Humic substances complexing sites

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.)...

Typical values of pK[nt and pfor a humic acid are 2.67 and 4.46. The introduction of the electrostatic factor into the equilibrium constant is analogous to the coulombic term used in the definition of the intrinsic surface complexation constants. In addition another binding site (WAH) is recognised which is thought to behave as a weak acidic phenolic functional group. Although this site does not contribute to the titratable acidity and, therefore, no pK is needed for proton dissociation, it is involved in metal complexation reactions. The total number of the three monoprotic sites is estimated from titratable acidity and then paired to represent the humic substance as a discrete non-interacting mixture of three dipro-tic acids, which act as the metal complexation sites. The three sites are... 

Tipping, E., Backes, C.A. and Hurley, M.A. (1988) The complexation of protons, aluminium and calcium by aquatic humic substances a model incorporating binding-site heterogeneity and macroionic effects. Water Res., 22(5), 597-611. 

Schnitzer, M. and Khan, S. "Humic Substances in the Environment," 327 p. Marcel Dekker, New York, 1972. Broadbent, F. and Lewis, T. Soil organic matter-metal Complexes 4. Nature and properties of exchange sites. Soil Sci. 91, 393-399 (1961). 

The first involves a known functional group derivatization to quantify specific types of carbons found in humic substances, the derivatization being carried out by chemical reactions with l C-labeled reactants.( d-27) For example, methylation with C-labeled diazomethane or methyl iodide has been used to distinguish between and quantify hydroxyl functionalities in humic acids. The second labeling methodology involves the use of C-labeled reactants to follow the course of a complex reaction or association such as the interaction of pollutants with humic acids.(6-9,22,2J) The first structural evidence for the type of interaction of pollutants with humic substances was provided using and site specific labeling in combination with and NMR, respectively. 

The single site model is used to describe the interactions between the cation and the humic acids (11). Since humic substances are heterogeneous and complex molecules, the ligand is defined as a monodentate site (A) with no particular assumption on its chemical nature. Complexes of 1 1 (metalication) stoichiometry are assumed to be formed according to the equilibrium ... 

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