Humic substances molecular structure

Frimmel, F. H., Lankes, U., Ltidemann, H.-D., and Muller, M. B. (2005). Structural characterization of humic substances from waste water. In Humic Substances—Molecular Details and Applications in Land and Water Conservation, Ghabbour, E. A., and Davies, G., eds., Taylor and Francis, New York, pp. 3-22. 

Perminova I, Grechishcheva N, Peterosyan V (1999) Relationships between structure and binding affinity of humic substances for polycyclic aromatic hydrocarbons relevance of molecular descriptors. Environ Sci Technol 33 3781-3787... 

The basic structure of humic substances involves a backbone composed of alkyl or aromatic units crosslinked mainly by oxygen and nitrogen groups. Major functional groups attached to the backbone are carboxylic acids, phenolic hydroxyls, alcoholic hydroxyls, ketones, and quinones. The molecular structure is variable as it is dependent on the collection of DOM available in seawater to undergo the various polymerization, condensation, and oxidation reactions and reaction conditions involved in humification, as well as the ambient physicochemical reaction conditions, such as temperature and light availability. 

Humic substances High-molecular-weight organic compounds that are variable in composiUon, have complex structures, and are relaUvely inert. They comprise a large fracUon of the DOM. Found in sods, sediment, fresh, and seawater. 

A reevaluation of molecular structure of humic substances based on data obtained primarily from nuclear magnetic resonance spectroscopy, X-ray absorption near-edge structure spectroscopy, electrospray ionization-mass spectrometry, and pyrolysis studies was presented by Sutton and Sposito (2005). The authors consider that humic substances are collections of diverse, relatively low molecular mass components forming dynamic associations stabilized by hydrophobic interactions and hydrogen bonds. These associations are capable of organizing into micellar structures in suitable aqueous environments. Humic components display contrasting molecular motional behavior and may be spatially segregated on a scale of nanometers. Within this new structural context, these components comprise any molecules... 

Stumm W, Morgan JJ (1995) Aquatic chemistry, 3rd edn. WUey, New York Sutton R, Sposito G (2005) Molecular structure in soil humic substances The new view. Environ Sci Technol 39 9009-9015... 

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. 

Duxbury, J. M. (1989). Studies of the molecular size and charge of humic stubstances by electrophoresis. In Humic Substances II In Search of Structure, Hayes, M. H. B., MacCar-thy, R, Malcolm, R. L., and Swift, R. S., eds., John Wiley Sons, Chichester, pp. 593-620. 

Sutton, R., and Sposito, G. (2005). Molecular structure in soil humic substances the new view. Environ. Sci. Technol. 39, 9009-9014. 

Three different viewpoints on the humic substances structural conformation are actually reported in the literature. One suggests that HS are macromolecular and assume random coil conformations in solution (Swift, 1999) a second proposes that HS are molecular associations of relatively small molecules held together by weak interaction forces, thus forming a supramolecular structure (Piccolo and Conte, 1999) a third considers that HS are in solution as micelles or pseudomicellar structures (Wershaw, 1999). Viewpoints two and three could be broadly considered to be under the same umbrella (Clapp and Hayes, 1999). 

More recently, Muscolo et al. (2007b), in order to understand if the biological activity of humic substances may be related to their molecular weight and/or chemical structure, compared the activity of two humic substances derived from an uncultivated couch grass and a forest soil, each separated in fractions with low (<3500 Da)... 

Hertkorn, N. (2006). Molecular level structural analysis of natural organic matter and of humic substances by NMR spectroscopy. Habilitation thesis, Technical University of Munich, Munich, Germany, 285 pp. 

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