Polymeric fulvic acid

Manganese(ii) forms both inner- and outer-sphere complexes in hydrochloric acid solutions. (279) For 1-5mF[C1, the species Mn(H20)e and Mn(H20)6 Cl are present in solution in relative amounts 3 2. An outer sphere complex has been suggested by the spectral data for the binding of Mn with the polymeric humic acid, fulvic acid. (280) This is in contrast to the corresponding Fe complex which is of the inner sphere type. 

In contrast to the selective preservation theory, the condensation pathway proposes that humic substances are derived from the polymerization and condensation of low-molecular-weight molecules that are products of the partial microbial degradation of organic residues (Kogel-Knabner, 1993). Under this scheme of increasing complexa-tion, fulvic acids would be the first humic substances synthesized, followed by humic acids and then humin (Stevenson, 1994). The two commonly accepted condensation models are the polyphenol theory and the sugar-amine or mela-noidin theory. 

Soil DOM comprises a wide variety of organic substances. Specific microbial or plant exudates are released as a consequence of metabolic activity of soil biota. Transformations of biogenic compounds, through partial degradation and polymerization, result in the formation of a structurally ill-defined mixture of humic substances, a term used to include both humic and fulvic acids (20, 21). Nonuniform effects of DOM components on metal mobilization may be anticipated because of the heterogeneity of their chemical structures and properties. 

Schnitzer and Khan (1972) concluded that fulvic acids consist in part of phenolic and benzenecarboxylic acids, held together through hydrogen bonds to form a polymeric structure of considerable stability (Fig. 5). Buffle s (1977) model structure of fulvic acid (Fig. 6) contains aromatic and aliphatic components extensively substituted with oxygen-containing functional groups. Both structures show an abundance of COOH groups. 

The abundance of polymethylene chains in humic acid and humin cannot be explained by the formation of these humic substances by polymerization of fulvic acid which has an extremely small amount of polymethylene chains. It is therefore concluded that, as far as polymethylene chains are concerned, the mechanism of sequential formation of humic substances proposed by Nissenbaum and Kaplan (1972) is not valid for humic acid and humin. 

A fifth prevalent theory is that stream humic acids are formed by continuation of the polymerization process to form larger molecular units of fulvic acid which are called humic acid (Beck et al. 1974). 

A systematic approach to the chemical structure of humic or fulvic acid could be patterned on the historical approach to the structure of lignin. Like humic acid, the composition of lignin varies with its source. By common agreement, lignin chemists have adopted Norway spruce as a reference source for native or milled wood conifer lignin. Not only were proposed structural studies based on this lignin material itself, but also on its precursors. Many important structural features were gleaned from a study of precursors and their polymerization products. 

Pichon, V., Coumes, C. C. D., Chen, L., Guenu, S., and Hennion, M. C. 1996. Simple removal of humic and fulvic-acid interferences using polymeric sorbents for the simultaneous solid-phase extraction of polar acidic, neutral, and basic pesticides,... 

Although this scheme of separation appears to be somewhat crude and arbitrary, it does achieve a degree of segregation of polymeric materials as gauged by their more important chemical properties. Table 2.5 summarizes some of these properties, showing that the fulvic acid-humic acid-humin sequence represents a continuum of... 

Whether a ligand increases or decreases Al solubility depends on the particular Al-ligand complex and its tendency to remain in solution or precipitate. Ligands that increase the overall solubility of Al include F", oxalate ", citrate ", fulvic acid, and silicate(monomeric). Ligands that decrease the overall solubility of Al include phosphate, sulfate, silicate(polymeric), and hydroxyl. Figure 5.9 summarizes the influence on Al of common soluble ligands that may be encountered in soil solution. 

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