Molecular heterogeneity, humic substance

Figure 17.13. Stacked C (Is) NEXAFS spectra of NOM from various sources and environments showing molecular-level structural and compositional heterogeneity. The spectra were recorded in transmission mode (fungi, bacteria, fresh charcoal, black C particle from Liang et al., 2006 black-C rich humic substance from Solomon et al., 2007b litter J. Lehmann, unpubl. data).

Researchers have devised numerous extraction and fractionation schemes to deal with the heterogeneous nature of humic substances. Traditionally, the operational definition of humic substances as used by the International Humic Substances Society (Hayes et al., 1989) is based on the solubility in a series of acids and bases. In this scheme, humic substances are classified into three chemical groupings (1) fulvic acid, soluble in both alkali and acid solutions, has the lowest molecular weight and is generally considered the most susceptible to microbial degradation (2) humic acid, soluble in alkali but not in acid, is intermediate in molecular weight and decomposability and (3) humin, insoluble in both alkali and acid solutions, is the most... 

The fact that a humic substance is not a pure compound, but is a heterogenous mixture of many compounds with generally similar chemical properties, places an important constraint on all these characterization methods. Examples of the multicomponent mixture problem are presented in the chapters discussing interpretation of elemental analysis, determination of molecular weight, analysis of potentiometric data, and interpretation of infrared and other spectroscopic data. 

Due to a high content of acid carboxy and phenolic groups, the humic substances may be dissolved in alkaline solutions. In the soil, the humic substances behave as natural ion exchangers, molecular sieves and as material able to accumulate mineral substances, particularly cations. At the present time it is assumed that macromolecules of humic substances, in spite of being a chemically very heterogeneous mixture, have in common a certain general molecular structure. The basic scheme and approximate molecular structure of humic substances are shown in Fig. 7.8. 

They constitute 75% w/w of the organic matter in most soils and 50% of the organic carbon in surface waters. Humic substances are composed of complex heterogeneous mixtures of organic compounds and are characterized as being yellow to brown in color, of high relative molecular mass, and refractory. Unlike many other natural organic products, they cannot be described in terms of unique chemical structures and are operationally defined by the technique used for their extraction and fractionation. 

Humic materials substances consist of a heterogeneous mixture of compounds for which no single structural formula will suffice. In fact, extracted humic materials are different from place to place. Therefore, no unique exact value could be represented the molecular weight of the extracted humics. Some researcher (Stevenson F.J., 1982) proposed that the molecular weight of humics are ranged from 2000 (fulvic acids) to 300000 (humins). Based on this point, a medium value of 150000 was used as the molecular weight in our quenching calculation. 

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