Generic NICA-Donnan Model Parameters for Proton Binding by Humic Substances

Forty-nine datasets consisting of literature and experimental data for proton binding by fulvic and humic acids have been analyzed using the NICA-Donnan model. The model successfully described the behavior of the individual datasets with a high degree of accuracy and highlighted the differences in site density and binding affinity between fulvic acids (FA) and humic acids (HA) while demonstrating their strong similarities. The data have also been used to derive generic model descriptions of proton binding by FA and HA that can be used for modeling in the absence of specific parameter sets for the particular humic substance of interest. These generic parameters can provide estimates of the amount of proton binding by a wide variety of humic substances to within approximately ±20% under any given conditions. The maximum site density for protons was 7.74 and 5.70 equiv kg-1 for a generic FA and HA, respectively. The recommended generic NICA-Donnan parameter values for FA are b = 0.57, Q max1,H = 5.88, log K̃ H1 = 2.34, m H1 = 0.38, Q max2,H = 1.86, log K̃ H2 = 8.6, and m H2 = 0.53; for HA the values are b = 0.49, Q max1,H = 3.15, log K̃ H1 = 2.93, m H1 = 0.50, Q max2,H = 2.55, log K̃ H2 = 8.0, and m H2 = 0.26.