Molecular composition humics

These considerations allow us to link the time required for humification (always directed to an increase in entropy) to the type of chemical transformations in humic system, which best suit this demand The system of NOM and HS should unavoidably evolve toward molecular compositions with the maximum number of isomers. Given that the overwhelming part of humic matter is being formed under oxic conditions, these structures are represented by low-molecular-weight aromatic and alicyclic acids. This suggests that under the same environmental constraints, the humification of NOM should lead to the formation of structures with an increased content of aromatic structures (or more precisely, the amount of DBE) and with a decrease in size similar to what was revealed by the results of data analysis on size-fractionated samples of humic materials shown in Figures 13.14A-D. 

Simpson, A. J., Boersma, R. E., Kingery, W. L., Hicks, R. P, and Hayes, M. H. B. (1997). Applications of NMR spectroscopy for studies of the molecular compositions of humic substances. In Humic Substances in Soils, Peats, and Waters Environmental and Health Aspects, Special Publication, Royal Society of Chemistry, vol. 172, pp. 46-62. 

Fractionation, on the other hand, is the subdividing of humic substances according to some property related to their molecular composition. Because humic substances are ill defined there will inevitably be some confusion between these processes of purification and fractionation, especially since the same or very similar techniques are used in both cases. Nevertheless, the distinction between the two should be clearly made and understood and adhered to by research workers in this field. 

GM permeation chromatography has been extensively and successfully applied to studies of humic substances. However, a number of problems are encountered which, if not overcome, can invalidate the results. For instance, the gel material should be inert to the solute molecules so that there are no chemical or physical interactions between gel and solute. When any adsorption of the applied polymer molecules by the gel takes place the observed retention by the column is not solely caused by penetration into the pores, and the resulting separation cannot be entirely attributed to molecular. veight differences. Because of their chemical composition, humic substances tend to be readily adsorbed by gel materials. Adsorption behavior... 

The molecular composition of fulvic acid and humic acid fractions from a German natural lake have been characterized by solid-state NMR. ... 

About half of the dissolved organic carbon may appear in humic or fulvic acids. These are high-molecular weight organic compounds of a composition which is somewhat uncertain. They contain aromatic hydroxyl and carboxyl groups which have the ability to bind to metal ions. Rivers and estuaries typically contain 10 mg/liter of acid with an exchange capacity of 5-10 mmol/g, mainly due to carboxylic... 

In spite of this variation in molecular weights and solubilities humic acid and fulvic acid have a very similar chemical composition. These acids consist of aromatic moieties such as phenols, benzenepolycarboxylic acids, hydroxybenzenepolycarbo-xylic acids, 1,2-dihydroxybenzene carboxylic acids, together with more complex condensed structures and polycylic compounds. It is conjectured that these various units are joined together by aliphatic chains (45, 54) the distribution of functional groups is presented in Table 5. 

As with the bulk POM and DOM, the operationally defined fractions of UDOM and humic substances are quantified by elemental analysis and via broad molecular-class detection. Other strategies involve measurement of the natural isotopic composition, both stable and radioactive, of the various fractions. Efforts are underway to develop more sophisticated techniques, such as solid-state NMR and high-resolution mass spectrometry, far identification of specific bonds and functional groups. 

Jince the time of Berzelius, chemists have proposed structures for the amorphous, black substance known as humic acid. In the past 150 years, much experimental work has appeared on the nature of humic acid, most of it based on classical chemical and microbiological studies. Very little information about the molecular structure of humic add has resulted from these studies however. Some of the problems plaguing investigators in this field have been (a) variation in the source of humic acid, (b) variation in the definition of humic fractions of soil and coal, (c) lack of crystallinity of the samples, (d) uncertainty of molecular weight measurements, (e) variation in extraction techniques, and (f) variation in elemental composition. The little unambiguous information that exists today is based on extensive degradation of the humic acid polymer and represents only a small fraction of the total molecule. 

A problem for both humic- and yellow substances is that for these groups of experimentally defined components of different sources, each analysis will be ambiguous in terms of relative composition and molecular weight distribution. Additionally it appears that almost every scientist working in this field has developed his own extraction procedure (Weber and Wilson, 1975 Mantoura and Riley, 1975 a Schnitzer, 1976 Stuermer and Harvey, 1977). Different extraction times and -procedures result in different compositions of the organic constituents (Laane and Kramer, 1984). Soil humic-and fulvic acids, often used for studies on the interaction with trace elements, and those derived from water have certainly not the same composition and contain not the same distribution of functional groups. Therefore, results should be compared with care (Buffle, 1980 Buffle et al., 1984). 

Chemical differences between these fractions of DOM are apparent, but there is considerable compositional overlap as well. Extraction of humic substances or ultrafiltration removes most of the colored DOM from water samples and most of the dissolved lignin (Ertel et al., 1986 Opsahl and Benner, 1998). This is consistent with the observation that many humic and fulvic acids have molecular masses greater than the cutoff (1000 Da) of the membranes used for DOM isolation by ultrafiltration (Thurman, 1985). 

Humic acid is composed of aromatic, aliphatic and carbohydrate carbon compounds. An average humic acid s elemental composition is 55.1% C, 5.0% H, 3.5% N, 35.6% O, and 1.8% S (Rice and MacCarthy, 1991). Its molecular weight distribution is typically broad, and it is a relatively high-molecular-weight material relative to the fulvic acid isolated from the same soil or sediment. It s predominantly functionalized by carboxylic acid and phenolic groups. At least some components of humic acid are surface-active, and these components have been shown to form micelles in concentrated, alkaline aqueous solutions (Piret et al., 1960 Visser, 1964 Wershaw et al., 1969 Tschapek and Wasowski, 1976 Chen et al., 1978 Rochus and Sipos, 1978 Hayano et al., 1982 Hayase and Tsubota, 1984 Guetzloff and Rice, 1994). 

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