Humin studies

A fractionation procedure has been established and widely applied to studies of humic materials [42-44]. The procedure begins with natural OM (i.e., humus) and uses an aqueous basic solution (e.g., 0.1-0.5 mol/1 NaOH and Na2C03) to solubilize a fraction of the OM. The basic extract is then acidified which causes a precipitate to form, i.e., humic acids (HA). The fraction, which remains in solution, is called fulvic acids (FA). Humin is the name given to the insoluble organic fraction that remains after extraction of humic and fulvic acids. At nearneutral pH (pH 5 - 8), which is characteristic of most natural water, the FA are the most water soluble of these three fractions. HA are somewhat less soluble, with their solubility increasing as the pH increases. Humin is insoluble at all pH values. 

Because the vast majority of the sedimentary organic compounds are not amenable to direct study by current analytical techniques, organic geochemists rely on operational approaches to characterize them, such as measuring the %OC, %ON, OC/ON ratio and humin content. Other strategies include molecular analysis of the small fraction of sedimentary organic compounds that are detectable. Some of these compounds make... 

Melanoidins (36, 48-50) are different from melanins, humins, and caramels, but similar to humus (37), according to Maillard (4, 1 1, 19-21). Kato and Tsuchida (51) studied the possible chemical structure of melanoidins. 

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. 

Hatcher, P. G. D., VanderHart, L., and Earl, W. L. (1980). Use of solid-state 13C NMR in structural studies of humic acids and humin from Holocene sediments. Org. Geochem. 2, 87-92. 

Shah, R. K., Choski, M. R., and Joshi, B. C. (1975b). Development studies on soil organic matter Humin. Chem. Era 6,1-3. 

Campbell et al. (1967) applied sequential extraction to characterize FA, HA, and humin from gray podzolic and chemozemic soils. The fractions of FA and HA extracted by 0.5 M NaOH without acid pretreatment, which they called mobile humates (since the researchers assumed that they are not bound to minerals), had a lower mean residence time (ranging from 85 to 785 for HA, respectively, in the chernozemic and gray podzolic soils) as compared to Ca-humates extracted from humin (1410 years in the chemozemic soil) and to the total FA and HA extracted after acid pretreatment (195-1235 years for HA). This study showed that in the chernozemic soil, Ca-humates and clays play an equally important role in the stabilization of HS, whereas in the podzolic soil the oldest fraction was associated with clays. 

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... 

Saab, S. C., and Martin-Neto, L. (2004). Studies of semiquinone free radicals by ESR in the whole soil, HA, FA and humin substances. J. Braz. Chem. Soc. 15,34-37. 

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. 

Waksman (1936) recommended abandonment of the whole nomenclature of humic acids, beginning with humins and ulmins, through the whole series of humus, hymatomelanic, crenic, apocrenic, and numerous other acids, and ending with the fulvic acid and humal acids . Notwithstanding, terms such as humic acid, humin, fulvic acids, and others have survived and will undoubtedly continue to be used in the future. Most studies on humus chemistry involve preliminary separations on the basis of solubility characteristics, and abandonment of these terms would cause even greater confusion than their continued use. For example, reference to the alkali-soluble, acid-insoluble material as humic acid is considerably less cumbersome than repeated reference to the alkali-soluble, acid-insoluble fraction. ... 

By using methods analogous to those in the above studies Zelazny and Carlisle (1974) found that oxygen-containing functional group levels in humin and humic and fulvic acids from all series and layers of the Florida mucks were similar (Table 13), irrespective of their extents of humification or rates of subsidence. 

Several authors (Ishiwatari et al., 1966 Ishiwatari, 1970a Kemp and Mudrochova, 1973 Bourbonniere and Meyers, 1978) have published infrared spectra of fulvic acids. These studies show that absorption spectra of fulvic acids extracted from the same sediment sample are not necessarily the same. This may be due to the difficulty of removing inorganic materials and to the existence of many kinds of organic materials in the fraction. However, absorption bands for most fulvic acids appear essentially the same as those for humic acids except for a carboxyl band at 1740 cm for fulvic acids and at 1720 cm for humic acids. To date no reliable data on humin have been obtained by infrared spectroscopy. 

In the seven lakes studied by Yamamoto (1983), amino acid distribution of fulvic acid, humic acid, and humin resembled each other. However, after detailed examination of amino acid distribution, the following regularities were found to exist in almost all humic substances studied ... 

Several ideas have been presented in the literature on the formation process of humic substances in marine sediments. Abelson (1967) claimed that polymerization of unsaturated fatty acids in phytoplankters after their death accounts for the formation of kerogen in marine sediments. Abelson and Hare (1971), Hoering (1973), and Hedges (1976) studied reactions between carbohydrates and amino acids under laboratory conditions as a possible formation reaction of humic acid and humin in sediments. They prepared a number of artificial humic acids by reacting glucose with amino acids. The synthetic products resembled natural humic acid and humin. A comprehensive review was published by Abelson (1978). 

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