Rationale: Understanding REE speciation in different natural environments is important to evaluate their environmental risks since different chemical species of an element may have different bioavailability and toxicity. REEs have a great affinity for particulate- and dissolved organic matter, particularly fulvic and humic acids (HA). Thus, the use of humic ion-binding models may help to understand and predict the behaviour and speciation of these species in surface waters, groundwaters, and soils.Methodology: In this work, we used previously published experimental datasets to parametrize the NICA-Donnan (ND) model for REEs binding with HA, using the model optimization tool PEST-ORCHESTRA. We propose to use linear-free energy relationships (LFER) to constrain the number of parameters to optimize.Results: We determined a coherent ND parameter set for the whole REEs series being compatible with available generic ND parameters for other metals. The impact of pH, ionic strength, REE/HA ratio as well as the presence of competitors (Fe3+, Al3+ and Cu2+) on model results is analysed.Discussion: We consolidate the confidence in our derived ND parameters for REE by comparing with the Irving-Rossotti LFER. We also showed the general applicability of this relationship to predict and constrain metal-binding parameters for the NICA-Donnan model. We discuss observed shortcomings and come with suggestions for potential improvement of NICA-Donnan modelling.< Réduire