in sufficient quantities in the soil solution under most soil conditions to affect tranguranic solubility over the long-term. MICROBIAL TRANSFORMATION OF THE TRANSURANIC ELEMENTS IN SOIL Potential Mechanisms of Transformation From the results of limited atudies of soil chemistry, microbiology and plant availability of transuranics in soils, and by inference from studies ef complexation of other trace metals in soils (as discussed above) it may be concluded that the soil microflora will play a significant role in transformations governing the form, and ultimately, the long-term solubility and behavior of transuranic elements in soil. There are four general mechanisms whereby microorganisms may alter the form of trace metale in soil (Alexander, 1961; Wood, 1974}. These include (1) indirect mechanisms resulting from metal interactions with microbial metabolites, or changes in pH and Eh, (2) direct transformations such as alkylation and alteration of the valence state through microbial oxidation (use of the metal as an energy source) or microbial reduction (use of the metal as an electron acceptor in the absence of oxygen), (3) immobilization by incorporation 143