Comparison of two models for metal-humic interactions

Humic acid has been shown to play a significant role in the environmental behavior of many metal cations as it can function as both a complexant and a redox agent. A number of models have been proposed to explain their complexing role, but most use quite different chemical descriptions of the metal-humate interactions. Two of these models which have been applied to humic acid interaction with actinide cations are briefly discussed In one model in which humics are treated as anionic polyelectrolytes, cations can bind to specific anionic donor sites (site binding) as well as be attracted by the net anionic charge of the macromolecules (the Polyelectrolyte Model). In the second model (the Charge Neutralization Model), the binding for each cation is assumed to be associated with a number of carboxylate groups equal to the cationic charge. It is concluded that the Charge Neutralization Model is more useful in geochemical calculation codes, whereas the Polyelectrolyte Model can provide more insight into the chemical behavior of the humic acids.