310 TRANSURANIC ELEMENTS IN THE ENVIRONMENT 



information concerns micronutrients of greatest agronomic importance (B, Co, Cu, Fe, 

 Mn, Mo, Se, and Zn) and has been the subject of a number of excellent reviews over the 

 last two decades (Mitchell, 1964; 1972; Mortensen, 1963; Hodgson, 1963; Stevenson and 

 Ardakani, 1972). Earlier studies generally emphasized metal interactions with intact soil 

 or with the higher molecular-weight humic components of soil, whereas recent studies 

 emphasize the more soluble components of soil. 



It is practical to categorize metal complexes in soil in terms of their solubility since, 

 in general, it is this factor, as previously noted, that most influences their mobility and 

 plant availability. Three principal categories have been proposed (Hodgson. 1963), 

 althougli the complexity of the soil system results in considerable overlap between 

 categories. These categories include the (1) relatively high-molecular-weight humic 

 substances containing condensed aromatic nuclei in complex polymers derived from 

 secondary syntheses which have a high affinity for metals but are largely insoluble in soil, 

 (2) low-molecular-weight organic acids and bases classified as nonhumic substances and 

 derived largely from microbial cells and metabolism which demonstrate relatively high 

 solubility in association with metals, and (3) soluble ligands that are precipitated on 

 reaction with metals. 



Humic Substances. Humic substances are generally divided into three categories based 

 on their solubilities (Felbeck, 1965). The humin (alkaU and acid insoluble) fraction is 

 soluble only under drastic conditions and is apparently of the highest molecular weight. 

 The humate (alkali soluble and acid insoluble) and fulvate (alkali and acid soluble) 

 fractions of soil may constitute up to 90% of the soil organic fraction (Kononova, 1966). 

 The humates and fulvates are characterized, in part, by a high charge density due to acidic 

 functional groups (Stevenson and Ardakani, 1972; Felbeck, 1965). This property leads to 

 a high degree of reactivity, and these materials exhibit a strong pH-dependent affinity for 

 cations in solution and are likely strongly bound to soil minerals and other organic 

 constituents in soil (Greenland, 1965). The acidic functional groups consist principally (in 

 general order of acidity) of carboxyl, hydroxyl (phenolic and alcoholic), enolic, and 

 carbonyl groups (Broadbent and Bradford, 1952; Felbeck, 1965; Schnitzer, Shearer, and 

 Wright, 1959). Total acidity has been estimated to range between 500 to 900 and 900 to 

 1400meq/100g for humic acids and fulvic acids, respectively (Stevenson and Butler, 

 1969). The acidic hydrogen of humic acids was differentiated by Thompson (1965) into 

 diree groups at 100 to 200, 500 to 700, and 1000 to 1200 meq/100 g using nonaqueous 

 titration methods. Basic functional groups, likely amides and heterocyclic nitrogen 

 compounds (Bremner, 1965), probably also contribute to retention of metals but are of 

 much less importance than acidic groups at most soil pH values. 



In batch equilibration studies (Bondietti, 1974), calcium-saturated humates removed 

 greater than 94% of the Pu(IV) from pH 6.5 aqueous solutions (compositions not given). 

 It is unclear whether the humates represented a surface for precipitation of hydrolyzed 

 species or were involved in complexation of plutonium. However, in studies of plutonium 

 desorption from humates and reference clays, citrate removed 10 to 30% of sorbed 

 plutonium from the clays but less than 1% of that from the humic acids. Ligands forming 

 stronger complexes with plutonium [DTPA and ethylenediaminetetraacetic acid 

 (ETDA)] were required to remove significant quantities (up to 30%) of the plutonium 

 from the humate complex. 



Although humic and fulvic acids likely account for most of the metal immobilization 

 attributed to the soil organic matter (e.g., Hodgson, 1963; Stevenson and Ardakani, 



