Modelling proton and metal binding to humic substances with the NICA-EPN model

The mathematical modelling of metal cation-natural organic matter interactions is a fundamental tool in predicting the state and fate of pollutants in the environment. In this work, the binding of protons and metal cations to humic substances is modelled applying the Elastic Polyelectrolyte Network (EPN) electrostatic model with the Non-Ideal Competitive Adsorption (NICA) isotherm as the intrinsic part (NICA-EPN model). Literature data of proton and metal binding to humic substances at different pH and ionic strength values are analysed, discussing in depth the model predictions. The NICA-EPN model is found to describe well these phenomena. The electrostatic contribution to the Gibbs free energy of adsorbate-humic interaction in the EPN model is lower than that predicted by the Donnan phase model; the intrinsic mean binding constants for protons and metal cations are generally higher, closer to independent estimations and to the range of acid-base and complexation equilibrium values for common carboxylic acids. The results for metal cations are consistent with recent literature findings. The model predicts shrinking of the humic particles with increased metal binding, as a consequence of net charge decrease.