Application of the NICA-Donnan model for proton, copper and uranyl binding to humic acid

Summary Humic acids are natural organic materials that play an important role in the migration of heavy metal and actinide ions in aquatic and soil systems. In the present study, the binding of protons, copper ions and uranyl ions to the purified Aldrich humic acid (PAHA) is investigated and the results are modeled with the Non-Ideal Competitive Adsorption (NICA) model extended with electrostatic interactions according to the Donnan model (NICA-Donnan model). The NICA part of the model enables one to describe the competitive ion binding to a heterogeneous substrate taking into account a different stoichiometry per ion. The NICA-Donnan model can describe the binding of the ions to PAHA in large concentration ranges (3<pH<11; 3<pCu<13; 4<pUO 2 <18) and over a wide range of environmental conditions (pH, ionic strength). From the model description it follows that the affinity distribution of PAHA is bimodal with about equal numbers of "carboxylic"- (low log K H ) and "phenolic"-type (high log K H ) sites. The stoichiometry parameters of the binding reactions for copper and uranyl indicate both mono- and bi-dentate binding. Some specific attention is paid to the distribution of the bound uranyl ions over the "carboxylic" and "phenolic"-type sites. Such information is useful for comparison with spectroscopic information on the binding.