Humic acids degradation products

The effect of soil composition on the photolysis of soil-adsorbed niclosamide using a xenon lamp was reported [86]. The study was conducted with both moist and air-dried soils fortified with sodium nitrate (commonly used as a fertilizer), iron (a natural soil component) or humic acid. Degradation in soils fortified with iron was slower than in that in soils fortified with sodium nitrate and no extractable degradation products were observed in any of the soils fortified with humic acid. Soils with higher organic or iron contents or that are exposed to fertilizers do not affect as dramatically the half-life of pesticides as does the presence of moisture in the soil. The maintenance of moisture was found to be crucial to the reliability of soil photolysis studies. 

In all aerobic and anaerobic sediments examined, selective preservation of biomolecules appears to be the mechanism by which humin evolves. It is likely that humic acids are products of the degradation of humin rather than precursors. 

Phenolic compounds have also been oxidatively polymerized to humic substances by clay minerals (29) and by the mineral fraction of a latasol (66). After a 10-day equilibration period, montmoril-lonite and illite clay minerals yielded 44 to 47% of the total added phenolic acids as humic substances whereas quartz gave only 9%. Samples of a latasol yielded over 63% of the total amount, from mixtures in varied proportion, of mono-, di- and trihydroxy phenolic compounds as humic substances (66). Extractions of the reaction products yielded humic, fulvic, and humin fractions that resembled soil natural fractions in color, in acid-base solubility, and in infrared absorption spectra. Wang and co-workers (67) further showed that the catalytic polymerization of catechol to humic substances was, enhanced by the presence of A1 oxide and increased with pH in the 5.0 to 7.0 range. Thus the normally very reactive products of Itgnin degradation can be linked into very stable humic acid polymers which will maintain a pool of potentially reactive phytotoxins in the soil. 

Figure 4. Degradation products of humic acid A—Schnitzer (25), B—Flaig (il), C-Ku-mada (17)...

In this scheme, Reaction 1 represents a direct (pre-ignition) combustion reaction which may or may not be accompanied by formation of carbon monoxide Reaction 2 describes the oxidation reaction (and its sequences) examined in the present study. B and C in Reaction 2 denote degradation products of humic acids (e.g. hymatomelanic, fulvic, and/or so-called water soluble coal acids), and ki,, etc. represent the corresponding rate constants. 

Thus put, details of the individual reactions—which are, in any event, certain to be complex—remain as undetermined and debatable as before. What becomes clear (and consistent with experiment) is that (a) product gases such as carbon dioxide can form via two fundamentally unrelated paths (b) humic acids can be abstracted by secondary degradation or by stripping reactions such as decarboxylation (i.e. by reactions respectively characterized by kn> fe, etc. and by k) (c) in a sequential reaction series such as Reaction 2, a zero rate of humic acid formation denotes establishment of a steady state condition rather than formation of a simple equilibrium of the type coal humic acids. 

The production of humic substances by microorganisms is an extracellular process, because the enzymes are secreted into the external solution that contains the phenolic compounds derived from lignin and tannic acid degradation and microbial and plant metabolites. These phenolic compounds can then be enzymatically oxidized to quinones, which can undergo further polymerization or polycondensation reactions with other biomolecules (e.g., amino acids) to form humic polymers (Stevenson, 1994 Bollag et al., 1998 Burton, 2003). 

Prosen, H., and Zupancic-Kralj, L. (2004). Evaluation of photolysis and hydrolysis of atrazine and its first degradation products in the presence of humic acids. Environ. Pollut. 133, 517-529. 

Rook JJ. Possible pathways for the formation of chlorinated degradation products during chlorination of humic acids and resorcinol. In Jolly RL, Cumming RB, Jacobs VA, eds. Water Chlorination Environmental Impacts and Health Effects. Vol. 3 Ann Arbor, MI Ann Arbor Science, 1980. 

Burdon542 has surveyed the current hypotheses for the structure of humic substances and has concluded that the various products from chemical degradations and NMR data are all consistent with their being mixtures of plant and microbial materials and their microbial degradation products. The examination of soil carbohydrates, proteins, lipids, and aromatics supported this view the presence of colour, fluorescence, ESR signals, mellitic acid, and other features do not contradict it. Regarding the Maillard reaction, some free monosaccharides and the necessary amino species are present in soil, so it may proceed, but only to a small extent it is not a major process. However, in marine environments, the relative abundance of carbohydrates and proteins makes them more probable precursors of humic substances than lignin or polyphenols. 

Soil. Strongly absorbed by soil/humic acid complex and is virtually immobile in soil. Rapidly degraded in soil, with a half-life of <7 days. The principal degradation products are 4-chlorophenylurea and 2,6-difluorobenzoic acid... 

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. 

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