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A Comparative Evaluation of Metal Humic and Fulvic Acid Predictive Models

Research into the safe disposal of nuclear waste prompted a comparative study of the predictive capabilities and limitations of metal humic and fulvic acid binding models. Three approaches were investigated: (i) A simple ligand mixture approach; (ii) a continuous distribution approach; and (iii) an electrostatic discrete site approach. Appropriate geochemical equilibrium codes, incorporating the models designated, PHREEQESP, MINTEQA2 and PHREEQEV, were applied to three unrelated test problems, (1) Ni interactions with fulvic acid. (2) Eu interactions with fulvic and humic acids and (3) actinide speciation in Mol groundwater. The model predictions were compared with experimental data. Only PHREEQEV provided results in all three test cases. Overall it was concluded that of the approaches tested PHREEQEV currently offers the most advantages for predictive modelling.