Origin of humic substances

This chapter discusses and reviews the amount of organic carbon and humic substances in groundwater and the characterization and origin of humic substances in groundwater. Because the concentration of organic carbon in groundwater is commonly less than 1 mg carbon per liter, it is difficult to isolate and characterize humic substances, and there are relatively few studies of these substances in groundwater. 

From the data and conclusions presented in the characterization section, it is obvious that humic substances in groundwater are different from humic substances in surface water. At least two hypotheses may be proposed for the origin of humic substances in groundwater. 

POSSIBLE ORIGINS OF HUMIC SUBSTANCES FROM MARINE... 

Synthetic humic acids have been prepared by a number of investigators over the past 50 years. Most of these preparations are based on a preference for a specific hypothesis for the origin of humic substances (1) partial degradation of lignin (2) chemical polymerization of microbial phenols or lignin monomers and amino acids (3) cell autolysis products (4) microbial synthesis (see Stevenson, 1982, Chapter 8 for review). 

The isotopic ratio provides information on the origin of humic substances. During biochemical and chemical reactions, the isotopes of a given element undergo fractionation, which depends not only on their mass, but also on the nature of the reaction. Isotopic ratios, in particular depend on the organic... 

Table 1. Origin and nature of humic substances used with corresponding codes, abbreviations and concentrations...

Origin and nature of humic substances IHSS Code Abbreviation Concentrations used (mgL-1)... 

In natural water, singlet oxygen originating from humic substances has been shown, for example, to oxidize thioether pesticide contaminants such as disulfoton (Zepp et al. 1981). Irradiation of dilute hydrogen peroxide in the presence of various non-sunlight-absorbing herbicides results in enhanced oxidation of these substances (Draper and Crosby 1981). 

Hayes, M. H. B. (1991). Concept of the origin, composition, and structure of humic substances. In Advances in Soil Organic Matter Research The Impact on Agriculture and the Environment, Wilson, W. S., ed., Royal Society of Chemistry, Cambridge, UK, pp. 3-22. 

Various separation methods have been used to isolate, fractionate, and characterize humic materials. Originally it was fractionation, based on solubility differences of humic components in diluted alkalis and acids, which laid the ground work for the first classifications of humic substances (HS) in the 19th century (Mulder, 1861 Sprengel, 1837) and provided for operational definition of HS (Kononova, 1966). And now, alkali extraction is the method of choice for isolating HS from solid humus-containing substrates like soil, peat, coal, and so on (Swift, 1996), while hydrophobic resins (e.g., Amberlite XAD resins) are typically used to extract HS dissolved in natural waters (Aiken, 1985). Initial research on HS began with the used simple separation methods to prove, examine, and define characteristics of components of humic matter (Oden, 1919).Today, however, advances in HS research require ever more sophisticated techniques of separation combined with structural analysis (Orlov, 1990 Stevenson, 1994). 

Uyguner, C. S., and Bekbolet, M. (2005a). A comparative study on the photocatalytic degradation of humic substances of various origins. Desalination 176,167-176. 

Schulten H.-R. and Gleixner G. (1999) Analytical pyrolysis of humic substances and dissolved organic matter in aquatic systems structure and origin. Water Res. 33, 2489-2498. 

The purposes of this paper are then (a) to refer on some additional results obtained, in order to confirm the previously suggested [9, 10 ] involvement of humic free radicals in the cross coupling reactions of humic substances with xenobiotic phenoxy radicals and (b) to discuss comparatively the different behaviours of humic acids in these reactions, as a function of their different origin and chemical properties. 

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