Humic acid complexing capacity

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. 

Recently, Weber and co-workers reported on the application of dialysis to the determination of the complexing capacity of fulvic acid for a series of metal cations (19, 20). In the present paper we report a further elaboration on the dialysis technique to allow simultaneous determination of complexing capacity for the humic acid sample, and interaction constants for the metal ion - humic acid complexes, as well as the stoichiometry of the latter. The technique furthermore opens up a possibility for an indirect determination of apparent ionization constants for the humic acids. 

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 ... 

The present paper has demonstrated the versatility of the dialysis technique in studies of interactions between metal ions and humic acid samples. The method allows facile determination of interaction constants, as well as of complexing capacities of the humic acid samples. It is noteworthy that the method described here, without modifications, can be applied to other areas of complex chemistry involving macromolecular ligands. 

Itabashi, H., Kawamoto, H. and Akaiwa, H. (1997) Contribution of humic acid to the cop-per(II) complexing capacity of natural waters. Anal. Sci., 13, 411-412. 

The importance of complex formation is exemplified by the binding of Mg, Na, Co, Mn, Fe, Cu, and Zn to fulvic acids (FA), and of Fe to humic acids. The binding capacity of these natural acids for metal ions is within the range of 0.2-0.6 mmol/g, and the order of stability of complex formation (M-FA) with some key metals is Fe2+ > Al3+ > Cu2+ > Ni2+ > Ca2+ > Zn2+ > Mn2+ > Mg2+(see Schnitzer, 1970). Interestingly, some cryptogams (i.e., mosses and lichens) capture part of their essential minerals by secreting... 

Organic Ligands. Investigations of organic compounds dissolved in surface waters have largely dealt with identification of specific compounds or groups (e.g., phenols, polyaromatic hydrocarbons, etc.), determination of complexation capacity, or separation—with variable types of characterization—into fulvic-and humic-acid fractions. The information developed in these studies has not been shown to be particularly useful to quantitative chemical modeling. The analyses of specific compounds or... 

Table L Values of conditional interaction constants P (in l/mol) of tri-, penta- and hexavalent actinides with Aldrich humic acids and maximum complexing capacities (W in mmol/g)).

Fulvic acids are organic substances which remain in the solution after acid precipitation of humic and hymatomelanic acids from the extract of humic acids. The chemical structure and physico-chemical properties of these substances are similar to those in humic acids. The aromatic character of the molecule is less distinct, and side chains predominate. They also differ from humic acids in their lower molecular weight, yellow to light-brown colour, better solubility in water and easier hydrolysis in acid media. They are easily eluted from soils and peat. They are more acid than humic acids (pH of solution with c = 0.01 mol 1 is about 2.6). They manifest complex-forming properties and exchange capacity. 

Humic acids are soluble in weak alkaline solutions and are essentially insoluble in water and mineral acids. They may be precipitated from solution by the action of mineral acids and bivalent or trivalent cations, however, they are fairly resistant to the acid hydrolysis. They are dark spherocol-loids with a cross-linked structure which plays a part in their high sorption capacity. They exhibit different degrees of a tendency to aggregation and very different degrees of dispersion. In comparison with other types of natural organic substances, the humic acids are characterized by their extraordinary stability in the soil. This stability is due to their ability to form organomineraJ complexes, particularly with clay minerals and with aluminium and iron hydroxides. 

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