Humin paraffinic structures

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. 

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... 

It is strikingly apparent that all spectra of aquatic kerogen are predominantly aliphatic with a major peak centered at 30 ppm. This peak is also the most intense peak in the spectrum of humin from Mangrove Lake also shown in Figure 9. Paraffinic structures of the type shown to be present in... 

In peat, humin is also composed of the three structural entities mentioned above. The anaerobic nature of peat precludes the extensive decomposition that occurs in aerobic soils, and biomolecules are likely to be better preserved. Carbohydrates are major components of humin in near-surface intervals but are decomposed and lost with depth in the peat. Lignin and the paraffinic structures are selectively preserved with depth. When the humin of peat is delignified, the paraffinic structures remain. These components are likely to be derived from nonvascular plant contributors to the peat, namely, algae. 

In an algal sapropel and also in marine sediments, humin and the HF/ HCl-treated humin are predominantly composed of the paraffinic structures that evolve as a result of selective preservation with depth in the sediments. In this case as well, the paraffinic structures are most likely derived from algal or microbial sources and could very well be original biomolecular components of the algae. 

Humin varies widely in composition. Sediments derived from algal/microbial biomass have humin with paraffinic structures resembling those of corresponding humic acids. Estuarine or coastal marine sediments examined in this study have humin with highly aromatic structures which resemble coal-like materials rather than modern plant residues. In these latter sediments no structural correspondence exists between humin and humic acids which appear to more nearly reflect the nature of modern plant... 

Comparison of this region of the spectrum with that of the respective humic acids shows that the humin contains significantly less carboxyl (peak at 175 ppm). The peaks at 250 and 0 ppm are spinning sidebands of the aromatic carbon peak. In summary, the primary difference between the spectra of humic acids and humin is the presence of a paraffinic carbon peak at 30 ppm in the spectrum of humin. There is little indication that lignin-like structures exist in either spectrum due to the lack of significant peaks at 55 and 150 ppm for methoxyl and phenolic carbons, respectively. 

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. 

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. 

The problem of foremost interest in future studies should be to ascertain whether operationally defined humin contains significant amounts of clay-humic acid complexes and to determine the genetic relationship between humic acids and humin. Another problem would be to define structurally the paraffinic components of humin and to ascertain whether they are truly biomolecules or are rapidly formed products of decomposition of vegetal matter. Finally, the hypothesis of selective preservation should be tested in other environments. 

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