Humic acid complexing sites

Dialysis has been used to study the interaction between radionuclides and humic acid. The technique gives information on 1) the complexing capacity of the humic acid samples, i.e. the concentration of complexing sites, and 2) the radionuclide - humic acid complexes by a) its stoichiometry (i.e. metal ion/ligand ratio), and b) interaction constants. 

Abate G, Masini JC. Complexation of Cd(II) and Pb(II) with humic acids studies by anodic stripping voltammetry using differential equilibrium functions and discrete site models. Org. Geochem. 2002 33 1171-1182. 

Tables 7.6 and 7.7 compare the binding affinities of sites on the a-Al203 (0001), a-Al203 (1-102), and a-Fe203 (0001) surfaces and those in various types of bacteria, natural organic matter, and polyacrylic acid (PAA). The metal oxide substrates and humic acid are characterized by two binding sites, one strong and one weak, for Pb(II). The stability constant for the PAA-Pb(II) complex (i.e., ML2 species) is typically larger than those of higher affinity mineral surface binding sites (i.e., Mj).

Quantification of metal ion binding and the determination of the nature of the binding sites are extremely difficult for humic acid because of its polymeric (molar masses ca. 5 x 104) and heterogeneous nature. The results of such attempts are often contradictory.15,16 Frequently a 1 1 metal humic acid ratio is assumed for the complexes, a very surprising assumption for such large molecules with high concentrations of carboxylic acids. There is evidence for other stoichiometries, including... 

Dialyses are normally carried out with four different humic acid concentrations, each of which is combined with seven different metal ion concentrations (up to 2 x 10 5 mol/L) For the determination of the complexing capacity a further series of three metal ion concentrations (up to ca. 10"1 mol/L) was added in the case of the lowest humic acid concentration, in order to occupy all sites available ... 

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... 

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