Humic acids class structure

Recall from Chapter 23.2.4 that humic substances are isolated from seawater by adsorption on a hydrophobic resin followed by elution using solvents of varying pH. The desorbed compounds are fractionated into two classes, humic acids fulvic acids based on their solubility behavior. A model structure for a humic acid is illustrated in Figure 23.10a in which fragments of biomolecules, such as sugars, oligosaccharides. 

The mechanism of formation and class structure of marine humic substances presented here was first proposed by Harvey et al. (1983). Since that time the proposed pathway and structures of marine fulvic acid and marine humic acid have been tested and the predicted chemical and physical behaviors of marine humic substances have been supported in several experiments. 

Humic acids are ubiquitous in soils and are among the most important compounds that coat clay mineral surfaces. For this reason, some time ago we started MD simulations of humic acid/clay mineral complexes. It is a difficulty of such modeling calculations that the structures of humic acids are ill defined and highly variable from one molecule to the next, so that it is difficult to design a humic molecule that is representative of the entire class. 

The designation of certain classes of organic materials as humic and fulvic acids unfortunately implies a certainty and regularity of structure which... 

The classical definition of fulvic acids is not very specific. Many biochemical substances such as proteins, sugars, and fatty acids would fall under this classification. These substances, in many instances, can hardly be considered "humic" in nature. But, if one uses the classical definition of fulvic acids these substances are included. No doubt the observations made above that polysaccharidelike substances constitute the major components of sedimentary fulvic acids is partly attributable to the fact that the operational definition classes polyuronides as fulvic acids. It is not the intent of this paper to discuss the merits of using the classical operational definition for fulvic acids as opposed to one s perception of what true fulvic acids are. He know far too little about the composition of humic isolates and about their origin to begin discussions of whether they are humified or not. He therefore chose to use operational definition with recognition that well defined structural entities can sometimes be a part of what is isolated. 

LMW organic compounds produced by irradiation of DOM cover a wide range of compound classes, but are generally carbonyl compounds such as aldehydes and carboxylic acids, compounds readily available to aquatic microbes [26-28,30,33,35,38,95-111]. These compounds appear to arise from the degradation, or fragmentation, of larger humic structures into its component molecules by either direct photolysis or indirect secondary reactions discussed in Section 6.3. Measurement of these photoproducts usually involves gas chromatography/mass spectrometry of derivatized compounds or capillary ion electrophoresis (e.g., [37,38,52,112-114]). 

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