Of humin

Malekani K., J.A. Rice, and J.-S. Lin (1997). The effect of sequential removal of organic matter on the surface morphology of humin. Soil Science 162 333-342. 

The mechanism of acid-catalyzed decarboxylation of hexuronic acids has been the subject of many investigations.231,232 The formation of carbon dioxide is accompanied by the formation of 2-furaldehyde, C5H402 (82) as the main product, along with considerable amounts of humins however, both 5-formyl-2-furoic acid (83) and reductic acid (84) have been isolated as end products from treatment of hexuronic acids with strong acid. 

Humin has been regarded as the most intractable component of SOM. It must be considered to be a very important component, however, because typically it represents more than 50% of the organic carbon in a soil (Kononova, 1966 Stevenson, 1982,1994) and more than 70% of the organic carbon in unlithified sediments (Durand and Nicaise, 1980 Rice, 2001). The definition of humin (Section 1.3.3) is similar to that of a protokerogen (Calvin and Philip, 1976 Rice, 2001), which is often used in petroleum geochemistry to describe insoluble organic matter in unlithified sediments. 

Solid-state NMR has done much to dispel the mysteries of humin compositions, and significant advances have recently been made using proton NMR in the liquid state (see Section 15.3.3 of Chapter 15). Based on solid-state 13C NMR spectra, Hatcher et al. (1980) concluded that a repeating aliphatic structural unit, possibly attributable to branched and cross-linked algal or microbial lipids, is common to both soil and sediment humin samples. Hatcher et al. (1983) viewed the increase in humin relative to the other humic fractions as a selective preservation of the aliphatic compounds of the sediments and did not support condensation theories. 

There is a need to resume studies of soil saccharides and peptides. These can compose as much as 30-40% (when account is taken of the compositions of humin materials). Much is known about how polysaccharides of known structures interact with soil colloids, but it has not been possible as yet to know in sufficient detail the structures of the polysaccharides that persist in the soil. Hence we do not know the mechanisms of their binding to soil mineral colloids. The same applies for the peptide materials, though it is clear that polysaccharides and peptides have important roles in soil structure formation and stabilization. 

Figure 2.15. FTIR spectra of the humic-like substances produced by (1) C. maxima, (2) C. maxima + C. hirsutus, and (3) C. hirsutus. Reprinted from Yavmetidinov, I. S., Stepnova, E. V., Gavrilova, V. R, et al. (2003). Isolation and characterization of humin-like substances produced by wood-degrading white rot fungi. Appl. Biochem. Microbiol. 39, 257-264, with permission from Springer.

Cloos, R, Badot, C., and Herbillon, A. (1981). Interlayer formation of humin in smectites. 

Figure 15.9. 13C CPMAS NMR spectrum of humin extracted from a brown chernozem soil from Western Canada. The characteristic doublet in the unsubstituted aliphatic region is characteristic of methylene carbon (28-34 ppm) and shows the presence of both amorphous (soft) domains at 29 ppm and crystalline (rigid) domains at 33 ppm in soil humin. Reprinted from Simpson, M. I, and Johnson, R C. E. (2006). Identification of mobile aliphatic sorptive domains in soil humin by solid-state 13C nuclear magnetic resonance. Environ. Toxi. Chem. 25, 52-57, with permission from the Society of Environmental Toxicology and Chemistry.

Besides applications with whole soils, LIF can be applied to study physical and insoluble chemical fractions of soil. Gonzalez-Perez et al. (2006b) carried out studies about fractions of SOM under sewage sludge application. In this work, the high fluorescence contribution of humin fraction to the fluorescence of whole soils was... 

A successful technique applied for the analysis of humin and humic acids was the pyrolysis with on line methylation followed by GC/MS analysis [6,9]. In one such study [9] humin deashed by treatment with HCI and FIF was pyrolysed and compared to humic acid obtained from the same soil, showing that humin contains larger amounts of carbohydrates and aliphatic compounds. This type of study also revealed the presence in the humin and humic acid pyrolysates of monocarboxylic acids with up to 32 carbon atoms, dicarboxylic acids, methoxymonocarboxylic acids with up to 26 carbon atoms, triterpenoid acids, etc. These compounds were not reported in other studies (e g. [2]) where the chromatographic separation did not allow the detection of compounds difficult to elute due to their high boiling point and polarity. 

Publications on humic substances are dominated by discussions on humic acids and fulvic acids, with relatively little discussion of humin. The former two fractions can be dissolved in aqueous media which facilitates their isolation and study. The geochemistry of humin is discussed by Hatcher et al. in Chapter 11 the presence and nature of humin in various environments are also discussed in a number of other chapters in this book. For example, Stevenson (Chapter 2) discusses humin from soils, Ishiwatari (Chapter 6) provides a rather extensive discussion of humin from lake sediments, and Vandenbroucke et al. (Chapter 10) consider humin in marine sediments. 

The relationship of humin to kerogen, and the role of these substances as precursors to coal and petroleum are discussed by Hatcher et al. in Chapter 11. Schnitzer discusses the nature of nitrogen in humic substances in the last chapter (Chapter 12) of the geochemistry section. 

With regard to the designation of humin as a separate fraction, it is possible that this material consists of portions of other fractions so intimately associated with mineral matter that they cannot be solubilized by extraction with alkali. Also, it is not known whether hymatomelanic acid is a distinct chemical entity. This material may be an artifact produced from humic acid during fractionation. The simple process of redissolving the alcohol-insoluble material in alkali followed by reprecipitation with acid results in a further increase in alcohol-soluble material. 

Determination of the amounts of humic and fulvic substances in peats and mucks of various botanical origin and mineral content to establish whether advances in humification preferentially favor the increase of one or the other would therefore be achieved only when proper methods of extraction and fractionation are established and followed in full cognizance of the problems discussed in this and other chapters. The fractionation of humin from min-... 

As minor oxidation products, C1-C3 monocarboxylic acids and C4 a,w-dicarboxylic acid were obtained (1-2% of the initial humin carbon). The humin was then hydrolyzed by 6N HCl at 110°C for 24 hours, and the unhydrolyzable part of humin was oxidized. Interestingly, degradation products of the unhydrolyzable part of humin were almost the same as those of the original humin although 30-49% (by weight) of the original humin was released into solution on HCl hydrolysis. 

Gas chromatography was used to analyze amino acids in 6N HCl hydrolysates of fulvic acid, humic acid, and humin from lake sediments (Lakes Suwa, Nakanuma, Yunoko, Haruna, Shoji, Motosu, and Biwa) (Yamamoto, 1983). Table 8 gives an example of analytical results of amino acids (Lake Haruna). The total amino acids for the seven-lake sediments accounted for 3-16% of humin, 11-21% of humic acid, and 4-24% of fulvic acid. The percentage of amino nitrogen in the total nitrogen in each fraction was 20-44% for humin, 21-36% for humic acid, and 4-30% for fulvic acid. 

A portion of these biochemical compounds may be associated with the extracted humic substances. However, as already described, the humin which had been hydrolyzed by 6N HCl at 110°C for 24 hours gave essentially the same oxidative (KMn04) degradation products (aliphatic C4-C14 a,co-dicarboxylic acids as major products) as untreated humin. Moreover, stepwise (eight steps) oxidative (KMn04) degradation of humin produced similar degradation products (aliphatic Cs-C 8 monocarboxylic and C5-C16 a,co-dicarboxylic acids and small amounts of benzenecarboxylic acid Machihara and Ishiwatari, 1980). These facts indicate that the major part of humin forms aliphatic structures with biochemical compounds distributed uniformly in the humin matrix. These compounds are firmly linked within the humin matrix by unknown bonds. 

Depth (m) Total Organic Matter Concentration (mg/g) Abundance (% of Total Organic Matter) Elemental Analysis of Humin ... 

In summary, fulvic acid and humic acid decrease gradually with depth and the relative abundance of humin increases in this stage of diagenesis. 

Sample Number IN Hydrolysate Fulvic Acid Humic Acid Hydrolyzable Part of Humin Stable Residue Total Hydrolysate Total Humic Acid... 

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