Humin in peat

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. 

Other major peaks in the spectra of humin are those derived from lignin (150, 130, and 55 ppm). In contrast to the humin from soil shown earlier, these peaks are much better resolved, especially the peak at 55 ppm for methoxyl carbon. No doubt, lignin is a major component of humin in peats, and it is likely that it exists in a relatively unaltered state in the peat, even at depth. 

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-... 

Although some soil scientists have long considered it to be a humic acid-clay complex, those scientists dealing with humic substances in peats con-Hder humin to be a macromolecular condensate of humic acids (Flaig, 1972 Spackman et al., 1974 Casagrande et al., 1980). Evidence for this relation-... 

Coal is formed from peat and the vascular plant remains that accumulate in peat bogs. Anaerobic conditions are considered mandatory for the accumulation and preservation of peat and the formation of coal. Two major types of coals are known humic coal and sapropelic coal (see Breger, 1963, 1976). The former are formed from peat accumulations rich in humic substances derived predominantly from vascular plant remains. The latter represent coal formed from algal (boghead coal) or spore (cannel coal) accumulations. In many respects, sapropelic coal can be considered to have an aquatic origin similar to that of humin of aquatic sediments which forms from the accumulation of aquatic nonvascular plant debris in clastic sediments. Conversely, kerogen can also have the properties of humic coals (Breger and Brown, 1962) is the source materials to the sediment at the time of deposition are predominantly derived from vascular plants. 

In this chapter we recognize essentially two soil classes aerobic soils that are well drained and subjected to continued availability of oxygen, and peat that represents subaqueous deposits of vegetal matter subjected to an early oxidative stage of decomposition in upper layers but to an anaerobic stage of decomposition in the lower layers. Differences have been noted in the composition of humic substances in both of these soil classes (Kononova, 1966 Manskaya and Drozdova, 1968). Because of the differences in depositional environments, it is likely that different processes are operative in the formation of humin in these two classes of sediment. 

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. 

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... 

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 occurrence of a predominantly aliphatic humin in marine sediments has been known for some time (Stadnikov, 1930 Breger, 1960 Cane, 1976 Stuermer et al., 1978). This humin was believed to be the precursor of aquatic kerogen in ancient shales and as such has been extensively studied. However, the discovery that a structurally similar aliphatic component exists in humin from peats and even soils is a major new finding that has demonstrated the usefulness of solid-state C NMR. The analytical visibility of this component in soil humic substances has been masked by the over-... 

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. 

Figure 15.8. 13C CPMAS NMR spectra of the IHSS Pahokee Peat and a Canadian Grassland (black chernozem) soil and their corresponding humin samples. Reprinted from Simpson, M. J., and Johnson, P. 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.

Elemental composition, functional group analyses, spectral properties, and characterization of acid hydrolysates have shown that peat humic acids tend to be similar to those from mineral soils. NMR spectroscopy has revealed that peat fulvic acids are largely carbohydrate in nature while the residue of alkali extraction is not all humin. 

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. 

FIGURE 2. NMR spectra of humin isolated from various depth intervals in cores from two peats. The Everglades peat was collected from a sawgrass area in Conservation District lA west of Fort Lauderdale, Florida. The peat core from Mangrove Lake, Bermuda, was collected and described by Hatcher (1978). 

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. 

FIGURE 4. C NMR spectra of humin isolated from section 14 (65-70 cm) of the peat core from Conservation District 1A in the Everglades, Florida, and of the same humin hydrolyzed by refluxing in 6N HCI for 2 hours. 

Grasset, L., Guignard, C., and Ambles, A., Free and esterified aliphatic carboxylic acids in humin and humic acids from a peat sample as revealed by pyrolysis with tetramethylammonium hydroxide or tetraethylammonium acetate, Org. Geochem., 33, 181-188, 2002. 

Soil organic matter (SOM) is often referred to as humus and is derived primarily from the degradation of plant material lignin, carbohydrates, protein, fats, and waxes. Mineral soils may contain 0.5-3.0% of soil organic matter while muck soils and peat contain 50% and higher. Operationally, the material that cannot be extracted by alkaline agents is called humin. The material that precipitates from the alkaline extract on acidification is called humic acid, and what remains in solution fulvic acid. Felback summarized some of the properties of these complex polymeric materials as follows ... 

Figure 4. Contribution of hole-filling to total sorption calculated by the Freundlich slope method for 1,3-dichlorobenzene in (a) Cheshire fsl. and (b) Pahokee peat soil and its humin and humic acid fractions for a 48-h equilibration time. Data taken from isotherms in Figures 2 and 5.

Figure 5. Isotherms of 1,3-dichlorobenzene in whole Pahokee peat soil (ATp = 340, N =0.8501) and its derivatives, humin (K = 464, N =0.7662) and humic acid ( p = 161, AT =0.936). Equilibration period, 48 h. The soil was extracted with sodium pyrophosphate. Adapted from data in ref. 11.

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