Simplified modelling of the complexation of humic substance for equilibrium calculations

For the prediction of chemical species distribution in systems containing humic substances (HS), a simplified complexation model was proposed in our previous study. In the model, the logarithm of the apparent complexation constant is described using the equation log K^c_app-log K_c + a_c log α-b_c log I-m_c log [M] consisting of four HS specific parameters, log K_c, a_c, b_c, and m_c, which are the minimized number of parameters to reproduce the complexation as a function of α (degree of dissociation of functional groups of HS), I (ionic strength of bulk solution), and [M] (free metal ion concentration). In this study, the applicability of this model was demonstrated by applying the equation to the complexation data of Ca(II) obtained by potentiometric titration in a wide range of pcH (=-log [H ⁺]), Ca(II) concentration, and ionic strength, as well as to the complexation data of Al(III), Ca(II), Cu(II), Pb(II), Am(III), Cd(II), Cm(III), Co(II), Dy(III), Eu(III), and U(VI) reported in the literature. The proposed equation fairly well reproduced a variety of complexation data with the minimum specific parameters, indicating the suitability of the proposed model for the purpose of equilibrium calculation of metal ion species in the system containing HS as a soluble ligand.