Purpose : Humin, which is the insoluble part of humic substances, plays an important role in regulating the behavior and fate of metallic trace elements at the soil/water interface. However, compared to the two other fractions of humic substances, humic and fulvic acids, humin has been the least studied due to its insolubility and difficulties in purification. Materials and methods : In this study, humin was extracted from a peat soil following the International Humic Substances Society (IHSS) protocol. The adsorption behavior of copper(II) and lead(II) onto humin as a function of pH was studied at different solid charges and metallic cation concentrations and was described by the NICA–Donnan model. The proportion of the two types of functional groups considered in this model, carboxylic and phenolic groups, was determined by cross polarization/magic angle spinning solid-state 13 C nuclear magnetic resonance spectroscopy ( 13 C CP/MAS NMR). Results and discussion : The NICA–Donnan model was applied to describe the adsorption of copper(II) and lead(II) onto humin using generic parameters previously defined for humic acids with only few parameter adjustments. This approach allowed a good description of experimental adsorption data at the different tested conditions with the main involvement of carboxylic moieties in the metallic cation adsorption. Conclusion : This study highlights the reactivity of humin, an important fraction of soil organic matter little studied until now, toward copper(II) and lead(II) and proposes, for the first time, a simple and pertinent approach using the NICA–Donnan model to successfully describe and predict the binding of these two metallic cations onto humin.