Humic substances theories

Field JA, Cervantes FJ (2005) Microbial redox reactions mediated by humus and structurally related quinones. In Perminova IV, Hatfield K, Hertkom N (eds) Use of humic substances to remediate polluted environments from theory to practice, vol 52. Springer, Dordrecht, pp 343-352... 

A modification of the lignin theory of the formation of humic substances is the polyphenol theory, which considers the key interactions to be those between quinones, derived either from polyphenols or lignin, and amino compounds. 

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. 

According to one theory, the synthesis of humic substances is supposed to be brought about, primarily, by the condensation of the autolysis products from micro-organisms growing on carbohydrates (mostly cellulose). The biosynthesis of aromatic compounds from carbohydrates " may be of importance in the soil. 

It is my opinion that the theory of humic substances coagulation calls for a more intensive chemical approach rather than a physical approach. Let us consider... 

The mechanism(s) for the formation of stream humic substances is not known. Several theories of formation of these substances have been formulated, but none have been supported with adequate systematic data. Most of the theories of formation are common to soil humic substances and few are unique to stream humic substances. Five of the general or overall theories are the following ... 

A second theory, closely aligned with the first, assumes stream humic substances to be formed within the stream by the same processes as soil humic substances, whatever they may be. 

A third theory speculates that stream humic substances are soil fulvic acids leached from soil in the initial stages of humification and then modified, transformed, or aged by stream humification processes which result in humic substances unique to this aquatic environment. 

A fourth theory postulates that stream humic substances are formed by a unique stream humification process, whereby simple reactive moieties are polymerized and condensed into humic substances unique to the stream environment. 

Another common theory of formation for stream and soil humic substances is a modified remnant of the old ligno-protein theory of Waksman 11938). This theory states that soil and stream humic substances are degradation products of lignin from terrestrial plants which are composed primarily of lignin and celluloses. This theory has been especially promoted by many... 

Other possible theories of stream humic substance formation are the browning reactions (the black substances produced by the action of acids on sugars) (Haworth, 1971) and the melanoidin reaction (the dark pigments produced when glucose, alanine or ammonia, and phenolic substances react with one another) (Hoering, 1973 Filip et al., 1974 Ertel and Hedges, 1983). These theories are discussed in detail by Stevenson in Chapter 2 of this book. 

An intensive research effort should be focused on the mechanisms of formation of humic substances to evaluate the merits of the various theories. Unfortunately, these types of studies are very time consuming and require highly trained biochemists and process biologists. 

Stream ecologists are the major proponents of two recent theories, the river continuum concept (Vannote et al., 1980) and the nutrient spiraling concept (Elwood et al., 1983), which they believe may be important frameworks for studying stream humic substances. These theories emphasize the diverse and dynamic aspects of different reaches within the same stream, the differences among streams, and the differences in seasonal patterns among streams in various geographical regions. The theories predict enormous vari-... 

Pyridine, followed by water, extracted more humic substances than did the DMF, DMSO, or sulfolane systems (Table 4). The enhanced solubilization by pyridine could be attributed partially to a pH effect. When pyridine (eight parts) was diluted with water (one part) to simulate the composition of the solvent in the air-dried soil, the pH of the mixture was 11.6. Because of the low buffering capacity of the solvent system, the pH of the extract was only 4.2 (Swift, 1968). Theory suggests that substantially more humic materials can be brought into solution by pyridine if the pH of the medium is maintained at 9.0 (the pAT for pyridine is 8.96). 

Applications of polymer solution theory to the studies of the dissolution of humic acids and of their extraction from soils suffer most because interactions betvi een each pair of components must be known (criterion 4, p. 343). Unfractionated humic and fulvic acids and humic substances in the soil are parts of multicomponent systems, and interactions between the different components are unknown. 

The closest approach to successful applications of polymer solution theory to humic substances was made by Chiou et al. (1983) when studying the binding of small organic chemicals by soils. They considered the soil sorbent substances to be amorphous macromolecular humic substances, and they adapted the Flory-Huggins theory to a study of the sorbate species solubilized in the amorphous macromolecules. 

Anion-exchange resins have been used (Wright and Schnitzer, 1960) in an attempt to fractionate soil humic substances. Some of the humic material is readily retained and a fractionation can be achieved by elution with a salt gradient and/or an alkaline reagent (usually NaCl and NaOH, respectively). In theory the anion-exchange technique should work well, but in practice the... 

Acid hydrolysis has been used extensively to remove ash, protein, and carbohydrate from humic substances (see reviews by Schnitzer and Khan, 1972, 1978 Stevenson, 1982 Kononova, 1961). In theory, this method should remove peripheral groups and co-precipitated materials and leave the core of humic or fulvic acid. What actually happens may be quite disconcerting. Structural changes may take place, as well as the removal of contaminants. Depolymerization may occur, but acid-catalyzed condensation may also take place. These changes can be seen by the loss of hydrogen content and by a decrease in the E IEf, ratios. All of these events are best summarized in Table 5 (Stevenson, 1982). 

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