Complexing properties, humic

Humic substances can form complexes with metals, including cationic micronutrients (36), thanks to the presence of electron-donor functional groups in these molecules. It therefore appears evident that due to these properties, humic substances can contribute to the regulation of the chemical balances of metals, thus influencing their solubility (5). With regard to plant availability, the molecular dimension and solubility of humic substances are very important. 

Buffle, 3., Greter, F.L. and Haerdi, W., 1977. Measurement of complexation properties of humic and fulvic acids in natural waters with lead and copper ion selective electrodes. Anal. Chem., 49 216-222. 

Tipping, E., and M. A. Hurley. 1988. A model of solid—solution interactions in acidic organic soils, based on the complexation properties of humic substances. Journal of Soil Science 39 505-519. 

Complexing properties of humic substances can also have a great importance in P nutrition (Stevenson, 1991). Indeed Fe3+ and Al3+ (acidic soils) and Ca2+ (calcareous soils) complexed by humic molecules can bound phosphates to humic substances thus making P in a plant-available form (Gerke and Hermann, 1992). Complexation of cations by carboxylates such as those released by plant roots (e.g., citrate) can increase phosphate availability (Gerke, 1993). 

Lund, W., Helbak, I.A. and Seip, H.M. (1990) Studies on the complexation properties of aquatic humic material by differential pulse polarography. Sci. Total Environ., 92, 269-281. 

Nearly every method available to the analytical chemist has been used in an attempt to unravel the complex properties and behavior of humic substances. Some of the more widely used methods are listed in Table I. 

Although solution-state NMR has revealed much regarding the chemical structural composition of humic substances and dissolved organic matter (5), the development of more sophisticated software, pulse sequences, probes in recent years have not been exploited much in the field of soil science. Most of the novel applications have strictly relied on conventional one-dimensional spectroscopy. Two-dimensional NMR methods have become the mainstay of the chemical research field, especially for the structural elucidation of complex soluble biopolymers. The complexity of humic substances along with other inherent physical properties such as limited solubility and the presence of paramagnetics... 

Francko and Heath (1979) studied the influence of UV radiation on the phosphate complexation properties of dissolved humic substances from Crazy Eddie Bog in the central United States. In this bog, filterable phosphorus compounds were largely associated with humic substances of high apparent molecular weight. These fulvic acid-phosphorus associations resisted hydrolysis by alkaline phosphatase, but released orthophosphate upon irradiation with low doses of UV radiation, " e turnover time of the fulvic acid-phosphorus compounds was calculated to be less than 1 hour at the surface of a lake on a cloudless day. 

Gustafsson, J.P, Modehng the acid-base properties and metal complexation of humic substances with the Stockholm humic model, J. Colloid Interf. Sci., 244,102, 2001. 

This expression is used to characterize the complexation properties of macroligands such as humic acids, proteins, etc. (see for instance [15-17]). There are many formalized models of complexation [18-20] in which expressions for the constant of formation considering the different forms of complexes are suggested. However, they are inconvenient for practical use and require a multiplex calculation procedure. 

The evaluation of interactions between trace metals and humic substances has received considerable attention. Evidence for strong complexation has been demonstrated under controlled experimental conditions. Howeveq the complexity of fulvic and hiunic compounds is often at odds with the amoiuit of information contained in the experimental data, thus making it very difficult to obtain a rigorous thermodynamic description of their complexing properties. 

Clay minerals, oxides, and humic substances are the major natural subsurface adsorbents of contaminants. Under natural conditions, when humic substances are present, humate-mineral complexes are formed with surface properties different from those of their constituents. Natural clays may serve also as a basic material for engineering novel organo-clay products with an increased adsorption capacity, which can be used for various reclamation purposes. 

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