Humic substances complexants

Senesi, N. (1992a). Metal-humic substance complexes in the environment. Molecular and mechanistic aspects by multiple spectroscopic approach. In Bio geochemistry of Trace Metals, Adriano, D. C., ed., Lewis Publishers, Boca Raton, FL, pp. 425-491. 

On the other hand, at a pH value of 5.0 there were no large differences in the strength of bonds between humic acids and metals such as Ca, Mg, Mn, Co, Ni, and Zn, whereas bonds with Pb, Cu, and Fe were stronger than with other metals (Schnitzer and Kahn, 1972) this behavior indicates that at different pH values, metal humic substance complexes of different stability are formed in the soil. This aspect is of particular relevance in an environment such as the rhizosphere, where dynamic pH gradients are present mainly due to the availability of nutrients and to their selective uptake by roots. With regard to plant availability, great importance lies in the molecular dimension and solubility of humic substances (Briimmer and Herms,... 

Bolea, E., Bouby, M., Laborda, F., Castillo, J. R., and Geckeis, H. (2006). Multielement characterization of metal-humic substances complexation by size exclusion chromatography, asymmetrical flow field-flow fractionation, ultrafiltration and inductively coupled plasma-mass spectrometry detection A comparative approach. J. Chromatogr. A 1129, 236-246. 

Perminova, I. V. (1999). Size exclusion chromatography of humic substances complexities of data interpretation. Soil Sci. 164, 834-840. 

Martin-Neto, L., Nascimento, O. R., Talamoni, J., and Poppi, N. R. (1991). EPR of micronu-trients-humic substance complexes extracted from Brazilian soil. Soil Sci. 151, 369-376. 

Humic substances complex assemblages of molecules that have a yellow-to-brown color and are derived from plants and soils. 

Figure 3.19 Schematic presentation of metal-humic substances complexation (adapted and modified from Stevenson, 1982, with permission). 1, Electrostatic interaction 2, inner-sphere complexation 3, weak water bridging.

Mn in estuarine or seawater-humic substance systems might be partly attributed to the formation of the Mn-humic substance complex (besides positively charged hydrolytic products of Mn(III) which are present to some extent in the estuarine or diluted seawater without the addition of humic substances). 

For trivalent metals the influence of humic substances present in diluted seawater is shown by the formation of negatively charged species, which might be attributed to metal-humic substance complexes (especially pronounced for fulvic acid). The presence of humic substances, even at lower concentrations, solubilizes iron. This is partly evident by the formation of the cationic species, which are not found in estuarine or seawater unless humic substances are added. These effects might also be attributed to the stabilization of iron colloids in water rich in humic substances. 

Figure 4.7. XANES spectra (a) and their first derivatives (b) of Cu"+ -humic substance complexes. (From Xia et al., 1997a.)...

Figure 4.8. Experimental radial stmcture function (RSF) for Cu -humic substance complexes at pH 4, 5, and 6 (dots) and EEEE simulations (solid line) for an adjusted model of the coordination site derived from bond network analysis. The inset shows plots of experimental (dots) and fitted (solid line) inverse Fourier-transformed scattering curves for the first atomic shell (Cu-O) and second atomic shell (Cu-C). (From Xia et al., 1997a.)...

Now consider the situation in the presence of humic substances that can complex Fe(III). The total amount of dissolved Fe(III) must be greater than in Jhe absence of humic substances because, in addition to the Fe(III) in equilibrium with Fe(OH)3[s), we have in solution the Fe(III)-humic substance complexes. Let us also assume that the Fe III)-humic substance complexes are labile—that is, they will dissociate rapidly to release Fe(III). In this situation we have not only seen an increase in the total dissolved iron but also an increase in the pool of readily available Fe(III). Observations made by Schnitzer discussed previously have indicated... 

Navratilova Z, Kula P (1993) Modified carbon paste electrodes for the study of metal-humic substances complexation. Anal Chim Acta 273 305-311... 

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