Generic NICA−Donnan Model Parameters for Metal-Ion Binding by Humic Substances

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Abstract
A total of 171 datasets of literature and experimental data for metal-ion binding by fulvic and humic acids have been digitized and re-analyzed using the NICA−Donnan model. Generic parameter values have been derived that can be used for modeling in the absence of specific metal-ion binding measurements. These values complement the previously derived generic descriptions of proton binding. For ions where the ranges of pH, concentration, and ionic strength conditions are well covered by the available data, the generic parameters successfully describe the metal-ion binding behavior across a very wide range of conditions and for different humic and fulvic acids. Where published data for other metal ions are too sparse to constrain the model well, generic parameters have been estimated by interpolating trends observable in the parameter values of the well-defined data. Recommended generic NICA−Donnan model parameters are provided for 23 metal ions (Al, Am, Ba, Ca, Cd, Cm, Co, CrIII, Cu, Dy, Eu, FeII, FeIII, Hg, Mg, Mn, Ni, Pb, Sr, ThIV, UVIO2, VIIIO, and Zn) for both fulvic and humic acids. These parameters probably represent the best NICA−Donnan description of metal-ion binding that can be achieved using existing data.