Humin aromatic 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. 

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. 

Humus is an amorphous, hydrophillic, acidic, partly aromatic, generally dark colored, and structurally complex material, resulting from the microbial degradation of plant detritus. Humus can be further classified as follows (a) humic acids a fraction that is soluble in alkali but precipitates on acidification of the solution, (b) fulvic acids a fraction that remains in solution after the extraction is acidified, and (c) humin a fraction that cannot be extracted by either alkali or acid (see Chapter 5 for details). 

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