Humin polysaccharides

Soil polysaccharides can amount to as much as 20% of the humic fractions isolated in aqueous media, and identification of the classes of components in humin materials in associations with the soil clays indicates that carbohydrates contribute significantly to those isolated in the DMS0/H2S04 medium (see Sections 1.4.7 and... 

Qualitative and quantitative identification of the sugars in the hydrolysates of these humin isolates may give indications about their origins (plant or microbial), and identification of the configurations of the sugar linkages could lead to deductions about the sorption mechanisms. To achieve the latter objective will require isolation of the polysaccharide and/or mucopolysaccharide components. That will not be an easy task, but it is doable. 

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. 

Humic substances are those organic compounds found in the environment that cannot be classified as any other chemical class of compounds (e.g., polysaccharides, proteins, etc.). They are traditionally defined according to their solubilities. Fulvic acids are those organic materials that are soluble in water at all pH values. Humic acids are those materials that are insoluble at acidic pH values (pH < 2) but are soluble at higher pH values. Humin is the fraction of natural organic materials that is insoluble in water at all pH values. These definitions reflect the traditional methods for separating the different fractions from the original mixture. 

NMR spectra of humin from three major types of depositional environments, aerobic soils, peats, and marine sediments, show significant variations that delineate structural compositions. In aerobic soils, the spectra of humin show the presence of polysaccharides and aromatic structures most likely derived from the lignin of vascular plants. However, another major component of humin is one that contains paraffinic carbons and is thought to be derived from algal or microbial sources. Hydrolysis of the humin effectively removes polysaccharides, but the paraffinic structures survive, indicating that they are not proteinaceous in nature. The spectra of humin differ dramatically from that of their respective humic acids, suggesting that humin is not a clay-humic acid complex. 

In this chapter, we consider humin to be the residue after successive extraction of sediments by benzene/methanol to remove lipids, dilute acid IN HCl), and 0.5N NaOH. In marine sediments, further treatment with concentrated HF/HCl (1 1 v/v) is required to concentrate the organic matter by removal of mineral matter. This treatment will partially or totally hydrolyze polysaccharides and proteins while probably having little effect on the humic material (Hatcher et al., 1983a). 

The C NMR spectra of humin from the Georgia soil are notably different from the spectrum of humic acids (Fig. 1). In the surface interval, polysaccharides are dominant with NMR peaks at 72 and 106 ppm. However, at depth in the soil, the peaks for polysaccharides diminish in relative intensity as can be expected. Polysaccharides and carbohydrates, in general, are known to be degraded rapidly in soils (Lowe, 1978). 

The comparison between NMR spectra of humin and humic acids in this aerobic soil bears out the fact that some basic structural differences exist between these two soil fractions. The lower polysaccharide content of humic acids compared to humin is expected. The greater relative proportion of carboxyl (or amide) carbons (peak at 175 ppm) in humic acids is another minor difference that was noted. The most important difference is the relative concentration of paraffinic carbons with humin having a much greater concentration than humic acids. Excluding the presence of polysaccharides, it is difficult to imagine that humin, in this instance, is a clay complex of humic acids. If this were the case, the spectra would be nearly identical except for the presence of carbohydrates in humin. It is also difficult to imagine that humin is a condensation product of humic acids. Rather, the comparisons show that either humic acids are decomposition products of the humin (where decomposition selectively alters the structure of individual precursors in humin), or humic acids are genetically unrelated to humin. 

Many C NMR spectra have been published for humin in peat (Hatcher et al., 1980c, 1983a Preston and Ripmeester, 1982 Dereppeetal., 1983) and all appear to contain peaks for carbohydrates and aromatic, carboxyl, and paraffinic carbons, the proportions of which vary considerably. Hatcher et al. (1983a) pointed out that, while the presence of carbohydrates (polysaccharides) and hgnin was expected, the discovery of significant quantities of paraffinic carbons by C NMR has made a major contribution to our knowledge of the components of peat humin. 

On the basis of these studies on woody tissues, it seems that lignin from vascular plants can be selectively preserved compared to biologically degradable polysaccharides when buried. The same can be expected for the lignin in humin from peat the spectra shown in Figure 2 consistently demonstrate this selective preservation with increasing depth. 

Several factors lead us to believe that this paraffinic component of peat is macromolecular and nonproteinaceous. First, the peat was treated with a benzene/methanol mixture prior to isolation of humin. Thus, it is unlikely that the paraffinic structures have a significant contribution from lipids. Second, when hydrolyzed in refluxing 6N HCl, the humin lost some paraffinic carbons, but mostly its polysaccharides as demonstrated in Figure 4 which shows C NMR spectra of humin and its hydrolyzed residue. The paraffinic carbons survive the hydrolysis, demonstrating their resistance. It is unlikely that proteinaceous material would survive such a treatment as an insoluble residue. 

By examination of the spectra in Figure 5, it is clear that polysaccharides (holocellulose, peaks at 72 and 106 ppm) are dominant in the delignified humin in the upper layers of peat but diminish in relative concentration with depth. This trend was also observed in the spectra of humin in Figure 2. At depth, the polysaccharides are minor compared to the paraffinic carbons (peak at 30 ppm). Thus, the paraffinic structures in humin are resistant to sodium chlorite oxidation, and their relative increase in concentration with... 

An example of this selective preservation is shown in Figure 6 with the NMR spectra of the algal sapropel from a near-surface and a 2.8 m interval. In the surface layers, the organic-rich sapropel is dominated by polysaccharides denoted by peaks at 72 and 106 ppm in the spectrum, and humin accounts for less than 20% of the organic matter. At depth, the polysaccharides and other labile substances are decomposed, and the sapropel contains mostly paraffinic macromolecular humin (approximately 70% of the organic matter). Hatcher et al. (1983b) have shown that the loss of labile... 

A similar selective preservation was observed in peat as discussed earlier where an additional component, lignin, was also preserved selectively. However, the major component of humin from Everglades peat was the paraffinic component that also appeared to be selectively preserved relative to the polysaccharides. It is interesting to note the similarity between the spectra of delignified humin at the 15-16 cm interval in peat (Fig. 5) and that of the algal sapropel from Mangrove Lake at the 272-290 cm interval. The similarity between these two spectra infers that similar structural entities are present in these two depositional environments, and it is probable that the two similar structural components are from a common source, namely, algal and microbial remains. 

Humic and fulvic acids as well as humin were isolated from the samples described in Table 1 by standard methods ( ). In short, humic and fulvic acids are extracted with 0.5 N NaOH under N2. Humic acids are protonated on an ion exchange resin, precipitated by acidifying to pH 2, separated by centrifugation, and lyophilyzed. The soluble fulvic acids are concentrated by ultrafiltration and lyophilyzed. Humin, the residue after treatment with NaOH, is treated with concentrated HC1 HF to remove a large portion of the mineral matter and hydrolyzable substances such as proteins and polysaccharides. 

Polysaccharides are present in fulvic acid, humic acid, and humin fractions of soil organic matter. Extractants remove only a small percentage of the total present and hence determinations must be made by means of hydrolysis followed by analysis of the products. These include hexoses, pentoses and uronic acid. There seems to be no doubt that soil polysaccharides are important constituents of soil organic matter. They have a role in... 

Polymers belong to the group of macromolecular substances. They are comprised of one or several kinds of repeating units called mers interconnected with chemical bonds and ordered according to certain statistics. Some macromolecular substances are built from the low-molecular moieties, mutual arrangement of which can be hardly described by any statistics (for example humin substances). The latter should not be designated polymers . In other words, all polymers are macromolecular substances but not all macromolecular substances are polymers. In this chapter, we will deal almost exclusively with synthetic and man-made polymers. The important exceptions represent polysaccharides, especially cellulose and its derivatives. Cellulose is the most abundant organic polymer on the earth. The behavior of polysaccharides in many aspects resembles that of synthetic polymers. Polysaccharides are often chemically modified to adjust both their solubility and utility properties. Chemical modification of polysaccharides results in the specific class of natural/synthetic polymers. 

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