Humic core” material

There are no accurate determinations of the overall contributions of peptide components to the compositions of HS and of SOM. The difficulties arise from the problems of separating these from other components of the SOM mixtures. Use of DMSO + 1% 12M HC1 and XAD-8 resin [(poly)methylmethacrylate] technology enabled Appelqvist et al. (1996) to decrease the amino acid content of a HA by 23%, and the decrease was uniform for the different amino acid groupings. This still left considerable amounts of peptide material in association with the HAs. Some may well have been in association with the humic matrix, and some may have been covalently linked to the humic core through, for example, the formation of Schiff... 

Structural representations of humic and fulvic acids are purely hypothetical and should just be considered as an attempt to rationalize chemical behavior. Several structural models have been proposed based on data gathered from degradative and nondegradative models. The different proposed models should be considered as complementary since each emphasizes certain particular properties. Briefly, humic acids are macromolecular and complex, and composed of substituted aromatic and aliphatic hydrocarbon core materials. It is probable that some aliphatic and aromatic ring compounds are heterocyclic with N, O, or S as the heteroatom. Segments or some side-chain components in the structures are substantially hydrophobic. In general, however, the... 

The northern portion of Chesapeake Bay was studied. This upper section of the bay can be classified as a classic salt-wedge type estuary (15). The humic materials used in this study were extracted from several cores of sediments. Two one-meter cores gravity corer. The cores of sediments were then divided into sections and squeezed under nitrogen to remove most of the interstitial water. The squeezed sediments were then dried, weighed and washed with distilled water. The sediments were then washed with 0.5N KCl to remove iron hydroxides, carbonates and exchangeable cations from the clays. Some of the fulvic acids are lost in this acid wash procedure. The humic materials were then extracted from the remaining solids by a O.IN NaOH solution. 

A large volume of concentrated humic material solution was needed to perform the ion exchange experiments. The UV spectra of the different humic material solutions were found to be similar. The extinction coefficients, as measured at 270 mp, were also found to be identical. Since the samples seemed to be of a similar nature, a mixture of the samples from the four cores was made. The mixture was then dialyzed against distilled deionized water for a period of three weeks. The outer solution was replaced periodically. The molecular vjeight cutoff value for the dialysis membrane (gpectraporj 6) was approximately two thousand. The concentration of the interior humic material solution was monitored by UV absorption. Less than five percent of the original humic m.aterial solution passed through the membrane and was lost to the outer solution. 

Acid hydrolysis has been used extensively to remove ash, protein, and carbohydrate from humic substances (see reviews by Schnitzer and Khan, 1972, 1978 Stevenson, 1982 Kononova, 1961). In theory, this method should remove peripheral groups and co-precipitated materials and leave the core of humic or fulvic acid. What actually happens may be quite disconcerting. Structural changes may take place, as well as the removal of contaminants. Depolymerization may occur, but acid-catalyzed condensation may also take place. These changes can be seen by the loss of hydrogen content and by a decrease in the E IEf, ratios. All of these events are best summarized in Table 5 (Stevenson, 1982). 

This structure does not account for the nitrogen and sulfur content of humic substances. It has been suggested that these elements are derived from parts of other types of molecule, for example, proteins, which are associated with the humic substances. Indeed it has been proposed that humic substances consist of an aromatic core to which peptides, carbohydrates, metals, and phenolic acids are chemically or physically attached. It can be seen that the structure in Fig. 5-9 is an "open" network. In fact, there have been suggestions that organic and inorganic materials associated with humic substances are trapped inside these "holes" in the humic substance structure. " ... 

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