Aliphatic structures, humic acids from

Volk and Schnitzer (1973) concluded that variations in the functional group components and spectral properties of humic acids from a group of Florida mucks indicated that higher rates of humification were related to (1) greater amounts of carboxyl, phenolic hydroxyl, quinone, and ketonic carbonyl groups (2) fewer alcoholic hydroxyl groups and aliphatic structures, as per IR evidence and (3) increments in EJEf, ratios and free-radical contents as revealed by ESR spectroscopy (Table 12). 

They also concluded that HAN and HAL samples have higher molecular weights than the HAM sample, and that HAN and HAL samples are of marine origin. This is also supported by the findings of Hatcher e al. ( 1 9) showing that humic acids from marine sediments are predominantly composed of highly branched, unsubstituted aliphatic structures. 

The structural proposal for humic acids by Stevenson (1982) had significant logic, based on the state of information at that time. The structure (Figure 1.3) has compositional aspects of phenols derived from lignin and from tannins, and paper chemistry exercises would allow reactions and interactions needed to give structures of the type predicted. The structure shown is highly aromatic, and the proposed aliphatic moieties are saccharide- and peptide-derived. Modern analytical procedures invariably show peptide and saccharide components to be associated with... 

Figure 15.5. Two-dimensional spectrum produced from an F1-F2 slice through the 3-D HMQC-TOCSY spectrum of a pine forest soil fulvic acid at 1.3 ppm on the F3 (proton) axis (Figure 15.2). Labels on cross-peaks correspond to the C-H structures in the aliphatic structures shown. The full 3D cube is superimposed onto the example slice. Reprinted from Simpson, A. I, Kingery, W. L., and Hatcher, R G. (2003a). The identification of plant derived structures in humic materials using three-dimensional NMR spectroscopy. Environ. Sci. Technol. 37,337-342, with permission from the American Chemical Society.

Some of the first NMR spectroscopic applications in humic substance research involved the use of NMR for the examination of soluble humic substances.(5) The information obtained from NMR spectra of humic and fulvic acids dissolved in alkaline or neutral aqueous solutions has been quite useful in characterizing the aromatic and aliphatic structures of these complex materials. In as much as humic materials are inherently complex, the spectra show only broad signals with some fine structure which can be related to specific methyl, methylene, methine, and aromatic/olefinic hydrogens. Because NMR spectroscopy contains inherently more structural information for humic substances, the use of NMR has been limited since much of the work conducted prior to 1987. This is... 

Pyrolysis in the presence of TMAH has also been applied to the structural characterization of HAs isolated from low-rank coals. Figure 6 shows the chromatogram of the compounds released after the TMAH/pyrolysis of the HA isolated from a humic coal from Konin (Poland). A large variety of components were released, the lignin-derived phenol derivatives and aliphatic acid methyl esters being the most prominent. A series of fatty acid methyl esters were identified in the range from Cio to C34, with maxima at C16 and Cig showing an even-over-odd... 

Hatcher et al. (1981) pointed out that the aliphatic region of terrestrial humic acids is very similar to that of marine humic acids and that the only difference is the presence of aromatic bands in the terrestrial humic acid spectra. In previous work, Hatcher (1980) and Hatcher et al. (1980b) concluded from the H/C ratio of 1.5 and presence of a strong terminal methyl band at 0.9 ppm that marine humic acids have highly branched and cross-linked paraffinic carbon atoms. These structures appear to arise from algal and microbial lipids. The similarity in the aliphatic region in terrestrial humic acids suggests that soil microbial lipids may be the source of the aliphatic structures in terrestrial humic acids. 

Fulvic acids. Marine sedimentary humic substances soluble in base and acid (fulvic acids) have previously been examined by and NMR (12). The dominant structural components were postulated to be polysaccharide - like substances, probably polyuronic acids. Solid-state NMR spectra of fulvic acids isolated from a number of marine and estuarine sediments are shown in Figure 1. Major peaks at 72 and 106 ppm betray the overwhelming presence of polysaccharide -like substances, and, as shown by Hatcher and others (12.), the moderate peak for carboxyl or amide carbon at 175 ppm suggests that these polysaccharides are more like polyuronides. Aromatic carbons (110 to 160 ppm) are decidedly minor components. Aliphatic carbons (0-50 ppm) are also minor components. H NMR spectra shown by Hatcher and others (12) indicate that these aliphatic structures are highly branched. 

The study of humic substances by nuclear magnetic resonance of the isotope and the proton suggests that aliphatic structures prevail over aromatic structures for the fulvic acids extracted from marine water, contrary to the case for soil fulvic acids. The high values of the H C ratio in marine fulvic acids also point to an aliphatic nature (Stuermer and Payne, 1976). 

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